Impact of the equivalence ratio and the mass flow rate on turbulent lean premixed prevaporized combustion

Transcription

1 Avalable onlne at Energy Proceda 6 () 5 6 MEDGREEN -LB Impact of the equvalence rato and the mass flow rate on turbulent lean premxed prevaporzed combuston Feth Bouras a, Azeddne Soudan a * a Laboratore de Physque Energétque Applquée Département de Physque Faculté des Scences Unversté HL Batna Algére Abstract Our contrbuton conssts of the demonstraton of the advantages of LES-WALE coupled wth PDF approach ncludng progress varable (c) n reactng flow, by usng the Fluent-CFD. The confrmaton s based on the comparson of three parameters: mean longtudnal velocty, ntensty of longtudnal velocty and lengths of recrculatons zones. In lne wth what was observed by the expermental reference study, the smulaton succeeds to detect the flame zone of the recrculatng regon and shows the dfferences between the dfferent cases of flows n varable equvalence rato ( and.75) or n varable mass flow rate (Q= and 95 g/s). All cases of study consdered the dentcal condtons from the tow supply channels of the burner. The man ssue s the modelng of the closure of the turbulent combuston. In addton, the numercal smulaton predcts as well as the asymmetry and the symmetry flow, respectvely, for nert and reactng flows. * Correspondng author. Tel.: ; fax: E-mal address: azeddne.soudan@unv-batna.dz. Publshed by Elsever Ltd. Selecton and/or peer-revew under responsblty of [name organzer] Keywords: Turbulent combuston; Chaotc moton; Probablty densty functon; Computatonal flud dynamcs; Large eddy smulaton; ORACLES confguraton. Nomoclature c Progress varable C w D WALE -model constant Dffuson coeffcent Publshed by Elsever Ltd. do:.6/j.egypro..5.9

2 5 Feth Bouras and Azeddne Soudan / Energy Proceda 6 () 5 6 I k P S t T u x Y Z t LES LPP LZR Intensty turbulent Subgrd knetc energy Pressure Tensor velocty of deformaton Tme Temperature Velocty component Spatal coordnate mass fracton Mxture fracton. Kronecker delta Thermal conductvty Molecular vscosty Turbulent vscosty Knematcs vscosty Mass densty Equvalence rato Vscous stress tensor Large eddy smulaton. Lean Premxed Prevaporzed Length of Zone of Recrculaton Introducton Lean premxed prevaporsed (LPP) combuston s consdered practcal n the development of systems of aeronautcal propulson and many aspects of the nteracton wth turbulence [-]. Most numercal computatons of turbulent combuston gases are currently performed wth the use of Large Eddy Smulaton (LES) model [-7]. Consequently, the gaseous mxture n the combuston chamber may be feature by the equvalence rato that may compromse the targeted objectve n terms of pollutants emsson reducton and also the control of the combuston stablty [, 8-]. Besson et al. have developed an ORACLES benchmark (One Rg for Accurate Comparsons wth Large Eddy Smulatons), whch s a Lean premxed prevaporzed combuston suppled by two dentcal flows of premxed ar and propane []. The mportant results obtaned allowed to characterzaton of the average structure and unsteady nert and reactve flows developed n downstream

3 Feth Bouras and Azeddne Soudan / Energy Proceda 6 () of the sudden expanson. Ths shows the asymmetry of nert flow for the dentcal nlet condtons. However, symmetry gven by the presence of the combuston when the mass flow rate and equvalence rato of the supply flows s dentcal [-]. Nguyen et al.[, ] contnued ther works n the framework of the European program modelng of low emsson combustors usng Large-Eddy smulatons (MOLECULES) amed at promotng the extensve use of LES to model low emssons combustors. The goals of the study are the nfluence on the characterstcs of all flows wth varaton of the thermal power wth change mass flow rate or equvalence rato and the determnaton of extncton characterstcs of lean combuston, accordng to dfferent parameters defnng the flow achevable wth ORACLES benchmark [, ]. KURENKOV and OBERLACK [] used the Reynolds Averaged Models coupled wth the G- for the valdaton. All the obtaned results based on D smulatons, present two cases: The frst s purely devoted to the nert flow for the mass flow rate Q= g/s. The second one concerns the combuston, for same mass flow rate from nert case and equvalence rato =.75. The results are computed for the half of the chamber and then mrrored wth respect to the symmetry plane. Ths dd not allow the vsualzaton of asymmetrcal flows and the dfference between the two zones of recrculaton n nert case. However, Reynolds Averaged Model s applcable only enough far of the wall whch s needed on the logarthmc law o whch nfluences the results []. Our work s focused on applyng the model of Wall Adaptng Local Eddy Vscosty (WALE) based on probablty densty functon of the progress varable (c) n D smulaton. Our dea s to use ths model mplement the auxlary model to calculate velocty n the zones close to the walls. It necessary to know any source term of spces producton, n order to resolve the equatons speces. Agan, wth PDF approach we can surmount problem wthout need to defnton ths term, but wth new concepton about equatons. Computatons are performed usng the FLUENT_CFD. The study ncludes nert and reactve cases of flows. The confrmaton of the calculated parameters (mean longtudnal velocty and fluctuaton of longtudnal velocty and length of recrculaton zones) gven by expermental data of Nguyen et al. [, ] are obtaned n MOLECULES EU- research program.. Expermental reference case and computatonal doman A bref descrpton wll be gven on the ORACLES-Burner; more detals are n references [-]. In the expermental set-up, both the fuel (commercal propane) and the ar are suppled from hgh pressure storage reservors; afterwards they are premxed and homogenzed durng ther flows n two separate dentcal channels before enterng the combuston chamber. Both channels have constant rectangular

4 5 Feth Bouras and Azeddne Soudan / Energy Proceda 6 () 5 6 cross secton and have 7. mm n length, needed to obtan a fully developed turbulent channel flow. A schematc vew of the ORACLES-burner s shown n Fg.. The combuston chamber, mm n length, s thermally nsulated. We recall that the Reynolds number s calculated by Re= U b H/. The heght of the nlet channel H =. mm and the vscosty of fresh mxture (propane/ar) at T = 76 K. The dfferent labels and parameters that characterze the dfferent flows are lsted n Table. Fg. Schematc of the entre ORACLES. Table Man parameters of the test cases (atmospherc pressure, temperature of the ncomng flows = 76K) [, ]. Case Chanel Re Q(g/m) U b(m/s) Thermal power (kw) nc Upper Lower c, Upper Lower c Upper Lower m Upper Lower Governng equaton The fltered governng equatons n LES for compressble flow can be wrtten n Cartesan coordnates as [5,7]: Contnuty: ( u ) t x Momentum: u t p j ( u u j ) [ ( uu j u u j )] x x x xj () ()

5 Feth Bouras and Azeddne Soudan / Energy Proceda 6 () t x Mxture fracton Z ( u Z ) ( D Z ) () x Progress varable c ( u c ) c.( D c ) c t x x x x () Where: =,, and j=,, Thermodynamc state p RmT (5) x The majorty of the subgrd models are based on the assumpton of Boussnesq whch present the tensor of the unsolved constrants t j to the tensor velocty of deformaton S j by the ntermedary of a turbulent vscosty. The small scales nfluence the large scales va the subgrd-scale stress [5,7]: j t Sj kll j (6) Where kll s the subgrd knetc energy. The fltered stran rate tensor s defned by [6, 9]: S u u j j ull j (7) x j x We selected the WALE (Wall-Adaptng Local Eddy-Vscosty) eddy vscosty model from Ncoud and Ducros [5] to represent the eddy vscosty term n Eq8. The man dea of ths model s to recover the proper behavor of the eddy vscosty near the wall n case of wall-bounded flows, whle preservng nterestng propertes such as the capacty to provde no eddy-vscosty n case of vanshng turbulence (property requred for the transton from lamnar to turbulent states). The major nterest of ths model frst reles n on the fact that t needs no nformaton about the drecton and dstance from the wall (avodng the use of any dampng functon) thus beng really sutable for unstructured grds, where evaluatng a dstance to the wall s precarous. The resdual stress tensor of the WALE eddy vscosty model can be found as [5]: And Where t ( s s d d / ( ) 5 / ( j j Cw d d sj sj ) ( sj sj ) (8) s d j ( g j g j ) g kk ) j 5 / (9) u g j () x

6 56 Feth Bouras and Azeddne Soudan / Energy Proceda 6 () 5 6 Cw: WALE model constant (Cw =.9). The s the spatal flter wdth, whch s generally related to the grd sze of the resolved feld.. Results and dscusson The results are normalzed by the step heght h=9.9 mm and the nlet bulk velocty each for correspondent flow case, U b =and m/s. All the smulaton and the expermental results are presented and compared usng these unts.. Inert flow We gve n frst the results of the nert flow n order to use t n the comparson wth each case of reactve flow consdered n ths smulaton study. Fgures (a) present the comparson for LES_WALE model and expermental [, ] mean longtudnal velocty and fluctuaton of longtudnal velocty profles. The results show adequate agreement wth the expermental data. Except n the frst staton (x=) n fgure where the computaton gves the velocty values lower than the experment ones, n the remanng statons, t also gve almost the same evolutons of velocty (x=h and x=8h). In partcular, the poston of the tow peaks of velocty, whch s well reproduced by the present numercal computaton. These two peaks are attenuated downstream of the burner (x=8h) and t s observed that velocty feld ncrease wth longtudnal dstance [, ]. Moreover, the smulaton detects, as observed n the experment, two recrculaton zones, n the upper and lower corners. Both zones are caused by the edges between the supply channel and the combuston chamber, these facng step make backward the flow n these zones [-, 5, ]. The most promnent feature n nert case s the asymmetrc mean flow wth two unequal reattachment lengths on the top and bottom walls. We determne the average lengths of the recrculaton zones. The obtaned results, presented n table, show that the length of the upper zone of recrculaton.h.h), ths s n agreement wth the reference expermental data. It s mportant to note that n Besson and Duwg study smulaton for dfferent condtons of flows, they had obtaned an opposte stuaton wth a zone of lower average recrculaton longer than the hgher zone [, 5]. In agreement wth the expermental observatons, the smulaton detects the asymmetry n the longtudnal velocty fluctuatons, as shown n fgure for all the statons. We note that both n the experment and n the computaton the velocty fluctuatons are developed n regons wth a strong mean velocty gradent. From x=, x=h and x = 8h statons, we deduce that the turbulent fluctuaton values are well reproduced by the present computaton. Partcularly, t s worth to notce that the velocty fluctuatons n the nert case exhbt always mportant ampltudes, partcularly around.u b [, ]

7 Feth Bouras and Azeddne Soudan / Energy Proceda 6 () x= x=h x=8h a x= x=h x=8h b u/u, I/U u/u, I/U u/u, I/U x= x=h x=8h c x= x=h x=8h d u/u, I/U u/u, I/U u/u, I/U Fg. Comparson of the longtudnal velocty and the longtudnal velocty fluctuaton. LES mean longtudnal velocty; Expermental mean longtudnal velocty; LES longtudnal velocty fluctuatons; X Expermental longtudnal velocty fluctuaton.

8 58 Feth Bouras and Azeddne Soudan / Energy Proceda 6 () 5 6. Reactve flows Fgs (b), (c) and (d) presents the profles of the component averages longtudnal velocty and the assocated profles of fluctuaton for c., cand m presented n table. Mean longtudnal velocty profles appear beng perfectly symmetrcal compared to the horzontal medan plane located on y=,8h. The profles of mean longtudnal velocty obtaned for the low equvalence rato are lghtly compressed compared to the profles obtaned for the nert flow (nc), so the poorest flow s strongly of less acceleraton. The double peak n the graphs presents the shear layers formed by the nteracton wth the wake generated by the tralng edge of the spltter plate whch ntally separates the two supply channels. The flame front stablzed at the level of shear layers [-, ]. The peaks that are present for the flows c., c and m are almost absent n the last staton x=8h and beng to dsappear downstream from the combuston chamber. It s easy to see that at x = 8h, the combuston s far from over [, ]. The experment and the computatonal model show that n the burnt zone, the velocty feld takes hgher values, where the burnt gas expands and accelerates the flow. For example, we calculate the velocty correspondng to the burned gas flow n the burner for c flow. We note that at x = 8h the maxmum value of the report u/u b barely reaches and that the central area of the flow between h <y <h s such that the reduced velocty rato u/u b s between and. The acceleraton depends sgnfcantly the flow consdered. For example, a forte ncreases n the mean longtudnal velocty for the c flow. Whle the nfluence of the combuston s a lttle slow to be faster n the flow m. Note also that the acceleraton of the velocty feld nduced by the development of combuston leads to a behavor already observed n expermental study of Nguyen et al. and Besson et al. [-]. Fgs (b), (c) and (d) show a mean relatve uncertanly about %. The nfluence of the value of equvalence rato s sgnfcant, when we consder the changes n velocty (Fgs., and ). The fluctuatons longtudnal of velocty may to dstngush two classes of the behavors of the reactve flows. The lean flows c. flow (Fgs. (b) for whch one observes that the fluctuatons of the longtudnal velocty reman the low then obtaned for the flow nc nert and are characterzed by a maxmum approxmately of.u b. The second class s made of flows c wth rchness of, 75. Ths flow s characterzed by the feld of the longtudnal velocty fluctuaton (Fgs (c)), reachng levels of very hgh value, wth more than.5 U b. The longtudnal fluctuaton values n frst staton x= become senstve to the value of the rchness of the ncdental mass flow rate wth a clear ncrease. The assocated symmetry of the profles s notced. The maxmum values are observed n the two layers of shearng between the hgh velocty and the zones of recrculaton. In the zone close to wdenng, the double peak assocated wth the wake s qute present and s reabsorbed at x = h. For the maxmum values of the fluctuaton whch are observed n fgs (a), (b) & (c). From x=8h, a clear reducton n the values of the fluctuatons velocty compared to what s observed for the nert flow nc [7,

9 Feth Bouras and Azeddne Soudan / Energy Proceda 6 () ]. The profles of the fluctuatons longtudnal of velocty gve average relatve uncertanty approxmately of %. The presence of the two zones of recrculaton s attested by the negatve values of the mean longtudnal velocty. To clarfy ths aspect, we made a determnaton of mean length of the recrculaton zones by measurng, for each of them, the longtudnal velocty to mm near the walls (top and bottom wall) for the all flows [, ]. The fgs (b), (c) and (d) show values of mean longtudnal velocty, the symmetry of the velocty profles as well as the equal n the average lengths of recrculaton zones. The obtaned results of comparson that concern LES_WALE and experment, llustrated n table, show that h) s roughly twce to thrce larger than the length of reactng flows. In addton, lengths of zones of recrculaton of reactng flow reman lower than h) for the nert case. The combuston causes an mportant reducton of the lengths of the recrculaton zones comparatvely to the nert flow. The length of the zones of recrculaton s a strongly decreasng when the value of the equvalence rato ncrease (Table ). That s an nverse relatonshp between the length of recrculaton zone and equvalence rato. Thus accordng to Nguyen et al. and Besson et al. [-], for example for the flow c, a reducton length of more than 5% for the lower zone and 7% for the hgher zone s observed compared to that obtaned for the nert flow. However, ths reducton s slghtly less mportant for the flow of heght velocty nlet (m). Thus, the relatonshp between the mass flow and the average length of the zones of recrculaton s a proportonal. Table Length of the recrculaton zones for the two flows consdered Length of the lower recrculaton zone Length of the upper recrculaton zone Case Smulaton Expermental Smulaton Expermental nc 6.h 5.5h.h 8.h c,.5h.9h.5h.9h c.h.h.h.h m.h.h.h.9h Concluson Lean premxed prevaporsed turbulent combuston s a complex phenomenon that nvolves mult dscplne physcs to descrbe and understand t, hence the acqurng of knowledge useful for the desgn of relable LPP combuston chambers. The present work amng to testng numercal smulaton models based on LES-WALE model coupled on PDF approach. We have performed a D numercal smulaton usng the commercal CFD package Fluent. The valdaton was based on the comparson wth expermental data Nguyen et al. [, ]. The followng conclusons may be drawn from ths study:

10 6 Feth Bouras and Azeddne Soudan / Energy Proceda 6 () 5 6 Applcaton of LES_WALE for reactve flow gves the ablty to detect the morphology of flow,.e. flame and zone of recrculaton presented n expermental data of Nguyen n MOLECULES framework, LES-WALE eddy vscosty makes t possble to calculate velocty drectly at the zone near the wall. However, the other dynamc models requre the nterventon of the auxlary models, Verfcaton of symmetrcal and asymmetrcal flow n the dfferent cases of flow that are proved expermentally by Besson and confrmed later by Nguyen. Results obtaned n ths work allow us to explot them drectly n other domans: Exergy, envronment. References [] Nguyen, P.D., Bruel, P., Rechstadt, S.: An Expermental Database for Benchmarkng Smulatons of Turbulent Premxed Reactng Flows: Lean Extncton Lmts and Velocty Feld Measurements n a Dump Combustor. Flow, Turbulence and Combuston. 8,558(9). [] Nguyen, P. D. : Contrbuton expérmentale à l'étude des caractérstques nstatonnares des écoulements turbulents réactfs prémélangés stablsés en aval d'un élargssement brusque symétrque. Ph.D Thess, Unversté de Poters, France (). [] Besson, M., Bruel, P., Champon, J.L., and Deshaes,B.: Expermental analyss of combustng flow developng over a plane symmetrc expanson; Journal of Thermophyscs and Heat Transfer.,59-67(). [] Vreman A.W., Albrecht B.A., van Ojen J.A., de Goey L.P.H. and Bastaans R.J.M.: Premxed and nonpremxed generated manfolds n large-eddy smulaton of Sanda flame D and F. combuston and flame 5, 9-6 (8). [5] Duwg, C., Fureby, C.: Large eddy smulaton of unsteady lean stratfed premxed combuston; Combuston and Flame. 5, 85- (7) [6] Wang, P.and Ba, X.S.: Large eddy smulaton of turbulent premxed flames usng level-set G-equaton.Proceedngs of the Combuston Insttute., 58-59(5) [7] Ptsch, H.and Duchamp de Lageneste, L.: Large-eddy smulaton of premxed turbulent combuston. Computatonal Flud and Sold Mechancs. Proceedngs Second MIT Conference on Compuratonal Flud and Sold Mechancs June 7,, () [8] Huang, Y. and Yang, V.: Dynamcs and stablty of lean-premxed swrl-stablzed combuston. Progress n Energy and Combuston Scence.5, 9-6(9) [9] Hwang, C. H., Lee, S., Km, J. H. and Lee, C. E.: An expermental study on flame stablty and pollutant emsson n a cyclone jet hybrd combustor. Appled Energy. 86, 5-6(9) [] Kurenkov, A.and Oberlack, M.: Modellng Turbulent Premxed Combuston Usng the Level Set Approach for Reynolds Averaged Models; Flow, Turbulence and Combuston.7, 877(5)