Adiabatic and diabatic measurements of the liquid film thickness during spray cooling with FC-72

An experimental investigation was performed to measure the liquid film thickness created by both a low flow rate single nozzle and a high flow rate four-nozzle array spray design using FC-72. Measurements were made using a non-intrusive optical technique based on total internal reflection. Reflected light rings that formed on a test die were photographed, and an automated program measured and recorded the radii, from which the thickness of the film was calculated using fundamental equations of geometric optics. This measurement technique was used to investigate the dependence of the film thickness on location and applied power. For a four-nozzle array, regions of the die that have been previously shown to exhibit the poorest heat transfer performance were found to have the thickest films, in agreement with qualitative imaging. With the single-nozzle spray, it was found that adding a heat load did not affect the film thickness. Away from the spray impact region, the applied heat caused a significant amount of localized dryout.