Suppression and Enhancement of Boiling Associated with Multiple Droplet Impingement

Spray cooling has proven to be efficient in managing thermal load in high power applications. Reliability of electronic products lies on the thermal management and understanding of heat transfer mechanisms of the most commonly used thermal management schemes such as spray cooling. Many experiments have been done to understand the heat transfer mechanisms associated with spray cooling. However, most of them have relied on comprehensive spray cooling experiments where multiple physical variables are at play simultaneously. Furthermore, experiments with single streams of droplets have not been able to elucidate the effects of the onset of boiling (ONB) during the droplet impingement process. Therefore, efforts have been undertaken to consider the effects of using three droplet streams arranged in a triangulated fashion. The effects of using triangulated multiple droplet impingements on the suppression or enhancement of boiling on heated surfaces has been investigated. Moreover, the effects of using screen laminated on the suppression of ONB during the droplet impingement process has been studied in detail. The main goal of this project is to study the effects of multiple droplet impingement on the flat heater surface in the spray cooling with and without the use of metallic screen laminates. Single and triple droplet impingement experiments have been performed to understand the droplet behavior in spray cooling systems where multiple droplets simultaneously impact a heated surface.

The experiments consisted of using a stainless steel screen laminate over a sample surface to observe the suppression or enhancement of pool boiling which tends to occur at the periphery of each droplet impingement zone. An infrared-based imaging technique was used to measure surface temperature during droplet impingement. The heat transfer performance has been evaluated in terms of heat flux, droplet frequency and volume flow rate. The results indicate that droplet stream spacing and the use of copper meshes can enhance surface cooling significantly. Specifically, droplet stream spacing of 1000 ?m with copper meshes with a 6 mm hole and gap of 0.2 mm lead to enhanced surface cooling during the multiple droplet impingement process. It is expected that the results and conclusions of this study will be useful in understanding the physics of spray cooling which should help design better spray cooling system.