Extinction timescales of periodically strained, lean counterflow flames

The extinction limits of unforced and periodically forced turbulent counterflow flames have been measured with equivalence ratios of relevance to lean-burn gas turbines. Thus, the opposed flows comprised mixtures of methane and air with the same equivalence ratios in the two streams in the range of 0.5 to 0.7 and also mixtures with equivalence ratios less than 0.7 in one flow and the other with an equivalence ratio of 0.9. The oscillations were imposed by loudspeakers and forced flame extinction was shown to depend on the total duration of pulsation. Extinction times were measured by forcing the flow with a sinusoidal signal of specified frequency, amplitude, and duration and, if extinction did not occur, the time of pulsation was increased and the procedure was repeated until extinction took place. Extinction strain rates increased with equivalence ratio and were greater for asymmetric than symmetric flames with the same total quantity of fuel and total equivalence ratios below 0.7, based on the fuel and air mixtures of both streams. The forced flames withstood instantaneous strain rates larger than the critical values for unforced flame extinction and survived for up to 100 cycles in the frequency range from 200 to 1000 Hz with instantaneous strain rates equal to the unforced extinction limit. Symmetric flames had shorter extinction timescales and were more sensitive to changes in the equivalence ratio than asymmetric flames of the same total quantity of fuel. (Author)