04. Boiling and Multi-Phase Flow

Gas-liquid turbulent flows with phase change are ubiquitous in a variety of natural and engineering processes, such as breaking waves, sprinkler irrigation, printing, and spray combustion in propulsion devices. Among them, spray combustion features a sophisticated process that involves liquid-fuel atomization, droplet evaporation, fuel-vapor and air mixing, and fuel-vapor combustion. The droplet evaporation is significantly important as it is a precursor of combustion, and the accurate prediction of droplet evaporation is crucial to maximize the combustion efficiency and minimize pollution formation. In this work, we firstly develop a DNS method to consider the deformable gas-liquid interface and the heat and mass transfer across the interface. We consider a gas-liquid flow with phase change in the low Mach number approximation and solve the governing equations for mass, momentum, level set function, species, and temperature in both phases. The present framework has been validated in several cases. Direct numerical simulation of the evaporation of interface-resolved liquid droplets in decaying homogeneous isotropic turbulence is then performed in this work. The effect of the evaporation on droplet dynamics and turbulence is investigated by examining the turbulence kinetic energy, the dissipation rate, the power of surface tension and evaporation. It is shown that evaporation could hinder the coalescence of droplets qualitatively. The reason is that the Stefan flow induced by the evaporation process generates repulsion between droplets. The contours of temperature and vapor mass fraction are self-similar because they are both passive scalar. The non-uniform evaporation rate at the interface implies that the point-particle assumption is not valid for the present finite-size droplet evaporation. This work gives us a better understanding of the interaction between evaporating droplets and turbulence through using a high-fidelity interface-resolved method and potentially serves as the database to improve the droplet evaporation models in the dense spray zone.