Cooling effectiveness of a water drop impinging on a hot surface

We studied, using both experiments and a numerical model, the impact of wat er droplets on a hot stainless steel surface. Initial substrate temperature s were varied from 50 degreesC to 120 degreesC (low enough to prevent boili ng in the drop) and impact velocities from 0.5 to 4 m/s. Fluid mechanics an d heat transfer during droplet impact were modelled using a "Volume-of-Flui d" (VOF) code. Numerical calculations of droplet shape and substrate temper ature during impact agreed well with experimental results. Both simulations and experiments show that increasing impact velocity enhances heat flux fr om the substrate by only a small amount. The principal effect of raising dr oplet velocity is that it makes the droplet spread more during impact, incr easing the welted area across which heat transfer takes place. We also deve loped a simple model of heat transfer into the droplet by one-dimensional c onduction across a thin boundary layer which gives estimates of droplet coo ling effectiveness that agree well with results from the numerical model. T he analytical model predicts that for fixed Reynolds number (Re) cooling ef fectiveness increases with Weber number (We). However, for large Weber numb ers, when We >> Re-0.5, cooling effectiveness is independent of droplet vel ocity or size and depends only on the Prandtl number. (C) 2001 Elsevier Sci ence Inc. All rights reserved.