A series of experimental measurements are presented of the liquid film thickness for a dense water spray impacting onto an unheated, smooth, flat impact surface. These data have been acquired using a non-contacting confocal chromatic optical thickness probe originally developed to measure the central thicknesses of precision optics. The measured average liquid film thickness is approximately 160 μm. This local residual liquid layer thickness increases slightly versus radial location at each spray flow rate, and also decreases slightly as the spray flow rate is increased. Also, backlit high-speed video imaging through the glass impact plate from below is used to observe the time variation of the diameter of individual spray droplet impact cavities. The spray droplet impact cavity diameter results indicate a time variation that is proportional to (t)~(0.2) during the cavity growth phase, rather than the (t)~(0.5) behavior that is commonly observed for isolated single drop impacts into a static liquid film. Generally, the details of the cavity retraction phase could not be observed in the current experiments. Measured maximum cavity diameters ranged from around 0.4 mm to 1.6 mm. It is estimated that the corresponding impinging droplets had diameters of between 80 μm and 320 μm. This range of estimated droplet diameters is consistent with a series of Phase Doppler Particle Analyzer measurements that were made for the same spray nozzle and flow conditions. These results are found to be consistent with the predictions from simple time scale estimates of the cavity formation time and cavity lifetime. Based on these observations and the time scale analysis, it is reasoned that transient conduction to the sub-cavity liquid film beneath spray droplet impact cavities enhances the overall spray cooling heat transfer, but resulting dry out of these cavities could then contribute to onset of critical heat flux.
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