Studies on the water cooling of hot surfaces. (Analysis of spray cooling in the region associated with film boiling)

An analytical study has been performed on the spray cooling of a hot surface in the region associated with film boiling under the assumption that there will be two parallel conductances of heat transfer. One is of radiation from the hot surface to the environment, the correlation of which is cited from the authors' preceding report. The other is heat transfer to the sprayed water droplets by heat conduction through the hypothetical uniform vapor layer underneath the droplet which is in the spheroidal state with a flat bottom. First, we deduce analytically an expression for heat transfer per droplet by solving the equations of momentum in the vapor layer, energy balance at the interface and balance of static forces exerting on a droplet simultaneously. Then a semiempirical correlation of the mean lifetime of the droplets is estimated using the experimental data with surface superheating below 500 K at atmospheric environmental pressure, mass velocity of water between 0.0162 and 0.174 kg/(m²·s), volume mean diameter of droplets between 130 and 550 μm and Weber number between 10 and 120. The procedure we propose predicts the heat transfer data of spray cooling within the experimental range mentioned above fairly well.