Fuel Lewis number effects in unsteady Burke-Schumann hydrogen flames

Flame response (as determined by temperature and flame thickness) to unsteady hydrodynamics has been measured in acoustically pulsed Burke-Schumann hydrogen flames at two different oscillation frequencies and amplitudes. The effect of fuel Lewis number (Le(F)) on flame dynamics is isolated by investigating steady and unsteady 40% H-2/60% He (Le(F)> 1) and 40% H-2/60% Ar (Le(F) < 1) flames. For a given flame with Le(F) < 1, local temperature was found to increase with stretch imparted on the reaction zone by the unsteady flow, whereas the opposite trend was observed for the LeF > 1 flame. Unsteadiness might qualitatively alter the effect of the fuel Lewis number. Notably, for Le(F) < 1 flames under oscillations Of SUfficiently high frequency and amplitude, the temperature at the flame tip is higher than that in the shoulder regions, and is different from the temperature field of both steady and low-frequency oscillation flames. This suggests that the effect of unsteady flame stretch may overwhelm that of the flame curvature for sufficiently high unsteadiness.