Nonpremixed n-heptane autoignition in unsteady counterflow

The effect of unsteady strain on n-heptane autoignition at elevated pressure is studied numerically with detailed chemistry and transport in the counterflow configuration. Specifically, fuel and air temperatures are chosen so that two-stage ignition occurs, as is typical, in the negative temperature-coefficient regime, and the strain perturbations are selected to rapidly rise and fall to mimic rapid turbulent strain-rate fluctuations. For these rapid fluctuations, the response of the mixing layer is unsteady. Small to moderate strain-rate fluctuations result in a small increase in the ignition delay. For sufficiently large strain rates such that the critical scalar dissipation rate is exceeded, the ignition delay is substantially greater. The latter delay results from both the time required for the dissipation rate to decay to below its critical value and the longer ignition delay time for systems with dissipation rates near the critical dissipation rate. For all magnitudes of strain-rate fluctuation, there is evidence that the second stage of the two-stage ignition is more sensitive to inhibition by higher dissipation rates. For sufficiently large dissipation rates, the heat losses force the second stage to the border between the low- and intermediate-temperature chemistries.