Effects of low-temperature chemical reactions on ignition kernel development and flame propagation in a DME-air mixing layer

Abstract

The impact of chemical reactions at low-temperature (i.e., low-temperature chemistry, LTC) and LTC-induced cool flames on autoignition and premixed flame propagation has been investigated extensively. However, much less analysis is made to explore the role of LTC in forced ignition of non-premixed fuel/oxidizer systems. The objective of this work is to assess and interpret the effects of LTC on ignition kernel development and subsequent flame transition in a quiescent DME-air mixing layer. A series of two-dimensional simulations are conducted for forced ignition by a hot spot. It is found that under elevated initial temperatures and pressures, a cool flame or a warm flame can be directly ignited depending on the hot spot temperature Tig. When Tig is relatively low, a three-staged ignition process is observed where the cool, warm and hot flames are initiated sequentially. A novel penta-brachial flame structure is identified consisting of a trailing warm flame and a trailing cool flame attached to the hot triple flame. A parametric study is conducted to examine the effects of mixture layer thickness and hot spot size and location on ignition kernel development. It is found that the mixture layer thickness has little influence on the cool flame initiation but it substantially affects the subsequent warm flame or hot flame initiation. It is demonstrated that the mixture fraction range covered by the hot spot has a strong impact on subsequent flame initiation. Moreover, different ignition modes (e.g., ignition failure, only hot flame initiation and only cool flame initiation) can be achieved via changing the hot spot configuration. Furthermore, the analysis of cool flame displacement speed indicates that the cool flame initiated by the hot spot is a self-sustained partially premixed flame. Its density-weighted displacement speed changes linearly with flame stretch. These results provide useful insights into how LTC affects non-premixed ignition.