Molecular Transport Effects of Hydrocarbon Addition on Turbulent Hydrogen Flame Propagation

1. Molecular Transport Effects of Hydrocarbon Addition on Turbulent Hydrogen Flame Propagation Siva P R Muppalaand 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, 57076 Siegen, Germany.

2. Contents Motivation Flames previously investigated Molecular transport effects in premixed turbulent combustion Outwardly propagating spherical flames Various approaches to modelling of H2+HC flames Algebraic flame surface wrinkling model Results Conclusion

3. Motivation: Safety considerations in hydrogen usage Study: • Influence of molecular transport (preferential diffusion and Lewis number) effects in H2+HC mixtures • Quantitative evaluation of flame speed for mixed fuels

4. Hydrocarbon flames previously investigated Different configurations studied numerically using the Algebraic Flame Surface Wrinkling Premixed Turbulent model Tohoku University University of Orleans

5. Modelling Turbulent Premixed Combustion • Fuel Variation(Lewis Number) Reaction zonePreheat zone Products a Reactants: Fuel + Air Dminor a a Dminor Dminor Le>1 Le=1 Le<1 Molecular Transport Effects • Pressure: • Fine scale structures that can wrinkle the flame decreases Aluri NK, Doctoral Dissertation, University of Siegen 2007

6. Flame Curvature, Mass Flow & Turbulent Flame Speed Unburned gas consumed by a turbulent flamelet A • Premixed gas flows along marked streamlines • Streamlines ┴ to flamefront • Ratio of mass flow flowing into the convex ‘BC unburned‘ / to convex ‘AC burned‘ ~ 3:1 • The convex part of flamelet towards the unburned mix. affects the turbulent flame speed predominantly Burned Reaction Sheet C A' B B' Stream Lines Unburned ToConvexFlamelettoward theBurnedMixture ToConvexFlamelettoward theUnBurnedMixture

7. Preferential Diffusion and Lewis number Effects

8. H2/O2/N2; f=1.2; Le=1.29 f=0.8; Le=0.42 u’/SL0=1.4 SL0=25cm/s Le decrease Outwardly Propagating Spherical flames – Kido database Mixture data: Hydrogen-methane & Hydrogen-propane lean mixtures Equivalence ratio: 0.8 Turbulent velocity = 2 m/s (max) Measured data: SL= Mean local burning velocity; = f(PD=Df/Do) ST = Turbulent flame speed Spherical gaseous (H2) explosion

9. Lewis Number Effects – DNS investigations • DNS by Trouvé and Poinsot 1994 on lean H2/O2/N2 flames……………………………….. …………………………………………………… and DNS of lean H2 flame by Bell et al. 2006 (not depicted here), confirm the Le influence on turbulent flame speed, especially in lean H2 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

10. Critically curved flamelets imposed on flame-ball concept by Zel’dovich Based on exp’tal data Analytical Methods (a submodel for preferential diffusion effect) Weakly stretched flamelet models based on Ma Submodel for time scale exponential (Le-1) relation Modelling approaches to multi-fuelpremixed turbulent flames

11. Measured Ma for CH4- and H2-air mixtures for f=0.40, 0.43 and 0.50 are 0.7 and -0.3, respectively. Bechtold and Matalon Combust. Flame 1999 Weakly stretched flamelets concepts SL= SL0(1 – Maּ ּtc) SL= stretched laminar flame speed substituted for SL0 Ma = Markstein number = f(flame stretch, curvature) Chemical time scale ּtc = laminar flame thickness/ (unstretched laminar flame speed)2 is stretch rate ּtc is commonly simplified to Ka.Ma or Ka.Le

12. Analytical method – A submodel for preferential diffusion effect SL(alp) = Mean local burning velocity at the leading point of the flamelet, substituted for SL0 in the turbulent flame speed model alp is (1/ local equivalence ratio) at the leading point Assumption: DO,u = ku mass stoichiometric coefficient = ratio of diffusivities of fuel and oxidant 1/Initial equivalence ratio Kuznetsov, V.R., and Sabel'nikov, V.A., Turbulence and Combustion, Hemisphere, 1990.

13. 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

14. AT Fuel+Air  because of Damköhler hypothesis Premixed Turbulent Combustion submodel • General reaction rate expression • Folding factor S = Flame surface area / Volume Turbulent flame surface area Surface density function Averaged flame surface area Muppala, Aluri, Dinkelacker -- Comb Flame 2005 Aluri, Pantangi, Muppala,, Dinkelacker – Flow, Turb Comb 2005

15. Algebraic Flame Surface Wrinkling model: two forms Model predictions {without Preferential Diffusion effect} {without the Lewis number effect}

16. Results1 – Hydrocarbon + Hydrogen mixtures: CH4 + H2 50%H2-50% CH4 100%H2-0% CH4 0%H2-100% CH4 Turbulent flame speed vs. turbulence intensity for lean CH4–H2 flames. Model predictions based on SL0.

17. Results2 – Hydrocarbon + Hydrogen mixtures: CH4 + H2 100%H2-0% CH4 50%H2-50% CH4 0%H2-100% CH4 CH4–H2 mixtures. Model predictions are based on SL (mean local burning velocity with preferential diffusion) and Lewis number effect.

18. Results1 – Hydrocarbon + Hydrogen mixtures: C3H8 + H2 50%H2-50% C3H8 100%H2-0% C3H8 Le = 0.70 Le = 0.42 0%H2-100% C3H8 Le = 1.57 Turbulent flame speed vs. turbulence intensity u’ for lean C3H8–H2flames. Model predictions based on SL0.

19. Results2 – Hydrocarbon + Hydrogen mixtures: C3H8 + H2 50%H2-50% C3H8 100%H2-0% C3H8 0%H2-100% C3H8 C3H8–H2 mixtures. Model predictions are based on SL (mean local burning velocity with preferential diffusion) and Lewis number effect.

20. Correlation plots – turbulent flame speed: Exp vs. Model Correlation plot for turbulent flame speed ST : Experimentally measured vs. model predicted, estimated based on SL for CH4–H2 and C3H8–H2 mixtures.

21. Summary • 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 (CH4–H2 and C3H8–H2) mixtures. • The model predictions were in good quantitative agreement with the corresponding experiments, if either mean local burning velocity SL 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 H2 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 H2 flames.

22. Thank you for your attention