Fundamental Heat-Transfer Processes Related to Phase-Change Thermal-Storage Media.

摘要

Research performed between June and December 1982 on fundamental heat transfer processes which occur in phase-change thermal storage systems is described. The research encompasses both freezing and melting, and includes both experiment and analysis. The experimental portion of the work in progress is concerned with phase change which occurs within a closed cylinder or tube. In separate but interrelated freezing and melting experiments, the effect of the inclination of the tube in the gravity field was investigated. For freezing, it was found that despite local variations, the global (i.e., surface-averaged) freezing rate was virtually independent of the inclination of the tube. On the other hand, for melting, different regimes characterized by different melting rates were encountered as a function of the tube inclination. The rate of melting is least when the tube is vertical. At moderate angles relative to the vertical, gravity presses the melting solid against the tube, thereby accelerating the rate at which melting occurs. Numerical solutions obtained with an implicit-explicit finite difference scheme were employed to explore the effects of solid-solid phase transitions on freezing which occurs on a plane wall or on the outside surface of a cylinder. Another focus of the work was to take account of heat transfer at the solid-liquid interface due to natural convection in the adjacent liquid melt. In the presence of interfacial convection, freezing ceases when a critical thickness of the solidified layer is attained. The greater the interfacial convection, the smaller is the critical thickness and the shorter is the time required for its attainment. The heat liberated by the solid-solid transition reduces the thickness of the solidified layer but increases the rate of heat extraction at the cooled wall. (ERA citation 08:012452)