Study on Themo-Fluiddynamic Coupling involved in Supercritical Droplet Combusion and Vaporization/Combustion Characteristics Prediction

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The effects of thrmo-fluidynamic coupling phenomena which take place near the critical mixing surface of liquid fuel have been examined for droplet vaporization and liquid fuel jet flow.(1) Our previous study on supercritical droplet vaporization has been extended to reveal new features which intrinsically appear in such a gasification regime that the droplet experiences a transition to continuous phase change, showing that (a) The gasification lifetime becomes very short and comparable to the temperature relaxation time of the droplet. (b) The heating of droplet is indespensable for the transition. (c) The fuel diffusivity is greatly increased by the loss of phase equilibrium constraint. (d) A moving droplet without surface deforms to enlarge its surface area, thus enhancing vaporization and mixing rates. (e) Sprays confined in a chamber may experience an acoustically resonant vaporization mode.(2) The atomization, vaporization and mixing characteristics of a liquid fuel issued into an inert gas whose pressure and temperature exceed the critical values of the fuel have been examined by means of linear stability analysis and TVD simulation. Main findings are ; (a) The reduction of surface tension at elevated pressures requires the formation of thinner liquid fragments, for which Rayleigh instability effectively works to atomize liquid. (b) The inviscid analysis conducted for the linear stability of shear flow in vicinity of the jet surface shows that the maximum growth rate rapidly increases to a saturated value due to density variation and vanishing surface tension as the critical mixing condition is approached. (c) Since the sound speed takes a small value near critical conditions, the effect of compressibility (supercompressibility) must be taken into account for the numerical simulation of supercritical fluid flow- a new flow regime in which both Reynolds number and Mach number may take large values.