Time-dependent melting and freezing heat transfer in multiple phase change materials.

摘要

Based on an analysis of the causes of non-convergence in the effective heat capacity methods a new conservative effective heat capacity method is developed for general phase change heat transfer problems. Numerical experiments verified the accuracy and efficiency of the new method.; A multi-layer phase change material (PCM) heat transfer module is proposed for latent heat energy storage. Cyclic heat transfer in the module was modelled using the finite element technique. A parametric study was performed to investigate the energy charge/discharge rates for the new design.; A second-law thermodynamic analysis was carried out for thermal energy storage using multiple PCMs. The exergy efficiency of energy storage units using two, three as well as five different PCMs was analyzed and compared with that using a single PCM.; A novel cone-cylinder design configuration is proposed for a shell-and-tube latent heat energy storage exchanger. A finite element model was developed to simulate the coupled convection and cyclic melting/freezing phase change heat transfer occurring outside the tube. The advantages of the new configuration are examined and discussed with the help of numerical experiments. Following the new design configuration a novel multi-exchanger energy storage system is proposed. Finite element simulation results validated and extended the thermodynamic analytical results.; A new solar thermal storage unit using multiple PCMs was proposed and analyzed by a finite element model. A parametric study was carried out to investigate the advantages of the new design when compared with conventional single PCM designs.; Finally, a finite element model for melting and freezing heat transfer including free convection in the melt region was developed. The streamline upwind/Petrov-Galerkin method was employed to enhance both the stability and accuracy of the numerical solution. Using this finite element model simulations were carried out for melting of a PCM in a rectangular cavity heated from below. Flow patterns and local heat flux distributions at the heating surface are presented and discussed. In addition, melting of a PCM in a rectangular cavity with an isothermal vertical wall was simulated. To enhance the heat transfer rate during the last stage of the melting process, inverting the PCM container is shown to be an effective technique; this idea was examined with a parametric study.