Film boiling of subcooled liquids. Part I: Leidenfrost phenomenon and experimental results for subcooled water

Film boiling of subcooled liquids is important, at least, for two applications: for systems of post-accident cooling in nuclear power plants and for quenching technology. An analysis of the previous results shows that at high water subcooling film boiling presents a particular mode of boiling heat transfer, dissimilar greatly to saturated film boiling mainly by high intensity of heat transfer; this statement appears to be true, even though the researchers themselves do not recognize it. As at the temperature much higher than that of homogeneous nucleation, liquid-vapor phase transition occurs practically instantly, an actual temperature at the liquid/solid interface cannot be higher than the attainable limiting temperature of liquid. The possible mechanisms of high intensity heat transfer between the surface and subcooled liquid at the absence of liquid/solid contact and the conditions of this regime incipience require experimental and theoretical investigations. The present paper describes the method and the results of experimental study of heat transfer during cooling the spherical patterns from nickel, stainless steel, and copper with initial temperature above 700 °C in water at subcoolings up to 70 K. In distinction to all the previous studies temperature was measured in several points of the spheres that gives new information on the cooling process. It is revealed that at high cooling rates the temperature field can lose its spherical symmetry, high temperature gradients are observed not only in the radial direction, but also along the surface. At high subcoolings the heat transfer regime of high intensity arises at the surface temperature 600-700 °C that excludes a possibility of liquid/solid direct contact; heat flux density can be as high as 5-7 MW/m2.