Sub-cavity liquid volume beneath spray droplet impacts into static liquid layers, and initial estimates of the heat flux required to dry out this volume

• Single drop impacts were studied for Weber numbers between 140 and 1000.
• Time-resolved droplet impact cavity film thickness was measured vs. cavity radius.
• Liquid volume beneath droplet impact cavity was computed.
• Heat flux to dry out the cavity liquid volume was estimated to be 400–800 W/cm2.

The time variation of the sub-cavity liquid volume beneath individual droplet impact cavities was measured for ranges of drop Weber and Reynolds numbers that match those for a full cone spray nozzle of interest. Cavity lifetime was also measured. These results will be used in a Monte-Carlo model of the spray cooling process that is currently under development. Droplet Weber numbers were varied between 140 and 1000. Corresponding Reynolds numbers ranged between approximately 1200 and 3600. These ranges matched Phase Doppler results that were obtained for the water spray under study. Thickness of the static liquid layer into which the single droplets impacted was varied between 0.2 and 1.0 times the droplet diameter.The measured sub-cavity liquid volume generally was between 60% and 80% of the original droplet volume over much of the cavity lifetime. Increasing the static liquid layer thickness increased this plateau value of the sub-cavity liquid volume between these lower and upper bounds, for Weber numbers greater than around 400. Sub-cavity liquid volume also increased somewhat as Weber number was increased. The cavity lifetime increased significantly as Weber number increased.The sub-cavity liquid volume and cavity lifetime results were scaled to values for the corresponding spray droplets at equal (We, Re). These were then used in an energy balance between the energy transferred through a heated wall to the sub-cavity liquid and the sum of the sensible heating and latent heat required to dry out the spray drop sub-cavity liquid volume within the cavity lifetime. These computed heat fluxes to dry out the drop impact cavities were then used to estimate the overall average heat flux for a heated surface that would dry out these individual spray droplet impact cavities. The resulting average heat flux values were between 400 W/cm2 and 800 W/cm2. These predictions are similar to the range of CHF values reported in the literature for water, of 500 W/cm2 to 1000 W/cm2.

Measured sub-cavity liquid volume vs. time for h0/d = 1.0 (left), and estimated average heat flux required to dry out the droplet impact cavity (filled symbols) and to initiate boiling (open symbols) (right).Figure optionsDownload as PowerPoint slide