Numerical simulation of a finned-tube LHTES system: Influence of the mushy zone constant on the phase change behaviour

This work presents a numerical investigation on latent heat thermal energy storage (LHTES) systems during the phase change process. The numerical analysis, based on the apparent heat capacity formulation, was carried out through a two-dimensional axisymmetric numerical model developed by means of the COMSOL Multiphysics software. A thermal energy storage system based on the configuration of a double tube heat exchanger with finned surfaces was used as an experimental test case and the commercial paraffin RT35 was selected as phase change material (PCM). The influence of the heat transfer by convection, in particular the influence of the term describing the mushy zone in the momentum equation, was investigated during the whole charge and discharge processes. Three different values of the constant Amush, equals to 104, 106and 108were selected as well as two different values of the HTF volumetric flow rate were adopted in order to reproduce both laminar and turbulent flow regimes. The results are reported in terms of temperature, melting fraction and phases evolution during the whole melting and solidification processes, and compared to previous experimental tests carried out in the laboratories of the University of Lleida, Spain. A good agreement with the experimental results was obtained showing that the mushy zone constant has a significant influence on the interface shape and motion. The results show that large values of Amushdetermine an increase of the mushy region reducing the natural convection effects during the charge phase. Thus, the proper evaluation of the mushy zone constant allows providing a deeper understanding of the phase change behaviour, resulting in an important parameter for accurate modelling of LHTES systems.