Thermal Performance Research On Unit Tube For Phase Change Heat Storage Embedded With Metal Foam

Heat Receiver is the key component of the space solar dynamic power systems (Space Solar Dynamic Power System——SSDPS), it adopts the method of high temperature phase change thermal storage, in which the molten salt is usually selected as PCM. However, solid density of PCM is different from its liquid density, which will cause cavity in the phase change process. The cavity will seriously affect the heat transfer performance and structure reliability of heat storage container. Therefore, PCM is encapsulated in many containers and metal foam is embedded in PCM to reduce the adverse effects of PCM. Metal foam, with its high thermal conductivity, light weight, high heat capacity and etc, is selected as a filling with PCM, and is becoming an effective way of strengthening the heat storage. Foam skeleton not only can enhance the PCM thermal conductivity, but also can disperse the cavity distribution and reduce the bad effects caused by the cavity concentration effectively. It will have broad application prospects in the field of aerospace, aviation and so on.The paper takes the phase change thermal storage unit tube used in Solar Dynamic Power System ground test as the research object. The molten salt LiF-CaF₂ is selected as thermal storage material in heat exchange tube, Co-based alloys Haynes188 is selected as material of PCM container and working fluid tube, and the porous nickel foam of 95% porosity is selected to strengthen heat transfer. The following topics are studied:(1)A numerical model of the single-tube phase change heat storage system is developed, based on enthalpy method, and cavity problem is also considered. The change of maximum wall temperature of PCM canisters, gas outer temperature and liquid PCM fraction with time in 5 orbital periods have been numerically calculated by FLUENT, and the periodic heat flux boundary condition is set by writing user-defined function (UDF). In order to prove the correctness of the mathematical model of this paper, numerical results are compared with the result in some literature and the trends are in close agreement.(2) The results indicate that metal foam dose could enhance the heat storage effect of the unit tube by a certain extent. Numerical results show that the greater heat transfer temperature difference will result in higher heat efficiency. At the same time, metal foam not only increases the heat release rate, but also distributes the temperature of the heat storage containers.(3) Working fluid inlet temperature, flow and wall heat flux must be selected reasonably to ensure the utilization of PCM and effect of heat transfer enhancement using metal foam. The numerical result predicts the thermal performance of the unit tube well, it proves that the thermal analysis model has a certain reference to the overall design of space station solar heat receiver.