Supercritical Liquid Fuel Combustion Mechanism

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A numerical simulation scheme was developed to disclose the unsteady evaporation and combustion processes of a single fuel droplet immersed in an otherwise quiescent gas at a various ambient gas state beyond the critical point of the fuel, which is necessary for the design of rocket motors and high speed Diesel engines. This study was carried out, based on the author's previous theoretical work on supercritical droplet combustion wherein it was found that the transition from sub- to supercritical evaporation regime can take place only if the binary diffusion coefficient relevant vanishes at the liquid gas interface in a mixture critical condition, an expression for the diffusion coefficient which satisfies this condition was proposed and the asymptotic transition feature was revealed on an analytical basis.The condition under which the droplet experiences the transition during its lifetime was identified in terms of ambient gas temperature and pressure. Other important knowledge obtained is as follows. Consistent with experimental results no singular behavior is observed in the droplet surface temperature at the transition. The core characterized by high fuel concentration preserves the liquid-like property even after the droplet has lost its liquid-gas interface. Since continuous phase change takes place in a thin layer, usual optical observation will give images as if there still exists the droplet with surface. Once the core is about to disappear, the concentration as well as the central temperature changes abruptly to assume the gaseous state. The transition is achieved far more quickly in the presence of flame which is located closer to the droplet at higher pressure. The transition epochs the change from vaporization- to diffusion-controlled combustion, accompanying an increase in flame temperature.