Stabilization of laminar nonpremixed DME/air coflow flames at elevated temperatures and pressures

Abstract

The structure and stabilization mechanism of laminar nonpremixed autoignitive DME/air coflow flames was investigated at elevated temperatures and pressures. Computations with detailed chemistry were performed for DME and heated coflow air at 30 atm with uniform inlet velocities (2.4, 3.2, and 8.0 m/s) imposed for both streams. The heat release rate profiles are first examined for each case to demonstrate a multibrachial thermal structure. Species concentrations and temperature were sampled along mixture fraction iso-contours, and Chemical Explosive Mode Analysis (CEMA) was performed to identify the controlling chemistry at representative points. One-dimensional Lagrangian Flamelet Analysis (LFA) was also performed and compared with the two-dimensional computations to elucidate the relative importance of diffusion processes parallel and normal to the mixture fraction gradient. Various coflow temperatures with different inlet velocities are examined to elucidate their influences on the multibrachial structure as well as the stabilization mechanism. NTC (negative temperature coefficient)-affected inhomogeneous autoignition and the coupled effects with premixed flame propagation on stabilization are further studied. It is found that, at high coflow boundary temperatures or low inlet velocities, the classical tribrachial flame structure is achieved, and autoignition contributes less to the stabilization due to reduced heat and radical accumulation. The kinematic balance between the local flow speed and flame propagation speed is the dominant stabilization mechanism. On the contrary, kinetic stabilization is achieved at lower coflow temperatures or higher inlet velocities as autoignition becomes dominant. Due to the transition of the dominant chemical pathways during autoignition, the kinetically stabilized structure is usually multibrachial. The transition of different stabilization mechanisms can be made by changing either the boundary velocity or temperature of the coflow. Based on these results and previous work (Deng et al., 2015) [12], a regime diagram is constructed that identifies the possible stabilization regimes: blow out, kinetically stabilized, autoignition-propagation-coupled stabilized, kinematically stabilized, and burner stabilized.