An experimental study of vortex-flame interaction in counterflow spray diffusion flames

The extinction behavior of methanol counterflow spray diffusion flames was investigated using a combination of formaldehyde planar laser-induced fluorescence (PLIF) and phase Doppler measurements. Extinction was brought about quasi-steadily, by progressively increasing the flow rates of both oxidizer and fuel side, and unsteadily, by generating a vortex on the oxidizer side. The unsteady experiments yielded values of extinction strain rates a factor of 2 larger than the quasi-steady values. The greater robustness of the spray flame under unsteady perturbation was explained phenomenologically by estimating the timescales involved in the process. It was found that the vortex introduces unsteady effects in the outer diffusiveconvective layer of the flame. The inner reactive-diffusive layer, on the other hand, behaves in a quasisteady manner, since the characteristic chemical time is much smaller than the characteristic unsteady time. As a result, even though the instantaneous strain rate is much larger than the quasi-steadyextinction strain rate, the flame is subject to a damped strain rate through the outer layer. An estimate of the thickness of the mixing layer, based on formaldehyde PLIF, provided a convenient means to compare the scalar dissipation rate and the Damköhler number between the two extinction modes, bypassing the need for detailed species measurements for the assessment of the mixture fraction and its gradient. Such a comparison showed that the difference between the two extinction modes was reduced to 25% on the average, consistent with expectations based on flame structure models from asymptotic theory. Spray flames exhibited longer time delays between the onset of extinction and reignition, as compared to gaseous flames. Estimates of the relevant Stokes number suggested that the difference may be attributed to droplet inertia effects.