A Computational Fluid Dynamics Study of Turbulence, Radiation, and Combustion Models for Natural Gas Combustion Burner

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1 Avalable onlne a BCREC Webse: hps://bcrec.undp.ac.d Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, Research Arcle A Compuaonal Flud Dynamcs Sudy of Turbulence, Radaon, and Combuson Models for Naural Gas Combuson Burner Y Sang Pang 1, Woon Phu Law 2, Kang Qn Pung 1, Jolus Gmbun 2,3 * 1Faculy of Engneerng and Bul Envronmen, Tunu Abdul Rahman Unversy College, Jalan Genng Kelang, Seapa, Kuala Lumpur, Malaysa 2Cenre of Excellence for Advanced Research n Flud Flow (CARIFF), Unvers Malaysa Pahang, Gambang, Pahang, Malaysa 3Faculy of Chemcal & Naural Resources Engneerng, Unvers Malaysa Pahang, Gambang, Pahang, Malaysa Receved: 26 h July 2017; Revsed: 9 h Ocober 2017; Acceped: 30 h Ocober 2017 Avalable onlne: 22 nd January 2018; Publshed regularly: 2 nd Aprl 2018 Absrac Ths paper presens a Compuaonal Flud Dynamcs (CFD) sudy of a naural gas combuson burner focusng on he effec of combuson, hermal radaon and urbulence models on he emperaure and chemcal speces concenraon felds. The combuson was modelled usng he fne rae/eddy dsspaon (FR/EDM) and parally premxed flame models. Dealed chemsry necs CHEMKIN GRI-MECH 3.0 conssng of 325 reacons was employed o model he mehane combuson. Dscree ordnaes (DO) and sphercal harmoncs (P1) model were employed o predc he hermal radaon. The gas absorpon coeffcen dependence on he wavelengh s resolved by he weghed-sum-of-gray-gases model (WSGGM). Turbulence flow was smulaed usng Reynolds-averaged Naver-Soes (RANS) based models. The fndngs showed ha a combnaon of parally premxed flame, P1 and sandard -ε (SKE) gave he mos accurae predcon wh an average devaon of around 7.8% of combuson emperaure and 15.5% for reacan composon (mehane and oxygen). The resuls show he mul-sep chemsry n he parally premxed model s more accurae han he wo-sep FR/EDM. Meanwhle, ncluson of hermal radaon has a mnor effec on he hea ransfer and speces concenraon. SKE urbulence model yelded beer predcon compared o he realzable -ε (RKE) and renormalzed -ε (RNG). The CFD smulaon presened n hs wor may serve as a useful ool o evaluae a performance of a naural gas combusor. Copyrgh 2018 BCREC Group. All rghs reserved Keywords: Combuson; Parally Premxed; Radaon; Turbulence; compuaonal flud dynamc How o Ce: Pang, Y.S., Law, W.P., Pung, K.Q., Gmbun, J. (2018). A Compuaonal Flud Dynamcs Sudy of Turbulence, Radaon, and Combuson Models for Naural Gas Combuson Burner. Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1): (do: /bcrec ) Permaln/DOI: hps://do.org/ /bcrec Inroducon The combuson sysem nvolvng naural gas or mehane s commonly used n he ndusry, especally n he power generaon plan. Assessmen of he combusor performance and * Correspondng Auhor. olus@ump.edu.my (J. Gmbun), Telp: , Fax: polluon generaon can be performed expermenally usng a combnaon of equpmen such as onlne gas chromaography, arrays of emperaure sensors and advanced nonnrusve laser-based measuremen. However, he cos o se such an expermenal rg s ofen prohbvely oo hgh for mos researchers. In addon, he combusor chamber wall mus be made of a specal maeral such ransparen bcrec_1395_2017

2 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 156 quarz o enable laser-based measuremen. In addon, measuremen a hgh emperaure (>1000 K) s poenally dangerous [1-2]. Alernavely, CFD can perform a dealed evaluaon of he combuson sysem, bu need o be valdaed before can be rounely used. A smple cylndrcal burner s ofen used as a es bed o evaluae he accuracy of he CFD modellng sraegy of a naural gas combuson sysem. One of hose expermenal wors n a cylndrcal combuson chamber wh dealed measuremen of gas composon and emperaure profles was presened by Garreon and Smonn [3]. A CFD sudy on he cylndrcal burner has been performed by several researchers [4-10]. They emphaszed on he chemsry reacon, urbulence mxng, hermal radaon, polluon formaon and buoyancy effec nsde he burner. Mos of he early wor evaluaes he effec of models used o he predcon of hea ransfer and chemcal speces profle. The fndngs obaned from pror wors concluded ha he modellng approach s val o he predcon accuracy n cylndrcal burner. Hence, he am of hs wor s o develop an accurae modellng sraegy by evaluang he effec of dfferen urbulence, combuson and radaon models o he emperaure and gas speces profle n he burner. I s mporan o predc he urbulen flow nsde he combuson chamber accuraely o enable beer predcon of he dealed reacon chemsry nvolved n he combuson process. RANS-based urbulence model, such as: he SKE model s commonly used o resolve he urbulen flow due o s robusness and lesser compuaonal demand [4,7-8]. Ronche e al. [9] performed a comparson of dfferen urbulence models on he predcon of emperaure and carbon monoxde mass fracon. They found ha no subsanal dfference was obaned beween he -ε and -w urbulence models. SKE s nown o provde a reasonable predcon on he emperaure and chemcal speces concenraon for naural gas combuson, alhough has a nown ssue o manan a posve urbulence sresses besdes gvng a poor predcon of roaonal and sraned urbulence flow. The newer -ε varan,.e. RKE and RNG, are nown o address he aforemenoned ssues. However, no prevous wor deals wh varous -ε based models, such as: SKE, RKE, and RNG for he mehane combuson n a cylndrcal burner. Hence, he effec of SKE, RKE, and RNG on he predcon of emperaure and he gas speces profle was assessed n hs wor. Combuson models, such as: he eddy dsspaon concep (EDC) [4], eddy brea-up (EBU) [10], presumed probably-dsrbuonfuncon (PPDF) [10], fne rae/eddy dsspaon model (FR/EDM) [8-9], have been wdely employed o predc he naural gas combuson. Among all he aforemenoned combuson models, EDM s he mos commonly used due o s reasonable predcons for mehane combuson [11]. Therefore, a wo-sep EDM was consdered n he presen wor. Karm e al. [10] compared PPDF and EBU combuson models. They repored no sgnfcan dfference beween he predcon usng PPDF and EBU models. I was found ha a dealed mul-sep chemsry model whch ncludes he nermedaes s more accurae han he global chemsry model le EDM. The naural gas combuson nvolves a number of chemcal reacons ncludng nermedae speces. In addon, no prevous wors ha employed flamele model for naural gas combuson smlar o he presen case. Hence, he parally premxed flame model assocaed wh mulple reacons was used and compared wh he FR/EDM n hs wor. Thermal radaon domnaes he hea ransfer process n mos combuson sysems le he naural gas combuson burner. Pror wor has shown ha hermal radaon accouns for 96% of he oal hea ransfer n he combuson sysem [12]. Earler wors by da Slva e al. [6] on he smlar case focused on he effec of hermal radaon on he emperaure and chemcal speces concenraon dsrbuon. Ther wor ndcaed ha he ncluson of hermal radaon gave a more unform and accurae hea ransfer predcon nsde he combuson chamber. Wang e al. [13] repored ha he combuson smulaon whou radaon model over-predcs he emperaure feld. Hence, s val o consder he radaon model o he hea ransfer model for naural gas combuson. Mos of he CFD sudes dealng wh he naural gas combuson employed dscree ransfer radaon model (DTRM) for radaon [4-5,7-8]. I has o be noed ha DTRM does no nclude he effec of radaon scaerng and can only be accurae when a large number of rays s modelled (CPUnensve). In addon he reflecon of ncden radaon a he surface s soropc wh respec o he sold angle, whch s quesonable, snce he radaon should be a funcon of sold angle. All he aforemenoned ssues are addressed n he DO and P1 models. However, no prevous wor used DO and P1 models for he case suded n hs wor, herefore, DO and P1

3 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 157 models wh gas absorpon coeffcen WSGGM were assessed n hs wor. In hs wor, he modellng sraegy was developed by evaluang he effec of dfferen urbulence, radaon and combuson models n a successve way. The predcon was valdaed wh he expermenal daa from Garreon and Smonn [3]. 2. Compuaonal Mehod 2.1 Geomery and compuaonal grd The geomery n hs wor s smlar o he one measured by Garréon and Smonn [3]. A wo-dmensonal axsymmerc cylndrcal combuson burner was prepared by usng GAM- BIT as shown n Fgure 1. The cylndrcal burner has 170 cm of lengh and 25 cm n radus. The burner consss of wo ducs of nle whch s ar and fuel nle. The naural gas (0.232 m 3 /s) s neced no he burner from he fuel nle wh a radus of 3 cm, whle he ar (0.728 m 3 /s) eners he chamber hrough a cenered annular duc havng a spacng of 2 cm. The ouflow of he chamber s 12.5 cm n radus. The whole doman was prepared by quadrlaeral mesh. Four dfferen grds, (.e. 140, 335, 560 and 650), were esed n hs wor. 2.2 Combuson modellng In hs wor, combuson of naural gas was modelled by FR/EDM [14] and paral premxed flame model. The wo models chosen s suable for a fas reacon (Damohler»1) le he one n hs wor.e. Da 2.86 (volume averaged); alhough a he flame regon he Da can reach as hgh as Da 63. In EDM a fas chemcal reacon was assumed o be conrolled by he urbulen mxng, whle FR model abandoned he effec of urbulen mxng and compued he chemcal reacon rae accordng o he Arrhenus equaon. The FR/EDM model swches auomacally beween he wo mode usng he daa obaned from he CFD smulaon.e. daa on emperaure and urbulen flow s auomacally fed o he FR/EDM model durng Fgure 1. Two-dmensonal geomery of combuson burner CFD smulaon. The smplfed wo-seps combuson of naural gas s gven by Equaons (1-2). 2 CH. 4 3(O N2) 2CO 4H2O 11 28N2 2 CO 1(O N2) 2CO2 3 76N2 The speces ranspor equaon s gven by: ( x 2 u Y ) D m x 2, Y R S Sc (1) (2) (3) where ū s he mass-averaged velocy of mxure, Y s he mass fracon, D,m s a dffuson coeffcen for speces n mxure, μ s he urbulen vscosy, Sc s he urbulen Schmd number, R s he ne producon rae and S s he source erm. The ne producon rae s gven as: R M N R w Rˆ,, r r1 (4) where Mw, s he molecular wegh of speces, NR s he oal number of reacons, and R ˆ, r s he Arrhenus reacon rae. In EDM, he producon rae of speces s modelled accordng o Magnussen and Herager (Eqs. (5-6)) [14]: ', r R v R v ', r M M w, w, A mn AB R ( ' v R, r N '' v w, R (5) (6) ' '' where v R,r and v,r are he sochomerc coeffcens of reacan and produc, respecvely, A and B are he Magnussen consan for reacan and produc, respecvely, YP and YR are he mass fracons of he speces n produc and reacan, respecvely. Only wo smplfed nec mechansms were used n EDM o solve he naural gas combuson reacon rae. However, he combuson of naural gas s complex due o he mulple chemcal reacons ha occur smulaneously wh he urbulence and hea ransfer. Hence, an ncluson of a mul-sep reacon model s val n order o ge an accurae predcon. A mul-sep mechansm was nroduced no he flamele lbrary o accoun for p Y, r R M Y P M w, )

4 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 158 he urbulence and non-equlbrum chemsry. A deal CHEMKIN GRI-MECH 3.0 reacon mechansm [15] whch conssed of over 325 reacons and 53 speces equpped wh assocaed rae and hermodynamc daa was used for parally premxed urbulen combuson model. In hs wor, he combuson occurs n boh he non-premxed and premxed mode. Inally, naural gas and ar are nroduced separaely no he burner (non-premxed). The naural gas and ar are parally premxed a he base of he lfed dffuson flame. The nhomogeneous urbulen mxng separaes he mxure no fuel-rch and fuel-lean regons. As he flame fron propagaes, he hn flame shee separaes he regons no unburn and burn mxure regons. Therefore, parally premxed combuson was consdered by combnng he boh flamele models from non-premxed combuson and premxed combuson, respecvely [16-17]. The mxng of naural gas and ar n a urbulen flow feld s descrbed usng he mxure fracon model. The ranspor equaon of Favre mean and varance of mxure fracon s modelled n Equaons (7-8). ( f ) ( v f ) ( f ) Sc ' 2 ' 2 ( f ) ( v f ) ' 2 2 ' 2 ( f ) Cg( f ) Cd f Sc (7) (8) where f s mean mxure fracon and Sc s urbulen Schmd number. In premxed flame model, he reacve flows dvded no burn and unburn regon, whch s separaed by he flame shee. In premxed combuson model, a progress varable s used o model he flame fron propagaon (Equaon (9)): ( c) ( vc) ( c) uu c Sc (9) where c s mean reacon progress varable, μ s urbulen vscosy, Sc s he urbulen Schmd number, ρu s he densy of unburn mxure and U s he urbulen flame speed. The progress varable s compued as n Equaon (10): c n 1 Y n 1 Y, eq (10) where n s he number of producs, Y s he mass fracon of produc speces, and Y,eq s he equlbrum mass fracon of produc speces. I s gven ha c = 0 for unburn mxure and c = 1 for burn mxure. In parally premxed flame model, he reacon behnd he flame fron s modelled by mxure facon model and he flame fron poson s deermned usng progress varable. I s bes sued for a fas reacon (.e. Da» 1) especally for he case of chemcal equlbrum or moderaely nonequlbrum flamele srucure. 2.3 Radaon modellng As dscussed earler radave hea ransfer accoun for abou 96% of he oal hea ransfer n he combuson sysem and hence mus be modelled accordngly. The P1 model s he smples radaon model generae by he P-N model whch s based on he expanson radaon nensy no an orhogonal sphercal harmonc [18]. Only zeroh and frs order momens of he nensy are consdered n he P1 model. P1 model solves soropc radave hea ransfer and requres low compuaonal demand [19]. Smulaon va P1 model accouns for scaerng effec and s applcable for large opcal hcness. The ranspor equaon for P1 s modelled as [20] (Equaon (11)): ( ) ag 4aT q r (11) where qr s radave hea flux, a s he absorpon coeffcen, G s an ncden radaon flux, σ s he Sefan-Bolzmann consan, and T s a emperaure. The radave hea flux a wall s gven n Equaon (12): q r 1 G 3( a ) C s (12) where σs s he scaerng coeffcen and C s a lnear-ansoropc phase funcon coeffcen (beween -1 and 1). A bacward scaerng s gven a a negave value, a forward scaerng s n posve value and a zero value s denoed for an soropc scaerng. The DO model ulzes a dfferen approach o solve he radaon ransfer equaon (RTE) compared o P1 model. The sold angle a a ceran pon of doman s spl up no a number of dscree drecons and he radave nensy s assumed o be consan whn each dvson of he sold angle. DO model s more me consumng han he P1 model due o he soluon s requred for many dfferen drecons [19]. The RTE s modelled n Equaon (13): s 4

5 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 159 ( I ( r,s )s ) ( a a n I 2 b s 4 4 (13) where Iλ s he specral radaon nensy, λ s a wavelengh, aλ s a specral absorpon coeffcen, s s he pah lengh, Ibλ s a blac body nensy, n s a refracve ndex, ϕ s a phase funcon, s ' s a scaerng drecon and dω s a sold angle Turbulence modellng 0 )I ( r,s ) Flud flow n a combuson process s usually urbulen whereby he velocy and pressure flucuae chaocally. Turbulen flows can affec he hea ransfer and chemcal reacon n he combuson process, and hence mus be ncluded n he CFD model. The accuracy and relably of a CFD smulaon s sgnfcanly depends on he model used. Mlner e al. [21] and Ilbas e al. [22] repored ha no sngle urbulence model can be unversally appled n all cases. Therefore, hree RANS urbulence models, namely: SKE, RKE, and RNG, were compared n hs wor. The RANS ranspor equaons are gven n Equaons (14-15). u u x 0 (14) (15) where ū s mean velocy, ρ s flud densy, f s exernal forces, p s mean pressure, μ s flud vscosy, S s mean sran ensor rae, and u ' ' u s Reynolds sresses ensor. The SKE s he mos used -ε urbulence model as s easer o converge and requres relavely low compuaonal demand. The urbulen nec energy equaon for SKE s modelled n Equaon (16). ( ) x Tme dervave u u x (16) where ρ s he flud densy, s urbulen nec energy, μ s flud vscosy, μ s urbulen vscosy, σ = 1.0 s Prandl-Schmdl number, G and Gb are he producon rae due o mean velocy graden and buoyancy, respecvely, ε s dsspaon rae, and YM s he dlaaon dsspaon erm accouns for compressbly effec. The urbulen vscosy s compued by Equaon (17). s ' ' ' I ( r,s ) ( s s )d u G Gb YM Convecon f x x Dffuson ' ' p 2S uu x Pr oducon Desrucon C 2 (17) where Cμ s gven as The producon rae of SKE s gven n Equaons (18-19). G G b u u ' ' u x T g Pr x ' ' u u (18) (19) where s normal sresses, Pr s urbulen Prandl number wh a consan value of 0.85, and g s componen of gravaonal vecor n - h drecon. The desrucon rae (urbulen dsspaon rae) s gven n Equaon (20). u v x '' u x '' (20) The ranspor equaon of dsspaon rae n SKE s modelled n Equaons (21-22). x u Tme dervave 2 C1 ( G C3 G ) C2 x b x Dffuson C Convecon 3 anh v u Producon Desrucon (21) (22) where v and u are he componen of velocy parallel and perpendcular o gravaonal vecor, respecvely. The model consans are σε = 1.3, C1ε = 1.44, and C2ε = 1.92 [23]. The SKE model ofen gave a poor predcon of flow wh a srong sreamlne curvaure, graden flow and roaon owng o s consan eddy vscosy formulaon whch can lead o a negave normal sresses compuaon under ceran crcumsances. I was nown o gve a poor predcon on he speces concenraon owng o he consan value of Cε n he ranspor equaon of dsspaon rae [24]. Hence, oher -ε varan such as RKE [25] and RNG [26] were nroduced o overcome he lmaon of SKE. RKE dffers from he SKE because s urbulen vscosy s no longer consan. The urbulen vscosy coeffcen of RKE s compued as a funcon of local saes of he flow o ' ' ensure a posve normal sresses ( u u ) under all flow condons. Therefore, hs model can provde a beer predcon of he roaon, vorces and separaon flows feaures [27]. The Cμ for RKE s compued by Equaon (23):

6 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, C U Ao As (23) where Ao = 4.04, As= 6 cos φ, φ = (cos -1 ( 6w))/3, w=(sss)/ŝ 3, U S S ~ ~, and Ŝ= (SS). The ranspor equaon of dsspaon rae n RKE model s modelled as (Equaon 24): C1 Producon x x x u Tme dervave Convecon 2 C2 C1 C3Gb v Desrucon Buoyancy Dffuson (24) where he model consans are σε = 1.2, C2 = 1.9, C1ε = 1.44, and Pr = The coeffcen, C1 s gven by (Equaon (25)): C max 0.43, 1 5 (25) RNG s derved from he renormalzed group heory by Yaho and Orszag [26]. In RNG, he smaller scale eddes are elmnaed and he ranspor coeffcen s renormalzed. An analycal equaon for urbulen Prandl number (Pr) and an addonal erm (Rε) were nroduced o he dsspaon rae ranspor equaon o accoun for he neracon beween urbulen dsspaon and mean shear. The Rε allows a slgh reducon n dsspaon rae, subsequenly, he effecve vscosy s reduced. Thus, RNG can provde a good predcon for rapdly sraned flow and srong sreamlne curvaure [26]. The producon rae due o buoyancy effec (Gb) n RNG dffers from boh he SKE and RKE models because he urbulen Prandl number s no consan bu nsead calculaed by Pr=1/α. The α coeffcen s obaned from (Equaon (26)): o o (26) where αo s gven as 1.0. n hgh Reynolds number lm, he μmol/μeff s less han 1.0. Boh nverse effecve Prandl number are approxmaely The model consan are C1ε = 1.42 and C2ε = 1.68, whle he C3ε s compued usng Equaon (22). The addonal erm s formulaed as (Equaon (27)): R 3 C ( 1 / ) 2 o ( ) 8 1 (27) The model consans are Cμ = , ηo = 4.38, and β = mol eff 2.5 Modellng seup The smulaon of naural gas combuson n a wo-dmensonal cylndrcal chamber was performed usng ANSYS FLUENT 16.2 nsalled on he HP Compaq Pro 6300 MT worsaon wh a Quadcore processor (3.40 GHz) and 4 GB RAM. The smulaon was frsly performed usng frs-order upwnd scheme, seady-sae SKE urbulence, DO radaon, and FR/EDM. The unseady-sae solver and hgher-order dscrezaon scheme was hen enabled afer a converged soluon was acheved. The hermophyscal properes (.e., specfc hea, dynamc vscosy and hermal conducvy) of each chemcal speces a emperaure range from 300 o 2500 K were nroduced as a pecewse lnear funcon. In parally premxed flame model, he GRI-MECH 3.0 assocaed wh 325 mechansms was used for more deal predcon. NASA polynomals (Thermochemcal Daa for Combuson Calculaons) were used o model he gas properes as a funcon of emperaure. The daa were recorded for over 1000 me seps afer a pseudoseady soluon was acheved and he value repored n hs wor s a sascal meaveraged. The smulaon seup used n hs wor s shown n Table 1. The CFD predcons from varous model combnaons (.e., urbulence, radaon and combuson models) were compared wh he expermenally measured emperaure and chemcal speces concenraon [3]. 3. Resuls and Dscusson 3.1 Grd densy analyss A wo-dmensonal axsymmerc burner was prepared by quadrlaeral meshes. Four dfferen grds of combuson burner (.e., 140, 335, 560 and 650) were esed n hs wor. Table 1. CFD seup Solver Dscrezaon Combuson Model Radaon Model Gas Absorpon Turbulence Model Tme Sep Sze Absolue Resdual Boundary Condon : Fuel Ar Wall Transen Second-order upwnd FR/EDM and parally premxed flame DO and P1 WSGGM SKE, RKE and RNG s K; 7.76 m/s CH4 (0.9), N2 (0.1) K; m/s O2 (0.23), N2 (0.76), H2O (0.01) K; 0.01 m hcness of seel; 0.7 W/m 2 K of overall hea ransfer coeffcen; Emssvy: 0.6

7 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 161 SKE urbulence model, DO radaon model, and parally premxed combuson model were employed for he grd denses comparson. The predcons for he four dfferen grds were compared wh he expermenal daa [3]. Fgure 2 shows he emperaure dsrbuon a dfferen poson of combuson burner for he four dfferen grds. In Fgure 2(A), all grd ypes yelded smlar predcons of emperaure a 0 m < X < 0.9 m along he cenre of he burner. However, he wo coarser grds, namely 140 and 335 grds under-predcs he emperaure a 0.9 m < X < 1.7 m, whereas, he wo fner grds (560 and 650) showed a far predcon. Fgures 2(B) o 2(D) show he emperaure profles along he radal poson a hree dfferen axal posons. No subsanal dfference of predcons obaned from he four dfferen grds n Fgure 2(B). However, Fgures 2(C) and 2(D) clearly showed ha he fner grds (560 and 650) yelded a beer predcon compared o he wo coarser grds. Fgure 3 showed ha he 560 and 650 grds provded more accurae predcons on he chemcal speces concenraon, excep for he Fgure 3(D). Hence, he wo fner grds are he suable choce n hs wor. However, he hgher grd denses need longer compuaonal me, as shown n Table 2. The wo coarser grds (.e. 140 and 335) gave a poor predcon, alhough hey requre much lesser compuaonal me. The 560 grd was seleced, nsead of he fnes grd (650) for he res of hs wor o mnmze he compuaonal demand, snce he predcons obaned by he 560 grd s comparable o he one by 650 grd. 3.2 Effec of radaon model Ths wor ams o nvesgae he mporance of ncludng radaon model n he naural gas combuson smulaon. Therefore, he smulaon wh P1 radaon model was compared wh he one whou radaon model and whou weghed-sum-of-gray-gases model (WSGGM). The predcons of emperaure and Table 2. Grd densy analyss Grd CPU Tme (s/eraon) Fgure 2. Grd densy analyss on emperaure dsrbuon along he (A) symmery lne; (B) radal poson a m; (C) radal poson a m; (D) radal poson a m

8 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 162 chemcal speces concenraon were shown n Fgures 4 and 5, respecvely. I was clearly shown ha he smulaon s n beer agreemen wh he expermenal daa [3] when he radaon s ncluded and WSGGM s enabled. Whereas, he smulaon whou he radaon model enabled produce a relavely poor predcon of emperaure and chemcal speces concenraon. Ths s because he ncluson of radave hea ransfer enhanced he homogenzaon of emperaure nsde he combuson burner by ransferrng he hermal hea from ho gas regon o burner s wall and oule [6]. Therefore, he emperaure becomes more unform, unle he one whou a radaon model. For nsance, Fgures 4(A-C) show ha he emperaure s over predced n he core regon, bu under-predced near he oule n Fgure 4(D) due o lac of radave hea ransfer homogenzaon. Incluson of radaon model whou he WSGGM o accoun for he gray gas absorpon coeffcen also produces a less accurae predcon on emperaure and speces profle. The absorpon of gas speces n combuson chamber s no consan, bu s depends on he emperaure. WSGGM was nroduced o resolve he specral gas absorpon, and herefore s mporan o be ncluded for he radave hea ransfer le he smulaon n hs wor. Ths wor showed ha he ncluson of radaon provded more accurae predcon of emperaure and chemcal speces, and hence radaon model wh WSGGM mus be used. The smulaon usng P1 radaon model was hen compared wh he one usng he DO model. Fgures 6 and 7 are he emperaure and chemcal speces concenraon profles, respecvely, usng wo dfferen radaon models (.e. DO and P1). Parally premxed flame and SKE models were employed. The CFD predcon n hs wor shows a reasonable agreemen wh he expermenal daa from Garreon and Smonn [3]. Alhough boh DO and P1 models over-predcs he emperaure along he radal poson a m from he nle of he burner as shown n Fgure 6(C) and had a relavely poor predcon of carbon monoxde concenraon as shown n Fgure 7(D). The large devaon of he predced emperaure a radal poson of X = m s a follow hrough of poor gas fracon predcon n he same regon (see Fgure 7 a X ~ 0.9). The radaon hrough he gas nsde he chamber s modelled usng a WSGGM. The WSGGM uses a number of grey gases and weghng facor polynomals o model gas radave properes,.e. emssvy. Thus error n gas composon predcon may affec he radaon hea ransfer rae, snce radaon accoun for abou 90% of hea Fgure 3. Grd densy analyss on chemcal speces concenraons along he symmery lne: (A) mehane; (B) oxygen; (C) carbon doxde; (D) carbon monoxde

9 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 163 Fgure 4. Comparson beween wh radaon model, whou radaon and whou WSGGM on emperaure dsrbuon along he (A) symmery lne; (B) radal poson a m; (C) radal poson a m; (D) radal poson a m Fgure 5. Comparson beween wh radaon model, whou radaon and whou WSGGM on chemcal speces concenraons along he symmery lne: (A) mehane; (B) oxygen; (C) carbon doxde; (D) carbon monoxde

10 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 164 Fgure 6. Comparson of radaon model on emperaure dsrbuon along he (A) symmery lne; (B) radal poson a m; (C) radal poson a m; (D) radal poson a m Fgure 7. Comparson of radaon model on chemcal speces concenraons along he symmery lne: (A) mehane; (B) oxygen; (C) carbon doxde; (D) carbon monoxde

11 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 165 ransfer n a combuson chamber. In addon, P1 s nown o over-predc he radave fluxes from localzed hea sources,.e. combuson flame. I was found ha he smulaon usng DO and P1 models yelded a smlar rend on he emperaure and chemcal speces concenraon profles a varous posons of he burner. Theorecally, DO solves a fne number of dscree sold angles and hence DO requres hgher compuaonal demand han he P1 model. In he smplfed wo-dmensonal case le he one presened n hs wor, he dfference beween he P1 and DO models s no pronounced. Ths s arbued by he lmed sold angle avalable for radaon n wodmensonal smulaon. I s worh nong ha he DO s more accurae han P1 for 3D smulaon le he one presened n our prevous wor [2]. Therefore, he P1 model was used for he remander of hs wor o provde a quc esmaon of hea ransfer n he combuson burner. 3.3 Effec of urbulence model The effec of urbulence model on he predcon of emperaure and chemcal speces concenraon nsde he burner was evaluaed usng an unseady-sae, P1 and parally premxed combuson models. The hree dfferen RANS urbulence models are SKE, RKE. and RNG. The predcons were aen along he symmery lne and he radal poson of he burner. Fgures 8 o 9 shows he comparson of he predced emperaure and chemcal speces mass fracon nsde he burner obaned usng he hree urbulence models wh he expermenal daa [3]. The resuls clearly showed ha he smulaon va SKE yelded he bes agreemen wh he expermenal daa [3], whereas he RKE gave he poores predcon. Ths may be arbued by he fac ha he flud flow nsde he burner s mosly soropc and homogenous urbulence, whch favours SKE. The flud mxng n he burner dd no feaure a srong swrlng flow, whch s sued he RKE and RNG as shown n Fgures 10 and 11. Only a mnor recrculang flow appeared n he regon us above he nle and oule, respecvely (refer Fgure 11). Therefore, he SKE model s suffcen for he naural gas combuson modellng n a cylndrcal burner. 3.4 Effec of combuson model The SKE urbulence model was hen used o evaluae he effec of combuson models on he emperaure and chemcal speces concenra- Fgure 8. Comparson of urbulence model on emperaure dsrbuon along he (A) symmery lne; (B) radal poson a m; (C) radal poson a m; (D) radal poson a m

12 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 166 Fgure 9. Comparson of urbulence model on chemcal speces concenraons along he symmery lne: (A) mehane; (B) oxygen; (C) carbon doxde; (D) carbon monoxde Fgure 10. Flud pahlnes coloured by velocy magnude for he burner Fgure 11. Vecor plo of velocy magnude for he burner

13 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 167 on of he burner. Two dfferen combuson models (.e. FR/EDM and parally premxed flame) coupled wh he P1 radaon model were employed for he naural gas combuson n he burner. The predcons were compared wh he expermenally measured daa [3], as shown n Fgures 12 and 13. I was found ha he parally premxed flame model wh dealed chemsry mechansm gave more accurae predcons han ha of FR/EDM. The smulaon va parally premxed flame model excellenly predcs he emperaure along he symmery lne (Fgure 12(A)) and he radal poson a m (Fgure 12(D)) of he burner, alhough a mnor devaon was shown n Fgures 12(B) and (C). However, he FR/EDM Fgure 12. Comparson of combuson model on emperaure dsrbuon along he (A) symmery lne; (B) radal poson a m; (C) radal poson a m; (D) radal poson a m Fgure 13. Comparson of combuson model on chemcal speces concenraons along he symmery lne: (A) mehane; (B) oxygen; (C) carbon doxde; (D) carbon monoxde

14 Bullen of Chemcal Reacon Engneerng & Caalyss, 13 (1), 2018, 168 shows a relavely poor predcon of he emperaure a varous posons of he burner. In Fgures 13(A) and (C), he mass fracon of mehane and carbon doxde are well resolved by boh FR/EDM and parally premxed flame models. Fgure 13(B) shows he predcon usng parally premxed flame model yelded a beer agreemen wh he expermenal daa [3], whereas he FR/EDM over-predcs he oxygen mass fracon. Ths s arbued by he dealed urbulen flame modellng of parally premxed flame model by combnng he modellng sraeges for non-premxed and premxed flame models. The parally premxed flame model consders he urbulen mxng for boh fuels-rch and fuel-lean regons and also deermnes he reacons for boh burn and unburn mxure regons. In addon, GRI-MECH 3.0 s opmzed for he urbulence-chemsry n mehane oxdaon a he wde range n emperaure, le he naural gas combuson n he presen wor. GRI-MECH 3.0 consders 325 reacon mechansms, ncludng he nermedae reacons and chemcal speces dssocaon. However, FR/EDM only consders wo-seps global mechansm. Therefore, he parally premxed flame wh mulple mechansms s more accurae compared o he FR/EDM. 4. Conclusons The effec of modellng mehods on he naural gas combuson n a wo-dmensonal cylndrcal burner was performed by evaluang varous urbulence, radaon and combuson models. The CFD smulaon was successfully valdaed wh he expermenally measured daa. Applcaon of he radaon model (.e. DO and P1) n conuncon wh he gas absorpon coeffcen model, WSGGM, mproved maredly he predcon accuracy of radaon domnaed he hea ransfer n naural gas combuson burner. I was found ha dealed mechansm GRI-MECH 3.0 provded more accurae predcon of he emperaure and chemcal speces concenraon (.e. error of 7.8% and 15.5%) han a wo-sep FR/EDM (.e. error of 17.5% and 31.4%). The fndngs obaned from hs wor showed ha parally premxed combuson model coupled wh he P1 radaon model and he SKE urbulence model s he bes combnaon for he modellng of naural gas combuson n he burner. The CFD smulaon presened n hs wor may serve as a useful ool o evaluae a performance of a naural gas combusor. Acnowledgmen W.P. Law hans Mnsry of Educaon Malaysa for he Mybran15 PhD scholarshp. We acnowledge fundng from Unvers Malaysa Pahang hrough GRS W.P. Law s he recpen of UMP Pos-Docoral Fellowshp n Research. References [1] Law, W.P., Gmbun, J. (2015). Influence of Nozzle Desgn on he Performance of a Paral Combuson Lance: A CFD Sudy. Chemcal Engneerng Research and Desgn, 104: [2] Law, W.P., Gmbun, J. (2016). Scale-Adapve Smulaon on he Reacve Turbulen Flow n a Paral Combuson Lance: Assessmen of Thermal Insulaors. Appled Thermal Engneerng, 105: [3] Garreon, D., Smonn, O. (1994). Aerodynamcs of Seady Sae Combuson Chambers and Furnaces. ASCF Ercofac CFD Worshop, Ocober 17-18, Org: EDF, Chaou, France. [4] Magel, H.C., Schnell, U., Hen, K.R.G. (1996). Smulaon of Dealed Chemsry n a Turbulen Combusor Flow. Symposum (Inernaonal) on Combuson, 26(1): [5] Isnard, A.A., Gomes, M.S.P. (1999). Numercal Smulaon of NOx and CO Formaon n Naural Gas Dffusve Flames. In Proceedngs of he 15h Brazlan Congress of Mechancal Engneerng. Aguas de Lndoa, Brazl. [6] da Slva, C.V., Franca, F.H.R., Velmo, H.A. (2007). Analyss of he Turbulen, Non- Premxed Combuson of Naural Gas n a Cylndrcal Chamber Wh and Whou Thermal Radaon. Combuson Scence and Technology, 179(8): [7] da Slva, C.V., Segao, C.A., de Paula, A.V., Ceneno, F.R., Franca, F.H.R. (2013). 3D Analyss of Turbulen Non-Premxed Combuson of Naural Gas n a Horzonal Cylndrcal Chamber. In Proceedngs of he Anas do 22nd Brazlan Congress of Mechancal Engneerng. Rberao Preo, SP, Brazl. [8] Poozesh, S., Aafuah, N., Sao, K. (2016). NO Formaon Analyss of Turbulen Non- Premxed Coaxal Mehane/Ar Dffuson Flame. Inernaonal Journal of Envronmenal Scence and Technology, 13(2): [9] Ronche, B., da Slva, C.V., Velmo, H.A. (2005). Smulaon of Combuson n Cylndrcal Chamber. In Proceedngs of he 18h Inernaonal Congress of Mechancal Engneerng. Ouro Preo, Brazl.

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