Effects of unsteady scalar dissipation rate on ignition of non-premixed hydrogen/air mixtures in counterflow

As a step toward understanding the effects of a turbulent environment on ignition delay, the effect of impulsive strain forcing on the autoignition of non-premixed hydrogen/air mixtures is studied numerically using a one-dimensional unsteady opposed-flow code with detailed chemistry. The sensitivity of ignition kernel growth to changes in the scalar dissipation rate during the ignition process is studied at conditions in the second and third ignition limits. The sensitivity of the kernel growth is quantified by examining the time evolution of key radical species as well as their reaction and flow flux balances over a range of impulse amplitudes and times. Results show that transient ignition in both the second and the third limits is sensitive to changes in scalar dissipation rate. Increases in ignition delay of as much as five times are observed, depending upon the impulsive forcing amplitude and timing. For a given impulse amplitude, kernels that have accumulated more radicals at a given time during induction are found to ignite much sooner, indicating that the time history of the kernel radical pool relative to the impulse time is important. Furthermore, kernels are found to be able to survive excursions in the scalar dissipation rate to values that far exceed the steady ignition state. The increase in ignition delay in both limits is attributed to a shorter residence time of radicals in the kernel as measured by an instantaneous Damköhler number. A new ignition criterion based on the instantaneous Damköhler is found to be an accurate measure of predicting the ignitability under highly transient conditions.