Transcritical vaporization of a liquid fuel droplet in a supercritical ambient

The transcritical vaporization of a fuel drop placed in supercritical ambient is reported. A detailed model based on the numerical solution of the time-dependent conservation equations for both liquid and gas phase is employed. The equations are solved using an arbitrary Lagrangian-Eulerian procedure, which allows a dynamically adaptive mesh to analyze interfacial time-history effects, and a comprehensive thermodynamic model that accounts for the variation of thermotransport properties and the gas-phase nonidealities in the transcritical regime. The transcritical vaporization behavior is shown to be characterized by two parameters. One is the minimum pressure (Pain) required for the drop surface to reach the critical mixing state, and the other is the time ratio parameter (t(r)), which is the ratio of the time to attain critical mixing state to the drop lifetime. For pressures below P-min, the droplet undergoes subcritical vaporization (t(r) = 1), characterized by a distinct liquid-gas interface whose regression rate is determined by the gas-phase mass diffusivity and the difference between fuel vapor concentrations at the surface and in the ambient. For pressures above P-min, the droplet undergoes transcritical vaporization, i.e., it attains the critical mixing state sometime during its lifetime. The instant at which the critical mixing state is reached determines t(r). The subsequent surface regression or "supercritical vaporization" rate is given by the inward velocity of the critical surface, which is determined by the gas-phase thermal diffusivity and the difference between critical mixing temperature and liquid temperature in the droplet interior. While both the subcritical and supercritical vaporization rates are enhanced as the pressure is increased, the latter is consistently higher than the former. The effects of ambient temperature and initial drop size on P-min and t(r) are quantified. The parameter P-min shows a strong dependence on ambient temperature (T-infinity), decreasing rapidly as T-infinity increases, but a weak dependence on drop size. The parameter t(r) is strongly sensitive to ambient temperature, pressure, and droplet size. It decreases as ambient temperature and pressure are increased, and as drop size is decreased.