Cooling effectiveness of a water drop impinging on a hot surface

We studied, using both experiments and a numerical model, the impact of water droplets on a hot stainless steel surface. Initial substrate temperatures were varied from 50 degreesC to 120 degreesC (low enough to prevent boiling in the drop) and impact velocities from 0.5 to 4 m/s. Fluid mechanics and heat transfer during droplet impact were modelled using a "Volume-of-Fluid" (VOF) code. Numerical calculations of droplet shape and substrate temperature during impact agreed well with experimental results. Both simulations and experiments show that increasing impact velocity enhances heat flux from the substrate by only a small amount. The principal effect of raising droplet velocity is that it makes the droplet spread more during impact, increasing the welted area across which heat transfer takes place. We also developed a simple model of heat transfer into the droplet by one-dimensional conduction across a thin boundary layer which gives estimates of droplet cooling effectiveness that agree well with results from the numerical model. The analytical model predicts that for fixed Reynolds number (Re) cooling effectiveness increases with Weber number (We). However, for large Weber numbers, when We >> Re-0.5, cooling effectiveness is independent of droplet velocity or size and depends only on the Prandtl number. (C) 2001 Elsevier Science Inc. All rights reserved.