Effects of Scalar Dissipation Rate Fluctuations on Autoignition of Hydrogen/Air Mixture

摘要

To understand the autoignition behavior in response to the flow turbulence, the effects of scalar dissipation rate fluctuation on the ignition of a nonpremixed hydrogen/air mixture is computationally studied using detailed chemistry in a counterflow configuration. A sinusoidal fluctuation of the scalar dissipation rate is imposed by oscillating the velocity at the nozzle inlet. Mean scalar dissipation rate is chosen to be close to the steady ignition limit, such that the instantaneous scalar dissipation rate can exceed the steady ignition limit during the oscillation. Response of the ignition delay to the frequency of the scalar dissipation rate oscillation is studied for two distinct cases, depending on whether the mean scalar dissipation rate at ignition kernel is less than (case A) or greater than (case B) the steady ignition limit. For low frequencies, the ignition delay response for both cases is quasi-steady in that it correlates well with the mean scalar dissipation rate up to ignition delay. At high frequencies, however, the ignition delay response is significantly different for the two cases: for case A, the ignition delay increases with frequency and levels off at higher frequencies, whereas for case B, the ignition delay increases monotonically with frequency up to a critical value, beyond which no ignition is observed. A newly defined ignitability parameter is proposed based on the ignition-kernel Damkohler number such that all of the unsteady effects of scalar dissipation rate oscillation on ignition delay can be uniquely mapped to this parameter. Subsequently, a new criterion for ignitability is proposed based on this parameter.