Forced ignition of cool, warm and hot flames in a laminar non-premixed counterflow of DME versus air

Abstract

Recently, there has been great interest in the ignition of cool and warm flames (controlled by low-temperature chemistry, LTC, and intermediate-temperature chemistry, ITC, respectively) as they may have significant influence on the ignition and subsequent propagation of hot flame. However, it is still unclear how the flow affects the ignition of the cool/warm flame and their transition to the hot flame in a non-premixed fuel/oxidizer system. In this study, we conduct 2D simulations on the forced ignition of cool, warm and hot flames in a laminar, axisymmetric, non-premixed counterflow of dimethyl ether (DME) versus air. The objective is to assess the effects of flow on the ignition of cool, warm, and/or hot flames and the transition among them. Different ignition energies and strain rates are considered. It is found that the transient ignition process is mainly controlled by the extinction/transition limits and the minimum ignition energies of cool, warm and hot flames. When the strain rate is lower than the transition limits of cool and warm flames, all three flame types can be directly initiated but they eventually evolve into a hot flame. At intermediate strain rate between the transient limit and the extinction limit, quasi-steady cool and warm flames can be obtained without further transition to a hot flame. When the strain rate exceeds the extinction limit of cool and warm flames, only the hot flame can be ignited. An ignition regime diagram in terms of ignition energy and strain rate is proposed, in which different regimes for cool, warm and hot flames as well as transition among them are identified. Furthermore, the ignition of cool and warm flames in a non-premixed counterflow is shown to be very sensitive to ignition position as well as strain rate and ignition energy.