Autoignition of non-premixed n-heptane/air counterflow subjected to unsteady scalar dissipation rate

Abstract

To provide insights into understanding the effects of turbulence on autoignition of practical hydrocarbon fuels that exhibit two-stage ignition behavior, computational studies are conducted to investigate the response of autoignition of a non-premixed n-heptane/air system subjected to harmonic oscillations in the scalar dissipation rate. The baseline ignition studies under various steady conditions show that the first stage ignition is insensitive to scalar dissipation rate, while the second stage ignition is affected significantly. While similar behavior was observed in previous studies, the present study offers an alternative explanation based on the instantaneous ignition kernel location and the associated local scalar dissipation rate during the evolution. When an oscillatory scalar dissipation rate is imposed, the response of the ignition delay response to frequency is found to be highly nonmonotonic. At low frequencies, the ignition delay response is quasi-steady and correlates well with the mean scalar dissipation rate at the ignition kernel during the induction period. At intermediate frequencies, however, the unsteady ignition behavior is better described by a newly proposed ignitability parameter which represents the combined effects of mean kernel Damköhler number in the induction period and fractional duration within the induction period during which the kernel Damköhler number exceeds unity. At very high frequencies, the ignition kernel no longer responds to the rapid unsteady fluctuations in scalar dissipation rate, and the system recovers the quasi-steady characteristics. The study also reveals that two-stage ignition can be observed even at significantly higher initial temperatures when the ignition kernel is subjected to unsteady scalar dissipation rates. In contrast to the two-stage ignition commonly observed in homogeneous systems, the mechanism for the reappearance of the two-stage ignition in unsteady conditions is not chemical but is attributed to the spatial broadening of the ignition kernel and subsequent radical losses. Implications of the present results in turbulent combustion modeling are briefly discussed.