Numerical analysis and model development for laminar flame speed of stratified methane/air mixtures

Numerical simulations of stratified methane/air flames with various stratification configurations are conducted using an 1-D unsteady reacting flow solver. Among all the stratified flame cases investigated, rich-to-lean stratified flames show significant departures from homogeneous flames, i.e., up to 50% increase in fuel consumption speeds, primarily due to preferential diffusion of lighter species and radicals from rich burnt products. A sensitivity study of transport properties further reveals that preferential diffusion of molecular hydrogen H2 along with radicals H, OH is the dominant factor in increasing stratified flame speeds, compared to the influence of heat diffusion and diffusion of H2O. Larger departure of stratified flames from homogeneous flames is observed in those cases with higher degree of stratification. In order to model transient behaviors of stratified flames with arbitrary stratification configurations, a local stratification level (LSL) model is proposed. LSL incorporates the effect of preferential diffusion by introducing a transfer function from molecular hydrogen concentration gradients to equivalence ratio gradients, and the memory effect by solving the transient model equation of LSL. The model results match well with directly simulated results quantitatively with error less than ∼10% for both lean and rich conditions.