Heat Transfer and Film Thickness Characteristics of Spray Cooling with Phase Change

Abstract

Two test facilities were used to study the characteristics and method of spray cooling. Spray cooling is a process that delivers liquid droplets from an atomizer to 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. Ten nozzle designs were tested, including arrays of between 1 and 16 nozzles. Each nozzle design was tested at a minimum of 4 flow rates. The emphasis of this study is to investigate the behavior of the film on the die surface to determine ways to improve on spray cooling designs for future designs. The first test facility used to investigate the limits and abilities of the nozzles was a heat transfer stand. Measurements of the applied heat load of each die and the junction tempera- tures at 8 locations per die were taken. Measurements were also recorded for the conditions of the fluid being delivered to the die. From these, values of critical heat fluxes, heat trans- fer coefficients, and surface temperature distributions were obtained. The measured critical heat fluxes of the nozzle designs that were tested ranged from 8.5 to 75.7 W/cm2 . Local heat transfer coefficients ranged from 0.1 to 1.4 W/cm2 ? K. Another heat transfer related property that was investigated was the efficiency of the design. The efficiency was defined as the critical heat flux divided by the sum of the available latent and sensible heat capacities of the fluid. Experimental results showed that nozzle designs with lower efficiencies could reach higher levels of critical heat flux and had more desirable heat transfer coefficients. Efficiencies at critical heat flux were calculated to be between 0.17 and 0.35. Lower values of efficiency mean that the system is less reliant on the two-phase component of heat transfer. Designs that are capable of removing heat with-out surface nucleate boiling were found to perform better. A visualization test stand was also designed and constructed for the visualization por- tion of the study. A 2000 ?A thick coating of indium tin oxide (ITO) on a glass die created a transparent, resistive heating element. Measurements of the film thickness under adia- batic conditions were taken using an optical technique based on the angle of total internal reflection. Experimental film thicknesses were measured to be 42 - 918 ?m. A numerical model was developed to predict the film thicknesses; values predicted were 79 - 280 ?m. The results of these two tests were correlated to understand how the spray distribution and patternization effects the heat transfer characteristics.