Numerical Analysis of Thermal Load Variation in a Commercial Engine during Dual Fuel Operation

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1 Proceedngs of Montreal 2018 Global Power and Propulson Forum 7 th 9 th May, GPPS-NA Numercal Analyss of Thermal Load Varaton n a Commercal Engne durng Dual Fuel Operaton Marco Konle MTU Aero Engnes AG Marco.Konle@mtu.de Munch, Germany Thomas Wattrant MTU Aero Engnes AG Thomas.Wattrant@mtu.de Munch, Germany Ludovc de Gullebon MTU Aero Engnes AG Ludovc.DeGullebon@mtu.de Munch, Germany ABSTRACT The numercal descrpton of combuston provdes a good understandng of the thermal boundary condtons of a combustor already n the early desgn phase. To predct accurately the thermal load on combustor walls, the numercal modelng of such components has to cover a lot of physcal aspects. Besdes the precse predcton of the reactng flow feld, the gaseous and soot radaton as well as the heat transfer onto the combustor walls have to be taken nto account. Due to the hgh demand of computatonal power for the descrbed modelng depth, MTU used the open source CFD software OpenFOAM and developed a mult-physcs solver to capture all these aspects. The valdaton of the solver was accomplshed wth dfferent academc and sem-techncal test cases n the past. These cases covered gaseous as well as lqud fuels. Ths paper descrbes the nvestgaton of the mpact of the fuel swtch from natural gas to lqud fuels for a commercal combustor. For ths purpose, the thermal load caused by soot radaton s compared for natural gas and Jet-A operaton. The detaled comparson of the combustor lners heat fluxes and temperature felds reveal sgnfcant dfferences. INTRODUCTION The formaton of soot n the combuston process s a very complex topc. Due to ths complexty, there are many numercal models for soot predctons publshed. Models as e.g. the Magnussen model of the 1970 s answer the queston of soot formaton wth moderate efforts, hgh sophstcated models predctng even soot partcle sze dstrbutons as e.g. Eberle et al ncrease the requrement of computatonal power sgnfcantly. Addtonally, there are only lmted valdaton data avalable to confrm the accuracy of the dfferent approaches wth respect to techncal applcatons as combustors nvestgated n ths paper. Intated n 2010 by the European research project FIRST, the combuston group of the DLR Stuttgart desgned a swrl stablzed combustor and analysed t wth the focus on the soot formaton durng C 2 H 4 operaton (Gegle et al. 2014). The wde range of operaton tested for ths combustor covers varatons of ar-fuel ratos, thermal power, amount of secondary coolng ar, and operaton pressure. Chosen as one test case for the Internatonal Sootng Flame workshop ISF 2016, the publshed data for ths combustor desgn ncludes velocty measurements, chemlumnescence pctures, CARS temperature data, and LII soot nformaton. Publcatons of numercal research dealng wth the FIRST data to valdate ther approaches are e.g. Eberle et al and Wck et al MTU used ths data set to valdate an n-house multphyscs solver mplemented n the open source CFD code OpenFOAM. Addtonal data of the European Projects TECC-AE and TIMECOP-AE were used to valdate the solver s accuracy for lqud fuel operated combustors. More detals about the valdaton were publshed n Konle et al Due to ts comprehensve accuracy, the valdated solver was appled for re-desgn studes of a commercal combustor (Konle et al. 2018). Well aware of the lmted valdaton for lqud fuel operaton wth respect to soot formaton, purpose of the study reported here s to analyze the mpact of dual fuel operaton on commercal combustors va purely numercal studes and back-to-back comparsons. METHODOLOGY To predct accurately the thermal load on combustor walls, the numercal modelng of such components has to cover a lot of physcal aspects. The transent character of the reactng flow feld of the combustor nvestgated n ths paper requres a temporal resoluton and, thus, the URANS approach (SST k-) s chosen to descrbe the flow feld and turbulence. The solver ncludes addtonally gaseous radaton as well as the radatve heat load contrbuted by soot formed n the combuston process. Fnally, the conjugate heat transfer provdes the nformaton requred to evaluate

2 the combustor desgn wth respect to thermal loads and structural lfe. Due to ts mportance, the modellng of combuston, radaton, and soot formaton are presented n the followng. For more detals see Konle et al Turbulence and Combuston Modelng The mass, momentum, speces, and enthalpy conservaton equatons below are solved to predct the flow behavor and are solved wthn the Unsteady RANS turbulence approach accordng Menter 1994, the SST k- model. t t t t u~ 0 (1) x p u u~ u~ T ~ (2) j j j x x x ~ ~ (3) x x Sc Sc t x ~ t Y k Yk Yk u~ k ~ ~ (4) x x t x Pr Pr ~ t h h h u ~ T rad where the subscrpts and j represent the x and x j drecton components, k the speces and t the turbulent part (Ponsot and Veynante 2005). u s the velocty, ρ the densty, p the pressure, Y k the mass fracton of speces k, and h the enthalpy. µ and µ t are the molecular and turbulent vscosty, Sc and Sc t the lamnar and turbulent Schmdt number, Pr and Pr t the lamnar and turbulent Prandtl number. τ j and T j symbolze the vscous and the Reynold stresses. Moreover, ω k, ω T, and ω rad are source terms for the equatons. The producton and consumpton of reactve speces due to combuston s descrbed by ω k and heat release due to combuston by ω T. ω rad represents the radaton effects. The solver used by the authors s ntended to calculate flow felds wth Mach numbers below 0.3. The combuston s solved usng the Partally Strred Reactor model (PaSR) and, wth t, the emprcal Arrhenus law (Kärrholm 2008). The PaSR model s a fnte rate chemstry method whch assumes that the real flame s much thnner than any computatonal cell. Thus, each cell s dvded nto a reactng part, n whch all present speces are homogeneously mxed and react together, and a non-reactng part. The reactve volume fracton κ, whch s a multplyng factor for ω k and ω T n Eq. (3) and (4), results of the turbulence-chemstry nteractons: mx j c (5) c mx t Cmx T C (6) mx where τ c represents the chemcal tmescale and τ mx the turbulent tmescale. τ η and τ T are the Kolmogorov and Taylor tmescale, respectvely, and C mx a model constant. Ths constant as well as the turbulent Schmdt number Sc t for the thermal dffuson were calbrated va measured temperature profles at the ext of the combustors used for the valdaton [2-4]. Two dfferent fuels has been used n the further nvestgatons: methane (CH 4 ) and Jet-A. The Jet-A has been supposed to be mmedately evaporated enterng the chamber, n order to handle t as gas phase only. That ths assumpton for the smulated boundary condtons (.e. nlet pressure and temperature) provdes stll acceptable results was proved by smulatons of the so-called Tmecop-AE combustor (Meer et al. 2012). The chemcal reacton was descrbed by the Arrhenus nputs for one step chemstry of methane. Radaton Modelng Hgh temperatures n combustors lead to sgnfcant radatve heat transfer. To cover the contrbuton of the hot walls as well as the gaseous radaton to the thermal loads on the combustor, ths phenomenon has to be taken nto account. To predct radaton, two aspects has to be consdered: the spatal and the spectral radaton. In OpenFOAM, the fvdom (fnte volume Dscrete Ordnate Method) solves a smplfed form (Eq. 7) of the radatve transfer equaton (Vskanta and Mengüç, 1987) for a fnte number of dscrete sold angles (16 rays n the present study) and returns the ω rad source term term nto the enthalpy equaton (Eq. 4). 4 I s I T. (7) x wth I the radaton ntensty, s the x -component of the drecton vector, Ω the sold angle consdered, T the temperature and σ the Stefan-Boltzmann constant. The absorpton coeffcent of gas, α, s calculated wth the spectral model. In ths study, the grey gas model s used and takes nto account the absorpton/emsson of the two speces CO 2 and H 2 O (Barlow et al. 2001). Due to the sgnfcance of soot radaton, the contrbuton of soot to the gaseous radaton were added to the standard mplementaton. The absorpton/emsson coeffcent s calculated as followng. 5 pk X k b, kt bsoot fv, soott. (8) k 1atm 0 wth X k the molar fracton of spece k, CO 2 and H 2 O, p k the partal pressure, b,k and b soot constant of the model and f v,soot the soot volume fracton. Soot Modelng For the soot formaton n the combustor, the twoequaton model of Magnussen/Tesner s mplemented (Magnussen 1989). Ths sem-emprcal soot model calculates n a frst step the specfc concentraton of radcal nucle X N and n a second step the formaton of soot Y S formed by these nucle, Eq. (9) and (10). The oxdaton of soot partcles ω S,o s modelled by scalng the reacton rate of the combuston model wth the rato of the soot and the fuel mass fracton. The lmtaton to a maxmum soot level s mplemented, by usng the total amount of carbon provded 2

3 by the fuel (Kleveland 2005). Y l represents ths lmter n Eq. (11) and (12). Besdes that, the mpact of turbulence on the mean reacton rate accordng the Eddy Dsspaton Concept lmts the soot formaton and s calculated va the coeffcent γ * n Eq. (13). (ρ X N) + (ρ X Nu ) = (( μ Pr + μ t ) X N ) + t x x Pr t x ω N,f + ω N,o. (9) (ρ Y S) + (ρ Y Su ) = (( μ Pr + μ t ) Y S ) + t x x Pr t x ω S,f + ω S,o. (10) g 0a ω N,f = γ (n 0 + (f g)ρ Y lx N ρ X NY S). (11) bf c Y fuel ω S,f = γ ((f c Y fuel Y S)bρ 2 Y lx N). (12) γ = 9.7 ( μ ε ρ k 2)0.75. (13) ω N,f, ω S,f, ω N,o and ω S,o are formaton and oxdaton source term for the nucle and the soot. n 0, the spontaneous formaton of radcal nucle of the fuel, s calculated usng the Arrhenus approach. Y fuel s the fuel mass fracton, whle f c s the carbon mass fracton of the fuel. The model constants a, b, f, g and g 0 are defned by Magnussen. The valdaton of the soot formaton was conducted wth the FIRST data (Konle et al. 2017). In fg. 1, the contrbuton of soot radaton to the overall radatve energy emsson s shown. As one can see, the soot radaton s about 50% of the radatve load. Ths number already shows for ths academc test case operated wth ethylene the mportance of the rght soot formaton predcton. Fg. 2: Cut plane of the computatonal doman of commercal combustor nvestgated n ths study. RESULTS AND DISCUSSION In the followng, frstly, the macroscopc feld nformaton of velocty, temperature, and reacton rate s dscussed for the operaton wth CH 4 and Jet-A, respectvely. Afterwards, the soot partcles radaton for these dfferent operatons wll be compared. Fg.1: Tme-averaged soot contrbuton to the absorpton/emsson coeffcent for the FIRST combustor APPLICATION TO A COMMERCIAL COMBUSTOR The objectve of the paper s the numercal analyss of the mpact of dual fuel operaton on a commercal engne desgn. Fur ths purpose, a combustor desgn studed durng re-desgn actvtes was chosen (Konle et al. 2018). In fg. 2, the numercal doman of ths combustor s shown. The doman covers the dffusor casng, the combustor head ncludng the swrler and njector, and the combuston chamber of the reverse flow combustor. Fg. 3: Tme-averaged numercal results for CH 4 operaton: a) velocty magntude, b) temperature feld, c) reacton source term d) soot volume fracton. 3

4 Flud Feld Analyss Fgure 3 (a-c) shows these three felds for the combustor operated wth natural gas. Due to the conventonal combustor desgn, the reacton zone s spread all over the axal length of the combuston chamber. The formaton of soot partcles (fg. 3d) s concentrated n the areas of hgh fuel concentraton (close to the combustor head) and n the regon of fnal fuel oxdaton drven by the dluton ar njected through the combustor lner segments. All felds n fg. 3 are tme averaged felds n the order of 20ms. Whle for the velocty and temperature felds ths tme frame s suffcent to judge the flame shape, the process of soot formaton wthn ths tme may not be representatve for a long-term operaton of the combustor. Therefore, the radatve energy emtted by the soot partcles wll be analyzed purely n a back-to-back comparson. cone of hgh velocty n the near feld of the combustor head, the lqud fuel from a mass perspectve due to the lower heat value even larger than the CH 4 flow breaks up more rapdly and, thus, changes the fuel dstrbuton n ths area sgnfcantly. As consequence, the local temperature feld (fg. 4b) caused by the changed reacton rate dstrbuton (c) leads to sgnfcant changes n the local thermal load on the combustor lners. Addtonally, the soot formaton for Jet-A operaton (fg. 4d) s sgnfcantly ncreased. Whle the volumetrc concentraton of soot partcles for the CH 4 operaton s n the order of ~1 ppm, the ncreased carbon content n the lqud fuel leads to concentratons 100 tmes larger. An ncrease of the soot partcles concentraton for the swtch of gaseous fuel to lqud fuel s not surprsng, however, ths factor ndcates that the mpact of radaton caused by soot partcles has to be analyzed. Fg. 4: Tme-averaged numercal results for Jet-A operaton: a) velocty magntude, b) temperature feld, c) reacton source term d) soot volume fracton. Soot Partcles Radaton For the numercal study presented here, the soot model s only adjusted wth the chemcal nformaton of C concentraton n the fuel. For the case of CH 4 operaton, the low concentraton of soot partcles n the flame leads to a very low radatve energy emsson. The contrbuton of soot radaton on the wall heat flux can be neglected. Calculatng the soot formaton for Jet-A operaton, however, the soot concentraton s sgnfcantly ncreased. As mentoned n prevous secton, the soot partcles concentraton nsde the combuston chamber s around 100 tmes hgher than n CH 4 operaton. Analyzng the contrbuton of soot radaton on the total radatve heat release shows smlar results as the FIRST combustor. Fg. 5 reveals also for the commercal combustor desgn operated wth Jet-A a soot contrbuton of ~50 % to the overall radatve heat load. On hgh level, n comparson to the CH 4 operaton, the Jet-A operaton ncreases the radatve heat load by 50% due to the massve presence of soot partcles. From techncal perspectve t s of nterest to determne f ths hgher radatve heat load leads also to a sgnfcant ncrease of wall heat fluxes and, thus, to hgher combustor lner temperatures. To answer ths queston, not only the emsson of energy va radaton, but also the absorpton of energy by the soot partcles were analyzed. Due to the strong absorpton coeffcent, the sootng partcles absorb also roughly the half of the emtted energy n the combustor. Assumng a comparable temperature feld for both, CH 4 and Jet-A operaton, by just summng up the dfferent contrbutons n emsson and absorpton, t can be estmated that the emtted and absorbed energy of the sootng partcles ncreases the radatve heat load on the combustor walls by approxmately 25 % n comparson to the CH 4 case. Fgure 4 presents n the same way as fg. 3 the feld nformaton for Jet-A operaton. Focused on the dfferences, one can see the mpact of the dfference n densty on the velocty feld (fg. 4a). Whle the gaseous fuel develops a 4

5 Fg. 5: Tme-averaged soot contrbuton to absorpton/emsson coeffcent for the commercal combustor. Fg. 6 shows the comparson of analyzed wall heat fluxes on the frst and second combustor lner segments for CH 4 (a) and Jet-A (b) operaton. As one can see, the mpact of soot radaton s lmted to the regon of hgh soot concentraton, manly on the frst segment and the combustor dome. Summarzng the wall heat fluxes onto these lners, the radatve wall heat flux for the CH 4 case s n the order of 2100W, whle the correspondng heat flux for the Jet-A operaton sums up to 5000W. Fg. 6: Wall heat flux on the frst and second combustor lners for CH 4 (a) and Jet-A (b) operaton To compare the resultng wall temperatures for the combuston lners segments, conjugate heat transfer (CHT) smulatons were carred out. Due to the hgh dfferences n tme scales of sold and flud response, the study here used the approach to de-couple the flud doman and sold domans by freezng the nner flow feld. The results are shown n fg. 7: Whle for CH 4 operaton (a) the combustor lners see moderate temperatures, the mpact of the soot radaton for the Jet-A operaton (b) leads to a temperature ncrease n the order of ~100K. The comparson presented here, however, s not separated nto the mpact of soot radaton and all other heat fluxes. The changes n the flame shape and the resultng temperature feld (fg. 3 and fg. 4) lead also to changes n the solds temperatures and, thus, overlay the effect of radatve loads. Nevertheless, the addtonal load revealed n ths study requres addtonal nvestgatons. In areas of hgh thermal loads on the combustor lners, the potental effect of ncreased radaton caused by soot partcles may have to be taken nto account. Fg. 7: Materal temperatures of the frst and second combustor lners for CH 4 (a) and Jet-A (b) operaton CONCLUSIONS The paper presented results of combustor smulatons wth a mult-physcs solver based on the open source CFD package OpenFOAM. The n-house solver was appled to a commercal engne combustor and was used to nvestgate the mpact of dual fuel operaton. Due to dfferences n the soot formaton, an ncreased heat flux nto the combustor walls for the operaton wth Jet-A could be quantfed. CHT smulatons confrmed the ncrease n wall temperatures. The presented study here s based only on a very smple soot model. The valdty of the soot concentraton for lqud fuels as Jet-A has stll to be shown. Internal studes are ongong to close ths gap. Addtonally, t s also obvous that transent effects of the flame behavor overlay the radaton effect. A more precse predcton of combustor lner temperatures va conjugate heat transfer (CHT) smulatons wth a teratve approach and suffcent temporal averagng (Konle et al. 2018) wll be carred out n the near future to further separate the dfferent contrbutons. The current results of back-to-back comparsons confrm the necessty to evaluate the mpact of soot radaton n more detal. NOMENCLATURE f v,soot Soot volume fracton - h Sensble enthalpy per unt mass [J.kg -1 ] I Radaton ntensty [W.m -2 ] k Turbulent knetc energy [m 2.s -2 ] n 0 Spontaneous formaton of radcal nucle [mol.m -3.s -1 ] p Pressure [Pa] Pr Lamnar Prandtl number - Pr t Turbulent Prandtl number - s Drecton vector - Sc Lamnar Schmdt number - Sc t Turbulent Schmdt number - t Tme [s] T Temperature [K] T j Reynold stress tensor [N.m -2 ] u Velocty [m.s -1 ] X k Molar fracton of speces k - X N Specfc concentraton of radcal nucle [mol.kg -1 ] Y k Mass fracton of speces k - Y S Soot mass fracton - 5

6 α Absorpton coeffcent for radaton - γ * Turbulence nfluence coeffcent - ε Turbulent dsspaton rate [m 2.s -3 ] κ Reactve volume fracton - µ Molecular vscosty [kg.m -1.s -1 ] µ t Turbulent vscosty [kg.m -1.s -1 ] ρ Densty [kg.m -3 ] σ Stefan-Boltzmann constant [W.m -2.K -4 ] τ c Chemcal tmescale [s] τ j Vscous stress tensor [N.m -2 ] τ mx Turbulent tmescale [s] τ T Taylor tmescale [s] τ η Kolmogorov tmescale [s] ω Turbulent eddy frequency [s -1 ] ω k Mass reacton rate of spece k [kg.m -3.s -1 ] ω rad Enthalpy source term from radaton [W.m -3 ] ω T Heat release due to combuston [W.m -3 ] ω N,f Nucle formaton term [mol.m -3.s -1 ] ω N,o Nucle oxdaton term [mol.m -3.s -1 ] ω S,f Soot formaton term [kg.m -3.s -1 ] ω S,o Soot oxdaton term [kg.m -3.s -1 ] Ω Sold angle [sr] ā ã Reynolds averagng mean part for scalar a Favre averagng mean part for scalar a [9] Kärrholm F., Numercal modelng of desel spray njecton, turbulence nteracton and combuston, PhD Thess, Chalmers Unversty of Technology, Göteborg, [10] Vskanta R., Mengüç M.P., Radaton heat transfer n combuston systems, Progress n Energy and Combuston Scence, 13(2):97-160, [11] Barlow R.S., Karpets A. N., Frank J. H., Chen J.-Y., Scalar profles and NO formaton n lamnar opposed-flow partally premxed methane/ar flames, Combuston and Flame, 127: , [12] Magnussen B.F., Modelng of NOx and Soot Formaton by the Eddy Dsspaton Concept, Internatonal Flame Research Foundaton Frst Topc Orented Techncal Meetng, Amsterdam, [13] Kleveland R.N., Modelng of soot formaton and oxdaton n turbulent dffuson flames, PhD Thess, Norwegan Unversty of Scence and Technology, Trondhem, [14] Moss A., Galarça M.M., Brttes R., Velmo H.A., França F.H.R., Comparson of Spectral Models n the Combuston of Radatve Heat Transfer n Partcpatng Meda Composed of Gases and Soot, J. of the Braz. Soc. of Mech. Sc. & Eng., Vol. XXXIV, No. 2, REFERENCES [1] Gegle K. P., Hadef R., and Meer W., Soot formaton and flame characterzaton of an aero-engne model combustor burnng ethylene at elevated pressure, Journal of Engneerng for Gas Turbnes and Power, Vol.136, 2014, pp [2] Fretag S., Meer U., Henze J., Behrendt T., Hassa, C., Measurement of ntal condtons of a kerosene spray from a generc aero engne njector at elevated pressure, ILASS Europe 2010, 23 rd Annual Conference on Lqud Atomzaton and Spray Systems. [3] Meer U., Henze J., Fretag S., Hassa C. Spray and flame structure of a generc njector at aero engnes condtons, ASME Journal Gas Turb. Power, 134, [4] Konle M., de Gullebon L., Cotter F., Mult-Physcs Smulatons of an Aero Engne Combustor wth OpenFoam, Proceedngs of 1 st Global Power and Propulson Forum, GPPF , Zurch, [5] Konle M., de Gullebon L., Beebe C., Mult-Physcs Smulatons wth OpenFOAM n the Re-desgn of a Commercal Combustor, Proceedngs of ASME Turbo Expo 2018, GT , [6] Puggell S., Bertn D., Mazze L., Andren A., Assessment of scale-resolved computatonal flud dynamcs methods fort he nvestgaton of lean burn spray flames, ASME Journal Gas Turb. Power, 139, [7] Menter F.R., Two-equaton eddy-vscosty turbulence models for engneerng applcatons, AIAA-Journal, 32(8), 1994, pp [8] Ponsot T., Veynante D., Theoretcal and numercal combuston, Edwards, second edton,