Numerical analysis of the hydrogen combustion in a double cavity Trapped Vortex Combustor

Transcription

1 Scuola Dottorale d Ingegnera sezone d Ingegnera Meccanca e Industrale XXI Cclo Numercal analyss of the hydrogen combuston n a double cavty Trapped Vortex Combustor PhD thess by Alessandro D Marco Advsor: Prof. R. Camuss PhD Coordnator: Prof. E. Bemporad Degree of Doctor of Phlosophy March 2009

2 In Memory of Prof. G. Guj (R.I.P.) 2

3 Contents Abstract... 5 Lst of symbols Introducton Background and motvatons Structure of the thess Theoretcal Background Revew of some property relatons Ideal-Gas Mxtures Stochometry Hydrogen propertes Prncples of combuston Conservaton equatons for multcomponent reactng systems Dmensonless governng parameters Basc flame types Premxed Flames Non-premxed flames Chemcal knetcs and reacton mechansms Rates of reactons Hydrogen Chemcal Mechansm Oxdes of ntrogen formaton Mld Combuston Trapped Vortex Combustor TVC concept TVC Development Ar Force Research Laboratory General Electrc Company DOE Natonal Energy Technology Laboratory Ramgen Power Systems (RPS) ALM Turbnes ENEA TVC Geometry Numercal Combuston Modelzaton Computatonal doman and mesh propertes Numercal model Realzable k-ε Radaton model Combuston Model Gas Mxture Propertes Boundary condtons Results Test Matrx Flow feld and flame Effect of mosture Effect of the prmary and secondary equvalence rato

4 5.5 Combuston nstabltes Conclusons and future perspectves References Appendx

5 Abstract In the present study numercal smulatons are performed to characterze the hydrogen combuston n a double cavty Trapped-Vortex Combustor (TVC). Ths combustor utlzes two trapped vortces n two cavtes to mprove flame stablty and to provde low pressure drop. Good performances characterstcs are obtaned njectng a suffcent amount of fuel and ar drectly nto the frst cavty. The two cavtes are obtaned mountng three axsymmetrc dsks on a tube passng through ther centrelnes. The geometry and the confguraton of ths TVC are very smlar to that studed by Hsu et al and refer to a faclty n the Casacca (Rome) research center of the Italan Natonal Agency for New Technologes, Energy and the Envronment (ENEA). The numercal studes were made usng a commercal 3-D CFD code. A turbulent steady-state model wth fnte rate chemstry and second-order accuracy s used to smulate the TVC flowfelds. The turbulence-chemstry nteracton s provded by the Eddy Dsspaton Concept (EDC). Ths model allows the ncluson of detaled chemcal mechansms n turbulent flows. In the present analyss of reactng flow the chemcal knetc model needed to smulate the hydrogen combuston conssts of 37 reactons nvolvng 14 speces. In order to evaluate the thermo flud dynamcs n the TVC a parametrc study has been conducted. Varable parameters nclude the length of the frst cavty, the power, the equvalence rato, the humdty of the ar and the nlet ar composton and temperature. Results from the analyss provde valuable nformaton on the flow and flame structure and on the combuston process demonstratng the versatlty and effcency of burnng hydrogen n a double cavty TVC. 5

6 Lst of symbols Upper Case: Symbol Descrpton Unts A Area [m 2 ]; C Molar concentraton [mol/m 3 ]; C p Constant-pressure specfc heat [J/(kg K)]; C v Constant-volume specfc heat [J/(kg K)]; D Dffusvty [m 2 /s]; Da Damkoelhler Number [-]; Ea Actvaton Energy [kj/kg]; F Force [N]; Fr Froude Number [-]; Gr Grashof Number [-]; H Enthalpy or dsk dstance [J] or [m]; HHV Hgher Heatng Value [J/kg]; Ka Karlovtz Number [-]; J Dffusve heat flux [kg/(s m 2 )]; L-J Lennard-Jones coeffcent [-]; LHV Lower Heatng Value [J/kg]; Le Lews Number [-]; M Number of chemcal reactons [-]; MW Molecular Weght [g/mol]; Ma Mach Number [-]; N Number of chemcal speces [-]; Pr Prandtl Number [-]; Ru Unversal Gas Constant [kj/(kg K)]; Re Reynolds Number [-]; R Rchardson Number [-]; S Burnng velocty [m/s]; Sc Schmdt Number [-]; T Temperature [K]; V Volume [m 3 ]; X Mole Fracton [-]; Y Mass fracton [-]; Lower Case: Symbol Descrpton Unts d Dameter [m]; e energy per unt mass [kj/kg]; f Frequency or mxture fracton [Hz] or [-]; g Gravtatonal acceleraton [m/s 2 ]; k Rate coeffcent of reacton [-]; l length [m]; m Mass [kg]; 6

7 t Tme [s]; p Pressure [Pa]; q Heat Flux [W/m 2 ], r,θ,z Cylndrcal coordnate system [m, or rad,m]; u velocty component n x- or axal drecton [m/s]; v velocty component y- or radal drecton [m/s]; w velocty component z-drecton [m/s]; x,y,z Cartesan coordnate system [m]; Greek symbols: Symbol Descrpton Unts α Thermal dffusvty [m 2 /s]; δ Thckness [m]; ε Dsspaton rate of turbulent knetc energy [m/s 3 ]; Φ Equvalence rato [-]; κ Turbulent knetc energy [m 2 /s 2 ]; λ Thermal conductvty [J/(s m K)]; μ Dynamc vscosty [N s/m 2 ]; ν Knematc vscosty [m 2 /s]; ρ Densty [kg/m 3 ]; σ Normal stress [N/m 2 ]; τ Shear stress [N/m 2 ]; ω Mass rate of producton of speces [kg/(s m 3 )]; Ω Vortcty [rad/s]; Acronyms: Symbol Descrpton AFRL APU ATS AVC CEC DLN DOE EDC GE ICAO IGCC LES LBO LECTR NETL PSR RPS Ar Force Research Laboratory; Auxlary Power Unt; Advanced Turbne System; Advanced Vortex Combuston; Calforna Energy Commsson; Dry Lean NOx; Department Of Energy; Eddy Dsspaton Concept; General Electrc; Internatonal Cvl Avaton Organzaton; Integrated Gasfcaton Combned Cycle; Large Eddy Smulaton; Lean Blow Out; Low Emsson Combuston Test and Research; Natonal Energy Technology Laboratory; Perfect strred Reactor; Ramgen Power Systems; 7

8 RQL Rch-burn, Quck-mx/quench, Lean burn; SERDP Strategc Envronmental Research and Development Program; TVC Trapped Vortex Combustor; WSGGM Weghted Sum of Gray Gases Model. 8

9 1 Introducton 1.1 Background and motvatons Combuston s a subject of great relevance from the scentfc and technologcal pont of vew. The scentfc nterest s assocated wth the varety of physcal processes whch are encountered and wth the nherent mult-dscplnary character of the subject, whch nvolves Flud Mechancs, Chemstry, Thermodynamcs and the envronmental scence. The technologcal relevance s nstead assocated wth the vast spectrum of ndustral and engneerng applcatons n whch combuston s used. In almost all cases, chemcal reactons,.e. the combuston process, through whch heat s produced, develop wthn a turbulent flow feld. The characterstcs and the evoluton of the combuston process s strctly dependent on the property of the turbulent feld and on the way fuel and oxdzer are mxed. The state of the mxedness of the reactants dvdes the combuston, and the flames, n two classes: premxed and non-premxed (or dffusve). In a premxed combuston fuel and oxdzer are mxed at the molecular level pror to the occurrence of any sgnfcant chemcal reacton. Contrarly, n a dffuson combuston, the reactants are ntally separated, and reacton occurs only at the nterface between the fuel and the oxdzer, where mxng and reacton both take place. In both classes of flames, dependng on the combuston devce s operatng range, mportant desgn crtera are the avodance of flashback and lftoff, two condtons of nstablty. Flashback occurs when the flame enters and propagates through the burner tube or port wthout quenchng; whle lftoff s the condton where the flame s not attached to the burner tube or port hub, rather, s stablzed at some dstance from the port. A poor stablty of the flame can be obtaned attemptng to burn fuel n lean mxture combuston regmes to reduce the consumpton or the pollutant emssons (furnaces, arcraft engnes, turbo-reactors, etc.). Combuston stablty s often acheved through the use of recrculaton zones to provde a contnuous gnton source whch facltates the mxng of hot combuston products wth the ncomng fuel and ar mxture. Swrl vanes, bluff bodes and rearward facng steps are commonly employed to establsh recrculaton zones for flame stablty. Each method creates a low velocty zone of suffcent resdence tme and turbulence levels such that the combuston process becomes self-sustanng. The challenge, however, s the selecton of a flame stablzer whch ensures both performance (emssons, combustor acoustc and pattern factor) and cost goals are met. As opposed to conventonal combuston systems whch rely on swrl stablzaton, the TVC employs cavtes to stablze the flame and grows from the wealth of lterature on cavty flows [Hsu et al., 1995, Sturgess and Hsu, 1997, Straub et al., 2000, Roquemore et al., 2001]. Much of ths effort examnes the flow feld dynamcs establshed by the cavtes, as demonstrated n arcraft wheel wells, bomb bay doors and other external cavty structures. Cavtes have also been studed as a means of coolng and reducng drag on projectles and for scramjets and waste ncneraton [Gharb and Roshko, 1987]. Very lttle work, however, exsts on studyng cavty flameholders for subsonc flow [Roquemore et al., 2001]. and none at all for lean premxed operaton for potental use n a land based gas turbne engne The actual stablzaton mechansm facltated by the TVC s relatvely smple. A conventonal bluff or fore body s located upstream of a smaller bluff body - commonly referred to as an aft body - at a prescrbed dstance commensurate wth cold flow stablzaton studes [Hsu et al., 1995, Sturgess and Hsu, 1997, Roquemore et al., 2001]. The flow ssung from around the frst bluff body separates as normal, but nstead of developng shear layer nstabltes whch n most crcumstances s the prme mechansm for ntatng blowout, the alternatng array of vortces are convenently trapped 9

10 or locked between the two bodes. The very stable yet more energetc prmary/core flame zone s now very resstant to external flow feld perturbatons, yeldng extended lean and rch blowout lmts relatve to ts smple bluff body counterpart. Due to ts confguraton, the system has greater flame holdng surface area and hence wll facltate a more compact prmary/core flame zone; whch s essental n promotng hgh combuston effcency and reduced emssons. Incorporaton of transverse struts (Roquemore et al., 2001), whch enhance the mxng/nteracton of hot combuston products wth the cooler premxed fuel and ar, further renforces the merts of the TVC as an excellent canddate for a lean-premxed combuston system. Furthermore, snce part of combuston occurs wthn the recrculaton zone, a typcally flameless (Mld or Flox) regme can be acheved. The objectves of ths thess are the evaluaton of the performances and the stablty regmes of a double cavty TVC by means of numercal smulatons. The smulatons are performed wth a commercal code snce ths combustor s used n the ndustral feld. Another am of ths research s to contrbute to mprove the knowledge and understandng of the physcs nvolved n the TVC combuston and cover the lack of results regardng the use of hydrogen as fuel wth ths knd of combustor. 1.2 Structure of the thess The approach taken wrtng the thess s to present and dscuss some of the theoretcal and practcal elements needed to understand the model used and to relate the results obtaned to practcal applcatons. The secton 2 s structured to gve all the flud mechancs and thermochemstry fundamentals useful to a better comprehenson of the arguments treated n the followng sectons. The secton 3 contans a bref revew of the TVC state of the art and development, followed by a descrpton of the TVC geometry taken nto consderaton. Furthermore an outlne of the TVC behavour under certan workng condtons s gven. The numercal combuston modelzaton s descrbed n secton 4. It deals wth a descrpton of the approach used to solve the physcal model needed to analyze the TVC performances. The computatonal doman and the mesh propertes are frst shown and then the man characterstc of the model wth some remnds to the orgnal artcles/books to gather further nformaton are gven. Fnally, n secton 5 and 6 the results of the smulatons and dscusson are presented. The results are ordered followng the phenomenologcal consequences due to the varaton of the varables reported n the test matrx. 10

11 2 Theoretcal Background In ths chapter, some mportant concepts related to combuston processes are examned. A revew of some essental topcs such as the basc propertes of the deal-gas mxtures, the stochometry and the thermo physcal propertes of the fuel (hydrogen) s brefly gven. Successve paragraphs deal wth the conservaton equatons for multcomponent reactng mxtures and relatves dmensonless governng parameters, the study of premxed and non-premxed flames and some basc chemcal knetcs concepts. Then an outlne of the elementary steps nvolved n the H 2 -O 2 chemcal mechansm and n the oxdes of ntrogen formaton s gven. Detals can be found n reference books [e.g. Kuo 1986, Wllams 1985 ] so only the man concepts are summarzed n the followng. 2.1 Revew of some property relatons Ideal-Gas Mxtures Two mportant and useful concepts used to characterze the composton of a mxture are the consttuent mole fractons and mass fractons. Consderng a multcomponent mxture of gases composed of N 1 moles of speces 1, N 2 moles of speces 2, etc.. the mole fracton of speces, X s defned as the fracton of the total number of moles n the system that are speces: N X = = N1 + N N +... N N tot 2-1 Smlarly, the mass fracton of speces, Y, s the amount of mass of speces compared wth the total mxture mass: m Y = = m1 + m m +... m m tot 2-2 Mole fractons and mass fractons are readly converted from one to another usng the molecular weghts of the speces of nterest and of the mxture: X MW Y =, MWmx Y MW mx X = 2-3 MW The partal pressure can be related to the mxture composton and total pressure as: P = X P 2-4 For deal gas mxtures, many mass- (or molar-) specfc mxture propertes are calculated smply as mass (or mole) fracton weghted sums of the ndvdual speces-specfc propertes. For example, mxture enthalpes are calculated as. 11

12 h mx = Y h, h mx = X h Stochometry The stochometrc quantty of oxdzer s just that amount needed to completely burn a quantty of fuel. If more than a stochometrc quantty of oxdzer s suppled, the mxture s sad to be fuel lean, or just lean; whle supplyng less than the stochometrc oxdzer results n a fuel-rch, or rch mxture. The stochometrc oxdzer- (or ar-) fuel rato (mass) s determned by wrtng smple atom balances, assumng that the fuel reacts to form an deal set of products. For a hydrocarbon fuel gven by C x H y, the stochometrc relaton can be expressed by the a global reacton mechansm (see 2.3) as Where a = x + y 4 y C x H y + a( O N2 ) xco2 + H 2O aN It s assumed, n the followng that the smplfed composton for ar s 21% O 2 and 79% N 2 (by volume).e. that for each mole of O 2 n ar, there are 3.76 moles of N 2. The stochometrc ar-fuel rato can be found as A F stoc m = m ar fuel stoc 4.76a MW = 1 MW ar fuel 2-7 Where MW ar and MW fuel are the molecular weghts of the ar and the fuel, respectvely. The equvalence rato, Φ, s commonly used to ndcate quanttatvely whether a fuel-oxdzer mxture s rch, lean, or stochometrc. The equvalence rato s defned as ( A ) F ( A ) ( F ) ( F A ) stoc stoc Φ = = 2-8 F A From the defnton: For fuel-lean condtons, we have 0<Φ<1, For stochometrc condtons, we have Φ=1, And for fuel-rch condtons, we have 1<Φ<. 12

13 2.1.3 Hydrogen propertes Hydrogen s the fuel used n the smulatons. In ths secton ts man thermo physcal propertes are reported. At standard temperature and pressure, hydrogen s a colourless, odourless, non-metallc, tasteless, hghly flammable datomc gas. Dhydrogen (hydrogen gas) s hghly flammable and wll burn at concentratons of 4% or more H 2 n ar. The enthalpy of combuston for hydrogen s -286 kj/mol; t burns accordng to the followng global reacton: ( 286kJ mol) H + O 2H O 572kJ / 2-9 When mxed wth oxygen across a wde range of proportons, hydrogen explodes upon gnton. Hydrogen burns volently n ar. It gntes automatcally at a temperature of 560 C. Pure hydrogenoxygen flames burn n the ultravolet colour range and are nearly nvsble to the naked eye, as llustrated by the fantness of flame from the man Space Shuttle engnes. Another characterstc of hydrogen fres s that the flames tend to ascend rapdly wth the gas n ar causng less damage than hydrocarbon fres. In the followng tables the hydrogen man propertes and a comparson wth other fuels of the gnton and flammablty propertes are reported. Phase Gas Meltng pont [K] Bolng pont [K] Heat of fuson [kj/mol] Heat of vaporzaton [kj/mol] Gas Constat R [kj/kg K] Densty ρ (300 K) [kg/m3] Specfc heat c p (300 K) [kj/kg K] Thermal conductvty k (300 K) [W/m C] Thermal dffusvty α [m2/s] 1.55E-04 Dynamc Vscosty μ [kg/m s] 8.90E-06 Hgher Heatng Value HHV [kj/kg] Lower Heatng value LHV [kj/kg] H 2 Tab. 2-1: Hydrogen Thermo Physcal propertes. H 2 CH 4 C 3 H 8 Mnmum gnton energy [mj] Ignton temperature [K] Adabatc flame temperature [K] Lmts of flammablty (% by volume n Ar) Maxmum lamnar flame velocty [cm/s] Dffusvty [cm 2 /s] Mnmum quenchng dstance at 1 atm [cm] Normalzed flame emssvty (200 K and 1 atm) Tab. 2-2: Ignton and flammablty propertes. 13

14 Fg. 2-1 shows the exploson lmts of a H2-O2 mxture. It can be notced that there are dstnct regons n temperature-pressure coordnates where a stochometrc mxture wll and wll not explode. The temperatures and pressures correspond to the ntal chargng condtons of a sphercal vessel contanng the reactants No Exploson Pressure [mm Hg] Exploson Temperature [ ] Fg. 2-1: Exploson lmts for a stochometrc hydrogen-oxygen mxture n a sphercal vessel. 2.2 Prncples of combuston Conservaton equatons for multcomponent reactng systems Turbulent combuston s a mult-scale problem where complexty les n the nteracton between flud dynamcs and chemstry. The man am of ths chapter s to provde a theoretcal background of the man conservaton equatons used to study turbulent reactng flows. The lamnar conservaton equatons may be summarzed as follows. Overall contnuty, equaton s: 14

15 = 0 + v t r ρ ρ 2-10 Whch s the equaton of contnuty for the mxture. For a multcomponent system the equaton of contnuty for a component s: V Y Y v t Y ω ρ ρ ρ = + + r r 2-11 In a general multcomponent system there are N equatons of ths knd (or N-1 f the equaton for the mxture s used). The addton of these equatons gves the equaton of contnuty for the mxture. The ω n each speces contnuty equaton s determned by the phenomenologcal chemcal knetc expresson (see sec for detals): ( ) = = = N j u j u a k k M k k k k j k R T p X R T E B T W 1 ' 1,,, exp ' '' ν α ν ν ω 2-12 Where M s the total number of chemcal reactons occurrng and N s the total number of chemcal speces present. For what concerns the partal dfferental momentum equaton, the basc assumpton s that we are dealng wth contnuous, sotropc, and homogeneous meda. We shall consder the specal case of a Newtonan flud, that s, a flud exhbtng a lnear relatonshp between shear stress and rate of deformaton, resultng n the Naver-Stokes equaton. The momentum equaton n ndcal notaton s: ( ) = + = + N k k k j j j j f Y x x u u t u 1 ρ σ ρ 2-13 σ j s the stress tensor, f k s the force per unt mass on the k-th speces. Substtutng the consttutve equaton: = j j j k k j j x u x u x u p μ δ μ μ δ σ 3 2 ' 2-14 n the momentum equaton, we obtan the Naver-Stokes equaton: ( ) = = + N k k k j j j k k j j j j f Y x u x u x u p x x u u t u ' ρ μ δ μ μ δ ρ 2-15 Where μ s the bulk vscosty, μ s the dynamc vscosty. The dfference μ μ λ 3 2 ' = 15

16 s the second vscosty. The usual practce s to employ the hypothess made by Stokes n 1845: '= 0 μ n combuston processes also. The law of conservaton of energy for a flud contaned wthn a volume element s: ( ) k k N k k j j t t f Y V u x u Q x q x e u t e, 1, = = + ρ σ ρ & 2-16 Where: The frst term on the left sde s the rate of accumulaton of nternal and knetc energy; The second s the net rate of nflux of nternal and knetc energy by convecton; The thrd s the net rate of heat addton due to the heat flux q; whch contans the conducton heat, the energy flux caused by nterdffuson processes and the Dufour effect (heat flux produced by concentraton gradents); The fourth s the rate of heat added by heat source; The last two terms are the net rate of work done on system by surroundngs. e t s the energy stored per unt mass defned as: 2 t u u e e + = 2-17 Summaton of the nternal and knetc/mechancal energy. V k s the mass dffuson velocty of the k-th velocty. The mechancal-energy equaton, obtaned from the momentum equaton multplyng t by u, s: = + = + N k k k j j j j u f Y x u x u u u t u u 1, ρ σ ρ 2-18 The nternal energy equaton, obtaned subtractng (2-16) from (2-18), s: = = + N k k k k j j V f Y x u Q x q x e u t e 1,, ρ σ ρ & 2-19 The Fck s frst law for a bnary system s: ( ) A AB A A A A A Y D V v v J = = = ρ ρ ρ 2-20 Where D s the mass dffusvty, v A s the velocty of the speces A wth respect to the statonary coordnate axes, v s the mass-average velocty of the system and V the mass dffuson velocty. The Fck s second law of dffuson states: A AB A C D t C 2 =

17 Ths equaton s generally used for dffuson n solds or statonary lquds and for equmolar counterdffuson n gases. Other necessary equatons n multcomponent systems are lsted below. The deal-gas equaton, statng that N Y p = ρ RuT 2-22 W = 1 The relatonshp between X and Y : X = N ( Y / W ) j = 1 Y / W 2-23 The multcomponent dffuson equaton obtaned through rgorous dervaton from the knetc theory for =1, 2,, N speces s: X = N j = 1 X D X j j r r N p ρ r r ( V V ) + ( Y X ) + YY ( f f ) j p p + j j j = 1 j = 1 N X X ρd j j α j α T Yj Y T 2-24 Where α j s the thermal dffuson coeffcent of speces j. Physcally ths equaton states that concentraton gradents may be supported by dffuson veloctes, pressure gradents, the dfferences n the body force per unt mass on molecules of dfferent speces, and thermal-dffuson effects. For what concerns the soluton of a multcomponent-speces system, f the dffuson veloctes can be substtuted by Fck s Law n the speces and energy equatons, then n a system wth N speces there are N+6 unknowns. The N+6 equatons to be solved are: 1 overall mass contnuty 3 momentum equatons 1 energy equaton N 1 speces equatons 1 equaton of state 1 equaton relatng all Y If the dffuson veloctes must be solved from the dffuson equaton for a multcomponent system, there wll be 5N+6 unknowns. Further there are 4N equatons: 3N dffuson equatons N equatons relatng X to Y In the precedng paragraphs, we have dscussed some mportant equatons for lamnar reactng flows. For lamnar flows, the adjacent layers of flud slde past one another n a smooth, orderly manner. The only mxng possble s due to molecular dffuson. The velocty, temperature, and 17

18 concentraton profles measured n lamnar flow wth a hgh-senstvty nstrument wll be qute smooth. At very hgh Reynolds or Grashof numbers the flow becomes turbulent. In turbulent flow, eddes move randomly back and forth and across the adjacent flud layers. The flow no longer remans smooth and orderly. It s very dffcult to gve a precse defnton of turbulence. In the followng some of the characterstcs of turbulent flows are lsted: Irregularty: all turbulent flows are rregular, or random. Ths makes a determnstc approach to turbulence problems mpossble; one reles on statstcal methods. Dffusvty: the dffusvty of turbulence, whch causes rapd mxng and ncreased rates of momentum, heat and mass transfer, s another mportant feature of all turbulent flows. Large Reynolds number: turbulent flows always occur at hgh Reynolds numbers. Turbulence often orgnates as an nstablty of lamnar flows f the Reynolds number becomes too large. The nstablty s related to the nteracton of vscous terms and non lnear nerta terms n the equaton of moton. Three-Dmensonal Vortcty Fluctuatons: turbulence s rotatonal and three dmensonal. It s characterzed by hgh levels of fluctuatng vortcty. For ths reason, vortcty dynamcs plays an essental role n the descrpton of turbulent flows. Dsspaton: turbulent flows are always dsspatve. Vscous shear stresses perform deformaton work whch ncreases the nternal energy of the flud at the expense of the knetc energy of the turbulence. Turbulence needs a contnuous supply of energy to make up for these vscous losses. Contnuum: turbulence s a contnuum phenomenon, governed by the equatons of flud mechancs. Even the smallest scales occurrng n a turbulent flow are ordnarly far larger than any molecular length scale. Turbulent Flows Are Flows: turbulence s not a feature of fluds but of flud flows. Most of the dynamcs of turbulence s the same n all fluds, whether lke they are lquds or gases, f the Reynolds number of the turbulence s large enough; the major characterstcs of turbulent flows are not controlled by the molecular propertes of the flud n whch turbulence occur. Even chemcally non-reactng turbulent flows are hghly challengng due to the above characterstcs. When chemcal reactons occur, the problems become even more complex, snce the turbulent flud flow s further coupled wth chemcal knetcs and qut often wth phase changes. Ths s why the study of turbulent reactng flows s one of the most challengng felds of engneerng scence. The turbulent flame s often accompaned by nose and rapd fluctuatons of the flame envelope. In addton to the characterstcs of non-reactng turbulent flows mentoned earler, some of the flames are descrbed brefly n the followng: The flame surface s very complex, and t s dffcult ot locate the varous surfaces that are used to characterze lamnar flames. Also due to the enhanced transport propertes the turbulent flame speed s much greater than the lamnar flame speed. The heght of a turbulent flame s smaller than that of a lamnar flame at the sme flow rate, fuel-ar rato, and burner sze. Ths s shown by comparson of drect photographs. At a gven velocty, the flame heght decreases as the ntensty of the turbulence ncreases. Unlke the lamnar flame, the reacton zone s usually qute thck. 18

19 As the turbulent moton s random and rregular, t has a broad range of length scales and can be treated statstcally takng nto consderaton some type of averaged quanttes. There are two dfferent averagng procedures commonly used: conventonal tme averagng (also called Reynolds averagng) and mass-weghted averagng (also called Favre averagng). The tme-averaged conservaton equatons can be obtaned by applyng the Reynolds averagng procedure. But Reynolds averagng for varable densty flow ntroduces many other unclosed correlatons between any quantty and densty fluctuatons then to avod ths dffculty, massweghted averages (called Favre averages) are usually preferred. We can defne a mass-weghted generc quantty as: ~ ρf f = ρ 2-25 Any quantty f may be splt nto mean and fluctuatng components as: ~ f = f + f '' 2-26 Usng ths formalsm, the averaged balance equatons become: Contnuty ρ + t x j ( ρu~ ) = 0 j 2-27 Momentum (assumng that the body force s neglgble) t ( ~ ) ( ~ ~ p ρu + ρu u ) = + ( τ ρu '' u '') x j j x x j j j 2-28 Where the last term on the rght-hand sde represents the turbulent stresses due to turbulent dffuson of momentum. Chemcal speces t ~ Yk Y k ( ρ Yk ) + ( ρu~ ~ '' Yk ) = Dρ + Dρ ρu'' Y k '' + ϖ& k x x x x ~ x 2-29 Energy (total enthalpy) t Where: ~ ~ p p p ( h ) + ( ρhu ~ ) = + u~ + u '' + ( q ρh'' u '') u~ ~ u '' ρ j j j j j + τ j + τ j 2-30 x j x x j x j x j x j x j τ j 2 u = μ' μ 3 x k k u δj + μ x j u + u j 19

20 q j T = λ x j The left-hand sde represents the average rate of change of ρh per unt volume per unt tme; The frst two terms on the rght-hand sde represent the pressure work due to macroscopc moton; The thrd term s the work due to turbulence; The forth term s the transport heat due to conducton; The ffth term s the turbulent dffuson of ρh; The last two terms represent the dsspaton due to molecular frcton Dmensonless governng parameters In flow problems, the conservaton equatons can often be wrtten n forms nvolvng dmensonless ratos of varous quanttes and coeffcents. The mean dmensonless ratos nvolved n the combuston and nvoked n the followng sectons are reported here. These parameters characterzes the fresh mxture, the reactng flow from a global pont of vew and the turbulencecombuston nteractons for premxed and non-premxed flames. The Reynolds number s defned as: ρul ul Inertal forces = = = μ ν Vscous forces Re 2-31 The turbulence Reynolds number based on ntegral length (l t ) scale s defned as: The Lews number (or Lews-Semenov number) s defned as: u ''lt Re T = 2-32 ν Le = λ ρ C D p α = = D Rate of energy transport Rate of mass transport 2-33 The Prandtl number s defned as: The Schmdt number s defned as: C p μ ν Rate of momentum transport Pr = = = α 2-34 λ Rate of energy transport Rate of momentum transport Sc = ν = 2-35 D Rate of mass transport Le and Sc may be defned for each par of speces n multcomponent mxtures. From the above we see that: 20

21 Sc Le = 2-36 Pr In the followng the Le number s approxmately assumed 1(where not dfferently specfed). An mportant parameter n combuston s the Damköhler number. Ths parameter appears n the descrpton of many combuston problems and s qute mportant n understandng turbulent premxed flames (see followng secton). It s defned by: Da τ τ ( l ) l / u'' ( l ) t m t t t = = = 2-37 τ c τ c δ L / SL It s defned for the largest eddes and corresponds to the rato of the ntegral tme scale (τ t turbulence tme, characterstc flow tme) to the chemcal tmescale (τ c flame tme). The subscrpt m stands for mechancal. δ L s the lamnar flame thckness, S L the lamnar flame velocty and u the velocty fluctuatons. From the defnton, the Da can also represent the product of the length scale rato and the recprocal of a relatve turbulence ntensty. Thus, once fxed the length-scale rato, the Da falls as turbulence ntensty ncreases. The Karlovtz number s defned as: K a = 1 Da = ( l ) τ ( l ) K τ m c K l t = δ L 1 2 u'' SL 3 2 δ L = lk It corresponds to the smallest eddes (Kolmogorov) and s the rato of the chemcal tme scale to the Kolmogorov tme Basc flame types In combuston processes, fuel and oxdzer are mxed and burned. There are several combuston categores based upon whether the fuel and oxdzer s mxed frst and burned later (premxed) or whether combuston and mxng occur smultaneously (non premxed). Each of ths categores can be further subdvded based on whether the flud flow s lamnar or turbulent. Next fgure shows examples of combuston systems that belong to each of these categores, whch wll be dscussed n the followng sectons. 21

22 Fg. 2-2: Example of combuston systems ordered wth respect to premxedness and flow types (Warnatz 1996) Premxed Flames The couplng of heat and mass transport and chemcal reacton leads to a spatal structure, the flame, whch determnes the path from reactants to products. The flame s caused by a self-propagatng exothermc reacton whch s accompaned by a reacton zone. The structure of a lamnar-premxed flame s schematcally shown n Fg. 2-3: 22

23 Fg. 2-3: Schematc representaton of premxed flames structure. Premxed flame structure can be dvded nto four zones: unburned zone (cold reactants zone), preheat zone, reacton zone and burned gas zone (products zone). The unburned mxture of fuel and oxdzer s delvered to the preheat zone at prefxed condtons, where the mxture s warmed by upstream heat transfer from the reacton zone. In the reacton zone, the fuel s rapdly consumed and the bulk of chemcal energy s released. The thckness of the flame front depends on pressure and on ntal temperature and equvalence rato. Ths thn flame front mples steep speces and temperature gradents, whch provde the drvng forces for the flame to be self-sustanng. In the reacton zone, temperature s hgh enough for creatng a large radcal pool. Fnally, n the burned zone, radcals recombne, and both temperature and major speces concentratons approach ther equlbrum values. However, the concentratons of mnor speces n ths regon can devate substantally from ther equlbrum values. The velocty wth whch a flame front propagates wth respect to the unburned fuel-oxdzer mxture s called the lamnar burnng velocty, S L, and s strongly dependent upon fuel and oxdzer type, equvalence rato and temperature of unburned fuel-oxdzer mxture. Consderng a premxed flammable mxture n a long tube, open at both ends, gnted from one end. A combuston wave wll travel down the tube startng from the gnton pont. The characterstc burnng velocty depends upon the fuel. For most hydrocarbon-ar stochometrc mxtures ths velocty s about 0.4 to 0.6 m/s, for hydrogen-ar mxures ths velocty s several meters per second. The velocty of ths wave s controlled by the dffuson of heat and actve radcals. Indcatng the unburned fuel-oxdzer mxture velocty wth v, the speed of the flame front v fr s v-s L. From the last relaton three possble dealzed stuatons may occur, dependng on the relaton between v and S L. Frst, f v>s L the flame wll move away from the burner,.e., the flame wll blow off. If v=s L, the flame wll keep ts poston relatve to the burner surface, and be aerodynamcally stablzed. In ths case, neglectng possble radatve losses from the flame to the surroundngs, the enthalpy n the fuel s solely manfest n the temperature of the burned gases, and the flame s referred to as an adabatc or free flame. The temperature correspondng to an adabatc flame s the maxmum flame temperature that can be acheved for a gven fuel-oxdzer composton. If v<s L, the flame wll move towards the burner and wll attempt to enter the burner (flash back). Snce the pores of the dealzed burner are assumed to prevent the flame from enterng the burner, the flame wll 23

24 transfer heat to the burner to lower the actual burnng velocty to the flow velocty; n ths condton the flame s referred to be as beng stablzed by the burner surface. Turbulent premxed flames are of practcal mportance, beng encountered n many useful devces (Spark-Ignton Engnes, Gas-turbne Engnes, Gas Burners ), whle, paradoxcally, ther descrpton s stll a matter of uncertanty, or at least controversy. Unlke a lamnar flame, whch has a propagaton velocty that depends unquely on the thermal and chemcal propertes of the mxture, a turbulent flame has a propagaton velocty that depends on the character of the flow, as well as on the mxture propertes. The turbulent flame speed, S T, s defned as the velocty at whch unburned mxture enters the flame zone n a drecton normal to the flame. Fg. 2-4: A superposton of nstantaneous reacton fronts obtaned at dfferent tmes (left) and a turbulent flame brush assocated wth a tme averaged vew of the same flame. From ths fgure can be notced the effect of the turbulence. The effect s to wrnkle and dstort an essentally lamnar flame front. Rememberng that n a turbulent flow varous length scales exst smultaneously and that the smallest scale, the Kolmogorov mcro scale, l K, represents the smallest eddes n the flow. These eddes rotate rapdly and have hgh vortcty, resultng n the dsspaton of the flud knetc energy nto nternal energy,.e., flud frcton results n a temperature rse of the flud. At the other extreme of the length-scale spectrum s the ntegral scale, l t, whch characterzes the largest eddy szes. The basc structure of a turbulent flame s governed by the relatonshps of l K and l t to the lamnar flame thckness, δ L. The lamnar flame thckness characterzes the thckness of a reacton zone controlled by molecular, not turbulent, transport of heat and mass. More explctly, three turbulent combuston regmes are defned usng the Damköhler and the Karlovtz numbers defned before, three turbulent premxed combuston regmes may be dentfed n terms of length (l t /δ L ) and velocty (u /S L ) ratos (Tab. 2-3). K a <1 (D a >1) K a >1 and D a >1 D a <<1 Wrnkled lamnar flames Flamelets n eddes Dstrbuted reactons Flame s thnner than all turbulent scales Small turbulent scales may enter the flame front Tab. 2-3: Combuston Regmes. All turbulent tme scales are smaller than the chemcal tme scale 24

25 When K a <1, the chemcal tme scale s shorter than any turbulent tme scales and the flame thckness s smaller than the smallest turbulent scale. In ths regme the flame front s thn, has an nner structure close to a lamnar flame and s wrnkled by turbulence motons. For τ k <τ c <τ t (K a >1 and D a >1) the turbulent ntegral tme scale s stll larger than the chemcal tme scale but the Kolmogorov scales are smaller than the flame thckness and are able to modfy the nner flame structure. The flame can no longer be dentfed as a lamnar flame front but s stll a wrnkled flame. For D a <1 turbulent motons have shorter characterstcs tmes than the chemcal reacton tme τ c : mxng s fast and the overall reacton rate s lmted by chemstry. 1.00E+08 Weak Turbulence E Reacton Sheets 1.00E+04 l K /δ L =1 Da 1.00E+02 u /S L =1 Flamelets n eddes 1.00E E E E E E E E E E+08 l 0 /δ L = E-02 Dstrbuted Reactons E-04 Re T Fg. 2-5: Important parameters characterzng turbulent premxed combuston. Condtons satsfyng the Wllams- Klmov crteron for the exstence of wrnkled flames le above the dash lne l K =δ L, and condtons satsfyng the Damkohler crteron for dstrbuted reactons fall below the dash dot lne l 0 =δ L. The flame regme that mght occur n practcal combuston can be estmated from Fg. 2-5 [Turns 2000] once provded suffcent nformaton characterzng the turbulent flowfeld, n partcular once 25

26 dentfed fve dmensonless parameters nterrelated n the fgure. The parameters are: l K /δ L, l 0 /δ L, Re T, Da and u /S L. There are three separated regons on the graph of Da versus Re T correspondng to the three regmes defned before. The three regons are separated by the dash dot lne l 0 /δ L =1 and the dash lne l K /δ L =1. Above the dash lne reactons take place n thn sheets, the wrnkled lamnarflame regme; below the dash dot lne reactons wll take place over a dstrbuted regon n space. The regon between the lnes s the flamelets n eddes regme Non-premxed flames In a non-premxed flame, combuston occurs at the nterface between the fuel gas and the oxdant gas, and the burnng process depends more on the rate of dffuson of reactants than on the rates of chemcal processes nvolved. It s more dffcult to gve a general treatment of dffuson flames because no smple measurable parameter analogous to the burnng velocty can be defned. Non-premxed flames nclude more complex chemstry than those of premxed flames, because the equvalence rato covers the whole range from zero (ar) to nfnty (pure fuel). Rch combuston occurs on the fuel sde, lean combuston on the ar sde. The flame front, whch s usually characterzed by ntense lumnescence, s fxed to regons near the locaton of the stochometrc composton, snce ths s where the temperature s hghest. Thus, unlke premxed flames, nonpremxed flames do no propagate and, therefore, cannot be characterzed by a lamnar flame speed. The flame cannot propagate nto the fuel wthout oxdzer or nto the oxdzer wthout fuel and, thus, s fxed to the nterface. The flame of a match, of a candle, and of the famlar gas-jet burner are all of ths type. To acheve an dea of the nature of the non-premxed combuston the most used example s the jet flame. The jet flame s relatvely uncomplcated, and, because of ths, t has been the subject of many theoretcal and expermental nvestgatons [Chger 1972]. A typcal flame of ths type can be produced readly usng coaxal cylndrcal tubes (crcular nozzles). As the fuel flows along the flame axs, t dffuses radally outward, whle the oxdzer dffuses radally nward. The flame surface s nomnally defned to exst where the fuel and oxdzer meet n stochometrc proportons. We have to note that, although the fuel and oxdzer are consumed at the flame, the equvalence rato stll has meanng snce the products composton relates to a unque value of Φ. The products formed at the flame surface dffuse radally both nward and outward. The regon where chemcal reactons occur s generally qute narrow. As seen n Fg. 2-6, where a typcal composton profle of a hydrogen-ar dffuson flame s shown, the reacton zone occurs n an annular regon. Ths s true untl the flame tp s reached. The observed shapes of non-premxed flames may be dvded nto two classes. If the rato of the duct rad s such that more ar s avalable than what s requred for completer combuston, then an overventlated flame s formed and the flame boundary converges to the cylnder axs. On the other hand, f the ar supply s nsuffcent for complete burnng, then underventlated flame s reduced n whch the flame surface expands to the outer tube wall. For smple lamnar dffuson flames on crcular nozzles flame heght s mostly used to characterze the flame. For an overventlated flame the flame length, L f s smply determned by the axal locaton where Φ ( r = 0, x = L ) = 1 f 26

27 Fg. 2-6: Dffuson flame structure: speces varatons through a dffuson flame at a fxed heght above the fuel jet tube [Gaydon 1960]. As reported n Kuo 1986, t can be demonstrated that the lamnar flame heght s proportonal to the volumetrc flow rate and nversely proportonal to the mass dffusvty. The varaton of the nonpremxed flame heght as a functon of jet velocty s shown n Fg. 2-7 whle for a turbulent nonpremxed flame the flame heght s proportonal to the port sze. Fg. 2-7: The varaton of flame heght and character as a functon of jet velocty [Gaydon 1960]. Non-premxed turbulent flames burn n a turbulent flow feld, and for low turbulence ntenstes the flamelet concept can be used agan. Examples of turbulent non-premxed flames are: Conventonal gas turbnes; B-propellant rocket engnes; Desel engnes; Cement klns, glass furnaces, boler furnaces; Turbojet afterburners; Flares n rafneres/ol felds; 27

28 Most fres (lke forest fres), pool flames; Coal/wood combuston; For any partcular applcaton, some of the ssues the desgner s faced wth are as follows (mportance of each may change dependng on the nature of the applcaton): combuston ntensty and effcency, flame stablty shape and sze, heat transport and pollutant emssons. Because of the safety consderatons mentoned above and the ease wth whch such flames can be controlled, non-premxed flames are mostly used n ndustral furnaces and burners, the majorty of practcal combuston systems. Wth current concern for pollutant emssons, however, ths advantage becomes somethng of a lablty n that there s also less ablty to control, or talor, the combuston process for low emssons. Unless very sophstcated mxng are used, non premxed flames show a yellow lumnescence (Fg. 2-8), caused by glowng soot partcles, whch are formed by fuel rch chemcal reactons n the rch doman of the non-premxed flames; the non-premxed turbulent jet flames have brushy or fuzzy edges smlar to premxed flames. Fg. 2-8:Turbulent non-premxed flames. As mentoned before, the heght of the lamnar flame depends on the flow rate. As the flow rate ncreases, turbulence begns to nfluence the flame heght and a transtonal regme can be seen. Over ths transtonal, the ncreasng turbulence levels wth flowrate result n the fully turbulent flames beng shorter than ther lamnar counterparts. Concernng flame stablty, at suffcently low flowrates, the base of a jet flame les qute close to the burner tube outlet and s sad to be attached. As the fuel flowrate s ncreased, holes begn to form n the flame sheet at the base of the flame, and wth further ncrease n the flowrate, more and more holes form untl there s no contnuous flame close to the burner port (Lftoff). At suffcently large flow rate, the flame blows out. 2.3 Chemcal knetcs and reacton mechansms Chemcal knetcs s the part of chemcal scence dealng wth the quanttatve study of the rates of chemcal reactons and of the factors upon whch they depend. It also deals wth the nterpretaton of the emprcal knetc laws n terms of reacton mechansms. A reacton mechansm s a collecton of elementary reactons necessary to descrbe an overall reacton. We remnd to e.g. Kuo 1986, Glassman 1996, Peters 1993, Smoke 1991, Turns 2000 for more detals. 28

29 Some of the essental features of chemcal knetcs whch occur frequently n ths work wll be revewed n the followng sectons Rates of reactons All chemcal reactons, whether hydrolyss or combuston, take place at a defnte rate, dependng on the condtons of the system. Some mportant condtons are concentratons of the chemcal compounds, temperature, pressure, presence of a catalyst or nhbtor, and radatve effects. The rate of reacton may be expressed n terms of the concentraton of any reactant as the rate of decrease of the concentraton of that reactant (the rate of consumpton of the reactant). It may also be expressed n terms of product concentraton as the rate of ncrease of the product concentraton. A conventonal unt for reacton rate s moles/m 3 sec. The rate law for an elementary reacton can be descrbed by the equaton: k A + B + C +... D + E + F Where A,B,C denote the dfferent speces nvolved n the reacton. A rate law descrbes an emprcal formulaton of the reacton rate,.e. the rate of formaton or consumpton of a speces n a chemcal reacton. The law of mass acton, whch s confrmed by numerous expermental observatons, states that the rate of dsappearance of a chemcal speces s proportonal to the products of the concentratons of the reactng chemcal speces, each concentraton beng rased to a power equal to the correspondng stochometrc coeffcent. Lookng at the consumpton of speces A, the reacton rate can be expressed accordng to [ A] d dt a b c [ A] [ B] [ C]... = k 2-40 Where a,b,c.. are reacton orders wth respects to the speces A,B,C and k s the rate coeffcent of the reacton (or specfc reacton rate constant). The sum of all exponents s the overall reacton order. For the reverse reacton (characterzed here by the subscrpt r) one obtans the rate law for the producton of A: [ A] d dt r d e f [ D] [ E] [ F]... = k 2-41 Snce mechansms may nvolve many elementary steps and many speces, the compact notaton can be used to represent both the mechansm and the ndvdual speces producton rates. For the mechansm, one can wrte: N j= 1 N ν ' A ν '' A for =1,2,,L j j j= 1 j j Where ν j and ν j are the stochometrc coeffcents on the reactants and products sde of the equaton, respectvely, for the jth speces and th reacton. A j s the arbtrary specfcaton of all chemcal speces and N the total number of compounds nvolved. 29

30 The followng three relatons compactly express the net producton rate of each speces n a multstep mechansm: [ A ] d dt j L = ω& = ν q for j=1,2,,n j = 1 j Where And ν j ( ν '' ν ' ) = 2-44 j j q = k N N j [ Aj ] kr [ Aj ] f j = 1 ν ' ν '' j = 1 j 2-45 The last equaton defnes the rate of progress varable, q, for the th elementary reacton. The specfc reacton-rate constant, for a gven reacton, s ndependent of the concentratons [A j ] and depends only on the temperature. In general, k s expressed as: E k = AT n a exp 2-46 RuT Where AT n represents the collson frequency and the exponental term s the Boltzmann factor, specfyng the fracton of collsons that have an energy greater than the actvaton energy E a. The values of B, n and E a are based on the nature of the elementary reacton. For gven chemcal changes, these parameters are nether functons of the concentratons nor of temperature Hydrogen Chemcal Mechansm The H 2 -O 2 reacton mechansm has been extensvely studed over the years and has already got wdespread applcaton such as n hgh energy rocket engnes. More recently t has become extremely evdent that combuston of hydrogen wth ar wll contnue to receve ncreasng applcaton prmarly because of the non-pollutng characterstcs of ths combuston process. In ths contest t s essental to dscuss the hydrogen oxygen combuston reactve mechansm. It has been observed that under ambent condtons of temperature, hydrogen and oxygen do not enter nto any drect reacton between them n absence of a catalyst. It s further seen f a mxture of hydrogen and oxygen gets exposed to lght oxygen gets actvated usually by way of dssocaton. In the presence of the senstzers of Cl, N 2 O and NH 3, a set of secondary reactons take place and form H atoms. These H atoms enter nto a reacton wth the actvated oxygen thus formng H 2 O. Relyng heavly on Glassman 1996, the oxdaton of hydrogen s as follows. The ntaton reactons are H 2 + M H + H + M (very hgh temperature) H 2 + O2 HO2 + H (other temperatures). 30

31 Chan-reacton steps nvolvng O, H and OH radcals are H + O 2 O + OH O + H 2 H + OH H 2 + OH H 2O + H O + H 2 O OH + OH. Chan-termnatng steps nvolvng O, H and OH radcals are the three-body recombnaton reactons: H + H + M H 2 + M O + O + M O2 + M H + O + M OH + M H + OH + M H 2 O + M. To complete the mechansm, the reactons nvolvng HO 2, the hydroperoxy radcal, and H 2 O 2, hydrogen peroxde need to be ncluded. When H O + M HO + M Becomes actve, then the followng reactons come nto play: HO 2 + H OH + OH HO 2 + H H 2O + O HO O O + OH And HO + HO + Wth 2 + HO2 H 2O2 O2 2 + H 2 H 2O2 H H 2O2 + OH H 2O + HO2 H 2 O2 + H H 2O + OH H 2O2 + H HO2 + H 2 H O + M OH + OH + M 2 2 Dependng upon the temperature, pressure and extent of reacton, the reverse reactons of all of above can be mportant; therefore n modellng H 2 -O 2 system as many as 40 reactons can be taken nto account nvolvng eght speces: H 2, O 2, H 2 O, OH, O, H, HO 2 and H 2 O 2. The H 2 chemcal mechansm used n the smulatons s reported n the followng table. 31

32 No. Reacton A n E a Rh.1 H+O 2 =OH+O 2.00E Rh.2 H 2 +O=OH+H 1.80E Rh.3 H 2 O+O=OH+OH 5.90E Rh.4 H 2 +OH=H 2 O+H 1.17E Rh.5 H+O 2 +M a =HO 2 +M a 2.30E Rh.6 H+HO 2 =OH+OH 1.50E Rh.7 H+HO 2 =H 2 +O E Rh.8 OH+HO 2 =H 2 O+O E Rh.9 H+H+M a =H2+M a 1.80E Rh.10 H+OH+M a =H 2 O+M a 2.20E Rh.11 HO 2 +HO 2 =H 2 O 2 +O E Rh.12 H 2 O 2 +M=OH+OH+M 1.30E Rh.13 H 2 O 2 +OH=H 2 O+HO E Rh.14 O+HO 2 =OH+O E Rh.15 H+HO 2 =O+H 2 O 5.00E Rh.16 H+O+M=OH+M 6.20E Rh.17 O+O+M=O 2 +M 6.17E Rh.18 H 2 O 2 +H=H 2 O+OH 1.00E Rh.19 H 2 O 2 +H=HO 2 +H E Rh.20 O+OH+M=HO 2 +M 1.00E Rh.21 H 2 +O 2 =OH+OH 1.70E Rh.22 O+N 2 =N+NO 1.82E Rh.23 O+NO=N+O E Rh.24 H+NO=N+OH 2.63E Rh.25 NO+M=N+O+M 3.98E Rh.26 N 2 +M=N+N+M 3.72E Rh.27 N 2 O+O=NO+NO 6.92E Rh.28 N 2 O+O=N 2 +O E Rh.29 N 2 O+N=N 2 +NO 1.00E Rh.30 N+HO 2 =NO+OH 1.00E Rh.31 N 2 O+H=N 2 +OH 7.60E Rh.32 HNO+O=NO+OH 5.01E Rh.33 HNO+OH=NO+H 2 O 1.26E Rh.34 NO+HO 2 =HNO+O E Rh.35 HNO+HO 2 =NO+H 2 O E Rh.36 HNO+H=NO+H E Rh.37 HNO+M=H+NO+M 1.78E a H2 =1.0 H 2 O=6.5 O 2 =0.4 N 2 =0.4. Unts are cm 3, mol, s, kj and K Tab. 2-4: Hydrogen chemcal mechansm. 32

33 2.3.3 Oxdes of ntrogen formaton Ntrc oxde s an mportant mnor speces n combuston because of ts contrbuton to ar polluton. The detaled ntrogen chemstry nvolved n the combuston of methane s reported n the GRIMECH mechansm [GRI-Mech 3.0]. In the combuston of fuels that contan no ntrogen, ntrc oxde s formed by three chemcal mechansms or routes that nvolve ntrogen from the ar: the thermal or Zeldovch mechansm, the Fenmore or prompt mechansm and the N 2 O ntermedate mechansm. There s growng evdence for the possblty of a fourth route nvolvng NNH. The thermal mechansm domnates n hgh-temperature combuston over a farly wde range of equvalence ratos, whle the Fenmore mechansm s partcularly mportant n rch combuston. It appears that the N 2 O-ntermedate mechansm plays an mportant role n the producton of NO n very lean, low-temperature combuston processes. Recent studes show the relatve contrbutons of the three mechansms n premxed and dffuson flames. The thermal or Zeldovch mechansm conssts of two chan reactons: O + N 2 NO + N N O NO + O + 2 Whch can be extended by addng the reacton N + OH NO + H Ths three-reacton set s referred as the extended Zeldovch mechansm. In general, ths mechansm s coupled to the fuel combuston chemstry through the O 2, O, and OH speces; however, n process where the fuel combuston s complete before NO formaton becomes sgnfcant, the two processes can be uncoupled. In ths case, f the relevant tme scales are suffcently long, one can assume that the N 2, O 2, O, and OH concentratons are at ther equlbrum values and N atoms are n steady state. These assumptons greatly smplfy the problem of calculatng the NO formaton. If we make the addtonal assumpton that the NO concentratons are much less than ther equlbrum values, the reverse reactons can be neglected. Ths yelds the followng rather smple rate expresson: [ NO] d dt N.1 f [ O] eq [ N2] eq = 2k 2-47 Wthn flame zones proper and n short-tme-scale, postflame processes, the equlbrum assumpton s not vald. Superequlbrum concentratons of O atoms, up to several orders of magntude greater than equlbrum, greatly ncrease NO formaton rates. Ths superequlbrum O (and OH) atom contrbuton to NO producton rates s sometmes classfed as part of the so-called prompt-no mechansm. The actvaton energy for the frst NO reacton s relatvely large; thus, ths reacton has a very strong temperature dependence. As a rule-of-thumb, the thermal mechansm s usually unmportant at temperatures below 1800 K. Compared wth the tme scales of fuel oxdaton processes, NO s formed rather slowly by the thermal mechansm; thus, thermal NO s generally consdered to be formed n the postflame gases. The N 2 O-ntermedate mechansm s mportant n fuel-lean (F<0.8), low temperature condtons. The three steps of ths mechansm are 33

34 O + N2 + M N2O + M H + N 2 O NO + NH O N O NO + NO + 2 Ths Mechansm becomes mportant n NO control strateges that nvolve lean premxed combuston, whch are currently beng explored by gas-turbne manufacturers. The Fenmore mechansm s ntmately lnked to the combuston chemstry of hydrocarbons [Lefebvre 1983]. Fenmore dscovered that some NO was rapdly produced n the flame zone of lamnar premxed flames long before there would be tme to form NO by the thermal mechansm, and he gave ths rapdly formed NO the appellaton prompt NO. The general scheme of the Fenmore mechansm s that hydrocarbon radcals react wth molecular ntrogen to form amnes or cyano compounds. The amnes and cyano compounds are then converted to ntermedate compounds that ultmately form NO. In the atmosphere, ntrc oxde ultmately oxdzes to form ntrogen doxde, whch s mportant to the producton of acd ran and photochemcal smog. Many combuston processes, however, emt sgnfcant fractons of ther total oxdes of ntrogen as NO 2. the elementary reactons responsble for formng NO 2 pror to exhaustng of the combuston products nto the atmosphere are the followng: NO + HO2 NO2 + OH (formaton) NO 2 + H NO + OH (destructon) NO + O NO + (destructon) 2 O 2 Where the HO 2 radcal s formed by the three-body reacton: H O + M HO + M The HO 2 radcals are formed n relatvely low-temperature regons; hence, NO 2 formaton occurs when NO molecules from hgh temperature regons dffuse or are transported by flud mxng nto HO 2 rch regons. The NO 2 destructon reactons are actve at hgh temperatures, thus preventng the formaton of NO 2 n hgh-temperature zones. 2.4 Mld Combuston One of the major advantages nto the applcaton of the TVC combustors s the possblty to acheve mld combuston that s one of the promsng technques proposed to control pollutant emssons from combuston plants. It s characterzed by hgh pre-heatng of combuston ar and massve recycle of burned gas before reacton. Real-sze burners acheve flameless condtons by feedng the combuston ar and the fuel through separated or coflowng hgh velocty jets nto the combuston chamber. The ar jets entran a large amount of burned gases from the combuston chamber before reactng wth the fuel. Consequently oxygen concentraton n combuston ar s lower than n tradtonal flames, whle turbulence s hgher. Chemcal reactons occur at lower rates (compared wth mxng ones) and are less lmted by mass dffuson. Consequently, the combuston regon s no longer concentrated close to the flame front but s extended over the whole combuston chamber, resultng n a volumetrc rather than superfcal combuston and n the complete dsappearance of the flame, hence the name flameless. As a result, mld oxdaton can easly control thermal gradents by avodng the formaton of hot 34

35 spots n the furnace and by enablng reducton of thermal NOx producton wthout compromsng combuston effcency. Another pecular characterstc s the reducton of combuston nose, whch s set to values smlar to those generated by a non-reactng jet [Wünnng 1997]. 35

36 3 Trapped Vortex Combustor Trapped Vortex Combuston technology holds postve promse for gas turbne applcatons wth mproved effcency, lower emssons, greater flame stablty, added fuel flexblty, ncreased durablty, and reduced captal costs. The TVC concept was orgnally conceved n the early 1990s for aero-propulson applcatons wth hgh through-put velocty requrements. It was not untl the early 2000s that research and development organzatons ntated the frst desgn concepts for ndustral applcatons. The TVC technology has the potental for product nserton nto a wde varety of ndustral applcatons ncludng gas turbne power generaton, manufacturng processng, chemcal process heatng, steam boler systems, and ncneraton. The requrements of low fuel consumpton and low pollutant emssons are paramount for all types of combustors, wth the combustor prmary zone arflow pattern of prme mportance to flame stablty, combuston effcency, and low emssons. Many dfferent types of arflow patterns are employed by non-tvc concepts, but one common feature to all s the creaton of a torodal flow reversal that recrculates (Fg. 3-1) and entrans a porton of the hot combuston products to mx wth the ncomng ar and fuel to stablze the flame. Fg. 3-1: Non-TVC Swrl Stablzed Combuston (Courtesy of Natonal Combuston Equpment, Inc.)) Although these desgns have long been used n many practcal combuston devces, there are lmtatons, especally for lean premxed applcatons. Flame stablty s acheved through the use of recrculaton zones to provde a contnuous gnton source whch facltates the mxng of hot combuston products wth the ncomng fuel and ar mxture. Swrl vanes are commonly employed (Fg. 3-2) to establsh the recrculaton zones. Ths method creates a low velocty zone of suffcent resdence tme and turbulence levels such that the combuston process becomes self-sustanng. The challenge, however, s the selecton of a flame stablzer whch ensures that both performance (emssons, combustor acoustc and pattern factor) and cost goals are met. Fg. 3-2: Industral Fuel/Ar Swrler 36

37 3.1 TVC concept Before outlnng the features of a TVC, t s useful to brefly descrbe essental feature of a typcal gas turbne combustor. The generc swrl- stablzed combustor shown n Fg. 3-3 has a prmary recrculaton zone that s establshed by swrlers located around the fuel njector. Dffuser Lner Ar Jets Prmary Zone Fg. 3-3: Gas Turbne Combuston Chamber. Ths zone transports some of the hot combuston products back towards the combustor face and gnte the ncomng fuel and ar as the are mxed n the combuston chamber, thus provdng a contnuously lt, stable flame. The recrculaton zones n the forward corners of the combustor also provde combuston stablty. If the velocty of the ar enterng the combuston chamber s excessve, the prmary recrculaton zone become unstable, resultng n poor flame stablty and low combuston effcency. An unstable prmary zone s avoded but the use of a dffuser do slow down the combustor nlet ar. Ar njected nto the combuston chamber through the lner ar jets s used to establsh the length of the prmary zone, and also used to mx and shape the temperature profle of the exhaust gases as the enter the turbne vanes. Although most arcraft and power generaton gas turbne combustor functon n ths way, many dfferent combustor desgns exst because of the need to satsfy specfc arcraft msson or power generaton objectves. The smlartes and sgnfcant dfferences between a TVC and a conventonal combustor are dscussed n the followng. The TVC s a staged combuston system wth a trapped vortex plot and a man combustor. The plot employs a cavty (Fg. 3-4) to provde a stable recrculaton zone over a wde range of man arflow condtons. Ths can be accomplshed wth a relatvely low-pressure drop across the combustor. The prmary fuel and ar are njected drectly nto the cavtes such that the vortex s renforced. Because the plot flame s shelded from the man flow by the cavtes, sable combuston can be acheved, even when the man ar veloctes are hgh. The TVC employs cavtes to stablze the flame and grows from the wealth of lterature on cavty flows. Much of the hstorcal effort examnes the flow feld dynamcs establshed by the cavtes, as demonstrated n arcraft wheel wells, bomb bay doors and other external cavty structures. Cavtes have also been studed as a means of coolng and reducng drag on projectles and for scramjets and waste ncneraton. 37

38 Fg. 3-4: Trapped Vortex Combuston. The actual stablzaton mechansm facltated by the TVC s relatvely smple. A conventonal bluff or fore body s located upstream of a smaller bluff body - commonly referred to as an aft body. The flow ssung from around the frst bluff body separates as normal, but nstead of developng shear layer nstabltes whch n most crcumstances s the prme mechansm for ntatng blowout, the alternatng array of vortces are convenently trapped or locked between the two bodes. In a TVC concept, the re-crculaton of hot products nto the man fuel-ar mxture s accomplshed by ncorporatng two crtcal features. Frst, a stable recrculaton zone must be generated adjacent to the man fuel-ar flow. If the vortex regon, or cavty regon, s desgned properly, the vortex wll be stable and no vortex sheddng wll occur. Ths stable vortex s generally used as a source of heat, or hot products of combuston. The second crtcal desgn feature nvolves transportng and mxng the heat from the vortex, or cavty, regon nto the man flow. Ths s accomplshed by usng wake regons generated by bodes, or struts, mmersed n the man flow. Ths approach gntes the ncomng fuel-ar mxture by lateral mxng, nstead of a back-mxng process. By usng geometrc features to gnte the ncomng fuelar mxture, nstead of pure aerodynamc features, the TVC concept has the potental to be less senstve to nstabltes and process upsets. Ths s partcularly mportant near the lean flame extncton lmt, where small perturbatons n the flow can lead to flame extncton. The very stable yet more energetc prmary/core flame zone s now very resstant to external flow feld perturbatons, yeldng extended lean and rch blowout lmts relatve to ts smple bluff body counterpart. Early research has demonstrated that the TVC confguraton can wthstand through-put veloctes near Mach 1. Ths unque characterstc of the TVC technology provdes a flud dynamc mechansm that can overcome the hgh flame speed of hydrogen-rch syngas and potentally allow gas turbnes to operate the combustor n premxed mode. Ths system confguraton also has greater flame holdng surface area and hence wll facltate the more compact prmary/core flame zone essental to promotng hgh combuston effcency and reduced CO emssons. An example of a pratcal applcaton of the concept s the annular verson of the TVC depcted n Fg. 3-5 [Roquemore 2001]. The dffuser conssts of one or more crcumferentally dstrbute spltter plates that drect the man ar through dfferent passages n the dffuser. The man fuel nozzles are located near the ext of the dffuser. The nlet to the man combuston chamber s a flat surface that s n the same plane as the front face of the cavtes. The combustor nlet plane conssts of passages where the fuel and ar enter the man combuston chamber and a matrx of radal and crcumferental struts. The struts act as conduts for transportng hot products out of the cavtes. Ignton of the man combustor s acheved as the man fuel and ar mx wth the hot products from the plot. The recrculaton zones behnd the struts serve as flameholders that are smlar to those used n afterburners. The rapd mxng n the wakes behnd the struts promote rapd combuston. The struts also dstrbute the ext gases such that an acceptable radal profle and pattern factor can be acheved wthout usng lner jets. 38

39 Fg. 3-5: Sketch of a annular verson of the TVC. Combuston took place wthn the cavty, to a degree determned by the cavty loadng and equvalence rato. The combuston effcency acheved and the gaseous emssons generated both depended strongly on these quanttes. However, cavty loadng and equvalence rato cannot be defned drectly from the metered fuel and ar masses suppled to the combustor. A quantty of manstream ar s entraned nto the cavty by the acton of the cavty vortex and by the addtonal pumpng acton of the prmary ar and fuel jets. The TVC s a smple desgn that has the potental to provde hgh performance and low emssons. Because of the stable recrculaton zones n the cavtes, one should expect a TVC to have a low Lean Blow Out (LBO) lmts and good alttude relght capabltes. Also f rapd mxng can be acheved n the cavtes, a TVC should have hgh combuston effcences over a wde operatng range. Low NOx should also results from the rapd mxng between the fuel and ar n the cavtes and the man. Low NOx emssons can also be acheved by operatng a TVC as a staged system wth lean or premxed combuston. If a TVC could be operated at hgh nlet veloctes, low Thermal NOx could be acheved as a result of the reduced resdence tme. A TVC can also be operated as a rch burn quck quench lean burn combustor (RQL) wth the cavtes provdng the rch-burn mode and the rapd mxng wth the man ar (no man fuel) provdng the quck quench lean burn modes. Thus several strateges can be employed to reduce NOx emssons n a TVC. In the followng ponts the man TVC technology potental are resumed: Burn a wde varety of medum and low-energy gases ncludng hydrogen-rch gasfed coal, bomass products, and landfll gas; Operate n a low NOx, lean premxed mode combustor envronment on hydrogen-rch syngas to accommodate the hgh flame speed that s a characterstc of these fuels; Acheve extremely low NOx emssons wthout the added expense of exhaust gas aftertreatment; Elmnate the costly requrement for hgh pressure dluent gas (ntrogen, steam or carbon doxde) for NOx emssons control; Accommodate more types of gas turbnes for IGCC applcatons by decreasng the mass flow through the turbne secton; Improve the overall cycle effcency of the gas turbne by decreasng the pressure drop through the combustor; and 39

40 Extend the lean blowout lmt offerng greater turndown, (load followng), wth mproved combuston and process stablty. 3.2 TVC Development Several TVC approaches from amercan government research organzatons and prvate companes are revewed. Each desgn shares n the common fundamental features of the TVC technology but also exhbt the unque features that set them apart from each other Ar Force Research Laboratory The Ar Force program started n the early 1990s at the Ar Force Research Laboratory (AFRL) n Dayton, Oho. The phenomena of locked or trapped vortces has been known to reduce aerodynamc drag for years, and the geometrc features requred to produce a locked or trapped vortex are the same features used to mnmze drag. Hsu et al. n 1995 was frst to report usng ths feature to stablze reactons n gas turbne combustors for aero-propulson applcatons. Snce then, several papers and patents have descrbed the results from usng ths TVC concept to acheve stable and low combustor emssons [Katta 1998]. The AFRL contnues to nvestgate potental TVC applcatons for advanced mltary gas turbne engnes. The AFRL TVC development efforts have focused prmarly on lqud fuel burnng aero-propulson applcatons and not on ndustral natural gas or syngas burnng applcatons. The developmental evoluton of the TVC concept at the AFRL s extremely well summarzed by Roquemore et al The frst generaton TVC s shown n Fg The cavty s formed between the two dsks n tandem. Katta and Roquemore used a tmedependent, axsymmetrc model to predct the results of reducng the drag of bluff-bodes n non-reactng flow and the expermental results of the frst generaton TVC [Katta 1998]. Fg. 3-6: AFRL Frst Generaton TVC. The second generaton TVC desgn, shown n Fg. 3-7, was an axsymmetrc can-type confguraton wth the cavty on the outsde of the man burner. The depth of the cavty was approxmately the same as that for the optmum frst generaton TVC. 40

41 Fg. 3-7: AFRL Second Generaton TVC. The thrd generaton TVC shown n Fg. 3-8 was a two-dmensonal sector desgned for easy replacement and optcal vewng of the cavtes. The objectve of the desgn effort was to develop a lqud fuel burnng TVC concept for gas turbne engne applcatons. Fg. 3-8: AFRL Thrd Generaton TVC. The development program at AFRL concluded that the TVC offers sgnfcant mprovements to arcraft gas turbne engnes n lean blow out (LBO) and alttude relght when compared to conventonal swrl stablzed combustors. Also, a wder operatng range and the potental to acheve lower NOx emssons were demonstrated. The TVC concept can operate n a staged, man-plot mode as well as n a rch burn quck quench lean burn (RQL) mode. Even though encouragng rg results have been obtaned to date, no full engne test as been completed wth an ntegrated TVC concept General Electrc Company The General Electrc (GE) Company has been developng TVC concepts for gas turbne engnes snce the md 1990 s. At least ten GE TVC patents have been ether fled and/or cleared snce The majorty of the patent work has been n the area of mltary gas turbne engnes. More recently, GE has fled two TVC patents for low NOx emssons ndustral gas turbne engne applcatons. The nventon was made wth support from the U.S. DOE. Arcraft Applcaton GE Arcraft Engnes and the AFRL have been jontly developng a novel TVC concept for mltary gas turbne engnes snce 1996 [Burrus 2001]. Ths effort represents an extenson of earler AFRL 41

42 research wth the thrd generaton TVC concept. The work led to the fabrcaton of a rectangular sector test rg shown n Fg. 3-9 wth a pressure capablty of up 20.5 atmospheres and temperatures as hgh as 900 K. Fg. 3-9: GE TVC Sector Rng. The performance evaluaton covered all aspects of a gas turbne engne. The operatng condtons wth JP-8 fuel provded smulatons of current commercal and mltary arcraft gas turbne engne cycles as well as some advanced cycles. Data was also obtaned at selected condtons for the LM2500 marne Navy duty cycle usng #1 Desel. The TVC test rg demonstrated that gnton, blow out, and alttude re lght were up to 50% mproved over current swrl stablzed combustors. The NOx emssons were n the range from 40% to 60% of the U.N. Internatonal Cvl Avaton Organzaton (ICAO) standard. The combuston effcency was mantaned at or above 99% over a 40% wder operatng range than a conventonal arcraft gas turbne engne combustor. Industral Applcaton GE Research s pursung the applcaton of TVC concepts to ndustral gas turbne engnes that can meet sub-9 ppmv NOx emssons. The objectve of DOE Contract s to explore advanced combustor concepts that show promse to meet future emssons requrements. The results of ths DOE program are not publshed at ths tme. Any further nformaton from GE regardng ther low emssons TVC development effort was unavalable DOE Natonal Energy Technology Laboratory The U.S. DOE s developng technologes for ultra-clean energy plants wth effcency and emsson goals that are well beyond the capablty of current gas turbne power plants. The DOE reports that nnety-percent of new power plants currently under constructon wll be fueled by a natural gasbased fuel. A key feature of these future power plants wll be fuel dversty and flexblty. The gas turbne combustor desgns wll requre the capablty to operate on a wde range of fuels ncludng hydrogen-rch synfuels. The DOE has also selected the TVC concept as a promsng technology for future gas turbne combustor desgns. A collaboratve effort began n 1999 between the AFRL and the NETL to evaluate the TVC concept for statonary power applcatons. The project was co-sponsored by the DOE Advanced Turbne Systems (ATS) program and the DOD Strategc Envronmental Research and Development Program (SERDP). 42

43 The prmary ntent was to assess the low-emssons capabltes of a novel RQL staged combustor shown n Fg [Straub 2000]. The goal was to acheve NOx and CO emssons that are comparable to other commercal natural gas burnng DLN systems. Hgh BTU-fuels and fuels contanng sgnfcant amounts of fuel-bound ntrogen were evaluated. NETL has contnued to pursue the development of TVC concepts and combustor confguratons wth ther own nternal research program and through separate collaboratve projects wth GE and Ramgen Power Systems (RPS). The recently released mult-year turbne development program s evdence that the DOE s commtted to the advancement of novel combustor desgns such as the TVC concept. Fg. 3-10: NETL RQL TVC. NETL and RPS wth support from the Calforna Energy Commsson completed n early 2005 a seres of rg tests that demonstrated less than 3 ppmv NOx at ndustral gas turbne operatng condtons wthout the need for a stablzng catalyst or exhaust aftertreatment Ramgen Power Systems (RPS) RPS s pursung the development of ts unque Advanced Vortex Combuston (AVC) technology that has postve promse for mproved effcency, lower emssons, greater flame stablty, fuel flexblty, ncreased durablty, and reduced manufacturng costs. RPS s evaluatng the potental for product nserton nto a wde varety of ndustral applcatons ncludng gas turbne power generaton and mechancal drve, manufacturng processng, chemcal process heatng, steam boler systems, and ncneraton. The mmedate AVC applcaton s for low emssons statonary gas turbne engnes. In 2002, RPS tested the frst AVC concept (Fg. 3-11) wth support from the DOE NETL and acheved 9 ppmv NOX emssons on natural gas n lean premxed mode [Bucher 2003]. Fg. 3-11: RPS AVC confguraton. The testng was conducted at the GASL faclty n New York. The development of AVC concepts for gas turbne applcatons has contnued snce the test program at GASL. RPS and NETL wth support from the CEC have recently completed a jont project that has shown tremendous promse for the AVC technology for extremely low emssons and acoustcally stable combuston. These test results ndcate the potental to acheve unprecedented emssons levels of 1 ppmv NOx and 9 ppmv CO at ndustral gas turbne operatng condtons. 43

44 The tests were conducted n the NETL Low Emssons Combuston Test and Research (LECTR) faclty at 10 atmospheres wth combustor nlet temperature of 329 C. The combustor s ar-cooled to closely smulate actual gas turbne condtons. The seres of parametrc tests allowed for varaton n nlet pressure, fuel flow, ar flow loadng, lean blowout and pressure dynamcs. A 4nch-by- 4nch quartz wndow allowed for flame vsualzaton up to 5 atmospheres. The test results can be summarzed as follows: Data taken at 10 atmospheres and 329 C; Ar-cooled combustor functoned as desgned and wthout ncdent; Combustor gnted at ambent condtons; System was stable throughout start-up and shut-down; Flame structure was observed and recorded by vdeo through the quartz wndow; Overall combustor pressure drop was less than 2.6%; NOx emssons are not pressure dependent under ultra-lean condtons; Lowest measured NOx wth acceptable CO emssons: 3 ppmv NOx and 20 ppmv CO; Combuston effcences greater than 99.9%; Recorded combuston pressure oscllatons wth hgh frequency probe were nsgnfcant; RPS wll contnue to advance the development of ts AVC technology and dentfy product nserton opportuntes n ndustral applcatons ncludng gas turbne power generaton and mechancal drve applcatons. The AVC technology s scalable to varous szes and heat load capabltes ALM Turbnes Many turbne and combustor experts, ncludng those at ALM Turbne Inc., are ncreasngly optmstc about the promse of TVC [Kaln 2005]. It s ALM s poston that the TVC concept has many real potental advantages over both dffuson flame and DLN combustors, ncludng lower emssons, mult-fuel capablty, better flame stablty, unformty of flame, better dynamcs, greater lean blowout lmt offerng greater turndown, hgher effcency due to lower combustor pressure drop losses, compactness, and lower manufacturng costs. Over the last few years, ALM has desgned, manufactured and tested a number of propretary prototype TVCs that have demonstrated many of the above mentoned advantages. ALM has been developng and testng ts own propretary verson of TVC for both mcroturbnes and large MW scale ndustral turbnes. In 2003, ALM and Alturdyne successfully desgned, manufactured, and ncorporated a TVC combustor nto a Sunstrand T-62 APU. ALM has also desgned, manufactured and rg tested two MW scale prototypes at ambent condtons that meet GE 7E 85MW operatng condtons. The ALM TVC conssts of two autonomous sectons a thermal nozze and vortex (Fg. 3-12). The tme of complete fuel burnng, at the prmary combuston temperature, s less than 2 mllseconds. ALM has acheved combuston at very low temperatures wthout use of any catalysts. The ALM desgn s notably dfferent from all other TVC concepts. The concept utlzes ts vortex and manages the recrculaton flows n a dfferent manner. The combuston manly takes place n the thermal nozzle and not n the vortex. 44

45 Fg. 3-12: ALM TVC Concept. ALM uses a lamnar boundary layer between the recrculaton vortex flow and the fuel ar mxture flow, to create certan desrable chemcal reactons, and n turn a thermal nozzle effect whch sgnfcantly mproves the combuston process over other combustor desgns. Ths lamnar boundary layer, along wth ALM s unque use of the vortex dstngushes the ALM TVC from other TVC desgns. 3.3 ENEA TVC Geometry A TVC combustor has been desgned and bult at ENEA n 2008 under the framework of a research program focused on mld combuston. The ENEA TVC combustor ( Fg. 3-13) conssts of two majors portons: a centerbody assembly and, a housng assembly. A secton vew of the combustor s gven n Fg The sketch shows the supplyng ducts and the two cavtes trapped-vortex. The fgure also shows the vortces locatons. The centerbody assembly s formed from a long central shaft of stanless steel that s made up of an nner fuel tube surrounded by a concentrc ar passage wth outer dameter, d, of 14 mm, and whch passes wth a sldng ft through a sold dsk of 100 mm dameter, d f, that forms a forebody. On the down stream half of the shaft s permanently mounted a short, hollow drum of 71.8 mm dameter, d a, that forms an afterbody. The drum contans two nner compartments that form respectvely, prmary ar and fuel manfolds (Fg. 3-15). These manfolds are suppled through the supportng double-tube shaft (Fg. 3-16). The adjustable spacng H, between the forebody, the afterbody and the shaft forms the frst annular cavty wthn whch a vortex may be trapped. On the down stream end of the shaft s mounted another dsk, the second afterbody. The second vortex s trapped n the cavty formed by the frst and second afterbody. Fuel s dscharged upstream nto the vortex cavty from the upstream face of the drum va a rng of 8 non-crcular jets (Fg. 3-17b) each of mm 2 area; tubes from the fuel manfold pass through the ar manfold to feed these jets. Ar from the ar manfold s dscharged upstream nto the vortex cavty from the upstream a face of the drum va two concentrc rngs of crcular jets each of 3.25 mm dameter. There are 8 ar jets n the nner rng and 16 ar jets n the outer. The rng of fuel jets s concentrc wth and between, the rngs of ar jets. The fuel jets are crcumferentally orented to place a fuel jet mdway between adjacent pars of nner and outer ar jets. The forebody s supported concentrc n a duct of 113 mm nternal dameter, d e, that supples man/secondary ar to the combustor from an ar condtonng unt. The combustor man body s a length of 113 mm nternal dameter Quartz tubng. The length of the combustor s 475 mm. The wake regon behnd the afterbody provde a low speed regon to consume the excess fuel, however, the vortex sheddng can also cause local quenchng. Ths can lead to reduced combuston effcency. Mar 1965 and Lttle and Whpkey 1978 had shown that addng a second afterbody resulted n a sgnfcant reducton n drag. Bascally, a second properly sxed cavty results n another trapped-vortex between the frst and second afterbody. The locaton of the second trapped- 45

46 vortex would normally be the unsteady wake regon of the afterbody, when only one afterbody s used. The second trapped-vortex reduces the drag because t reduces the unsteady wake moton. In a combustng flow, the second vortex should also reduce local quenchng of the flame by reducng the flame-vortex quenchng nteractons. Ths was the dea leadng to the two-cavty TVC. The space between the frst afterbody and the second afterbody s adjustable. The second afterbody has a 54 mm dameter. The complete centerbody assembly s cooled by the flows of ar and fuel nternally through t. The combustor man wall s protected from drect exposure to flame temperatures by the cool annular jet of man ar that s n contact wth t. No downstream external coolng of the man wall was provded. The forebody has no drect coolng, other than that provded by the annular flow of man ar passng ts outer edge. Fg. 3-13: ENEA TVC. The dsk wth the holes s for CH 4. 46

47 A B Fg. 3-14: ENEA TVC: A) Supplyng system ducts; B) Two cavtes Trapped Vortex. A B C Fg. 3-15:ENEA TVC. Two cavtes trapped-vortex: A) Forebody; B) Frst Afterbody; C) Second Afterbody. The ar path s reported n Blue, the fuel path n Red. 47

48 Fg. 3-16: ENEA TVC. Supplyng system. In Blue the ar, n Red the fuel. a) Plate for the CH 4 : 8 holes for the fuel (red), 24 holes for the ar (blue). b) Plate for the H 2 : 8 holes for the fuel (red), 24 holes for the ar (blue). Fg. 3-17: ENEA TVC. plates for the fuel and prmary ar conveyance. 48

49 4 Numercal Combuston Modelzaton A steady and unsteady, three dmensonal, turbulent CFD smulaton of the conservaton equatons for multcomponent reactng system has been performed consderng also the energy equaton to account for the temperature effects. The turbulence model adopted s a realzable k-ε whch s mplemented nto the used CFD code. The turbulent reactng flame has been modelled usng a fnte rate approach takng nto account detaled Arrhenus chemcal knetcs. The numercal scheme has been mplemented usng a parallel approach that can be opted n the commercal code presently used. The spatal dscretzaton s performed usng the fnte volume approach and a second order accuracy s accomplshed. The study has been performed on a cluster wth 16 processors for each test case allowng the management of a mesh wth sze optmzed n terms of soluton accuracy and reasonable calculaton tme. More detals about the mesh sze, the dscretzaton propertes and the combuston model are gven n the followng sectons. A commercal CFD package FLUENT s used as a solver to dscretze and solve the governng equatons. Assocated preprocessor GAMBIT s used for the constructon of the computatonal grd. 4.1 Computatonal doman and mesh propertes The geometry used for the calculaton s that descrbed n sec Only a quarter of t s drawn n GAMBIT. The sze of the frst cavty formed between the forebody and the dsk s vared by movng the dsk away from the forebody. In ths way fve geometres are obtanted. The cavty lengths of the fve geometres are: 60, 65, 70, 75 and 80 mm. The computatonal doman for one of them s represented n Fg It s dvded n many sub-domans to allow a better defnton and a more flexble constructon of the mesh. The length of the volume corresponds to the length of the TVC combuston chamber. 49

50 Fg. 4-1:TVC geometry drawn n Gambt. The grd generated from ths geometry s hybrd (Fg. 4-2 a). The zone, near the jet nlets on the frst afterbody, s meshed wth tetrahedral cells due to the partcular shape of the holes (Fg. 4-2 b). The remanng part of the doman s meshed wth hexahedral cells (Fg. 4-2 c). The total number of cell of the computatonal domans s about 220,000. Ths number has been obtaned after several tral, havng studed the soluton ndependence from the grd and checked the accomplshment of the stablty and convergence crtera for the numercal solutons. Furthermore t gves a good compromse between the tme needed for the calculaton, the soluton accuracy and the calculaton power of the cluster. The grd has been examned wth a tool of the program obtanng good values for the equangle skew (94 % of the elements le between 0 and 0.4), the aspect rato (96 % of the elements le between 0 and 30) and the EquSze skew (95 % of the elements le between 0 and 0.4). a) 50

51 b) c) Fg. 4-2: TVC dscretzaton of the computatonal doman: a) whole doman b) Tetrahedral grd near the jet nlets c) hexahedral grd for some sub-domans. For every case studed the same boundary condtons are adopted and are of the followng type: Mass Flow Inlet for the fuel nlet, the prmary ar nlet and the secondary ar nlet; Pressure Outlet for the flow ext; Perodc; Wall for all the TVC surfaces (forebody, frst afterbody, second afterbody and combuston chamber); The locaton of the boundary condtons are sketched n the followng fgures. Fg. 4-3: TVC Forebody, afterbody and second afterbody wall faces. 51

52 Fg. 4-4: Fuel nlets faces (n green). Fg. 4-5: Prmary ar nlets faces (n green). Fg. 4-6: Secondary ar nlet faces (n green). 52

53 Fg. 4-7: Perodc boundary faces (n cyan). Fg. 4-8: Combuston chamber wall faces and pressure outlet faces (n red). 4.2 Numercal model Ths secton examnes the numercal model, showng some steps n the setup and soluton procedure used for the TVC combuston computatons. The code uses a control-volume based technque to convert a general scalar transport equaton to an algebrac equaton. The control technque conssts of ntegratng the transport equaton about each control volume, yeldng a dscrete equaton that expresses the conservaton law on a controlvolume bass. The solver adopted s a pressure based solver wth an mplct formulaton. The pressure-based solver employs an algorthm whch belongs to a general class of methods called the projecton method. In the projecton method, wheren the constrant of mass conservaton (contnuty) of the velocty feld s acheved by solvng a pressure (or pressure correcton) equaton. The pressure equaton s derved from the contnuty and the momentum equatons n such a way that the velocty 53

54 feld, corrected by the pressure, satsfes the contnuty. Snce the governng equatons are nonlnear and coupled to one another, the soluton process nvolves teratons wheren the entre set of governng equatons s solved repeatedly untl the soluton converges. The governng equatons solved n the smulatons are the contnuty, momentum, energy, radaton, turbulence and speces conservaton equatons. The models for the, turbulence radaton, and combuston are reported n the followng sectons wth a bref dscusson on the theoretcal back ground. Then the mxture speces and materal propertes together wth the boundary condtons are explctly descrbed Realzable k-ε Ths secton presents the essental features of the turbulence model: the realzable k-ε model. The model s very smlar to the standard k-ε. It also have the two transport equatons for k and ε. The realzable k-ε model s a relatvely recent development and dffers from the standard model n two mportant ways: The realzable k-ε model contans a new formulaton for the turbulent vscosty. A new transport equaton for the dsspaton rate, ε, has been derved from an exact equaton for the transport of the mean-square vortcty fluctuaton. The term realzable means that the model satsfes certan mathematcal constrants on the Reynolds stresses, consstent wth physcs of turbulent flows. The other known models (standard or RNG) are not realzable. And mmedate beneft of the realzable k-ε model s that t more accurately predcts the spreadng rate of both planar and round jets. It s also lkely to provde superor performance for flows nvolvng rotaton, boundary layers under strong adverse pressure gradents, separaton, and recrculaton. The realzable k-ε model has shown substantal mprovements over the standard k-ε model where the flow features nclude strong streamlne curvature, vortces and rotaton. The modelled transport equaton for k and ε n the realzable k-ε are: t μ k ρ x x 4-1 σ k x t ( k) + ( ρkx ) = μ + + Gk + Gb ρε YM + S k and t x x μ ε σ ε x 2 ε k + νε t ( ρε ) + ( ρεu ) = μ + + ρc Sε ρc2 + C1 ε C3 ε Gb + Sε ε k Where η k C 1 = max 0.43,, η = S, S = S j Sj 4-3 η + 5 ε 54

55 In these equatons, G k represents the generaton of turbulence knetc energy due to the mean velocty gradents. G b s the generaton of turbulence knetc energy due to buoyancy. Y M represents the contrbuton of the fluctuatng dlataton n compressble turbulence to the overall dsspaton rate. C 2 and C 1ε are constants. σ k and σ ε are the turbulent Prandtl numbers for k and ε, respectvely. S k and S ε are the source terms. The model constants can be vared n order to ensure good performance wth the flow under consderaton. As n the other models the turbulent vscosty s calculated wth: μ ρc μ 2 k ε t = 4-4 but the C μ s no longer constant and t s computed as a functon of the mean stran and rotaton rates, the angular velocty of the system rotaton and the turbulence felds Radaton model In Fluent there are fve radaton models whch allow to nclude radaton n the heat transfer smulatons. Dependng on the problem one radaton model may be more approprate than the others. In the case take nto consderaton here the choce s the Dscrete Ordnates (DO) model. The DO model spans the entre range of optcal thcknesses, and allow to solve problems rangng from surface to surface radaton to partcpatng radaton n combuston problems. The DO radaton model solves the radatve transfer equaton (RTE) for a fnte number of dscrete sold angles, each assocated wth a vector drecton fxed n the global Cartesan system. The fnesses of the angular dscretzaton s controlled by the user. The DO model transform the RTE equaton: r r di ds ( r, s ) + 4 σt π σ 4π 4π 2 s ( a + σ ) I( r, s ) = an + I( r, s ) Θ( s s' ) s r r 0 r r r r dω' 4-5 Where: s the poston vector; r s s the drecton vector; r s ' s the scatterng drecton vector; s s the path length; a s the absorpton coeffcent; n s the refractve ndex; σ s s the scatterng coeffcent; σ s the Stefan-Boltzmann constant; I s the radaton ntensty; T s the local temperature Θ s the phase functon; Ω' s the sold angle; nto a transport equaton for radaton ntensty n the spatal coordnates. Thus, the equaton 4-5 can be wrtten as: 55

56 4 r r r r r σt σ r r r r, s dω' π 4π 0 4π 2 s ( I( r s ) s ) + ( a + σ ) I( r, s ) = an + I( r, s ) Θ( s s' ) 4-6 r The DO model solves for as many transport equatons as there are drectons s. As the geometry s 3D, 8 octants are solved, resultng n 8N θ M φ drecton s r. Where N θ are the Theta Dvsons and N φ are the Ph Dvsons. Those parameters defne the number of control angles used to dscretze each octant of the angular space. Fg. 4-9: Angular coordnate system. When Cartesan meshes are used, t s possble to algn the global angular dscretzaton wth he control volume face, as shown n Fg for generalzed unstructured meshes however, control volume faces do not n general algn wth the global angular dscretzaton as shown n Fg leadng to the problem of control angle overhang. Fg. 4-10:Face wth no control angle overhang. 56

57 Fg. 4-11:Face wth control angle overhang. In these cases t s mportant to correctly account for the overhangng fracton. Ths s done through the use of pxelzaton. Each overhangng control angle s dvded nto N θp x N φp pxels as shown n Fg Fg. 4-12: Pxelzaton of control angle. Computatonal cost s moderate for typcal angular dscretzaton, and memory requrements are modest but solvng problem wth fne angular dscretzaton may be CPU-ntensve. In the smulatons the angular dscretzaton has been ncremented from the default values verfyng that the new values dd not become too much expensve for the CPU and memory resources. Fnally the achevement of a rght compromse between the computatonal effort and the results accuracy leads to the choce of 3 angular dvsons and a 2 x 2 pxelzaton. 57

58 The DO model also allows the soluton of radaton at sem-transparent walls and opaque walls. In ths way the rght amount of absorbed, emtted and reflected energy from the wall surface can be taken nto account. The TVC forebody, frst afterbody and second afterbody walls are consdered as opaque walls made n steel whle the TVC combuston chamber, made n quartz, s consdered as sem-transparent. All the physcal and thermal propertes of the steel and quartz are properly set n the code (see next table). Steel Quartz Densty [kg/m 3 ] Specfc heat [J/kg-K] Thermal conductvty [W/m-K] Refractve ndex Emssvty Tab. 4-1:Steel and quartz thermal and physcal propertes Combuston Model The combuston process n the TVC s solved usng a fnte rate reacton model based on the Arrhenus equaton: the Eddy Dsspaton Concept (EDC). It s an extenson of the eddy dsspaton model to nclude detaled chemcal mechansm n turbulent flows. The detaled mechansm has been reported n sec The assumptons, on whch the model resdes, can be resumed n: In turbulent flow, the chemcal reactons takes place where reactants or reactants and hot products are molecularly mxed. In turbulent flow the molecular mxng takes place where dsspaton of turbulent knetc energy takes place. Turbulent knetc energy s transported from energy contanng large turbulent structures to ntermttently dstrbuted fne structures wth hgh mxng where the knetc energy s dsspated nto heat. The dsspatve fne structures are ntermttently dstrbuted. Only a fracton of the dsspatve structures s burnng, the burnng fne structures Thus the model assumes that chemcal reactons occur wthn the smallest turbulent structures, called fne structures. Due to ntense mxng n the dsspatve structure, these are treated as perfectly strred reactors (PSR) whch exchange mass wth the surroundng flud. The overall reacton rate n each PSR s controlled by chemcal knetcs. The propertes of the fne structures are derved from a step-wse energy cascade model and expressed wth quanttes related to the man flow, such as the turbulent knetc energy, k, and the turbulent dsspaton rate, ε. Due to the precedng assumptons, the reacton rate n the fne structures perfectly strred reactors can easly be modelled as the change n mass fracton through the fne structure reactors dvded by the resdence tme of these reactors. The mean reacton rates are then modelled as the reacton rates of the burnng fne structures tmes the mass fracton of the burnng fne structures [Magnussen]. The EDC model s needed to compute the reacton rate (R ) appearng as source term n: 58

59 t r ( ρ Y ) + ( ρvy ) = J + R + S r The conservaton equaton for the th chemcal speces. S s the rate of creaton by addton from the dspersed phase. J r s the dffuson flux of speces, whch arse due to concentraton gradents. In lamnar flows usng the dlute approxmaton t can be wrtten as: r J ρ = D, m Y D,m s the dffuson coeffcent for speces n the mxture. Whle n turbulent flows t s: r J = D ρ, m μt + Y Sc t Where Sct s the turbulent Schmdt number whch by default s 0.7. The EDC assumes that reacton occurs n small turbulent structures, called fne scales. The length fracton of the fne scales s modelled as: * = C νε 2 k ξ ξ 1/ 4 Where * denotes the fne scales quanttes, C ξ s the volume fracton constant and k and ε are obtaned from the turbulence model. Speces are assumed to react n the fne structures over a tme scale ν τ* = C τ ε 1/ 2 Where C τ s the tme scale constant. τ* s controlled by the chemcal knetcs. Usng the prevous defntons the source term n the conservaton equaton for the mean speces, s modelled as R 2 ( ξ *) ( ξ *) ρ = * τ * 1 [ ] ( Y ) Y 3 Where Y* s the fne scale speces mass fracton after reactng over the tme τ* Gas Mxture Propertes The gas mxture s composed by 13 speces: H H 2 H 2 O H 2 O 2 HO 2 HNO N N 2 N 2 O NO O O 2 OH. For each speces n the gas mxture the propertes n the followng tables are set: 59

60 C p [J/kg-K] Thermal Conductvty [W/m-K] Vscosty [kg/m-s] H H 2 H 2 O H 2 O 2 HO 2 HNO Pecewse Polynomal Knetc Theory Knetc Theory Molecular weght [kg/kgmol] Standard State Enthalpy [J/kgmol] 2.18E E E E E+07 Standard State Entropy [J/kgmol-K] Reference Temperature [K] L-J Characterst length [angstrom] L-J Energy parameter [K] Tab. 4-2: Speces propertes Pt.1. C p [J/kg-K] Thermal Conductvty [W/m-K] Vscosty [kg/m-s] N N 2 N 2 O NO O O 2 OH Pecewse Polynomal Knetc Theory Knetc Theory Molecular weght [kg/kgmol] Standard State Enthalpy [J/kgmol] 4.73E E E E E E E+07 Standard State Entropy [J/kgmol-K] Reference Temperature [K] L-J Characterst length [angstrom] L-J Energy parameter [K] Tab. 4-3:Speces propertes Pt.2. The thermochemcal data needed to compute the specfc heat coeffcent and the transport data contanng the Lennard Jones coeffcent are taken from [GRI-Mech 3.0] and reported n the appendx. The propertes of the gas mxture are resumed n the followng table: Property Densty Specfc Heat Coeffcent Thermal conductvty Vscosty Mass Dffusvty Absorpton coeffcent Set Value Ideal Gas Mxng law Ideal Gas Mxng Law Ideal Gas Mxng Law Knetc theory WSGGM doman based Tab. 4-4: Propertes of the gas mxture. Due to the presence of gas phase speces as combuston products the absorpton n the gas s sgnfcant. The WSGGM opton s used to take nto account a varable composton dependent absorpton coeffcent. WSGGM stands for Weghted Sum of Gray Gases Model. It s a reasonable compromse between the oversmplfed gray gas model and a complete model whch takes nto account partcular absorpton bands. The basc assumpton of the WSGGM s that the total emssvty over the dstance s can be presented as ε = I = 0 a κ ps ( T )( e ) ε, 1 60

61 Where a ε, are the emssvty weghtng factors for the th fcttous gray gas, the bracketed quantty s the th fcttous gray gas emssvty, κ s the absorpton coeffcent of the th gray gas, p s the sum of the partal pressure of all absorbng gases, and s s the path length Boundary condtons The man boundary condtons (BC) suppled n the smulatons are defned n ths secton. They refer to those set durng the constructon of the mesh (see sec. 4.1) and are: mass flow nlet, pressure outlet, wall and perodc. The perodc BC put on the symmetry planes does not need any further settng. The walls are set as opaque or sem-transparent radaton surfaces, for more detals on the materal propertes see sec The ext of the doman s set as pressure outlet; the default settngs are used here. For what concern the ar and fuel nlets, the condtons are resumed n the followng table. Mass Flow Inlet [kg/s] Turbulence ntensty [%] Hydraulc dameter [m] Speces composton Prmary Ar m Dry Ar Secondary Ar See Test Matrx Dry Ar 10 % In the appendx m Or Wet Ar Fuel m Pure Hydrogen Tab. 4-5: Mass flow nlet BC. Snce no turbulence data of the nflows are avalable the default value for the turbulence ntensty s retaned. The values of the mass flow nlets depend on the partcular test case consdered, the complete lst s reported n the appendx. The prmary ar enterng the doman s always dry because t comes from a compressor wth a dryng system. The secondary ar s dry or wet because at ths tme no nformaton s gathered about the source, probably t wll be fed from the exteror (wet ar) by a fan but t s not for sure. Furthermore, addng vapour water to the ar, the effect of the mosture on the pollutant emssons can be nvestgated. 61

62 5 Results The present chapter reports the fundamental results of ths research: the study of the performances of a double cavty trapped vortex combustor feed wth hydrogen. The man parameters vared for the smulatons are descrbed n the followng paragraph. The smulatons are made wth the am of understandng the behavour of the combustor n dfferent workng condtons, to verfy transent effects n the cavtes and to descrbe the pecular flow structures of ths knd of combustor. Therefore the analyss presented s focused on descrbng the essental features of the TVC wth partcular regard on the temperature, the emssons, the velocty flow felds and the spectra, the last obtaned analysng unsteady data from some probes put n the computatonal doman. 5.1 Test Matrx The man parameters nfluencng the workng condtons and vared durng the smulatons are: Fuel o Hydrogen; Power: three man powers 21, 42, 84 kw needed to make avalable data for the comparson wth another power plant n ENEA Casacca; Gven three fxed secondary ar veloctes the power has been reduced startng from 84 kw to some kw; Dstance between the forebody and the afterbody: 60, 65, 70, 75 and 80 mm; Equvalence rato Φ (prmary and secondary); the quantty changed are: o Fuel mass flow rate; o Prmary ar mass flow rate; o Secondary ar mass flow rate; Secondary ar composton: o Ar wth mosture; o Gas from a combustor; o Wthout prmary ar; Tme dependence: steady and unsteady. The complete lst of the steady test cases studed s reported n the appendx. The prmary ar and fuel jets control the local equvalence rato, mxng, resdence tme, and stablty n the prmary zone. The maxmum secondary (overall) equvalence rato s about 0.2 whch s below the LBO lmt of typcal swrl or bluff-body stablzed flames. The actual equvalence rato n the prmary zone may be less than that reported n the test matrx due to the entranment of annular ar. However, the amount of entranment changes wth prmary ar and fuel flows and has not been evaluated. Furthermore, some prelmnary analyss wth methane has been made but the results are not presented n ths thess. 62

63 5.2 Flow feld and flame The flowfeld n a small or large cavtes s qute complex as a result of the formaton of several vortces. Because of the presence of the cavtes, the secondary ar s expanded toward the centerbody, whch, n turn, results n flow separatons on the outer wall. The CFD solutons do provde valuable qualtatve nformaton to more fully understand the vortex structures n the cavtes and the flow separatons, but the present smulatons predct only the large-scale vortcal structures n the cavty, small scales mportant for mxng on a local level are not resolved. Fg. 5-1 shows the trapped vortces behnd the TVC dsks. As can be seen n the fgure, several dstnct vortces are predcted n the TVC cavtes. The velocty feld shows a strong recrculatng flow that s expected for torodal vortces. The man vortex n the frst cavty rotates wth the secondary ar flow drecton Fg. 5-2a due to the placement of the fuel and ar njecton ponts. But dependng on the locaton and flow rates of the njecton ponts, cavty vortces rotatng n opposton or wth the secondary ar flow drecton can be generated [Straub et al. 2000]. The vortces n the second cavty Fg. 5-2b and behnd the second afterbody Fg. 5-2c also rotate wth the annular ar flow. Fg. 5-1: TVC Velocty Pathlnes. Test case: TVC-1. a) 63

64 b) c) Fg. 5-2: Velocty vector plots of the flow n the TVC. Test case :TVC-1. In general the center fuel jet s mxed wth the neghbourng ar jets and burns as a sngular flame. The Fg. 5-3 suggests that flow n the frst cavty has a large vortex generated by the secondary ar flow and a secondary vortex that have developed prmarly from the nteracton of fuel and ar jets njected n the cavty. There s also a thrd vortex n the left corner due to the moton of the mxture flow. The vortex trapped n the cavty provdes a suffcently stable gnton source through rengeston of hot products and radcals back nto the recrculaton zone, as ndcated by the velocty feld. 64

65 Fg. 5-3: Man vortces n the frst cavty. The general structure of the vortex appeared to be the same for a wde range of fuel and ar flow condtons. Decreasng the secondary ar mass flow rate the large vortex reduces ts dmensons (Fg. 5-4) reducng the amount of re-ngested hot products. Instead, wth the secondary ar mass flow rate fxed, decreasng the prmary ar mass flow rate the vortex progressvely grows occupyng the entre cavty (Fg. 5-5). Best results n terms of mxng are obtaned when the vortex occupes the entre cavty. 65

66 Fg. 5-4: Effect of the secondary ar mass flow rate reducton on the trapped vortex dmensons. 66

67 Fg. 5-5: Effect of the prmary mass flow rate reducton on the trapped vortex dmensons. For what concern the flame front, t depends on the varous regmes establshed n the combustor. Increasng the power (Fg. 5-6) or decreasng the prmary (Fg. 5-8 and Fg. 5-9) or secondary (Fg. 5-7) ar flow rate the flame bulges out radally toward the annular ar, can reach the second cavty or pass the second afterbody. In the last case the flame can have a length comparable wth the TVC length (47.5 cm). Flame lengths are mportant n practcal combustng devces. Arcraft gas turbne combustors, for example, are typcally short and compact. Ths s very desrable because the combustor weght s proportonal to the sze. A lght-weght combustor results n a good thrust to weght rato. Modern combustors are 15 to 25 cm long. Ths means that the flame must be confned to ths length, otherwse, combuston wll take place around the nozzle gude vanes and n the turbne. Ths may lead to poor turbne performance and short operatng lfe. When the flame s confned n the frst cavty, the prmary zone may not be affected by entranment of cold annular ar, whch f entraned, would lower the flame temperature. The hgher temperatures at ths condton result n good combuston effcency, but hgher NOx. 67

68 TVC-50: P=1.313 kw TVC-49: P=2.625 kw TVC-48: P=5.25 kw TVC-47: P=10.5 kw TVC-1: P=21 kw TVC-16: P=42 kw 68

69 TVC-31: P=84 kw Fg. 5-6: Mass fracton of OH. Effect of the power on the flame structure. The prmary and secondary ar mass flow rate are always the same. The dsk dstance s set to 60 mm. TVC-59: Φ overall =0.054 TVC-65: Φ overall =

70 TVC-71: Φ overall =0.2 Fg. 5-7: Mass fracton of OH. Effect of lowerng the secondary ar on the flame structure. The power s 21 kw. The dsk dstance s 60 mm. The prmary ar flow rate s the same (Φ prm =2). TVC-65: Φ prm =2 TVC-66: Φ prm =4 70

71 TVC-67: Φ prm =8 Fg. 5-8: Mass fracton of OH. Effect of lowerng the prmary ar on flame structure. The power s 21 kw. The dsk dstance s 60 mm. The secondary ar flow rate s the same (Φ overall =0.11). TVC-71: Φ prm =2 TVC-72: Φ prm =4 71

72 TVC-73: Φ prm =8 Fg. 5-9: Mass fracton of OH. Effect of lowerng the prmary ar on flame structure. The power s 21 kw. The dsk dstance s 60 mm. The secondary ar flow rate s the same (Φ overall =0.21). 5.3 Effect of mosture The effect of mosture on the combuston characterstcs of the TVC has been tested addng a fxed value of water mass fracton n the secondary ar composton. The smulated cases nclude three powers: 21, 42 and 84 kw. For each power the dstance of the dsk s vared by 5 mm from 60 to 80 mm. The Fg to Fg show that the maxmum temperature and the mean ext temperature decreases addng mosture. The mean temperature varaton s very low due to the low rato of water njected wth the secondary ar. Addng more water the varaton wll be more sgnfcant. The combuston effcency n Fg shows that n almost all cases there s not a drect correlaton wth the added water. It s notced that for the 21 kw power as mosture s added, most of the fuel s reacted except for 70 mm dstance. For hgher powers the effcency varaton s too low to nfer a substantal concluson and the values of the effcency are hgher than 98%. Ths means that only a maxmum of 2% of the fuel s not consumed. The addcton of the ambent humdty to the ar decreases the NOx emsson ndces as shown n Fg [Barlow 1999]. 72

73 Maxmum Temperature Temperature [K] kw 42 kw 84 kw 21 kw dry 42 kw dry 84 kw dry Dsk Dstance [mm] Fg. 5-10: Maxmum temperature n the TVC as a functon of the dsk dstance for dry and humd ar. Mean Ext Temperature Temperature [K] kw 42 kw 84 kw 21 kw dry 42 kw dry 84 kw dry Dsk Dstance [mm] Fg. 5-11: Mean ext temperature as a functon of the dsk dstance for dry and humd ar. 73

74 Mean Ext Temperature Temperature [K] kw 21 kw dry Dsk Dstance [mm] Fg. 5-12: Mean ext temperature as a functon of the dsk dstance for dry and humd ar: 21kW power. Mean Ext Temperature Temperature [K] kw 42 kw dry Dsk Dstance [mm] Fg. 5-13: Mean ext temperature as a functon of the dsk dstance for dry and humd ar: 42kW power. 74

75 Mean Ext Temperature Temperature [K] kw 84 kw dry Dsk Dstance [mm] Fg. 5-14: Mean ext temperature as a functon of the dsk dstance for dry and humd ar: 84kW power. Combuston effcency as a functon of dstance % 99.50% Combuston effcency 99.00% 98.50% 21 kw 42 kw 84 kw 21 kw dry 42 kw dry 84 kw dry 98.00% 97.50% Dsk Dstance [mm] Fg. 5-15: Combuston effcency as a functon of the dsk dstance for dry and humd ar. 75

76 EI NOx at the ext EI NOx [g/kg fuel] kw 42 kw 84 kw 21 kw dry 42 kw dry 84 kw dry Dsk Dstance [mm] Fg. 5-16: NOx emsson ndces as a functon of the dsk dstance for dry and humd ar. 5.4 Effect of the prmary and secondary equvalence rato The Fg to Fg present data on mean and maxmum temperature, combuston effcency, unburned hydrocarbons (UHC), and oxdes of ntrogen (NOx) for the two cavtes combustor. The results are plotted as a functon of Φ p. Here, as mentoned before, the combuston effcency s calculated from the percentage of burned fuel [HSU 1995]. The quantty of fuel njected s fxed at a mass flow rate correspondng to a power of 21 kw and the annular / secondary ar flow s fxed at three quanttes correspondng to veloctes of 11, 22 and 44 m/s. Emssons data are then collected for dfferent prmary ar flow rates. The fgures show that the TVC works n a wde range of prmary equvalence rato even for very low quantty of prmary ar. When the prmary ar flow s low the ar needed to burn the fuel s entraned nto the trapped vortex from the secondary flow. The mean ext temperature, the values of the combuston effcency and the NOx emssons decrease as the secondary ar ncrease. For the cases analysed, there s not a drect correlaton wth lower value of the prmary equvalence rato. The values of UHC and NOx emsson ndex are very smlar to those reported n Hsu

77 Maxmum Temperature as a functon of φ p Temperature [K] m/s 22 m/s 11 m/s φ p Fg. 5-17: Maxmum temperature n the TVC as a functon of the prmary equvalence rato for three secondary ar flow veloctes. Mean Ext Temperature as a functon of φ p Temperature [K] m/s 22 m/s 11 m/s φ p Fg. 5-18: Mean ext temperature n the TVC as a functon of the prmary equvalence rato for three secondary ar flow veloctes. 77

78 Combuston effcency as a functon of φ p % 99.50% 99.00% UHC [ppm] 98.50% 44 m/s 22 m/s 11 m/s 98.00% 97.50% 97.00% φ p Fg. 5-19: Impact of prmary ar and annular ar on the combuston effcency at a fxed fuel flow. EI UHC emssons as a functon of φ p EI UHC [g/kg fuel] m/s 22 m/s 11 m/s φ p Fg. 5-20: Impact of prmary ar and annular ar on the percentage of UHC at a fxed fuel flow. 78

79 EI NOx emssons as a functon of φ p EI NOx [g/kg fuel] m/s 22 m/s 11 m/s φ p Fg. 5-21: Impact of prmary ar and annular ar on the percentage of the oxdes of ntrogen at a fxed fuel flow. 5.5 Combuston nstabltes Several nvestgators [Lttle 1979 and Mar 1965] demonstrated that mountng two dsk n tandem can reduce the drag resultng from the unsteady vortex moton behnd the dsks. Flow oscllatons, studed n works such as Gharb 1987, were found to be establshed under certan geometrc aspect rato and resulted from mpngement assocated wth shear layer and cavty corner. In the TVC, n addton to the oscllatory phenomena for non reactng flow t must be consdered nstabltes due to the combuston process. In Combuston, the nstabltes are a result of the resonant nteracton between two or more physcal mechansms. A drvng process generates the perturbatons of the flow; a feed back process couples the perturbaton to the drvng mechansm and produces the resonant nteracton that may lead to oscllatory combuston. The heat release can amplfy certan flow frequences [Hsu 1999]. The flame stablty n the TVC s drectly related to the annular ar and ts nteracton wth the cavty. There are two possble nteractons: The shear layer along the nterface between the annular and cavty regons; The mpngement regon near the afterbody. The frst physcal process ncreases wth annular ar velocty and can affect flame stablty n the cavty. If the cavty s small, the dsturbance n the shear layer s less lkely to have mpact on the cavty. Increasng the cavty length, the possblty of dsturbance from shear layer and flow mpngement at the upstream face of the afterbody become hgher. The last case results n hgh pressure fluctuatons and hgh drag, and reduces the stablty of the cavty combuston. 79

80 Three cases are smulated to study ths phenomenon. The prmary equvalence rato s not mantaned constant whle the power s set to 21 kw. In the frst two cases the dsk dstance s fxed at 60 mm whle n the thrd case at 70mm. For all cases the tme step s fxed at ms accordng to Katta (1996). The mass flow rates are set to obtan the prmary and overall equvalence ratos reported n Tab Test Case Power [kw] Dsk Dstance Φ prm Φ overall Tab. 5-1: Unsteady analyss test matrx. There s an addtonal test case not reported n the table havng the same BCs as the frst but wthout the reactons (cold case). The study of a cold case s of nterest to understand the nature of the observed nstabltes. Power spectral densty of the velocty, the pressure and the temperature for the frst case at one locaton nsde the cavty (Fg. 5-22) are shown n Fg to Fg. 5-25, respectvely. In every spectrum, as expected, there s a domnant frequency peak n the lower frequency range. The peak s placed at about 40 Hz correspondng to St= Accordng e.g. to Kya, Sasak 1983 the low value of St s not compatble wth the shear layer nstablty but t may be due to a flappng moton of the recrculaton bubble. In the cold case the shear layer s stable but n the hot case when the fuel jet reaches the forebody nsde the small cavty, establshes an oscllatng flame. It s therefore demonstrated that ths nstablty s due to the nteracton of the annular shear layer and the heat released n the combuston process. The frequency of the flame oscllaton corresponds to a tme delay of 25 ms. The oscllatng phenomena s vsualzed through a tme sequence of OH contour plots shown n Fg The tme nterval between two consecutve mages s 2 ms. It can be notced how ntally the flame s attached to the forebody, then as tme goes on the flame progressvely detaches from the forebody, reaches a maxmum dstance (t=12ms) and after about 25 ms t returns to the startng poston. The nstantaneous solutons of ths flow revealed that the vortces n the cavty are not sheddng but movng back and forth wthn the cavty. 80

81 Fg. 5-22: Poston of the probe n the computatonal doman for the frst case Velocty Spectrum 10 1 PSD [m 2 /(s 2 Hz)] Frequency [Hz] Fg. 5-23: Velocty spectrum for the frst case: Φ=1, P=21 kw, H=60mm. 81

82 10 7 Pressure Spectrum 10 6 PSD [Pa 2 /Hz] Frequency [Hz] Fg. 5-24: Pressure spectrum for the frst case: Φ=1, P=21 kw, H=60mm Temperature Spectrum 10 4 PSD [K 2 /Hz] Frequency [Hz] Fg. 5-25: Temperature spectrum for the frst case: Φ=1, P=21 kw, H=60mm. 82

83 t 0 =0 ms Δt=2 ms Δt=4 ms Δt=6 ms Δt=8 ms Δt=10 ms Δt=12 ms Δt=14 ms 83

84 Δt=16 ms Δt=18 ms Δt=20 ms Δt=22 ms Δt=24 ms Δt=26 ms Fg. 5-26: Tme sequence contour plot of OH concentraton for the frst case vsualzng the oscllatng phenomena due to the nteracton of the heat release wth the shear layer at 40 Hz. As the prmary ar flow s reduced, keepng the same dstance between dsks, the fuel jet s pushed further toward the centrelne of the burner, ncreasng the possblty of the flow mpngement. The combnaton of the ncreased level of flow mpngement and lack of fuel transport towards the edge of the forebody s beleved to be the reason of a reduced flame stablty. The mpngement of the flame on the frst afterbody s shown n Fg In the sequence after the mpngement, the flame returns to the ntal state (t 0 ). Ths oscllatng regme can lead to the flame blowout. 84

85 t 0 =0 ms Δt=2 ms Δt=4 ms Δt=6 ms Δt=8 ms Δt=10 ms Δt=12 ms Δt=14 ms Δt=16 ms Δt=18 ms Δt=20 ms Δt=22 ms 85

86 Δt=24 ms Δt=26 ms Δt=28 ms Δt=30 ms Δt=32 ms Δt=34 ms Δt=36 ms Δt=38 ms Δt=40 ms Δt=42 ms Fg. 5-27: Tme sequence of OH Contour plots showng the mpngement near the frst afterbody for the second case. Under certan flow condtons the flame can reach the blowout lmt. LBO s an mportant parameter because t determnes the lower operatng lmts of the engne. In gas turbne combustors, LBO s often preceded by an unstable flame. Ths s beleved to occur because of non unform mxng process n the swrl stablzed recrculaton zone. Ths results n ntermttent, flame-vortex nteractons that locally quench the flame. Ths ntermttency n combuston drectly mpact the gnton source, therefore, the unsteadness tends to grow untl the flame blow out. Durng blowout tests performed on a TVC (Hsu 1999), two types of blowout have been observed: one smooth and another one abrupt. The last one has been obtaned for the second case of Tab. 5-1 and s shown as a tme sequence n Fg Instead of a gradual dsappearance of the shear layer flame, as n the smooth blowout case, the flame becomes ntermttent and blows out abruptly. Ths 86

87 type of abrupt blowout s very mportant because t s preceded by the hgher level of audble nose and ntermttent combuston. t 0 =0 ms Δt=2 ms Δt=4 ms Δt=6 ms Δt=8 ms Δt=10 ms Δt=12 ms Δt=14 ms Δt=16 ms Δt=18 ms Δt=20 ms Δt=22 ms 87

88 Δt=24 ms Δt=26 ms Δt=28 ms Δt=30 ms Fg. 5-28: Tme sequence of OH contour plots showng a possble blow-out. In the thrd case under study the fuel s not consumed n the prmary vortex but s transported n the shear layer at the rm of the frst afterbody obtanng a flame that bulges out the frst cavty. The flame s anchored, not stable but t does not reach the blow out lmt. The oscllatons take place n the two trapped vortex cavtes and behnd the bluff body represented by the second afterbody. The second vortex n the second cavty serves as a secondary mxng and combustng stage because some fracton of the reactng mxture burns out when mxed wth the hot products that are transported out of the vortex. Ths s confrmed by the presence of OH radcals n the second cavty (see Fg. 5-36). The flame and the hot products are then perodcally convected downstream of the second cavty and entraned nto the wake regon behnd the second afterbody (Fg Δt=24ms to Δt=36 ms) or carred on downstream toward the TVC ext. Power spectral denstes of the velocty, pressure and temperature for the thrd case at locaton n 1 (Fg. 5-22) nsde the cavty are shown from Fg to Fg. 5-33, respectvely. Two addtonal probes have been added nto the doman (locaton n 2 and 3 n Fg. 5-22). For those probes only the temperature spectra are presented. In every spectrum there s not a domnant frequency peak n the lower frequency range as n the frst case but a seres of peaks due to the oscllatons of the vortces n the three cavtes (Fg and Fg. 5-35) Fg. 5-29: Postons of the temperature probe for the thrd case. 88