Steady hot and cool dimethyl ether premixed flames in channels with wall heat loss

Abstract

The present work numerically investigates the role of wall heat loss on the propagation of stoichiometric dimethyl ether (DME)/air premixed cool flames in channels, with emphasis on the extinction limits, transition from cool to hot flames and the possibility of observing both cool and hot flame propagation in a channel of a given width and wall thermal condition. The premixed DME/air flame is simulated in a two-dimensional (2D) semi-open narrow channel employing a detailed kinetic model. The study focuses on the impact of the channel width, heat loss intensity and the initial hot kernel temperature on the formation of a steady cool or hot flame, the relevant quenching channel half-width and the transition between cool and hot flames. The results indicate that, as the wall heat loss intensity is increased, the quenching channel width for the cool flames increases as well. Transition to hot flame is observed for larger channel widths. For such a transition, the propagation of a cool flame front starts first, then a hot flame front appears behind it, and subsequently catches up with it. The critical channel width for the cool to hot flame transition (two-stage ignition) increases with heat loss intensity. As for the effect of the initial kernel temperature on the flame regime, stable cool flames were obtained for a certain range of channel widths for lower initial kernel temperatures. A non-monotonic dependence of the critical channel width for the two-stage ignition on the initial kernel temperature is shown to be related to NTC effects. Initial kernel temperatures larger than 1100 K resulted in the direct formation of a hot flame. In a preliminary study for non-stocihiometric mixtures, it was demonstrated that double cool and hot flame configuration can be observed for very lean or rich mixtures.

Novelty and Significance Statement

Propagation of premixed DME/air flames ignited at a closed end of a micro-channel in the presence of wall heat loss was studied numerically. For the first time, it was observed that both stable cool flame and stable hot flame can be established independently in a channel of a given width and wall thermal conditions. Regime diagrams for the emergence of cool and hot flames were obtained numerically for different channel widths, thermal resistance coefficients, and initial kernel temperatures. Despite active research into cool flames in DME-air mixtures in the past years, there are very few studies on the regimes of cool and hot DME flame propagation in channels, both numerical and experimental. This is the first systematic parametric study of DME-air premixed flames in micro-channels of different widths.