Study of Transient Effects on the Extinction Limits of an Unsteady Counterflow Diffusion Flame

Extinction strain rates in unsteady methane- and propane-air counterflow diffusion flames were experimentally measured as a function of initial strain rate, oscillation frequency, and amplitude of the imposed fluctuation. The maximum strain rate was found to occur at a temporal phase corresponding to the maximum velocity for the diluted methane flame. However, for the propane flame the maximum strain rate occurred when the imposed velocity fluctuation was zero and decreasing. Above an oscillation frequency of 100 Hz, the diluted methane flame was able to survive peak strain rates exceeding the steady extinction strain rate. The minimum air velocity in the pure methane and propane flames was negative for all cases studied, which is most likely responsible for flame extinction at low frequencies and initial strain rates. However, at high initial strain rates and forcing frequencies peak unsteady strain rates at extinction approached the steady extinction strain rate and flow reversal was much less significant.