Effects of strain rate on high-pressure nonpremixed n-heptane autoignition in counterflow

The effect of steady strain on the transient autoignition of n-heptane at high pressures is studied numerically with detailed chemistry and transport in a counterflow configuration. Skeletal and reduced n-heptane mechanisms are developed and validated against experiments over a range of pressure and stoichiometries. Two configurations are investigated using the skeletal mechanism. First, the effect of strain rate on multistage n-heptane ignition is studied by imposing a uniform temperature for both the fuel and the oxidizer streams. Second, a temperature gradient between the fuel and the oxidizer streams is imposed. The global effect of strain on ignition is captured by a Damkohler number based on either the heat-release rate or the characteristic chain-branching rate. Results show that for low to moderate strain rates, both the low- and intermediate-temperature chemistries evolve in a manner comparable to that in homogeneous systems, including the negative temperature coefficient regime, but with somewhat slower evolution attributable to diffusive losses. At high strain rates diffusive losses inhibit ignition; for two-stage ignition, it is found that ignition is inhibited during the second, intermediate-temperature stage. The imposition of an overall temperature gradient further inhibits ignition because reaction zones for key branching reactions with large activation energies are narrowed. For a fixed oxidizer stream temperature that is not sufficiently high, a higher fuel temperature results in a shorter ignition delay provided that the heptyl radicals are mainly oxidized by low-temperature chemistry. As expected, an increase in pressure significantly increases reaction rates and reduces ignition delay time. However, with increasing pressure there is a shift toward single-stage low-temperature-dominated ignition which serves to delay ignition.