Numerical study of laminar flame speed of fuel-stratified hydrogen/air flames

Numerical studies on hydrogen/air stratified flames in 1-D planar coordinate are performed using a time-accurate and space-adaptive numerical solver A-SURF. A step change in equivalence ratio is initialized as fuel stratification. Flame characterizations including fuel consumption speed and flame front propagation speed are compared between stratified flames and corresponding homogeneous flames. Two transport models, with equal diffusivity and mixture-average diffusivity assumptions respectively, are considered. With equal diffusivity assumption and stratification thickness larger than flame thickness, local fuel consumption speeds of stratified and homogeneous flames are identical, indicating that neither thermal effect nor chemical effect is present in stratified flames. When stratification thickness is reduced to the order of flame thickness, the difference between local fuel consumption speeds of stratified and homogeneous flames is caused by chemical effect due to different level of H radical in burnt gas. The same mechanism also leads to the difference between local fuel consumption speeds with mixture-average diffusivity assumption. In addition, preferential diffusion of H radical further increases the difference. The difference between flame front propagation speeds of stratified and homogeneous flames is mainly caused by additional heat release in the burnt gas with equal diffusivity assumption, while the difference with mixture-average diffusivity assumption is mainly caused by local chemical effect. Hydrodynamic effect due to fluid continuity on flame front propagation speeds is observed in both transport models. Additionally, with increasing stratification thickness, both local chemical and hydrodynamic effect are reduced. No significant lean flammability extension of hydrogen/air mixture is introduced by fuel stratification.