Effect of Dimethyl Ether Addition on Soot Formation Dynamics of Ethylene Opposed-Flow Diffusion Flames

Soot formation dynamics in C₂H₄/dimethyl ether (DME) opposed-flow diffusion flames was numerically studied in this paper, employing detailed gas-phase and dispersed-phase chemistries and transport models. A wide range of DME additions from pure C₂H₄ to pure DME was involved to systematically examine its impact on the soot formation process. It was found that the flow field had an important impact on the soot structure inside the flame. The soot volume fraction reached a maximum at the stagnation plane due to the infinite residence time, and then vanished abruptly on the fuel side of the stagnation plane. In the opposed-flow diffusion flames, enhancements of nucleation, H-abstraction–C₂H₂-addition (HACA) reaction, and polycyclic aromatic hydrocarbon (PAH) condensation rates in the near-stagnation region disappeared, such that the local particle size distribution function (PSDF) curve became unimodal, which was rather different from the burner-stabilized stagnation premixed flame. A synergistic effect of DME addition on soot formation of the C₂H₄ opposed-flow diffusion flame was reported herein. Either the summary soot number density or average particle diameter increased considerably upon 5% DME addition, and then turned to decrease with further addition; they dropped to equivalent values with that of the pure C₂H₄ flame at 20% DME addition. The synergistic effect due to DME addition was enhanced along the convection flow direction. In the sooting area, the synergistic effect was maximum at 5% DME addition, while in the near-stagnation region, the synergistic effect was maximum at 60% DME addition. With respect to the nonpremixed combustion systems fueled by C₂H₄/DME blends, at least 20% DME addition is required to effectively reduce the soot number density and particle diameter.