A numerical study of the pyrolysis effect on autoignited laminar lifted dimethyl ether jet flames in heated coflow air

Abstract

The liftoff, autoignition, and stabilization characteristics of autoignited laminar lifted dimethyl ether (DME) jet flames in heated coflow air are numerically investigated by varying the fuel jet velocity, U0. The detailed numerical simulations are performed using the laminarSMOKE code with a 55-species detailed kinetic mechanism of DME oxidation. An unusual U-shaped liftoff height, HL, behavior under MILD combustion condition is observed from the simulations, which is qualitatively consistent with previous experimental results. From additional numerical simulations with modified mass diffusivity of hydrogen, it is verified that the decreasing HL trend of the lifted flames under relatively-low U0 conditions is mainly attributed to the fast diffusion of hydrogen generated from the DME pyrolysis. The species transport and displacement speed analyses verify that the differential diffusion effect renders the lifted flames to be leaner at the center of the jet, ultimately leading to the change of their stabilization mechanism from the autoignition to the autoignition-assisted flame propagation mode with increasing U0. The chemical explosive mode analysis (CEMA) identifies important variables and reactions contributing to the autoignition of the DME jet flames, through which the fast diffusion rates of small species are found to cause the deviation of 2-D autoignition characteristics from that of 0-D homogeneous ignition. The effects of DME pyrolysis on the characteristics of the autoignited laminar DME jet flames are further investigated by varying the fuel tube length, Lres. HL shows a non-monotonic behavior with increasing Lres because the flame structure changes from a MILD combustion to a tribrachial edge flame and to an attached flame while the stabilization mechanism also changes from the autoignition to the autoignition-assisted flame propagation mode as the degree of the DME pyrolysis increases.