Decreased mixture reactivity and hot flame speed in the products of diffusion-affected autoignitive cool flames in the NTC regime

Abstract Cool flames propagating in autoignitive mixtures may be characterized as deflagrations or spontaneous ignition fronts depending on the relative importance of diffusion. In this paper, it is shown that, for certain thermochemical conditions in the negative temperature coefficient (NTC) regime, including at the Engine Combustion Network’s (ECN’s) baseline Spray A conditions, the cool flame products’ reactivity and hot flame speed can vary significantly from one cool flame propagation regime to the other. Simulations of steady one-dimensional cool flames in fixed-length inflow-outflow domains are performed. By varying the inflow velocity and controlling accordingly the domain length, isolated cool flames ranging from spontaneous ignition fronts to deflagrations are obtained. Towards the deflagration regime, the increasingly important contribution from diffusion (mostly heat conduction) promotes intermediate or high temperature fuel oxidation channels at the expense of low-temperature chain branching. As a result, the cool-flame products’ composition and temperature are significantly affected, with the product-side’s mixture reactivity and hot flame speed significantly reduced. Qualitatively similar results obtained with four chemical kinetics mechanisms and two transport models (mixture-averaged and unity Lewis number) are presented. More specifically, in a lean n-heptane–air mixture (equivalence ratio of 0.7) at 650 K and 1 atm, the time to second stage ignition is increased by a factor of up to 6 following a cool flame deflagration as opposed to a spontaneous ignition. The peak heat release rate is also reduced by a factor of more than five. With n-dodecane–oxidizer mixtures (equivalence ratios of 0.7 to 1.3) at the ECN’s baseline Spray A conditions, the role of diffusion on cool-flame products is observed to increase the remaining time to second stage ignition by a factor of up to 2.5, reduce hot flame speed by up to 30% and decrease peak heat release rate by a factor of up to five. These effects are shown to lead to a significant alteration of the double cool-hot flame ignition and stabilization. This is in part due to the fact that the cool flames are found to be as fast, and faster than hot flames at these conditions, such that a deflagrative cool flame can play a significant role on both ignition and flame stabilization. Finally, it is found that the effect of diffusion on chemical pathways and peak heat release rate can be even more significant at the baseline Spray A conditions in rich mixtures beyond the NTC regime.