The role of global and detailed kinetics in the first-stage ignition delay in NTC-affected phenomena

Abstract

Using n-heptane as a representative fuel exhibiting NTC (negative temperature coefficient) chemistry, a comprehensive computational and mechanism study was conducted on the role and controlling chemistry of the first-stage ignition delay in the superficially dissimilar systems of the auto-ignition of homogeneous mixtures and the nonpremixed counterflow ignition of fuel versus heated air. It is first shown that the first-stage auto-ignition delay time, τ1, possesses a minimum value, τ1,min, with increasing temperature, and that for temperatures below the range corresponding to τ1,min, τ1 is largely insensitive to the equivalence ratio (ϕ) and pressure (p) of the mixture. 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 in this temperature regime. However, when the temperature approaches that of τ1,min, competition of QOOH decomposition and the β scission reactions of the alkyl radicals with the low-temperature chemistry chain reactions, as well as the equilibrium shift of the oxygen addition reactions, increases τ1 and consequently results in τ1,min. The corresponding global reaction order also increases, to about two, indicating the progressive importance of the oxygen addition reactions. Extracted values of the global activation energy are also close to those of the controlling reactions in these temperature regimes. Results from the counterflow show the same global kinetic responses by identifying the reciprocal of the counterflow strain rate as the relevant ignition delay time and the temperature of the heated air stream as the homogeneous mixture temperature. It is further found that in the temperature range corresponding to τ1,min, the diminished heat release causes the counterflow to lose its characteristic, non-monotonic S-curve response and consequently distinct, abrupt ignition–extinction transition events.