Droplet impact cavity film thickness measurements versus time after drop impact and cavity radius for thin static residual liquid layer thicknesses

Abstract

Comprehensive measurements are presented of the sub-cavity liquid film thickness for single droplet impacts into a static residual liquid layer, measured both versus the radius away from the cavity impact centerline and the local time since initial contact of the drop with the static residual liquid layer. Droplet Weber and Reynolds numbers are representative of the highest-energy droplets for a water spray of interest, with Weber numbers ranging between 140 and 1000. These high-impact-energy droplets create drop impact cavities with the longest cavity lifetimes, thinnest sub-cavity liquid films, and smallest sub-cavity liquid volumes, all of which are expected to contribute to the droplet impact cavities being both a significant source of enhanced transient local heat flux into the sub-cavity liquid volumes, as well as locations of early local surface dry out.Sub-cavity liquid film thickness is essentially constant over the inner radial portion of the cavity, but is significantly thinner at large cavity radius, somewhat inboard of the inner crown wall. The inner, constant thickness region ranges between 62% and 85% of the maximum cavity radius in extent, with an average size of 72% of maximum cavity radius. The thickness of this inner, constant thickness region ranges between 100μm and 162μm, with an average value of 126.5μm, or around 4% of the nominal droplet diameter of 3mm. This thickness varies by only around 3% of its average value for single values of the Weber number and static residual liquid layer thickness. At the end of cavity formation, the thinner regions farther outboard in the cavity are about 23% thinner than the sub-cavity film thickness in the inner region; but later, during the collapse of the crown, the thinner outboard region averages 34% less than the thickness in the inner, constant thickness region.Two major data accuracy limitations are discussed, and approximate error magnitudes are estimated. Drop-to-drop uncertainty in impact location and the uncertainty in the level of three-dimensionality of the cavity retraction process are both inherent limitations in the present liquid film thickness point-measurement method; these limitations could be avoided if a global thickness measurement over the entire cavity surface could be obtained; e.g., via stereo high-speed imaging.