BOUNDARY-LAYER STRIPPING EFFECTS ON DROPLET TRANSCRITICAL CONVECTIVE VAPORIZATION

Previous studies have shown that the surface temperature of a liquid oxygen (LOX) droplet vaporizing in a supercritical quiescent environment reaches the computed critical mixture value; therefore, a model for supercritical combustion is needed. The purpose of this work is to investigate convective effects. A spherically symmetric model is constructed with an extension of the film model for the gas phase. The energy equation in the droplet is solved numerically and the high-pressure phase equilibrium is computed from the chemical potential equality for each component in each phase, using the Redlich-Kwong equation of state. Because of the reduced surface tension, the droplet undergoes mass removal from its surface by aerodynamic shearing (boundary-layer stripping). It is shown that, in a rocket engine, a LOX droplet is likely to undergo boundary-layer stripping for most of its lifetime. A simple integral analysis of the coupled gas-liquid boundary layers at the droplet interface yields an expression for the mass removal rate that includes blowing effects and that is shown to be much larger than the vaporization rate. The effect of stripping on the heat transfer into the droplet is evaluated with a modified film model. Results show that, in most cases, stripping precludes criticality at the interface. Computations also show that stripping reduces the predicted droplet lifetime by at kast one order of magnitude, thus affecting significantly the range of frequencies in which the gasification process might be driving combustion instabilities. The computed droplet lifetime is compared to the expressions available in the literature. The sensitivity of the predicted droplet behavior to the threshold value of the stripping breakup criterion is demonstrated.