SM(1) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Molecular Transport Effects of Hydrocarbon Addition on Turbulent Hydrogen Flame Propagation Siva P R Muppala and Jennifer X Wen Fire and Explosion Research Group, Department of Mechanical Engineering Kingston University, London, UK Naresh K Aluri Gas Turbine Combustion group, ALSTOM (Baden), Switzerland F Dinkelacker Institut Fluid- und Thermodynamik, Universität Siegen, Paul-Bonatz-Str. 9-11, Siegen, Germany.

SM(2) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Contents 1.Motivation 2.Flames previously investigated 3.Molecular transport effects in premixed turbulent combustion 4.Outwardly propagating spherical flames 5.Various approaches to modelling of H 2 +HC flames 6.Algebraic flame surface wrinkling model 7.Results 8.Conclusion

SM(3) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Study:  Influence of molecular transport (preferential diffusion and Lewis number) effects in H 2 +HC mixtures  Quantitative evaluation of flame speed for mixed fuels Motivation: Safety considerations in hydrogen usage

SM(4) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Hydrocarbon flames previously investigated Flame configurations Effects investigated ApproachExperimental source Bunsen Burner High-Pressure, Fuel TypeRANS, LES Swirl Burner High-Pressure, Fuel TypeRANS Dump CombustorHigh-Pressure RANS, LES Swirl Burner Flame Stability & Dynamics RANS, LES Bunsen Burner High-pressure, H 2 -dopingRANS, LES Tohoku University University of Orleans Different configurations studied numerically using the Algebraic Flame Surface Wrinkling Premixed Turbulent model

SM(5) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Modelling Turbulent Premixed Combustion  Pressure:  Fine scale structures that can wrinkle the flame decreases  Fuel Variation (Lewis Number) Products Reactants: Fuel + Air Reaction zone Preheat zone Le<1 Le=1 Le>1 D minor    Molecular Transport Effects Aluri NK, Doctoral Dissertation, University of Siegen 2007

SM(6) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Flame Curvature, Mass Flow & Turbulent Flame Speed Reaction Sheet Burned Unburned A A'A' C B' B Stream Lines To Convex Flamelet toward the Burned Mixture To Convex Flamelet toward the UnBurned Mixture Unburned gas consumed by a turbulent flamelet 1.Premixed gas flows along marked streamlines 2.Streamlines ┴ to flamefront 3.Ratio of mass flow flowing into the convex ‘BC unburned‘ / to convex ‘AC burned‘ ~ 3:1 4.The convex part of flamelet towards the unburned mix. affects the turbulent flame speed predominantly

SM(7) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Preferential Diffusion and Lewis number Effects

SM(8) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Outwardly Propagating Spherical flames – Kido database Measured data: S L = Mean local burning velocity; = f(PD=D f /D o ) S T = Turbulent flame speed H 2 /O 2 /N 2 ;  =1.2; Le=1.29  =0.8; Le=0.42 S L0 =25cm/s Le decrease u’/S L0 =1.4 Spherical gaseous (H 2 ) explosion Mixture data: Hydrogen-methane & Hydrogen-propane lean mixtures Equivalence ratio: 0.8 Turbulent velocity = 2 m/s (max)

SM(9) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain  DNS by Trouvé and Poinsot 1994 on lean H 2 /O 2 /N 2 flames……………………………….. …………………………………………………… and DNS of lean H 2 flame by Bell et al (not depicted here), confirm the Le influence on turbulent flame speed, especially in lean H 2 mixtures  This substantial rise in flame speed may be due to sum of DL and PDT effects, or, can also be explained using Leading Point concept Lewis Number Effects – DNS investigations

SM(10) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Modelling approaches to multi-fuel premixed turbulent flames Weakly stretched flamelet models based on Ma Analytical Methods (a submodel for preferential diffusion effect) Based on exp’tal data Critically curved flamelets imposed on flame-ball concept by Zel’dovich exponential (Le-1) relation Submodel for time scale

SM(11) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Weakly stretched flamelets concepts S L = S L0 (1 – Maּ ּc)ּc) S L = stretched laminar flame speed substituted for S L0 Ma = Markstein number = f(flame stretch, curvature) ּ  c is commonly simplified to Ka.Ma or Ka.Le is stretch rate Chemical time scale ּ  c = laminar flame thickness/ (unstretched laminar flame speed) 2 Measured Ma for CH 4 - and H 2 -air mixtures for  =0.40, 0.43 and 0.50 are 0.7 and -0.3, respectively. Bechtold and Matalon Combust. Flame 1999

SM(12) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Analytical method – A submodel for preferential diffusion effect S L (  lp ) = Mean local burning velocity at the leading point of the flamelet, substituted for S L0 in the turbulent flame speed model  lp is (1/ local equivalence ratio) at the leading point mass stoichiometric coefficient 1/Initial equivalence ratio = ratio of diffusivities of fuel and oxidant Kuznetsov, V.R., and Sabel'nikov, V.A., Turbulence and Combustion, Hemisphere, Assumption: D O,u =  u

SM(13) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Critically curved flamelets: flame-ball concept by Zel'dovich  Asymptotically (activation temp  ∞ ) exact solution of stationary 1D balance equations for the temperature and mass fraction of the deficient reactant, for single-step single-reactant chemistry.  For Lewis numbers < 1, the flame ball temperature is given by  The chemical time scale for the highest local burning rate is Lipatnikov and Chomiak., Prog. in Energy, Comb Sci 2005 Aluri, Muppala, Dinkelacker Comb Flame 2006 Muppala et al. Comb Flame 2005

SM(14) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Premixed Turbulent Combustion submodel  General reaction rate expression  Folding factor  = Flame surface area / Volume ATAT Fuel+Air Turbulent flame surface area Averaged flame surface area  Muppala, Aluri, Dinkelacker -- Comb Flame 2005 Aluri, Pantangi, Muppala,, Dinkelacker – Flow, Turb Comb 2005 Surface density function because of Damköhler hypothesis

SM(15) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain {without the Lewis number effect} {without Preferential Diffusion effect} Model predictions Algebraic Flame Surface Wrinkling model: two forms

SM(16) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain 100%H 2 -0% CH 4 50%H 2 -50% CH 4 0%H % CH 4 Turbulent flame speed vs. turbulence intensity for lean CH 4 –H 2 flames. Model predictions based on S L0. Results1 – Hydrocarbon + Hydrogen mixtures: CH 4 + H 2

SM(17) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain 100%H 2 -0% CH 4 50%H 2 -50% CH 4 0%H % CH 4 CH 4 –H 2 mixtures. Model predictions are based on S L (mean local burning velocity with preferential diffusion) and Lewis number effect. Results2 – Hydrocarbon + Hydrogen mixtures: CH 4 + H 2

SM(18) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain 100%H 2 -0% C 3 H 8 50%H 2 -50% C 3 H 8 Le = 0.42 Le = %H % C 3 H 8 Le = 1.57 Turbulent flame speed vs. turbulence intensity u’ for lean C 3 H 8 –H 2 flames. Model predictions based on S L0. Results1 – Hydrocarbon + Hydrogen mixtures: C 3 H 8 + H 2

SM(19) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain 100%H 2 -0% C 3 H 8 50%H 2 -50% C 3 H 8 0%H % C 3 H 8 C 3 H 8 –H 2 mixtures. Model predictions are based on SL (mean local burning velocity with preferential diffusion) and Lewis number effect. Results2 – Hydrocarbon + Hydrogen mixtures: C 3 H 8 + H 2

SM(20) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Correlation plot for turbulent flame speed S T : Experimentally measured vs. model predicted, estimated based on S L for CH 4 –H 2 and C 3 H 8 – H 2 mixtures. Correlation plots – turbulent flame speed: Exp vs. Model

SM(21) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain  An existing algebraic flame surface wrinkling reaction model was used to investigate the quantitative dependence of turbulent flame speed on molecular transport coefficients for two-component lean fuel (CH 4 –H 2 and C 3 H 8 –H 2 ) mixtures.  The model predictions were in good quantitative agreement with the corresponding experiments, if either mean local burning velocity S L or an exponential Lewis number term of the fuel mixture is used in the reaction model. The latter approach is a generalisation of earlier findings for single fuels and shows the applicability of the exponential Le term for dual-fuel mixtures.  The hydrocarbon substitutions to H 2 mixtures are expected to suppress the leading flame edges, which are manifested by a decrease in mean local burning velocity, eventually preventing transition to detonation. Addition of hydrocarbons may also promote flame front stability of lean turbulent premixed H 2 flames. Summary

SM(22) Sep nd International Conference on Hydrogen Safety, San Sebastian, Spain Thank you for your attention