Liquid volume measurements in the cavity formed by single droplet impacts into a thin, static liquid film

Abstract The time variation of the liquid volume beneath the cavity formed by the impact of a single droplet into a static liquid film (termed the “sub-cavity volume”) over an unheated, horizontal surface has been measured for the first time, using water as the test liquid. Droplet Weber numbers, Reynolds numbers, and initial liquid film thickness-to-drop diameter ratios were studied that are representative of the impact conditions that are expected for typical sprays. The thin liquid film thickness beneath the droplet impact cavity (the “sub-cavity liquid film thickness”) was measured using a non-contacting optical thickness sensor, as a function of both time and radial distance away from the impact cavity centerline. These data have been numerically integrated to determine the time variation of the sub-cavity liquid volume. The measured liquid film thickness decreases away from the cavity centerline in the immediate vicinity of the inner crown wall. The sub-cavity volume is typically between 30% and 35% of the droplet volume, and remains near this plateau value over much of the cavity lifetime. The measured sub-cavity liquid volume and cavity lifetime are used for one example case to demonstrate the predicted values of both the local heat flux averaged over an individual cavity, and the overall heater average heat flux, that would be required to dry out the cavity prior to cavity fill in. The computed average heat flux value for this first example case, on the order of 4–6 MW/m 2 , is the same order of magnitude as the range of measured overall critical heat flux values referenced in the spray cooling literature.