Effects of initial temperature on ignition and flame propagation of dual-fuel mixture in mixing layer

Abstract

The study aims to identify the characteristics of ignition and flame propagation in an n-heptane/methane–air dual-fuel mixture based on a specified mixing layer scheme using a group of numerical simulations by incorporating a skeletal chemistry mechanism. The effects of different temperature stratifications and uniform temperature distribution on autoignition kernel development, flame propagation, and flame structure in the dual-fuel combustion have been comprehensively investigated. The results indicate that with the increase in the oxidizer stream temperature, the high-temperature ignition (HTI) location in the mixing layer changes non-monotonously, as determined by the most reactive mixture fraction in the mixture fraction–temperature (ξ−T) space obtained from zero-dimensional simulation. The HTI points located in the negative temperature coefficient (NTC) region shift to a richer mixture with temperature increase, whereas the points located in the high temperature region shift to the smaller mixture fraction region. Additionally, the results demonstrate that for the ignition points in the NTC region, the shift to the richer mixture weakens the combustion rate, resulting in a prolonged time interval for steady flame propagation although the oxidizer stream temperature is increased. Furthermore, the low-temperature ignition (LTI) front propagation is driven by the gradient of the ignition delay time, and the HTI propagates as a spontaneous ignition front at first and then transforms to a diffusion-driven flame. The results also demonstrate that the maximum temperature of the LTI propagates to the richer mixture with temperature stratification, in contrast to that with a uniform temperature. Lastly, the comparative analyses of chemical explosion mode (CEM) and flame structure are conducted for two representative cases. A wide non-CEM region induced by low-temperature chemistry (LTC) occurs in the oxidizer stream when its temperature is relatively lower. And double peaks of heat release rate (HRR), that represent the HTI of n-heptane and post-combustion of methane respectively, are observed within the ignition front at the oxidizer side.