Heat Transfer in Film Boiling of Flowing Water

Abstract

Film boiling is a post critical heat flux (CHF) regime with such a high surface temperature, that the wall can not contact with the liquid, but is covered by the vapor and thus has relatively low heat transfer efficiency due to poor heat conductivity of the vapor. The film boiling is encountered in various practices, e.g., the metallurgy, the refrigeration, the chemical and power engineering, etc.. In a postulated break loss of coolant accident of nuclear reactors the uncovered core would experience this regime, and the maximum fuel temperature would be primarily dominated by the heat transfer of film boiling. Due to its significant importance to the applications the film boiling has received extensive investigations both experimentally and theoretically. It was one of three subjects in a coordinated research program on Thermal-hydraulic relationships for advanced watercooled reactors, which was organized by the International Atomic Energy Agency (1994 – 1999). A comprehensive review on these investigations has been presented in the technical document (IAEA-TECDOC-1203, 2001) In film boiling the heat is transferred from the wall to the vapor, then from the vapor to the liquid, characterized by non-equilibrium. The interaction between two phases dominates the vapor generation rate and the superheat, associated with extremely complicated characteristics. This presents a major challenge for the estimation of heat transfer because of less knowledge on the interfacial processes. In particular, due to the peculiar feature of the boiling curve it is difficult to establish the film boiling regime at stable condition in a heat flux controlled system by using a conventional experimental technique. As shown in Fig.1, the stable film boiling regime can only be maintained at a heat flux beyond the CHF, which associates with an excessively high surface temperature for water. But for a heat flux, q, below the CHF, the regime can not be maintained stably at the post-CHF region (F or T), but at the pre-CHF region (N). The experimental data on film boiling were mostly obtained with refrigerant or cryogenic fluids, and the data of water were generally obtained in a temperature-controlled system or at transient condition with less accuracy. Since a so-called hot patch technique was developed for establishment of the stable film boiling regime (Groeneveld, 1974, Plummer, 1974, Groeneveld & Gardiner, 1978), a large number of experimental data have been obtained (Stawart & Groeneveld, 1981, Swinnerton et al., 1988, Mossad, 1988). Based on the data base various physical models have been proposed (Groeneveld & Snoek, 1984, Groeneveld, 1988, Mossad & Johannsen, 1989), and the tabular prediction methods have been developed for fully-developed film boiling heat transfer coefficients (Leung et al., 1997, Kirillov et al., 1996).