Numerical simulation of the combustion of strong swirling confined reacting flow (natural gas /air) in gas turbine combustor

Transcription

1 18 ème ongrès Franças de Mécanque Grenoble, 7-31 août 7 Numercal smulaton of the combuston of strong swrlng confned reactng flow (natural gas /ar) n gas turbne combustor. Khell 1, H. Naj & L. Loukarf 1 1 Unversté de hlef P 11, hlef, lgére Unversté de Llle 1, Polytechnque de Llle, LML, UMR 817, 96 Vlleneuve d scq, France khella@yahoo.fr bstract: Ths paper s focused on the numercal smulaton of the hghly swrled and confned natural gas dffuson flame. Several factors nfluencng the combuston process are examned. One of the ams here s to study the nfluence of the turbulence models and the reacton mechansm on the predctons of the chemcals speces concentraton and temperature felds. The numercal results are compared wth the work developed by Meer et al. () and Frassoldat et al. (). The numercal calculaton was performed usng the commercal code Fluent. The k-ε standard and RSM models are employed to descrbe the turbulent nature of the flow and the Eddy dsspaton model wth one and two-step knetc scheme to modulate the combuston process. Résumé : ette étude s ntéresse à la smulaton numérque des flammes de dffusons au sen des chambres de combuston des turbnes à gaz. L un des objectfs prncpaux auquel nous nous ntéressons c est d étuder l nfluence des modèles de turbulence, et de deux schémas réactonnels sur la prédcton du champ d écoulement et des concentratons des espèces chmques. L étude concerne la prédcton numérque d une flamme de dffuson d un écoulement turbulent confné de gaz naturel avec un fort Swrl à l ade du code Fluent. Les résultats numérques obtenus sont comparés à ceux de Meer et al. () et de Frassoldat et al. (). Les modèles de turbulences k-ε standard et R.S.M auxquels on a adjont le model de combuston dt «Eddy dsspaton» avec un schéma réactonnel d une ou deux étapes sont utlsées. Key-words: Turbulence-chemstry nteracton, ffuson flame, Swrl burner. 1 Introducton The modern desgn of techncal combuston systems, must take nto account three essental factors, the heat and mass transfer, optmzaton of combuston effcency and reducton of pollutants. The omputaton Flud ynamcs (F) has become popular and represents an economc and relable tool for facltatng combuston system desgn. Flame structure and stablty, and pollutant emssons strongly depend on the aerodynamc and mxng characterstcs of the fuel and swrlng combuston ar jets n the near burner regon (German et al. ()). Swrlng flows are wdely used n ndustral burners employed, to provde stable and hgh ntensty and short flame, wth a wde radal development whch are assocated wth good flud mxng and long reacton tmes. The applcaton of a tangental velocty component to the flow (W), gves the flow a rotatng component, represented by the non-dmensonal swrl number (S) defned as the rato of the tangental momentum flux to axal momentum flux, as: R R n n n S = ρuwr dr / R ρu rdr (1) 1

2 18 ème ongrès Franças de Mécanque Grenoble, 7-31 août 7 The phenomena of combuston (ar/gas) are hghly complex wth hot flow gas recrculaton, energy exchange and strong turbulence-chemstry effects whch are enhanced stll further n the case of swrlng confned combustors. For ths reason, solutons became an nterestng and dffcult task for commercal codes (Meer et al. ()). The objectve of ths nvestgaton s to study the nfluence of turbulence models and the reacton mechansm on the numercal predcton of the flow and temperature felds. Geometry and mathematcal model.1 Geometry of burner The expermental data nvolved a cylndrcal water-cooled chamber wth an nternal dameter =.m and a heght L = 1.m, equpped wth a swrl burner ( =.6 m), made of a central bluff body, surrounded by one.3m wde annulus for the fuel and another one for the combuston ar for more geometry detaled n Frassoldat et al. () and Meer et al. ().. Mathematcal model The Favre-averaged equatons for conservaton of mass and momentum for a steady, varable-densty turbulent flow can be expressed n terms of artesan tensor notaton as: x ( ɶ ) ρ u = ( u u ) P ρ j uɶ uɶ j ( ρ uɶɶ u j) = + µ + x j x x j x j x j x where u and u are the Favre-averaged and fluctuatng velocty components respectvely n the x drecton, P and ρ are the mean pressure and densty of the mxture, and µ s the lamnar vscosty. The Reynolds stresses, ρ u u j, are obtaned usng two dfferent closure models: k ε model (Launder and Spaldng (1974)) and the RSM model (Launder (1989))..3 ombuston model The turbulent non-premxed combuston process s smulated usng the wdely employed eddy-dsspaton combuston model (Magnussen and Hjertager (1976)). The fuel s assumed to burn by a one-step and two-step chemcal reacton shown n table 1: Table 1: Reacton scheme: Left sde: one-step and rght sde: two-step.4 oundary condtons H 4 + O O + H O 3 H + O O + H O 4 1 O + O O The oundary condtons used n ths work are that proposed by (Frassoldat et al. ()). The nlet velocty profle has been assumed as the one measured expermentally close to the burner head (1mm downstream). Tangental velocty (W) of the fuel s assumed equal. The ()

3 18 ème ongrès Franças de Mécanque Grenoble, 7-31 août 7 turbulence s expressed by specfyng the turbulence ntensty and the hydraulc dameter that are 8% and 3 mm for the ar, % and 6 mm for the fuel nlet, respectvely. Thermal boundary condtons were defned by settng the wall temperature at 8 K.. omputatonal procedure The Fluent computer code uses a fnte volume procedure to solve the Naver-Stokes equaton of flud flow. choce of soluton methods, turbulence and reacton model, s avalable n the code. The models used for the numercal calculaton are as follows: For turbulence, k-ε model and RSM model wth the axsymmetrc Swrl opton( S.6) are used. For chemcal reacton, the Eddy dsspaton model s chosen. The numercal predcton has been carred out wth the followng assumptons: The flow s steady, two dmensonal axsymmetrc and turbulent. The fuel s approxmated as 96% (vol) H 4, wth 1.8% (vol) O, and the remanng part N.The algorthms PRESTO, SIMPLE has been used for pressure nterpolaton and the couplng of pressure and velocty, respectvely. Thermal radaton s neglected and pressure = 11 Pa. The thermal propertes of the speces n the mxture are gven as a functon of temperature. The numercal smulaton has been carred out on a trangular and unstructured mesh composed of cells. r and fuel are fed at hgh temperatures (T= K) n order to allow the gnton of the system. 3 Results and dscusson The S level n partcular affects the turbulent-energy, dsspaton rate, turbulent stresses and the scale of turbulence (Naj et al. (198)). t hgh swrl number ( S.6), the flow feld can be dvded nto three separate zones, and as shown n Fg.1 and. For more detals, see (Repp et al. ()) and (Frassoldat et al. ()). The contour of temperature determned by RSM model shows better the nner recrculaton zone (), where hot ntermedate products are transported and the combuston process s completed, the outer recrculaton zone() formed by the burnt gas flow and the mxng zone between the oxdzer and fuel streams (). FIG. 1 Favre-averaged temperature. Left sde: k-ε model wth two step reacton scheme, rght sde: expermental measurements (Meer et al. ()). FIG. Favre-averaged temperature. Left sde: RSM model wth two step reacton scheme, rght sde: expermental measurements (Meer et al. ()). 3.1 Temperature feld The comparson of the expermental data wth the numercal predcton usng the RSM model and two-step reacton scheme s represented n Fg.1 and. One can see that the hot central core s located at mm from the nlet. The combuston process s completed n ths zone () and the agreement wth the expermental data s better accurate by RSM model as schown n Fg. The radal dstrbuton of the mean temperature s represented n Fg.3. s can be seen, the general features of the measured temperature profles are reasonably well predcted 3

4 18 ème ongrès Franças de Mécanque Grenoble, 7-31 août 7 by both turbulence models. long the radal coordnate, the temperature profles move from the hghest values, characterstc of the hot core of the flame to the decrease and then fnally to the asymptotc value whch refers to the recrculatng exhausted gas. The peak of the temperature reveals that the combuston occurs n nner recrculaton zone. t staton X/ = 1.16, only the RSM model wth two-step reacton scheme s able to gve a good agreement wth the expermental data. However predctons overestmate these expermental values. 1 1 T [ K ] X/= T [ K ] X/=.3 T [ K ] X/= FIG.3 Radal dstrbuton of mean temperature at dstance from the nlets: X/=.16,.3 and 1.16 : Expermental results, KE: k-ε model and RSM:RSM model ( R1, R one step and two-step reacton scheme respectvely). 3. Mean flow felds The predcton and expermental radal profles of the mean axal, radal and tangental velocty for the combustng flow at three locatons (X/=.16,.3 and 1.16) are shown n Fg.4. The presence of the nner recrculaton zone s ndcated by the negatve value of U profles. X/=.16 X/=. X/=1.16 U [ m/s ] U [ m/s ] U [ m/s ] V [ m/s ] 1 - X/=.16-3 V [ m/s ] X/= V [ m/s ] X/= W [ m/s ] X/=.16 - W [ m/s ] -1 3 X/= W [ m/s ] 3 X/= FIG. 4 Radal dstrbuton of mean axal (U), radal (V) and tangental (W) velocty. : Expermental results, KE: k-ε model and RSM:RSM model ( R1, R one step and two-step reacton scheme respectvely ) 4

5 18 ème ongrès Franças de Mécanque Grenoble, 7-31 août 7 s can be seen, the predcton shows that the axal velocty predcted by the k-ε (, ) and RSM (, ) models are n reasonably good agreement wth the expermental data. However, at poston X/=1.16, only the gves better results. For the radal velocty, the comparson between predcton and expermental data s overall satsfactory at X/=. and s for the tangental velocty, there s a good agreement wth expermental data except at the last poston (X/=1.16). The mean mass fracton of H 4 and O are shown n Fg.. The mass fracton of H 4 s slghtly overestmated by RSM model wth two-step reacton scheme n the nner recrculaton zone () where fuel s mxed nto the flow. Near the axs of the combustor, the results predcted by the RSM model are n better agreement wth expermental data compared to those of the k-ε model. The comparson between the predcted and measured radal profles of O reveals, that both predcted results by the two models are n good agreement wth the measurements. In the outsde regon whch s domnated by the combuston ar flow, the predcted and measured oxygen concentratons ncrease to the maxmum value. The use of the RSM model wth two-step reacton scheme enhance the qualty of O predctons as shown n poston X/=1..6 H4 X/=.16.6 H4 X/=.3.6 H4 X/= O X/=.16. O X/=.3. O X/= FIG. Radal dstrbuton of mean H 4 and O mass fracton : Expermental results, KE: k-ε model and RSM:RSM model ( R1, R one step and two-step reacton scheme respectvely ) O X/=.16.4 O X/= O X/= FIG. 6 Radal dstrbuton of mean O mass fracton The radal dstrbuton of mean O mass fracton s shown n Fg.6. The maxmal values of O are obtaned n the central zones of the flame. The profle predcted by the RSM model wth two-step reacton scheme s n a qualtatve accordance wth the measured data. Note that these maxmal values are overestmated by the k-ε model. The O and O profles (Fg.6 and 7) behave n a manner smlar to temperature Fg.3.

6 18 ème ongrès Franças de Mécanque Grenoble, 7-31 août 7.4 O X/=.16.4 O X/=.3.4 O X/= FIG. 7 Radal dstrbuton of mean O mass fracton 4 onclusons The nfluence of two turbulence models assocated wth a reacton mechansm on the numercal predctng of the flow and temperature felds of the non-premxed flame are presented n ths paper. In general, the predctons of the swrlng flow feld and flame propertes obtaned wth the RSM model are n better agreement wth the data compared to those of the standard k ε model. However, for the flow wth a hgh nlet swrl number, some dfferences between the predctons of the k ε and RSM models reman. The effects of combuston on the sze shape and the levels of gas temperature and oxygen concentraton are better predcted by the RSM model wth two-step reacton scheme at staton X / 1. References Frassoldat,. Frgero, S. olombo, E. Inzol, F. Faravell, T.. etermnaton of Nox emssons from strong swrlng confned flames wth an ntegrated F-based procedure. hemcal Engneerng Scence.6, German,.E. Mhamud, T.. Modellng of non-premxed swrl burner flows usng a Reynolds-stress turbulence closure. Fuel.84, Launder,.E. Spaldng, The Numercal computaton of turbulent flows. omputer Methods n ppled Mechancs and Engneerng. 3, Launder,. E Second-moment closure and ts use n modellng turbulent ndustral flows. Internatonal Journal for Numercal Methods n Fluds. 9, Meer, W. Noll,. ockhorn, H. Leuckel, W.Schulz,. Wolfrum, J. Schneder,. Repp, S. Sadk,. rezler,. Jancka, J. a. onfned TEFLM swrl burner: expermental nvestgatons and numercal smulatons. th Workshop /TNF.htlm Magnussen,.F. Hjertager,.H On mathematcal models of turbulent combuston wth specal emphass on soot formaton and combuston. Proceedngs of the ombuston Insttute.16, Naj, H The predcton of turbulent swrlng jet flow. Internatonal Journal of Heat and Mass Transfer. 9, Repp, S. Sadk,. Schneder,. Hnz,. Landenfeld, T. Jancka, J.. Predcton of swrlng confned dffuson flame wth a Monte arlo and a presumed-pf-model. Internatonal Journal of Heat and Mass Transfer. 4,