SIMULATION OF COMBUSTION AND THERMAL FLOW IN AN INDUSTRIAL BOILER

Transcription

1 Proceedings of 7h Indusrial Energy Technology Conference May 11-1, 5, New Orleans, Louisiana SIMULATION OF COMBUSTION AND THERMAL FLOW IN AN INDUSTRIAL BOILER Raja Saripalli, Ting Wang Benjamin Day Research Assisan Professor Engineer Energy Conversion and Conservaion Cener Venice Naural Gas Processing Plan Universiy of New Orleans Dynegy Midsream Services, LLP New Orleans, LA7148- Venice, LA 791 ABSTRACT Indusrial boilers ha produce seam or elecric power represen a crucial faciliy for overall plan operaions. To mae he boiler more efficien, less emission (cleaner) and less prone o ube rupure problems, i is imporan o undersand he combusion and hermal flow behaviors inside he boiler. This sudy performs a deailed simulaion of combusion and hermal flow behaviors inside an indusrial boiler. The simulaions are conduced using he commercial CFD pacage FLUENT. The 3-D Navier-Soes equaions and five species ranspor equaions are solved wih he eddy-breaup combusion model. The simulaions are conduced in hree sages. In he firs sage, he enire boiler is simulaed wihou considering he seam ubes. In he second sage, a complee inensive calculaion is conduced o compue he flow and hea ransfer across abou 496 ubes. In he hird sage, he resuls of he sauraor/superheaer secions are used o calculae he hermal flow in he chimney. The resuls provide insigh ino he deailed hermal-flow and combusion in he boiler and showing possible reasons for superheaer ube rupure. The exhaus gas emperaure is consisen wih he acual resuls from he infrared hermograph inspecion. INTRODUCTION Boilers [3] are commonly used in indusries o burn fuel o generae process seam and elecric power. Due o economic and environmenal demands, engineers are required o coninuously improve he boiler efficiency and in he meanime reduce he emissions [8]. Compuer simulaion [] has been employed o undersand he hermal-flow and combusion phenomena in he boiler o resolve operaion problems and in search for opimal soluions. A siuaion may arise where he superheaer ubes brea due o excessive heaing [9], which may lead o boiler shudown and hus increase he expenses incurred. Overheaing of he superheaer ubes is prevened by using he appropriae maerials and designing he uni o accommodae he hea ransfer required for a given seam velociy hrough he superheaer ubes, based on he desired exi emperaure. In real applicaions, however, he operaion of he superheaer for producing highpressure, high-emperaure seam may resul in problems frequenly caused by rupured superheaer ubes. The damage or rupure of he superheaer ubes may be caused by many possible reasons including, galvanic corrosion, hermal conracion and expansion, composiion of he combusion gases, accumulaions of soo ouside he pipes, accumulaion of slag inside he pipe or high emperaure disribuion above maerial yield emperaure and high hermal sress. The damage caused by high emperaure can be minimized by providing uniform combusion and emperaure disribuion, eeping fouling resisance low, insallaion of soo-blowers o remove accumulaed soo and oher pariculaes, and opimizing he combusion condiions. The objecive of his paper is o help indusry o improve boiler s efficiency, reduce emissions, avoid rupure of superheaer ubes, and o undersand he hermal flow ranspor in he boiler. This sudy employs he Compuaional Fluid Dynamics (CFD) scheme o provide an overall picure of wha is happening wihin he boiler, maing i easy in mos cases o idenify he problems and develop a soluion. A CFD analysis provides fluid velociy, pressure, emperaure, and species concenraions hroughou he soluion domain. During he analysis, he geomery of he sysem or boundary condiions such as inle velociy and flow rae can be easily changed o view heir effecs on hermal-flow paerns or species concenraion disribuions. CFD can also

2 provide deailed parameric sudies ha can significanly reduce he amoun of experimenaion necessary o idenify problems and o opimize he operaing condiions. However, i mus be noed ha he accuracy of he simulaed resuls depends on he accuracy of models and he resoluion of compuaional scheme adoped. Burner Gases o chimney Flame Boiler ouer wall made of sauraor ubes Boiler inner walls Sauraed waer ubes Flue Gases PROBLEM SETUP AND MODELING The overall design of he sudied boiler is shown in Fig. 1. The model basically consiss of four secions, burner, combusion chamber, sauraor/ superheaor, and chimney (exhaus). The op view of he boiler is shown in Fig.. The burner is provided wih hree inles, wo for air inle and one for fuel inle. Primary and secondary air eners he burner as shown in Fig. 3. The primary air eners wih a swirl and is direced ouward. The air enering around he ouside periphery of he swirl air is defined as he secondary air [1]. I conribues o a conrolled expansion in he swirl secion of he burner where i reacs wih he unburned fuel from he cener reacion o complee he combusion process. The fuel used is Mehane, CH 4, which is burned in he combusion chamber and he flue gasses pass hrough he sauraed and superheaer ubes and exhaused hrough he chimney. Gases o chimney Boiler Ouer Wall Superheaer ubes 6x19 Boiler wall made of alernaing array x four passes superheaer and sauraor ubes (see deails in Figure 3.).9f 4.15 f (a) Top View Oule: Consan pressure P = 1 am 11.1 f 11.8 f Chimney: adiabaic 9.33 f (b) Side View Burner: Mass flow raes Fuel:.855 g/s Primary air: 6.9 g/s Secondary air: 13.8 g/s Boiler ouer walls: Consan emperaure T = K Gases o amosphere Combusion chamber walls: Consan emperaure T=56.68 K Figure 1. A 3-D view of he sudied boiler (seam ubes are no shown) showing he compuaional domain and boundary condiions for sage 1: The enire boiler is employed as he compuaional domain excluding waer/seam ubes. Drum Combusion chamber Burner (c) End View Figure. (a) Top view of he horizonal midplane of he boiler (b) Side view (c) Compressed end view looing owards he burner.

3 Secondary Air Primary Air wih swirl Secondary Air Primary Air, eners wih a sw irl Fuel (mehane) Secondary Air (a) (b) In energy equaion E is given as, E = h p v ρ h is sensible enhalpy and for incompressible flow i is given as h = p Y h j and j j ρ T h j = c p, jdt T ref T ref is consan aen as 98.15K (76.5 o F). S h in he energy equaion (3) is he source erm, which is provided by he ne enhalpy formaion raes from he species ranspor reacions. Fuel Inle Figure 3. (a) Schemaic of he burner geomery (b) Compuaional model of he 3-D view of he burner exi. The full hree-dimensional Navier-Soes equaions are employed wih five species ranspor equaions. The problem is modeled wih he following general assumpions: 1. The flow is seady and incompressible.. Variable fluid properies. 3. Turbulen flow. 4. Insananeous combusion wih he chemical reacion much faser han he urbulence ime scale. 5. The seam emperaure is assumed as he ube wall emperaure. Governing Equaions The conservaion equaions for mass, momenum and energy in general form are shown below. ρ ( ρv) = (1) r rr r r ( ρv) ( ρvv) = p ( τ) ρg F () r ( E) (V( E p)) r r eff T hjj j ( eff v) (3) ρ ρ = τ Sh j τ, he sress ensor is given by r r τ = µ T r ( v v ) v I 3 where I is he uni ensor. Boundary Condiions The flow and hermal variables are defined by he boundary condiions on he boundaries of he sudied model. Mass-flow inle condiions are applied a he hree inles in he burner. Pressure oule boundary condiion is applied a he oule and he walls are reaed as consan wall emperaure or adiabaic wall emperaure. The walls are saionary wih no-slip condiions applied on he wall surface. The deailed boundary condiions are summarized below. Fuel inle: m& fuel =.885 g/s Air inle: m& primary air = 6.9 g/s, m& secondary air = 13.8 g/s Oule: Consan pressure a P = 1 bar Walls: No-slip condiion: u =, v =, w =. Temperaure: Walls a he surfaces inside he combusion chamber are covered by he sauraing seam ubes a consan emperaure, T = K All he superheaers are se a he consan emperaure, T = 67.3 K Sauraors: All walls are se a T = K Superheaer secion: The ubes are alernaing beween sauraor (T = K) and superheaer (T = 67.3 K). Chimney walls: Adiabaic The specific hea of he species is emperaure dependan and is defined as a piecewise-polynomial funcion of emperaure. The physical properies are defined for he mixure maerial and he consiuen species.

4 Compuaional Domain In view of he complex geomery of he boiler, he simulaion is conduced in hree sages. Sage 1: In his sage of sudy, he compuaional domain includes he enire boiler bu excludes waer/seam ubes. The compuaional domain wih all boundary condiions is shown in Fig. 1. condiion for he sauraing/superheaing regions (Fig. 4b). A oal of 3 sub-secions of similar compuaional domain as shown in Fig. 4(b) are simulaed. Each uses he oule profile soluion of he previous (upsream) sub-secion as he inle profile boundary condiion. In his approach he inle pressure informaion will be calculaed and updaed o saisfy overall mass and momenum conservaion. Sauraor wall, T=56.68 K Burner Drum Sauraor ubes, T=56.68 K Inner walls, T=56.68 K Combusion chamber walls, T=56.68 K Ouer drum wall, T=56.68 K Alernaing sauraor/superheaor ubes as he wall Flow Oule, pressure condiion Alernaing sauraor/superheaer walls Superheaer ubes, T=67.3 K Flow (a) Sage 3: In his phase of sudy, he oule resuls of sage are used o calculae he flow in he chimney secion. For his sage he compuaional domain and boundary condiions are shown in Fig. 5. The oule profile (velociy, emperaure and species concenraion) soluion of he las sub-secion of sage is aen as he inle profile condiion for his sage. Pressure oule condiion Consan wall emperaure condiion Adiabaic wall condiion Velociy inle profile condiion Ouer walls, T=56.68 K Oule, Pressure oule, P= 1 bar (b) Figure 5. Compuaional domain wih boundary condiions for he chimney secion in sage-3 simulaion. Figure 4. (a) Compuaional domain for combusion chamber wih boundary condiions for sage- sudy. (b) Compuaional sub-domain wih boundary condiions for he sauraing and superheaing regions in sage simulaion. Sage : In sage of sudy, deailed simulaion of he sauraing and superheaing regions is performed. Due o he large numbers of sauraor and superheaer ubes, he simulaion is broen down ino 33 sub-secions. Each sub-secion includes 4xx ubes. The geomery and boundary condiions are shown in Fig. 4. Iniially only combusion chamber par secion, as shown in Fig. 4(a), is modeled and simulaed. The oule profile soluion (velociy, emperaure and species concenraions) of he combusor secion is used as he inle profile Turbulence Model In view of he complex flow field in he boiler, his sudy selecs he sandard κ-ε model due o is suiabiliy and robusness for a wide range of wallbound and free-shear flows. The κ-ε model is a semiempirical model wih several consans obained from experimens. The urbulence ineic energy, κ, and is rae of dissipaion, ε, are obained from he following ranspor equaions: µ κ ( ρκ ) ( ρκu i) = ( µ ) G Gb ρε YM S (4) x i x j σ x j µ ε ε ε ρε ρεu µ ( ) ( ) = ( ) C ( G C G ) C ρ S (5) x i x σ x 1ε κ 3ε b ε κ ε i j ε j In hese equaions, G κ represens he generaion of urbulence ineic energy due o he mean velociy

5 gradiens and he Reynolds sress, calculaed as u ' ' j G = ρuiu. κ j x i Gb represens he generaion of urbulence ineic energy due o buoyancy, calculaed as µ T G = βg b i Pr x i Pr is he urbulen Prandl number and g i is he componen of he graviaional vecor in he i-h direcion. For sandard κ-ε model he value for Pr is se.85 in his sudy. β is he coefficien of hermal expansion and is given as 1 ρ β =. ρ T p In equaion (4), Y M represens he conribuion of he flucuaing dilaaion in compressible urbulence o he overall dissipaion rae, and is given as Y = ρεm M M is he urbulen Mach number, given as M =, where a = γ RT is he speed of sound. a The urbulen (or eddy) viscosiy, µ, is compued by κ combining κ and ε as, µ = ρc, where C 1ε = µ ε 1.44, C ε = 1.9, C µ =.9, σ κ = 1., σ = 1.3. These consan values have been deermined from experimens wih air and waer for fundamenal urbulen shear flows including homogeneous shear flows and decaying isoropic grid urbulence. They have been found o wor fairly well for a wide range of wall-bounded and free-shear flows. The iniial values for κ and ε a he inles and oule are se as 1 m /s and 1 m /s 3 respecively. The κ-ε urbulence model used in his sudy is primarily valid for urbulen core flows (i.e., he flow in he regions somewha far from walls). Wall funcions are used o mae his urbulence model suiable for wall-bounded flows. The wall funcions consis of he following: The law-of he-wall for mean velociy gives 1 U = ln(ey ) (6) κ where.5.5 ρc.5.5 y U C P µ P µ P P, U y τ µ w ρ T = von Karman consan (=.4) E = empirical consan (=9.793) U P = mean velociy of he fluid a poin P P = urbulence ineic energy a poin P y P = disance from poin P o he wall µ = dynamic viscosiy of he fluid The logarihmic law for mean velociy is valid for y > 3 o 6. The law-of-he-wall for emperaure is given as, = Pr ( T w 1 [ ln( T.5.5 P ) ρc pc µ P q = Pr y C Ey ) P].5ρ.5.5 µ P q C.5ρ Pr {Pr U where P is compued using he formula 3 P 4.7 Pr/ Pr P = 9.4 r 1 1.8e P r P.5.5 µ P q U P (Pr Pr ) U f = hermal conduciviy of he fluid ρ = densiy of fluid c p = specific hea of fluid q& = wall hea flux T p = emperaure a he cell adjacen o he wall T w = emperaure a he wall Pr = molecular Prandl number (µc P /κ f ) Pr = urbulen Prandl number (=.85 a he wall) A = 6 (Van Dries consan) κ =.4187 (von Karman consan) E = (wall funcion consan) U c = mean velociy magniude a y = y T c (7) For κ-ε urbulence model, wall adjacen cells are considered o solve he κ-equaion. The boundary condiion for κ imposed a he wall is κ n =, where n is he local coordinae normal o he wall. The producion of ineic energy, G, and is dissipaion rae, ε, a he wall-adjacen cells, which are he source erms in κ equaion, are compued on he basis of equilibrium hypohesis wih he assumpion ha he producion of κ and is dissipaion rae assumed o be equal in he wall-adjacen conrol volume. The producion of κ and ε is compued as U τ w (8) G τ = τ w y w κρc.5.5 y µ P P C µ p ε = (9) P κy P ( y ( y < y T ) > y T )

6 Radiaion Model The Rosseland radiaion model [6] is used in his sudy, which is valid for medium opical hicness. In Rosseland model i is assumed ha he inensiy is he blac-body inensiy a he gas emperaure. The radiaive hea flux in a gray medium is approximaed by he following equaion q = 16σΓT 3 T (1) r where Γ is given as 1 Γ = (3(a σ ) Cσ ) s s where, a is absorpion coefficien σ s is scaering coefficien Combusion Model In his sudy, combusion of mehane (CH 4 ) is modeled by a one-sep global reacion mechanism, assuming complee conversion of he fuel o CO and H O. The complee soichiomeric combusion equaion is given as: CH 4 (O 3.76 N ) CO H O 7.5 N (11) The mehane-air mixure consiss of 5 species (CH 4, CO, H O, O and N ). The mixing and ranspor of chemical species is modeled by solving he conservaion equaions describing convecion, diffusion, and reacion sources for each componen species. The species ranspor equaions are solved by predicing he local mass fracion of each species, Y i, hrough he soluion of a convecion-diffusion equaion for he i-h species. The species ranspor equaion in general form is given as: v r ( ρ Yi ) ( ρνyi ) = J i Ri Si (1) where R i is he ne rae of producion of species i by chemical reacion. S i is he rae of creaion by addiion from he dispersed phase plus any userdefined sources. J r is he diffusion flux of species i, i which arises due o concenraion gradiens. For urbulen flows, mass diffusion flux is given as r µ J i = ρ Di, m Yi Sc Schmid number given as µ /ρd, where µ is he urbulen viscosiy and D is he urbulen diffusiviy.. Sc is he urbulen The reacion rae ha appears as source erm in equaion (1) is given by he urbulence-chemisry ineracion. In his sudy a generalized finie-rae combusion model (eddy-dissipaion model ) is used o solve he species ranspor equaions. The eddydissipaion model compues he rae of reacion under he assumpion ha chemical ineics are fas compared o he rae a which reacans are mixed by urbulen flucuaions, and herefore, he overall rae of reacion for mos fas burning fuels is conrolled by urbulen mixing. The ne rae of producion of species i due o reacion r, R i,r, is given by he smaller of he wo given expressions below [7]: ε Y R = ν ' ρ R i, rm A min (13) i, r w,i κ R ν ' R, rm w, R R i, r P Y ' ε = ν P i, rm ABρ (14) w,i κ N " ν j j, rm w, j Y P is he mass fracion of any produc species, P Y R is he mass fracion of a paricular reacan, R A is an empirical consan equal o 4. B is an empirical consan equal o.5 ν' i,r is he soichiomeric coefficien for reacan i in reacion r. ν" j,r is he soichiomeric coefficien for produc j in reacion r In he above equaions (13) and (14), he chemical reacion rae is governed by he large-eddy mixing ime scale, κ/ε, and an igniion source is no required. This is based on he assumpion ha he chemical reacion is much faser han he urbulence mixing ime scale, so he acual chemical reacion is no imporan. COMPUTATIONAL METHOD The commercial sofware pacage Fluen (version 6.1.) from Fluen, Inc. [4] is used in his sudy. FLUENT employs a conrol-volume-based [1, 5] echnique o conver he governing equaions o algebraic equaions, which are solved numerically using he implici mehod. In he segregaed formulaion, he governing equaions are solved sequenially, i.e. segregaed from one anoher. The SIMPLE algorihm [5] is used o couple he pressure and velociy and solves he pressure-correcion implicily. Second order upwind scheme is used o spaial discreizaion of he convecive erms. The diffusion erm is cenral-differenced wih secondorder accuracy. Figure 6 illusraes he model geomery wih compuaional grid for he curren sudy. In his sudy boundary layer is no imporan in he combusion chamber and exhaus chimney secions bu is

7 imporan in he sauraing/superheaing secions. Wall funcion is used o lin he soluion variables a he near-wall cells and he corresponding quaniies on he wall. The wall boundary condiions for he soluion variables, including mean velociy, emperaure, species concenraion,, and ε, are all aen care of by he wall funcions. F ull Boiler m odel w ih m esh C om busion cham ber w ih end furnace w all m odel w ih m esh Superheaing/sauraion secion m odel w ih m esh Tubes presen inside he m odel secion C him ney m odel w ih m esh Top view secion of he ubes Figure 6. Compuaional model of he sudied boiler showing differen secions wih meshes

8 Numerical Procedure 1. Solve he coninuiy, momenum and κ-ε urbulence equaion using he SIMPLE algorihm (pressure-predicor-correcion mehod). Obain he velociy, pressure and urbulence disribuion 3. Solve he energy equaion and calculae he emperaure disribuion 4. Calculae he species producion using he eddydissipaion model 5. Transfer he species producion o he source erms of he species ranspor equaions 6. Solve he species ranspor equaion o obain species concenraion disribuion and enhalpy formaion 7. Transfer he enhalpy formaion energy o he source erm of he energy equaion 8. Updae he coninuiy wih new disribuion of mass from he soluion of species of ranspor equaion (sep 6) 9. Reurn o (1) and reierae unil convergence is achieved. In he sep (1) above, when he momenum equaions are solved, several ieraions of he soluion loop mus be performed o obain converged soluion, which will saisfy he coninuiy and pressurevelociy relaionship. Each ieraion of he momenum equaions consiss of he following seps: (i) Fluid properies are updaed firs, based on he curren soluion or on he iniialized soluion (ii) To obain updaed velociy field, he u, v, and w momenum equaions are solved using he curren values of pressure and face mass fluxes. (iii) Equaion for he pressure correcion is calculaed from he coninuiy equaion and he linearized momenum equaions, since he velociies obained from he above sep may no saisfy he coninuiy equaion. (iv) The pressure correcion equaion obained from above sep is solved o obain he necessary correcions o he pressure and velociy fields and face mass fluxes such ha he coninuiy equaion is saisfied (v) Appropriae equaions for scalars such as urbulence, energy, and species are solved using he updaed values of he oher variables. (vi) The equaion is checed for convergence. The above seps are coninued ill he convergence crieria are obained and reurn o sep in he main loop. Convergence The soluion convergence is obained by monioring he coninuiy, momenum, energy, urbulence and species equaions separaely. A convergence crierion of 1-3 is used for mass conservaion, 1-6 is used for energy conservaion, and 1-5 for velociies and urbulence values. The emperaure disribuion is deermined afer a converged soluion is achieved. The energy conservaion is made by enforcing he hermal energy ransfer ou of he domain equal o ha of ino he domain. The ne ranspor of energy a he inle and oules consiss of boh he convecion and diffusion componens. Grid Sensiiviy Sudy Grid sensiiviy sudy was conduced using a coarse grid (44 grid poins), a medium-densiy grid (95845 grid poins) and a fine grid ( grid poins) for he enire boiler. The comparisons are shown in Table 1. The differences of mos parameers beween he medium and fine grid are less han 1% excep he average oule emperaure differs % and he oal pressure losses differs 3.6%. Alhough he soluion is sill changing beween he medium and he fine grid cases, considering he small difference, he medium densiy grid is used for his sudy o obain resuls wih reasonable ime frame. Variables Oule average emperaure (K) Pea emperaure in he domain (K) Toal pressure losses (Pascal) Oule massfracion of CH 4 Oule massfracion of O Oule massfracion of CO Oule massfracion of H O Coarse grid (,44) Medium grid (95,845) Fine grid (198,751) Table 1. Grid sensiiviy sudy RESULTS AND DISCUSSION This sudy illusraes he analysis of simulaion of combusion and hermal flow behavior inside an indusrial boiler. The simulaion is conduced in wo sages. In he firs sage he enire boiler wih combusion chamber and furnace is considered. The flow and emperaure disribuion is simulaed wihou he inclusions of superheaer and sauraor ubes locaed amid he flow pah. In he second sage of he simulaion, he boiler is divided ino hree main secions. The firs main secion consiss of only he combusion chamber where he fuel is burned. The

9 second main secion which are furher divided ino 31 sub-secions including superheaer and sauraor ubes secions. The las main secion consiss of he chimney exhaus. The flow disribuion inside he enire boiler is simulaed in he firs sage of sudy. A mesh wih approximaely 95,845 grid poins is used for he simulaion. The grid incorporaed he fuel inle duc for injecing he naural gas, he primary inle duc for injecing he swirling air sream and he secondary inle duc for injecing he uniform air sream. Figure 7 shows he velociy vecor disribuion for he hree inles, where he primary air eners wih a swirl. The swirling flow is used for enhance mixing for a complee combusion of air and fuel and for sabilizing he flame fron. The swirl induces a recirculaion zone along he cenerline of he combusion chamber downsream from he burner oule. The mass weighed average velociies for he hree inles are given in Table. The oal air supplied is g/s, which is a 35 % more air han he soichiomeric value. abou 1/4 of he boiler lengh and is less han he adiabaic flame emperaure,6k (3,547 o F) for mehane combusion. Boh Figs 8 and 9 show he flame propagaion and he mixing and chemical reacions occurring closer o he inle. The horizonal plane in Fig.9 shows ha he ho flames urn around 18 degrees a he end and eners ino he superheaer secion a he righ passage. This flame is a abou 1,1K (15 o F), which is higher han he yield emperaure of he superheaer ube maerial (SA- 13-T1, emperaure of yield a 15 o F). This imposes a ris o he inegriy of he superheaer ube wall. Due o his high emperaure flow, he firs few rows of he super-heaer ubes near he end furnace wall are subjeced o he ris of rupure. Variables Average Velociies (m/s) Fuel Inle Primary Air Inle 75. Secondary Air Inle Table. Average velociies a he burner inles Figure 8. Conours of saic emperaure on he verical plane a x= (a) Y Z X (b) Figure 7. Velociy vecors for he hree inles in he burner(a) 3-D velociy profile ( b) A crosssecional view The emperaure disribuions on he verical cener-plane a x= and on he horizonal plane a y= are shown in Figs. 8 and 9, respecively. The emperaure conour clearly shows he pea emperaure is a abou 1,63K (,474 o F) occurs a Figure 9. Conours of saic emperaure on he horizonal plane a y= Composiions of emperaure disribuion on differen horizonal planes and verical planes are

10 shown in Figs. 1 and 11, respecively. From hese wo figures, i can be clearly seen ha he combusion sars as a ring, hen propagaes boh inward and ouward radially as well as longiudinally lie a cone. The flow-fields on he cener horizonal and verical planes in Figs. 1 and 13 show he flow passes from he combusion chamber hrough he superheaer secion and finally hrough he chimney and is exhaused o he amosphere. Figure 1 shows ha he flame impinges on he combusion chamber sidewalls a 1/3 of he combusion chamber lengh. Figure 9 shows ha he emperaure of his impinging flame is abou 15K (4 o F). Care mus be aen o frequenly examine his ho region in he real boiler. Figure 1 shows ha he flow separaes a he 18-degree urn a he end. These separaions will induce oal pressure losses and requires more fan power o drive he flow hrough he boiler. Figure 13 shows he flow is characerized by a flame je spreading over he combusion chamber surfaces, op and boom wih a emperaure of abou 15K (179 o F) a abou 1/3 of he chamber lengh. A recirculaion zone is seen surrounding he flame near he burner. Figure 14 shows profile of mass-weighed average emperaure a seleced axial planes. I can be seen from he figure ha maximum emperaure is observed a abou 1/3 rd of he combusion chamber lengh. Figure 11. Conour emperaure profile disribuion on differen verical z-planes Figure 1. Vecor plo of velociy on he horizonal plane a y= Figure 1. Conour emperaure profile disribuion on differen horizonal y-planes. Figure 13. Vecor plo of velociy on he verical plane a x=

11 1 Temperaure, T (K) Axial Disance, z (m) Figure 14 Profile of mass-weighed average emperaure on seleced axial disances Figures 15 and 16 show he species (CH 4, O, CO, and H O) concenraions in verical plane a x= and horizonal plane a y=, respecively. These wo figures show ha he fuel (mehane) is compleely burned near he burner. The figures show ha mehane diffuses rapidly oward he flame fron, where i is almos compleely consumed, bu a very small amoun diffuses and convecs ouward from he leading edge of he flame. The figures show he oxygen deplees in he core of he flame je and mainains higher oxygen concenraions in he recirculaion zones. Downsream of he oxygendepleed region, he oxygen mass fracion exhibis an increase due o replenish by advecion/diffusion from he surrounding air. The figures also reveal ha large quaniies of H O and CO are produced soon afer he mehane has been consumed. The mass fracions of H O and CO are low near he burner region wih higher values furher downsream. Figure 16. Conours of mass fracion of CH 4, O, CO, and H O on plane y= Profiles for CH 4 concenraions, O concenraions, H O concenraions and CO concenraions, a seleced axial locaions are presened in Figs. 17 and 18, respecively. The profiles clearly show he decrease in mehane concenraions, oxygen concenraions and nirogen concenraions while he concenraions of CO and H O are increased going furher downsream..45 CH4 Mass Fracin Axial Disance, z (m) O Mass Fracion Axial Disance, z (m) -8-1 Figure 15. Conours of mass fracion of CH 4, O, CO, and H O on plane x= Figure 17. Profiles of CH 4 and O concenraions along seleced axial disances

12 CO Mass Fracion sauraor and superheaer ubes secion. The pah lines of velociy vecor colored by velociy magniude near he superheaer ubes inside he secion model are shown in Fig Axial Disance, z (m).1.1 HO Mass Fracion Axial Disance, z (m) Figure 18. Profiles of CO and H O concenraions along seleced axial disances. In he second sage of his sudy, he second main secion conaining he sauraor and superheaer ubes is specifically zoomed in for a deailed simulaion. This second main secion is divided ino hiry-one sub-secions. The oule soluion of he firs main secion (he combusion chamber) is used as he inle condiion for he firs sub-secion of he second main secion. The firs main secion is simulaed wihou downsream secions. The inerface beween he firs and he second main secions is se a a consan pressure. Figure 19. Conour emperaure plo a oule of firs secion of he second sage. The conour emperaure profile a oule of he firs secion is aen as he inle profile condiion of sub-secion 1 of he main second sage as shown in Fig. 19. High emperaure of abou 1,K (1,34 o F) is seen near he cener of he flow passage. This could be he region where he maerial of he superheaer ubes may subjec o exreme high hermal load and prone o rupure. Near he inner wall, he emperaure is reduced o abou 97K (1,86 o F) due o he separaion bubble, which serves as a buffer zone o proec he inner wall from he ho flue gases. Figure shows emperaure disribuion on differen seleced horizonal y-planes. The flow beween he superheaer ubes is shown in he expanded view in Fig.. A recirculaion can be seen near he inner wall due o low pressure area creaed by flow separaion while aing a 18 degree sharp urn from he combusion chamber o ener ino he Figure. Temperaure conour plos on differen horizonal planes and velociy paern on one horizonal plane.

13 Pressure, P (Pa) Axial Disance, z (m) Temperaure, T (K) Axial Disance, -z (m) Figure 1. Velociy vecors inside he firs subsecion of he superheaer secion The oule profile condiions for velociy, emperaure and species mass-fracion of sub-secion 1 are aen as he inle profile condiions for he nex sub-domain (sub-secion-) and he simulaion is carried unil he calculaion of he las sub-domain in he second main secion is compleed. The emperaure conour plos of he enire second main secion on he horizonal cener-plane y =.3 is shown in Fig.. The profiles of mass-weighed average oal pressure and emperaure of he enire 31 sub-secions of second sage, on he horizonal plane a y=.3 are shown in Fig. 3. The plos show decreasing emperaure disribuion due o hea losses o sauraor and superheaer ubes. The pressure drop due o he seam ubes is abou 1,55Pascal (.153psi). Figure 3. Profiles of mass-weighed average oal pressure and emperaure of all he 31 subsecions on he horizonal plane a y=.3 In he hird sage of sudy, he simulaion is conduced focusing he chimney (exhaus) region. The oule of sub-secion 31 of secion- is aen as he inle condiion for he main secion-3. Pea emperaure in he order of 58K (584 o F) exiss a he oule of he superheaer secion. In he sauraor secion he pea emperaure is lower a abou 46K (368 o F). This is reasonable because more energy is ransferred o he sauraor. Also since he sauraor ube walls are mainained a a lower emperaure han he superheaer ube walls, he ho gasses loose more energy o he sauraor ubes han o he superheaer ubes. There is oal pressure loss due o he sauraor and superheaer ubes. The ho flue gasses loose energy o he walls covered by sauraor ubes below he seam drum and finally passing hrough he chimney and exhaused o he amosphere. The emperaure is reduced from abou 58K (584 o F) o abou 465K (377 o F) a he chimney exhaus (Fig. 4a). The exhaus emperaure is relaively high; a fair amoun of useful energy is wased. In he acual exhaus secion, an economizer is insalled o recover his energy. Due o he limied scope of his sudy, he economizer is no considered in his sudy. An infrared hermography inspecion of he boiler was conduced. I is ineresing o see ha he acual infrared image (Fig. 4b) shows he chimney exhaus gases a emperaure abou 461K (37 o F), which are remarably close o he CFD resuls a 465K (377 o F). Cono urs of saic em peraure K Figure. Temperaure conour plo of he superheaer ubes secion including all he 31 subsecions on he horizonal plane a y=.3 Figure 5 shows he pah lines colored by velociy magniude. A large re-circulaion zone is formed in he chimney secion before leaving he amospheric oule. This circulaion, which creaes aerodynamic losses, is believed o be caused by high flow resisance due o a small exhaus opening locaed only on one side of he chimney walls. I is recommended ha he exhaus be opened in all four walls a he end of he chimney. Figure 6 shows he vecor disribuion colored by emperaure a seleced

14 planes. The plo shows he flow reversal in he chimney a differen horizonal planes. he roof. The hoes region is a he cener of he boiler wih he pea emperaure around 1,63 K (,474 o F) Figure 5. Pah lines colored by velociy magniude on z-plane K 73 Figure 4. Conour plos of saic emperaure a he oule of he boiler chimney, (a) simulaed resuls (b) infrared hermography of he boiler. Noe, he color definiion is differen in hese wo figures. CONCLUSIONS The compuaional simulaion of combusion and hermal-flow behavior inside an indusrial boiler was performed in his sudy using he commercial code FLUENT. The simulaions were conduced wih and wihou considering he sauraor/superheaer ubes. The resuls provide comprehensive informaion concerning combusion and hermal-flow behavior inside an indusrial boiler. The emperaure disribuion shows combusion saring as a ring a he inerface of fuel and primary air and expanding rapidly boh inwards and ouwards. The flame propagaes as a conical je slighly bending oward Figure 6. Velociy vecor field colored by emperaure on differen horizonal planes

15 The swirling flow is used o enhance he mixing of air and fuel and o sabilize he flame fron. The gas flow disribuion shows he flow is characerized by a flame je spreading over he combusion chamber surface. A recirculaion zones in he upper and lower porion of he combusion chamber surface near he burner are observed. The combusion and hea ransfer efficiency in hese recirculaion zones are low. The flame impinges on he combusion chamber sidewalls a 1/3 of he combusion chamber lengh, wih he emperaure of he flame abou 1,5K (,4 o F). In he verical plane, he flame impinges on he floor and roof a abou 4% of he combusion chamber lengh a 1,5K (1,79 o F). The ho flames urn around 18 degrees a he end of he combusion chamber and ener ino he superheaer secion. A large separaion bubble forms a he urning locaion across a dozen rows of ubes. This separaion bubble remains relaively cool a 94K (1,3 o F) by enraining downsream cooler gasses. The separaion bubble pushes he ho flow owards he mid-secion of he passage walls and subjecs he superheaer ubes o high emperaure hermal sress a abou 1,6K (1,448 o F), which can lead o ube rupure. Compuaional Fluid Dynamics, Proceedings of ASME TURBO EXPO, GT-315. [3] Babcoc & Wilcox, Seam is generaion and use, 39h ediion. [4] FLUENT 6.1 User s Guide, February 3 [5] Paanar, S.V., 198, Numerical Hea Transfer and Fluid Flow, McGraw Hill. [6] Siegel, R. and. Howell, J. R, 199, Thermal Radiaion Hea Transfer, Hemisphere,Washingon D.C. [7] Turns, S. R.,, An Inroducion o Combusion, nd Ed., McGraw Hill. [8] Wlodzimierz, Blasia and Yang, W., Combusion Improvemen Sysem In Boilers and Incineraors," Proceedings of IJPGC 3, IJPGC [9] Woodruff, E. B. and Lammers, H. B., Seam Plan Operaion, McGraw-Hill, [1] Zin, J, 1, The John Zin Combusion Handboo CRC Press, New Yor. An inensive calculaion was conduced o compue he flow and hea ransfer across he 496 ubes. Wih he inclusion of he sauraed/superheaer ubes, he exi emperaure drops from 746K (883 o F) (wihou including sauraor/super-heaer ubes) o almos 465 K (377 o F). The decrease in emperaure is due o he hea ransfer ino he ubes. Wih he inclusion of ubes, he acual emperaure disribuion was simulaed, and he exi emperaure a he chimney exhaus is close o he acual emperaure measured by infrared hermograph a abou 455K (36 o F). The pressure drop, due o he seam ubes, is abou 1,55 Pascal (.153 Psi). The overall simulaion was successful and provides comprehensive informaion of combusion and hermal-flow in he sudied boiler. Several ideas were formed from his sudy o improve boiler efficiency and minimize he hermal sress problem imposed on he super-heaer ubes. Fuure wor will include NOx and CO predicions. REFERENCES [1] Anderson, John D. Jr, Compuaional Fluid Dynamics, he Basics wih Applicaion, McGraw-Hill Inc. [] Armand, S and Chen, M., A Combusion Sudy Of Gas Turbine Using Muli-Species/Reacing