Unsteady extinction behaviors of $(CH_4+N_2)$ /air diffusion flames were investigated in terms of the time history of the strain rate and initial conditions. A spatially locked flame in an opposing jet counterflow burner was perturbed by a linear velocity variation. To observe the whole process of the unsteady extinction, changes in a time-dependent flame luminosity and unsteady extinction points were measured by a high-speed ICCD camera. In addition, the transient maximum flame temperature ($T_{max}$) and the OH radical ($Y_{OH}$)were measured over time with the aid of the Rayleigh scattering method and OH laser-induced fluorescence, respectively. Unsteady flames survive at strain rates that are much higher than the extinction limit of steady flames, and unsteady extinction limits extend as the slope of the strain rate increases or the initial strain rate decreases. Changes in $T_{max}$ and $Y_{OH},_{max}$ with respect to the strain rate history provide details of the extinction process, and they show different characteristics for steady and unsteady flames. To investigate the difference between unsteady and steady extinction processes, the equivalent strain rate concept was used to subtract the time lag of the convective-diffusive zone. The process of the unsteady extinction then becomes almost identical to that of the steady extinction in the region where the velocity begins to change and the modified unsteady extinction limits become smaller than the original unsteady extinction limits. However, the modified unsteady extinction limits are still larger than the steady extinction limits. These results suggest that there exist the unsteady behavior of a diffusive-reactive zone near the extinction limit due to the chemical non-equilibrium states associated with unsteady flames. Consequently, the unsteady extinction limit is extended by the time lag of the convective-diffusive zone and the unsteady effect of the diffusive-reactive zone. The former is dominant in the starting region of the velocity change and the latter is dominant near the extinction limit. The results of this work help to explain the extension of the unsteady extinction limit and they have direct relevance to the understanding of the characteristics of turbulent combustion.
