Detailed Numerical Comparison of Laminar Burning Speed of Stratified Hydrogen-Air and Methane-Air Mixture With Corresponding Homogeneous Mixture Using Open-Source Code

A detailed numerical study of laminar burning speed for fuel-air mixture is conducted using laminarReactingLMFoam solver which is a modified version of reactingFoam solver based on openfoam code. It accounts for detailed mixture-averaged transport property calculation for reacting flow using low-Mach number governing equations. The effect of various equivalence ratio gradients is studied on stratified hydrogen-air and methane-air mixture with mixture-averaged transport model and unity Lewis number for all species, and corresponding laminar burning speed is compared with homogeneous mixture. For both the fuel-air mixture, rich to lean stratified mixture resulted in a higher laminar burning speed and no significant difference was noticed for lean to rich stratified mixture when compared with homogeneous mixture at same local equivalence ratio. Increased burning speed is explained based on higher burnt gas temperature and molecular diffusion of lighter species from burnt gas referred to "Chemical Effect" in this study. The effect of thermal and molecular diffusion from the burnt gas on laminar burning speed is studied for stratified and homogeneous mixture using mixture-averaged transport model and unity Lewis number for all species. It is shown that the molecular diffusion effect from burnt gas ("Chemical Effect") is more prominent as compared with the thermal diffusion effect. Extension in lean flammability limit for stratified mixture of both the fuel is shown based on higher heat release rate as compared with homogeneous mixture and extension in flammability limit for stratified mixture is explained based on higher Chemical Effect from burnt gas.