Film boiling influence on the spherical body’s cooling in sub-cooled water

Abstract

An article represents the results of the experimental investigation of the film boiling influence on the spherical body’s, made from the three different metals (steel, copper and aluminum), cooling process in sub-cooled water under the different temperatures of the body and water.It was stated that the minimal temperature of film boiling depended on the body’s surface temperature, which was influenced by the body’s thermal diffusivity (heat conductivity, heat capacity, density), water temperature and heat transfer rate between the body’s surface and the liquid. Bodies with higher density and higher heat capacity (made from copper and steal) accumulated more amount of heat, therefore during the cooling process the film boiling on the surface of such bodies was observed longer. From the other side, higher conductivity of the bodies’ made from the copper and aluminum influenced on the more intensive cooling process, consequently the film boiling for those bodies lasted shorter. Low conductivity of the body’s material sometimes (in a case of low temperature of water and high heat transfer rate) did not allow transferring sufficient amount of the heat from the internal layers to the surface of the body (steel body). Therefore the film boiling was running longer on the surface of the body which had better conductivity despite less amount of the heat accumulated in it (aluminum body). Body, made from the copper, accumulated bigger amount of the heat than the aluminum body, but cooled down faster due to the higher conductivity and higher thermal diffusivity of the copper. The fast cooling process (copper body) allowed reaching quite low Leidenfrost temperature (350°C) due to the biggest amount of the heat accumulated in it. Furthermore, big amount of the heat accumulated and high conductivity of the copper body determined that the film boiling existed on the body’s surface at the lowest water temperature (70°C). Steel body has low conductivity, therefore steam layer (film boiling) during cooling existed only for the quite high temperature of the body’s surface (440°C and higher). Aluminum body, cooling down in the water at the temperature 99°C, reached the lowest temperature (320°C) of the film boiling.