Influence of pressure on film boiling of subcooled liquid

Abstract

Film boiling of subcooled liquid as the most available and use-proven way of quick chilling of hardening pieces, ensuring required microstructure of metal is widely used in quenching technology. A vapour explosion is the other important process, in which this boiling regime is surely observed. Under high subcooling of liquid the cooling process of high temperature bodies is featured with very high intensity and can be considered as a particular heat transfer regime. This was revealed first in 1986 by G. Hewitt and D. Kenning, who used for this regime a term “microbubble boiling” [1]. Mechanisms of very intensive heat removal from the metal surface to subcooled liquid are not understood till now, because the surface temperature in this process is much higher than the temperature of homogeneous nucleation. Some specialists do not accept even an existence of the problem itself. For revealing regularities and mechanisms governing intense transfer of energy in this process, the present authors conduct systematic investigations of cooling of high temperature balls made of different metals in water with a temperature range from 20 to 100°C [2]. It has been determined that temperature field in the balls of diameter higher than 30 mm in the intense cooling modes loses its spherical symmetry. An approximate procedure for solving the inverse thermal conductivity problem for calculating heat flux density on the ball surface is developed. During film boiling, when the ball surface temperature is well above the critical level for water, so that the liquid cannot come in direct contact with the wall, the calculated heat fluxes reach 3–7 MW/m2. The main aim of this study was analysis of excess pressure effect on heat transfer during film boiling of different subcooled liquids. The experiments have been performed in the pressure range 0.1–1.0MPa at the coolant temperature -15 - +90°C for all the liquids used. The primary results of the research are the experimental thermograms of cooling the spheres under the elevated pressures.