Study of Diffusion Cool Flames of Dimethyl Ether in a Counterflow Burner under a Wide Range of Pressures.

Cool flames have been studied for more than three centuries since the first observation. However, there are few achievements on the effects of the pressure on cool flames. In this work, a diffusion cool flame has been for the first time established using a counterflow configuration under a wide range of pressures. Dimethyl ether is used as the fuel because its low-temperature chemistry has been well tested. The pressure range of the experiments is from 0.05 to 0.15 MPa. The extinction limits, flame temperatures, and combustion products have been measured and simulated. In general, the reactivity of cool flames is stronger with increasing pressure. Specifically, at a fixed fuel mass fraction, the cool flame has a higher extinction strain rate, temperature, and concentration of products under higher pressure. However, the enhancement effect decreases with the increase of pressure. Interestingly, it was observed that the flame became thicker when the pressure increased. Moreover, the cool flame would deflagrate and transform to a hot flame when the pressure exceeds a certain value. The model captures the trends well but underpredicts the extinction limits and overpredicts the flame temperatures and product concentration. Path flux analyses and heat release rate analyses were carried out. It was found that the main heat release reactions are the reactions with CHOCH radicals under low pressure, and CO prefers to form CO indirectly through HOCHO radicals. The study advances the understanding of cool flames in a wide range of pressures and provides experimental data for the improvement of the models.