Extinction timescales of periodically strained, lean counterflow flames

The extinction limits of unforced and periodically forced turbulent counter flow flames have been measured with equivalence ratios of relevance to lean -burn gas turbines. Thus, the opposed flows comprised mixtures of methane a nd 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 f low and the other with an equivalence ratio of 0.9. The oscillations were i mposed by loudspeakers and forced flame extinction was shown to depend on t he total duration of pulsation. Extinction times were measured by forcing t he flow with a sinusoidal signal of specified frequency, amplitude and dura tion and, if extinction did not occur, the time of pulsation was increased and the procedure was repeated until extinction took place. A form of chemi luminescence was used to observe the flame front with and without oscillati on and gated measurements of the axial and radial velocity components quant ified the phase lag between the input signal and the flow as a function of frequency. Extinction strain rates increased with equivalence ratio and were greater f or asymmetric than symmetric flames with the same total quantity of fuel an d total equivalence ratios below 0.7, based on the fuel and air mixtures of both streams. For example, asymmetric flames of 0.6 total equivalence rati o extinguished at bulk strain rates 70% higher than those of symmetric flam es. The forced flames withstood instantaneous strain rates larger than the critical values for unforced flame extinction and survived for up to 100 cy cles in the frequency range from 200 to 1000 Hz with instantaneous strain r ates equal to the unforced extinction limit. Symmetric flames had shorter e xtinction timescales and were more sensitive to changes in the equivalence ratio than asymmetric flames of the same total quantity of fuel. The visual isation showed that the light intensity emitted from CH radicals varied in phase with velocity signals so that it initially decreased with increasing strain and increased as the strain reduced, consistent with a tendency to e xtinguish and then re-light.