Extinction timescales of periodically strained, lean counterflow flames

Abstract

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. A form of chemiluminescence was used to observe the flame front with and without oscillation and gated measurements of the axial and radial velocity components quantified the phase lag between the input signal and the flow as a function of frequency.