Hot-gas ignition of non-premixed methane flames in the presence of inert particles

摘要

A detailed numerical study was conducted on the ignition of non-premixed atmospheric CH_4/air flames containing inert Al_2O_3 particles in counterflow configurations. The gas injection temperatures of the fuel-side, the air-side, or both sides were increased separately until ignition was observed. The coupled conservation equations were solved for both phases along the stagnation streamline, with detailed descriptions of gas-phase chemical kinetics, molecular transport, and radiative heat transfer. The reactant injection temperature, strain rate, orientation of particle seeding, and injection particle number density and temperature were varied. Results showed that particles could drastically modify the temperature field of the gas phase and with it the ignition temperature. It was found that when the particle number densities are relatively high, the velocity field is also affected due to the momentum exchange between the two phases. Thus, the local strain rates and ignition temperatures are modified. No table differences were observed among the three configurations considered, namely heated air-side, heated fuel-side, and simultaneously heated air- and fuel-sides due to the different temperature fields generated. The controlling chemical pathways were found to depend not only on whether particles are present or not but also on whether the particles were injected from the air- or fuel-side. When particles are injected from the hot air-side rather than the cold fuel-side, lower ignition temperatures were observed. It was also determined that ignition occurs more readily in air-side seeding compared to fuel-side seeding as the particle number density increases. The effect of the particle injection temperature on hot-gas ignition was also assessed. It was shown that the ignition process is controlled by the competition between heating and cooling of the gas by the particles in cases where only one reactant stream is heated and that the conditions under which the particles assist or retard ignition cannot be readily determined without performing a detailed analysis of the problem.