Characteristics of first-stage ignition delay in NTC-affected phenomena

Computation was performed on the first-stage ignition delay, τ₁, in the homogeneous autoignition of n-heptane/air mixtures. Results show that τ₁ possesses a minimum ignition delay, τ1,min, in that it first decreases and then increases with increasing temperature, and that τ1 is largely insensitive to the equivalence ratio (φ) and pressure (p) when the temperature is sufficiently below the initial temperature corresponding to τ _1,min. This is consistent with the experimental results from shock tubes and rapid compression machines. Furthermore, in this regime the global reaction order was found to be close to unity, hence supporting the notion that the limiting steps in this temperature regime are the RO₂ isomerization reactions, which in turn explains the insensitivity of τ₁ on φ and p. However, when the temperature approaches that of τ _1,min. competition of the β scission reactions of the alkyl radicals with the low-temperature chemistry chain reactions increases the first-stage ignition delay and results in τ_1,min. The corresponding global reaction order also increases, to about two, indicating the progressive importance of the oxygen combination reactions. Additional simulation of ignition in the nonpremixed counterflow showed that the characteristic time scale given by the critical strain rate, hence the reaction order, also follows the same density (pressure) dependence as those for τ₁ and τ_1,min, indicating the preservation and thereby essential role of the NTC-chemistry in transport-affected ignition.