A computational study on shock induced deformation, fragmentation and vaporization of volatile liquid fuel droplets

Abstract

This study investigates the fundamental mechanisms underlying the deformation, fragmentation, and vaporization of volatile liquid fuel droplets impacted by a normal shock wave using a high-fidelity, VOF-DIM (volume of fluid - diffuse interface method)-based framework. The theoretical and mathematical formulation of this multiphase, multi-fluid problem is based on a modified 5-equation Kapila formulation with pressure-relaxation, viscous, and surface tension effects. A thermal-mechanical-chemical equilibrium relaxation procedure is implemented to simulate vaporization. The framework is first validated against measurements of shock impact on a non-vaporizing water droplet; the computations agree well with the experimental data. Next, the vaporization model is validated against the d2 law, showing excellent agreement. This is followed by a systematic investigation of the atomization and vaporization physics of a n-dodecane droplet as it interacts with a shock wave traveling at a Mach number of 6.5. To compare and contrast the effect of vaporization on breakup physics, two numerical experiments were conducted with and without the vaporization model. It is found that when the vaporization model is not enabled, the gaseous and liquid phase dynamics are similar to that of non-vaporizing water droplet-shock wave interactions. However, when vaporization is enabled, aerothermal heating from the shock impact and high temperatures in the post-shock region provide sufficient heating for volatile liquid droplets to undergo phase change and the breakup behaviors are significantly different from the non-vaporizing counterpart. Furthermore, it is found that vaporization is a strong function of the shock strength – low Mach number shock waves lead to higher vaporization.