Heat transfer during cooling of high temperature spheres in subcooled water at different pressures

摘要

In our previous publications Yagov et al. (2016) we have confirmed an existence of a particular mode of film boiling heat transfer in subcooled water, featured with extremely high intensity in comparison to saturated film boiling. The experiments with other liquids (ethanol, isopropanol, perfluorohexane) did not reveal this film boiling regime even at huge subcoolings (up to 160 K). Basing on the analyzes of the revealed regularities it is possible to assume that the regime of intensive heat transfer in film boiling of subcooled water occurs only in cooling processes, when heat supply to the surface is controlled with a metal thermal effusivity. This regime appears easier during cooling of spheres with lower value of this property. Probably, some points of the surface (ledges of roughness) can protrude a vapor film and contact with liquid, when the average surface temperature is much higher than that of homogeneous nucle-ation. At present, there is no clear model of mechanisms of incipience of the regime discussed, and cumulation of the experimental facts continues to be necessary. The present paper sets forth the experimental results on heat transfer during cooling the spherical patterns from nickel, stainless steel, and copper in subcooled water at different pressures. The experiments at atmospheric pressure revealed marked influence of the metal thermal effusivity on the cooling process in highly subcooled water. The new experimental data evidence that the regimes of high intensity heat transfer are found under the conditions of impossibility of direct liquid/solid contacts at cooling water temperatures 100-150 °C, if the liquid subcooling to saturation exceeds 20 K. The cooling time decreases at the same water temperature with subcooling increase due to the pressure growth. The cooling process in saturated and slightly subcooled water occurs in the commonly known regime of film boiling independently on an absolute value of the saturation temperature. The approximate method based on averaging the surface temperatures and solving 1D inverse unsteady heat conduction problem was applied for calculations of heat transfer coefficient and heat flux density during cooling. The calculated HTCs in the regimes of intensive film boiling of subcooled water are an order of magnitude higher than those in the saturated and slightly subcooled liquid.