Fluid Property Effects on Spray Cooling: An Experimental and Numerical Study

Abstract

Spray cooling is a process where liquid is atomized into droplets and sprayed onto a surface that is hotter than the saturation temperature of the fluid. The droplets impact the surface and spread, causing a thin liquid film to form. This liquid film is capable of removing large heat loads from the surface. The purpose of this study is to investigate the behavior of the spray film on the chip surface. Both pure fluids and mixtures were analyzed in an effort to ascertain the effects of fluid properties and to increase the performance of traditional spray systems. Measurements of the applied heat flux and temperatures at 8 locations per chip were taken. Measurements were also recorded for the conditions of the fluid being delivered to the die. From these, heat transfer coefficients and surface temperature distributions were obtained. Separately, the first step toward a general model of spray cooling was developed. The velocity distribution was characterized by two layers: the viscous sublayer, characterized by a linear profile, and the fully turbulent region, characterized by a power law profile. A numerical model and correlation for the thin film were both derived based on the two layer theory using a velocity profile predicted by a computational fluid dynamics (CFD) model in an effort to ascertain the fundamental behaviors behind the spray cooling phenomena. The model included mass flow and momentum equations integrated through the thickness of the thin liquid film and was implemented in an iterative software program (EES) with good agreement to previous empirical data. The correlation was also in good agreement to the data obtained in this work.