Cooling Characteristics and Heat Transfer Coefficients during Fog Cooling of Hot Steel Plates

Abstract

In forced-cooling of hot steel products, air-stomized fog cooling is suitable for the soft cooling whose rate is between those of forced-air cooling and water spray cooling. In fog cooling both water and air considered to take part in the cooling. It is well known that when fog impinges a surface to be cooled at high speed, heat flux transferred from the surface to the fog increases with impinging speed because of unstable vapor films on the surface. It has not been clarified yet whether heat flux increases or not when fog impinges the surface at a slow speed.
The heat transfer phenomena have been studied when hot steel specimens are cooled with fog which is atomized with low speed air. The main results obtained are as follows:
(1) When fog impinges a surface at a slow speed, air makes water atomized but scarcely contributes to heat transfer.
(2) Cooling capability of fog is dependent on only the water flux to the surface.
(3) The cooling capability of air-atomized fog is nearly equal to that of pressure-atomized one, for the same water flux.
In this paper, in addition to the above results, the differences in heat transfer mechanism between the upper and the lower surface of steel plate positioned horizontally and the influences of surface condition of a steel plate on heat transfer are discussed.