Studies on Two-Mode Spray Combustion

Abstract

Up to now the mechanism of spray combustion is considered as the un-ignited droplets evaporating in continuous gas flames, both in RANS modeling, LES and point-particle DNS. However, still in the 50's of the last century, experiments indicated that the spray combustion has different modes; it may be brush-like droplet diffusion flame-lets or cold droplets evaporating in blue gas flames. In this paper preliminary studies are made for two-mode spray combustion. At first a simplified analytical 1-D model for two-mode spray flame propagation is presented. The predictions are in agreement with the experimental results. The results indicate that different pre-vaporization fraction and different droplet sizes produce different flame modes. Secondly, a preliminary LES of spray combustion accounting for some ignited droplet is made with the assumption that some droplets will be ignited if their life time is greater than the ignition delay of a premixed gas-fuel-air mixture and the assumption of a same gas temperature for the evaporation of ignited and un-ignited droplets. The results show that the predictions accounting for the effect of ignited droplets is in better agreement with the experimental results, but the agreement is not always good due to lack of an appropriate droplet ignition model. In order to more accurately simulate the two-mode spray combustion, a simplified droplet ignition model is proposed using Frank-Kamenetsky's approximation for treating the highly nonlinear reaction term. The result gives the droplet ignition limits as a function of the gas relative velocity, droplet size and ambient gas temperature. To validate the proposed droplet ignition model, experimental studies on droplet ignition of a suspension droplet are carried out. The results show that the proposed model is in agreement with experiments. Finally, more accurate modeling equations of two-mode spray combustion are suggested for future RANS modeling, LES and point-particle DNS.