Thermal Modeling of Spray Cooling: Gravitational Effect on Droplet and Bubble Dynamics

Abstract

As the need for thermal management of high-power density electronic systems on space-based platforms (i.e., laser diode arrays, multichip modules, etc.) grows, interest in spray cooling as a thermal management solution is also increasing. The present study investigates numerically the effects of microgravity and macrogravity on spray cooling heat transfer as well as the effect of droplet impact on vapor bubble growth and development in a liquid film at the heater surface. A two-dimensional, multiphase flow computer model has been developed that includes the effects of surface tension, viscosity, phase change, and gravity. The liquid-vapor interface is modeled using the level set method. Initially, vapor bubble growth is simulated as pool boiling in the film's macroregion (1 to 10 mm normal to the heated wall) for purposes of model verification. Then, bubble merger in a thin film is simulated under microgravity and macrogravity conditions. Finally, droplet impingement is included in the thin-film model, and gravitational effects on the transport properties are discussed. For the thin-film bubble merger and droplet impingement simulation studies, the liquid film adjacent to the heated wall has been approximated as 70 μm thick. Wall heat transfer during droplet impingement was computed in terms of the nondimensional Nusselt number for gravitational constants ranging from 0.0001 to 2g. Computed Nusselt number versus time is presented and explained using spatial velocity vector diagrams for each simulation case. All of the computational studies were performed using FC-72 as the simulated fluid.