Multi-phase flow and heat transfer characteristics of composite heat storage materials during melting process: simulation and optimization

The low thermal conductivity of phase change materials (PCMs) has limited their large-scale energy storage applications. This paper focuses on the rapid heat storage process of phase change energy storage. A composite heat storage structure is proposed, in which 50 % of solid-PCMs are filled at the bottom and water is at the top. During the melting process, heat is continuously exchanged through the mixture of water and liquid PCM to enhance heat transfer. A computational model (Case 1) is established using the computational fluid dynamics-volume of fluid (CFD-VOF) numerical method and compared with the Case 0 structure with 50 % water at the bottom. The reliability of the numerical model is verified through experiments, and the two-phase flow state of wax and PCM in the Case 1 structure is observed. Further, through numerical studies, the liquid phase evolution, temperature distribution, and internal flow velocity during the heat transfer process of different composite melting structures were analyzed, and the melting performance and energy storage performance were quantitatively evaluated. In addition, the melting characteristics of this novel energy storage structure at different wall temperatures are discussed in detail. The results show that compared with Case 0, the melting time of PCM in Case 1 is shortened by 51.75 %, while the average velocity of PCM and the average velocity of water are increased by 100.75 % and 67.33 % respectively, compared with Case 0. However, compared with Case 0, the total heat storage capacities of PCM and water in Case 1 are reduced by 3.17 % and 19.27 % respectively. Finally, the response surface method is used to optimize and predict the comprehensive heat storage rate and heat storage time, and the accuracy of the optimization results exceeds 98.995 %.