Effects of local flame displacement velocity on turbulent burning velocity for lean H₂-C₃H₈-Air mixtures

Abstract

This study was made to establish a prediction method of turbulent burning velocity for lean hydrogen-propane-air mixtures, with paying special attention to the importance of local burning velocity to turbulent burning velocity. In this study, lean hydrogen added propane air mixtures with different rates of addition of hydrogen δ_H and equivalence ratios Φ(0.5, 0.6 and 0.8) were prepared, where Φ includes fuel concentration below a lean flammability limit of propane-air mixtures. A two-dimensional sequential laser tomography technique was applied to obtain the relationship between the flame shape and the flame displacement in a constant-volume vessel. The local flame displacement velocity S_F of turbulent flames was quantitatively measured as a key parameter of local burning velocity. It was found that the mean values of S_F, S_F,m, at the same u'/S_L0 tended to increase as hydrogen was added to lean propane-air mixtures with Φ=0.8. The trends of S_F,m against u'/S_L0 also corresponded well to its turbulent burning velocities, regardless of δ_H and Φ. An attempt was also made to predict the turbulent burning velocity by some empirical equations. As a result, the predicted turbulent burning velocities, where the obtained S_F,m was employed instead of S_L0 as a local burning velocity, were in better agreement with experimental ones for lean hydrogen-propane-air mixtures.