Turbulent burning velocity and its related statistics of ammonia‐hydrogen‐air jet flames at high Karlovitz number: Effect of differential diffusion

To clarify the role of differential diffusion in highly turbulent premixed flames, a series of turbulent premixed ammonia/hydrogen/air flames were investigated using the NH-PLIF diagnostics. The investigated flames have almost the same laminar burning velocity, S_L, but are characterized by different Lewis number, Le, from 0.56 to 1.77. The Karlovitz number, Ka, of these flames ranges from 11 to 1052, and the turbulence intensity, u’/S_L, covers from 10 to 156. It is observed that the global consumption speed, S_T,GC/S_L, of sub-unity Le flames is much larger than that of super-unity Le flames at high Ka, indicating that the differential diffusion plays a significant role in highly turbulent flames. The flame surface density and the area ratio of turbulent flames with different Le are, however, similar under wide turbulent conditions. The stretch factor of sub-unity Le flames is estimated to be significantly larger than that of super-unity Le cases. The enhanced S_T,GC of sub-unity Le flames is suggested to be attributed to the promotion of local burning rates by the couple effect of differential diffusion and turbulent flame stretch within the flame brush, rather than the enlargement of flame surface area at high Ka. Furthermore, three correlations for the S_T,GC were developed based on Damkohler's second hypothesis with consideration of the Le effect. The correlation of S_T,GC/S_L ∼ (Re_T·Le^-2)^0.5 is further validated by using small-scale methane/air and large-scale ammonia/air flames at high Ka, where Re_T is turbulent Reynolds number. It suggests that the S_T,GC is roughly inversely proportional to the Le, and the differential diffusion effect should be included in the theoretical analysis and numerical simulation of highly turbulent flames.