Topology and distinct features of flashing flow in an injector nozzle

The effect of thermodynamic non-equilibrium conditions (liquid superheat) on the two-phase flow field developing inside an axisymmetric, single-orifice nozzle is numerically investigated by means of different variations of a two-phase mixture model. A number of "hybrid" mass-transfer models that take into account both the effect of inertial forces (cavitation) and liquid superheat have been proposed and evaluated against widely used, pure-cavitation models, in order to pinpoint the flow conditions necessary for flash boiling to occur and to elucidate the distinct features of the phase and velocity fields that characterize flashing flows. The effect of the number of nucleation sites, required as an input by the models, on the developing two-phase flow has also been looked into. The numerical results have shown that incorporation of an additional term corresponding to liquid superheat into the mass-transfer rate leads to increased evaporation rate, compared to pure-cavitation models with liquid vaporization taking place within the entire nozzle cross section. The cavitation nucleation sites have been confirmed to act as the necessary flow perturbations required for flash boiling to occur. In addition, the developing velocity field has been found to be in close correlation to the mass-transfer rate imposed. It has been established that increased liquid evaporation leads to choked-flow conditions prevailing in a larger part of the nozzle and accompanied by a more significant expansion of the two-phase mixture downstream of the injector exit that results to increased jet cone angle. Finally, the results demonstrated that liquid cooling due to the increased mass-transfer rate is not significant within the nozzle and thus consider that a constant liquid temperature produces adequately accurate results with a decreased computational cost