A MULTI-SECTION DROPLET COMBUSTION MODEL FOR SPRAY COMBUSTION SIMULATION

摘要

Since 1960s, from experimental observation, there are several stages for liquid droplets in multi-phase combustion: pure heating, pure evaporation without individual droplet combustion, and individual droplet combustion (burning) with individual flame around which enhances evaporation. As for individual burning droplets, they have envelope flame or wake flame regimes. From experimental and theoretical research, in liquid fuel combustion chambers, according to the space between the droplets, there are four typical droplet group combustion modes. Generally, the external group combustion mode and the internal group combustion mode are the two main modes, and there are individual burning droplets in multiphase combustion. Up to now, based on the literature information, no matter in Reynolds Averaged Navier Stokes (RANS) method, Large Eddy Simulation (LES) method or Point Direct Numeral Simulation (PDNS) method, droplets in multi-phase combustion are treated as pure evaporation points, corresponding to the bright blue flame condition, i.e., small droplets in hot gas mixture with the pure evaporation stage, so in species equation and energy equation, the droplet points are mass source and heat sink. As for the individual droplets burning mode, in point source simulation method, corresponding to the big droplet in cold gas phase mixture with yellow trail flame condition, the source terms in the species equation and energy equation should be the mass sink and heat source. The individual burning droplets affects the NO formation because the flame around the droplet has higher temperature than the local gas phase mixture mostly for the quasi-stoichiometric ratio reaction in individual droplet flame. In this paper, a new spray combustion model with the consideration of individual droplets combustion mode is brought forward. The existing modeling approach is extended with an ignition criterion and a droplet combustion correlation. The ignition criterion, which judges the droplet stage between pure evaporation and burning, and the droplet combustion correlation, which integrates the micro-scales droplet burning effects into point source term frame, are both selected from pioneer's experimental work and added to the present existing widely used multi-phase combustion model. Then a jet spray flame is tested by the new model with RANS and LES method. The prediction results are compared with the literature experimental results. New model gives better simulation results in downstream regions. According to the results, this model added the individual droplet combustion mode in multi-phase combustion model, while its combustion correlations are quite rough at present. More verify studies should be carried in the future.