Extinction limit extension of unsteady counterflow diffusion flames affected by velocity change

Abstract

The unsteady extinction limit of (CH 4 + N 2)/air diffusion flames was investigated in terms of the time history of the strain rate and initial strain rates. A spatially locked flame in an opposed-jet counterflow burner was perturbed using linear velocity variation, and time-dependent flame luminosity and unsteady extinction limits were measured with a high-speed intensified CCD (ICCD) camera. In addition, the transient maximum flame temperature and hydroxyl (OH) radical were measured as a function of time using Rayleigh scattering and OH laser-induced fluorescence, respectively. In this experiment, unsteady flames survive at strain rates that are much higher than the extinction limit of steady flames and unsteady extinction limits increase as the slope of the strain rate increases or as the initial strain rate decreases. We found that the equivalent strain rate represents well the unsteady behavior in the outer convective–diffusive layer of the flame. By using the equivalent strain rate, we were able to accurately estimate the contribution of the unsteady effect in the outer convective–diffusive layer to the extinction limit extension, and we also identified the unsteady effect in the inner diffusive–reactive layer of the flame. Consequently, the extension of unsteady extinction limits results from the unsteady effects of both the convective–diffusive layer and the diffusive–reactive layer. The former effect is dominant at the beginning of the velocity change, and the latter effect is dominant near the extinction limit.