Evaporation and ignition of droplets in combustion chambers modeling and simulation

Computer simulation of liquid fuel jet injection into heated atmosphere of combustion chamber, mixture formation, ignition and combustion need adequate modeling of evaporation, which is extremely important for the curved surfaces in the presence of strong heat and mass diffusion fluxes. Combustion of most widely spread hydrocarbon fuels takes place in a gas-phase regime. Thus, evaporation of fuel from the surface of droplets turns to be one of the limiting factors of the process as well. The problems of fuel droplets atomization, evaporation being the key factors for heterogeneous reacting mixtures, the non-equilibrium effects in droplets atomization and phase transitions will be taken into account in describing thermal and mechanical interaction of droplets with streaming flows. In the present paper processes of non-equilibrium evaporation of small droplets will be discussed. As it was shown before, accounting for non-equilibrium effects in evaporation for many types of widely used liquids is crucial for droplet diameters less than 100 μm, while the surface tension effects essentially manifest only for droplets below 0.1 μm. Investigating the behavior of individual droplets in a heated air flow allowed to distinguish two scenarios for droplet heating and evaporation. Small droplets undergo successively heating, then cooling due to heat losses for evaporation, and then rapid heating till the end of their lifetime. Larger droplets could directly be heated up to a critical temperature and then evaporate rapidly. Droplet atomization interferes the heating, evaporation and combustion scenario. The scenario of fuel spray injection and self-ignition in a heated air inside combustion chamber has three characteristic stages. At first stage of jet injection droplets evaporate very rapidly thus cooling the gas at injection point, the liquid jet is very short and changes for a vapor jet. At second stage liquid jet is becoming longer, because evaporation rate decreases due to decrease of temperature. But combustion of fuel vapor begins which brings to increase of heat flux to droplets and accelerates evaporation. The length of the liquid jet decreases again and remains constant slightly oscillating.