Numerical Study of Propane-Air Mixture Combustion in a Burner Element

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1 Defect and Dffuson Forum Vols (8 pp onlne at (8 Trans Tech Publcatons, Swtzerland Onlne avalable snce 8/Feb/11 Numercal Study of Propane-Ar Mxture Combuston n a Burner Element C.E.L. Pnho 1,a, J.M.P.Q. Delgado,b, R. Plão 3,c, J. Conde 1,d and C. Pnho,e 1 INEGI Rua do Barroco, nº 174; Leça do Balo, Portugal CEFT DEMEGI, Faculdade de Engenhara da Unversdade do Porto Rua Dr. Roberto Fras, s/n; Porto, Portugal 3 CIEA Insttuto Superor de Engenhara do Porto, Rua Dr. Antóno Bernardno de Almeda nº 341, 4-7 Porto, Portugal a cpnho@neg.up.pt, b jdelgado@fe.up.pt, c rmp@sep.pp.pt, d jsantos@neg.up.pt, e ctp@fe.up.pt Keywords: Heat transfer, Combuston, Turbulence, Computer Flud Dynamcs. Abstract. Ths study consders numercal smulatons of the combuston of propane wth ar, n a burner element due to hgh temperature and velocty gradents n the combuston chamber. The effects of equvalence rato (φ and oxygen percentage (γ n the combuston ar are nvestgated for dfferent values of φ between.5 and 1. and γ between 1 and 3%. In each case, combuston s smulated for the fuel mass flow rate resultng n the same heat transfer rate (Q to the combuston chamber. Numercal calculatons are performed ndvdually for all cases wth the use of the Fluent CFD code. The results shown that the ncrease of equvalence rato corresponds to a sgnfcantly decrease n the maxmum reacton rates and the maxmum temperature ncrease wth the ncreases of oxygen percentage. Mxng hydrogen wth propane causes consderable reducton n temperature levels and a consequent reducton of CO emssons. Introducton When early commercal CFD packages became avalable more than years ago, smulatng the complex physcs nsde combuston chambers was already one of the target applcatons. Of course, projects were often lmted by computer resources these days [1,]. Therefore n most cases reacton was taken nto account usng relatvely smple approaches such as the Eddy Dsspaton Model. Today, wth ncreasng maturty of CFD technque and computng power, one focus n numercal smulaton of combuston s n the area of non-equlbrum chemstry and multphase flow. New felds of applcaton, such as the formaton of pollutants n techncal flames or the optmzaton of combuston processes, can be tackled that way. Some of the elements contend by fuel form dangerous combuston products. The amount of these products s related wth ther percentage n the fuel. The complete combuston of the man elements of fuel ncreases the effcency of the boler. The constructon of burner defnes the combuston effcency. Turbulent combuston of hydrocarbon fuels s an ntegral part of many segments of the chemcal and power ndustres, and also n hot water bolers usually used as heatng source for resdences. Combuston phenomenon s a complex mxture of flud dynamcs and chemstry. The prmary objectves n burner desgn are to ncrease combuston effcency and to mnmze the formaton of envronmentally hazardous emssons, such as CO, unburned hydrocarbons (HC and NO x. Crtcal desgn factors that mpact combuston nclude: the temperature and resdence tme n the combuston zone, the ntal temperature of the combuston ar, the amount of excess ar and turbulence n the burner and the way n whch the ar and fuel streams are delvered and mxed. Therefore, CFD codes can serve as a powerful tool used to perform low cost parametrc studes. The CDF codes solve the governng mass, momentum and energy equatons n order to calculate the pressure, concentratons, veloctes and temperatures felds. Ths work consders the combuston of propane wth ar due to the hgh temperature and velocty gradents n combuston chamber usng a sngle burner element. In order to nvestgate the effect of All rghts reserved. No part of contents of ths paper may be reproduced or transmtted n any form or by any means wthout the wrtten permsson of the publsher: Trans Tech Publcatons Ltd, Swtzerland, (ID: /3/8,16:13:15

2 Defect and Dffuson Forum Vols oxygen percentage on the combuston, the combuston of fuel wth ar s examned at varous oxygen percentages n the ar by usng Fluent CFD code [3]. Mathematcal Model In ths work we used the followng models for the numercal calculatons: (a turbulent flow, wth turbulent model of RNG k ε appled [4] wth a standard wall functons for near wall treatment; (b for the chemcal speces transport and reactng flow, the eddy-dsspaton model wth the dffuson energy source opton. The followng assumptons are made: (a the flow s steady, turbulent and compressble; (b the mxture (propane-ar s assumed as an deal gas; (c no-slp condton s assumed at the burner element walls. The governng equatons for mass, momentum and energy conservaton, respectvely, for the two-dmensonal steady flow of an ncompressble Newtonan flud are: Mass conservaton equaton wth and.( ρ u J + S (1 J Y =. ( D,m + t / Sc t Y = ρ µ (a 1-Y D,m = (b Y / D j, j where ρ s the densty, u s the flud velocty, Y s the local mass fracton, J s the dffuson flux, S s the rate of creaton by chemcal reacton, turbulent vscosty and Schmdt number, respectvely. Momentum conservaton equaton wth u j =. P +. eff D, m s the dffuson coeffcent, µ t and Sc t are the.( ρu τ (3 τ eff T µ( u + u /3. uδ = (4 where τ eff s the stress tensor, µ s the molecular vscosty and δ s the unt tensor. Energy conservaton equaton wth. [ u ( ρ E + P ] =. keff. T h j J j + u j eff + Sh τ (5 j P E = h + ρ u (6 where E s the energy, P s the pressure, k eff s the effectve conductvty, S h s the source of energy and h s the sensble enthalpy. In ths work, the combuston of propane wth ar s modelled wth one-step reacton mechansm. The reacton mechansm takes place accordng to the constrants of chemstry and t s defned by

3 146 Dffuson n Solds and Lquds III C γ 5 1 γ 1 φ + (7 φ φ γ φ γ φ 3H 8 O + N 3CO + 4H O + N + 5 O where φ ( [ M + (1 γ / γ M m ]/( M m & = 5 O N fuel fuel ar s the equvalence rato and fuel are fuel and ar mass flow rates, respectvely and γ s the oxygen percentage n ar. Computatonal Model & m& and A two-dmensonal burner element (see Fgure 1 was desgned usng Gambt package (v..3, Fluent Inc. The skewness of the cells was mproved n Gambt and usng the grd refnement/adaptaton procedure of the Fluent code t was refned. In the numercal calculaton, two calculaton models, a lamnar flow L=.5 m model and a turbulent flow model, were used because the Reynolds numbers based on the hydraulc dameter and the flow velocty vary from 5 to 3. Ar r For flows of Reynolds number below ar= m Propane 1, a steady lamnar flow model was r fuel=.4m r wall=.6 m used. For Reynolds numbers over 1, a turbulent model of RNG k ε was Fgure 1 Scheme of the burner analysed. appled wth a standard wall functon for near wall treatment. m& ar r out= m Geometry and Mesh Generaton Grd generaton represents a major challenge for CFD analyss. It s a tme-consumng task and, n spte of steady advances n automatc mesh generaton, t stll requres the skll of a CFD practtoner to yeld a sutable mesh. The choce of the type of grd depends on geometrcal complexty and on physcs. The skewness of the cells was mproved n Gambt package and usng the grd refnement/ /adaptaton procedure of the Fluent code t was refned. The grd was smoothed usng the swap/smooth optons n both codes. Grd refnement was performed untl further refnement showed no notceable effects. The fnal unstructured grd that had 5 cells was used n the smulaton. Gas Flow Smulaton For gas smulaton a propane-ar mxture was used wth the followng physcal values: h W/m K, T = T = T 3 K 3, P = 1135 Pa and ρ = 1.5 kg/m and = amb n amb ref = ar 3 ρ C H = 1.91 kg/m at the ar and fuel nlet, respectvely. The thermal propertes ( p 3 8 c, µ and λ of the propane and speces are functon of temperature. The propane densty at the fuel nlet and the molecular weghts, enthalpes and lower heatng values of reactant and product speces are taken from the materal property database gven by Fluent Inc. [3]. The ranges of the smulaton values are: Q & = 1W, φ =.5,.7, 1. and γ = 1%, %, 3%. Fluent Modellng The Fluent modellng s based on the two-dmensonal conservaton equatons for mass and momentum. The dfferental equatons are dscretsed by the Fnte Volume Method and are solved by the SIMPLE algorthm. As a turbulence model, the k ε was employed [4]. The Fluent code uses an unstructured non-unform mesh, on whch the conservaton equatons for mass, momentum and energy are dscretsed. The k ε model descrbes the turbulent knetc energy and ts dsspaton rate and thus compromses between resoluton of turbulent quanttes and computatonal tme. No-slp

4 Defect and Dffuson Forum Vols condton s assumed at the burner walls. The model constants for the RNG k ε model are C =.845, 1. 4 µ C, C 1. 68, and wall Prandtl number of ε = ε = Grd ndependence/soluton adaptve refnement Whle fner meshes would yeld more accurate results, we were lmted to ths number of elements because of the computer speed and memory. However, doublng the total number of elements yelded less than % change n overall pressure drop and the solutons were consdered bascally grd-ndependent. The standard k ε model was used wth non-equlbrum wall functons. Nonequlbrum wall functons were preferred to the standard wall functons because non-equlbrum wall functons are better to deal wth complex flows nvolvng separaton, reattachment, other nonequlbrum effects and strong pressure gradents (see Fluent User s Gude [3]. Results and Dscusson Numercal modellng of the burner element was performed at dfferent stochometrc mxture ratos and oxygen percentage n the ar. Table 1 shows the nlet veloctes of ar and stochometrc ar/fuel ratos for Q = 1W and u fuel =.47 m/s, wth R sto and u ar gven by: R u sto O (1 / N 5 M M + γ γ = (8 R ρ A M C H 3 8 sto fuel fuel ar = ufuel (9 φ ρar Aar Table 1 Inlet veloctes of ar and stochometrc ar/fuel ratos, for u ar (m/s γ (% R sto φ =.5 φ =.7 φ = Q = 1W r (m ( φ =. 5 and γ = 1% r (m ( φ = 1. and γ = 1% r (m ( φ = 1. and γ = 3%.5 Fgure Reacton rates contours for dfferent equvalence ratos and oxygen percentage. Fgure presents the dstrbutons of reacton rates n the burner element for γ = 1% to 3% n the case of φ = 1.. The numercal results showed that the maxmum reacton rates decrease sgnfcantly wth the ncrease of equvalence ratos and wth the ncrease of oxygen percentage n ar, these regons contract n the axal drecton whereas they expand n the radal drecton. The 3 maxmum reacton rate profles obtaned were R = 1.14,.55 and.41 kmol/m s n the cases of φ =.5/ γ = 1%, φ = 1. / γ = 1% and φ = 1. / γ = 3%, respectvely. Fgure 3 shows the temperature dstrbuton n the burner element for the cases of γ = 1%, %, 3% and φ =.5, 1.. The numercal results llustrates an ncrease of temperature

5 148 Dffuson n Solds and Lquds III wth both ncreases of γ and φ. The maxmum temperature level was about 71 K ( φ =. 5 and γ = 3%. The average temperatures of the burner element ncreased from 13 to 71 K and from 151 to 65 K n the cases of φ =. 5 and φ = 1., respectvely, wth the ncrease of γ from 1 to 3,.e., the results shows that γ has more effect n temperature than φ. Temperature level s decreasng consderably from the maxmum zone trough the burner ext. r (m ( φ =. 5 and γ = 1% r (m ( φ =. 5 and γ = % r (m ( φ =. 5 and γ = 3% r (m r (m r (m ( φ = 1. and γ = 1% ( φ = 1. and γ = % ( φ = 1. and γ = 3% Fgure 3 Temperature dstrbuton for φ =. 5 and φ = 1. at dfferent values of γ. T (K γ = % T (K Seres1 γ = 1% Seres γ = % Seres3 γ = 3% Seres1 φ =.5 Seres φ =.7 Seres3 φ = 1. Seres4 φ = 1. (5%H +5%C 3H φ = Fgure 4 Axal temperature varaton at dfferent (a-equvalence rato and (b-oxygen percentage. Fgure 4 llustrates the axal temperature, along the axs of the burner, for dfferent equvalence ratos and oxygen percentage n ar, on propane combuston. In general, ncreasng the oxygen percentage ncreases the temperature. Ths trend s as expected and smlar to the works developed by other authors [5,6]. Moreover, ths computatonal flud dynamcs study also consdered the combuston of hydrogen propane mxture fuel wth 5%H +5%C 3 H 8. It s known [] that hydrogen reduces the emsson of some pollutants, pure hydrogen fuel combuston does not gve CO or unburned HC emssons. Fgure 4(a shows that mxng hydrogen wth propane causes consderable ncrease n temperature levels. The predcted maxmum temperature level, for pure propane combuston wth an equvalence rato of 1. and an oxygen percentage n ar of %, was about 199 K, whereas the

6 Defect and Dffuson Forum Vols predcted temperature dstrbutons for hydrogen propane (5%H +5%C 3 H 8 mxture combuston, at the same condtons, showed a predcted hgh temperature of 195 K. The overall flame temperature ncreases as hydrogen s added to the fuel due to the hgher energy nput and lower flame radaton. It must be stressed that although a hgher combustors temperature wll reduce CO and unburned HC emssons t wll, on the other end, rase NO emssons through the thermal or Zeldovch mechansms [7]. Conclusons The combuston of propane wth ar was analyzed n a burner element and the effect of the equvalence rato and oxygen percentage n ar nvestgated, for dfferent numercal values. Combuston was smulated for the fuel mass flow rate resultng n the same heat transfer rate to the combuston chamber n each case. The results shown that the ncrease of equvalence rato corresponds to a sgnfcantly decrease n the maxmum reacton rates and the maxmum temperature ncrease wth the ncreases of oxygen percentage. Mxng hydrogen wth propane causes consderable reducton n temperature levels and a consequent, expected, reducton of CO and unburned HC but hgher NO x emssons are expected. Acknowledgment J.M.P.Q. Delgado wshes to thank FCT for the grant Nº SFRH/BPD/11639/. Notaton A Area R sto Stochometrc ar/fuel rato c p Constant-pressure specfc heat Sc t Turbulent Schmdt number C Coeffcent n k-ε turbulence model S h Source of energy µ C 1ε Coeffcent n k-ε turbulence model S Rate of creaton by chemcal reacton C ε Coeffcent n k-ε turbulence model T Temperature D,m Molecular dffuson coeffcent u Velocty E Energy Y Local mass fracton of each spece h Sensble enthalpy γ Oxygen percentage n ar J Dffuson flux of speces δ Unt tensor k eff Effectve conductvty φ Equvalence rato m& Mass flow rate λ Thermal conductvty M Molecular weght of speces µ Molecular vscosty P Pressure µ t Turbulent vscosty Q & Heat transfer rate ρ Densty R Reacton rate τ eff Stress tensor References [1] G. Wecel and R.A. Bałeck: Combust. Sc. and Tech. Vol. 178 (6, p [] M. Ilbas, I. Yılmaz and Y. Kaplan: Int. J. Hydrogen Energ. Vol. 3 (5, p [3] Fluent Incorporated: FLUENT User s gude verson 6. (5. [4] V. Yakhot and S.A. Orszag: J. Sc. Comput. Vol. 1 (1986, p. 151 [5] M. Ilbas: Int. J. Hydrogen Energ. Vol. 3 (5, p [6] M. Ilbas: Studes of ultra low NOx burner (PhD thess, Cardff, Unversty of Wales, [7] S.R. Turns: An ntroducton to combuston - Concepts and applcaton (McGraw Hll, Inc., New York, 1996.