Dynamic flammability limits of methane/air premixed flames with mixture composition fluctuations

As a fundamental study in the application to direct-injection spark-ignition engines or gas turbines, in which mixture stratification and partial quenching are of serious concerns, unsteady premixed methane/air flames subjected to time-varying composition fluctuations are investigated computationally. The code OPUS employs an unsteady opposed-flow combustion configuration, including detailed chemical kinetics, transport, and radiation models, using an adaptive time integration method for a stiff system of differential-algebraic equations with a high index. The primary issue of the study is to establish the concept of the dynamic flammability limit, defined as the minimum equivalence ratio above which the unsteady flame can sustain combustion. For the weak and strong strain rate cases studied, it is observed that the dynamic flammability limit depends on the mean and frequency of the composition fluctuation. The parametric study demonstrated that the flammability limit of an unsteady premixed flame is further extended to a leaner condition as the frequency or mean equivalence ratio fluctuation increases. It is also found that, under all conditions, the mean equivalence ratio and the minimum flame temperature must be higher than those at the steady flammability limit to sustain combustion. It is further shown that the dynamic flammability limit is primarily determined by the instantaneous branching-termination balance at the reaction zone. The behavior of the flame response attenuation with increasing frequency is found to scale properly using the normalized frequency based on the imposed flow strain rate, which represents the characteristic time scale of the transport process within the flame.