Numerical investigation of flow boiling behavior in a vertical tube of a shell-and-tube phase change heat storage unit

Phase Change Materials (PCMs) demonstrate significant potential as latent heat storage systems for direct steam generation. This investigation explores the heat transfer dynamics of an upward-flowing heat transfer fluid (HTF) boiling within a vertical tube, concurrently examining the solidification process of PCM outside the tube. A two-dimensional model of a shell and tube heat storage device is established for this purpose. The simulation employs the Volume of Fluid (VOF) model coupled with the Lee model to simulate HTF boiling inside the tube. Concurrently, the enthalpy-porosity model is utilized to analyze the PCM solidification process. The findings reveal that PCM solidification and heat discharge are influenced by the temperature and thermal conductivity of the HTF, leading to uneven solidified PCM thickness along the tube's length over time. Additionally, variations in tube length, inlet velocity, and inlet temperature impact both the vapor quality of HTF and the thickness of solidified PCM. Tube length correlates with boiling time, affecting the accumulation of steam quality throughout the coupling process. Inlet velocity dictates the heat transfer of HTF with PCM within a limited distance, and insufficient heat exchange resulting in the cessation of HTF boiling. Inlet temperature determines the appropriate device size, lower inlet temperatures necessitate longer tubes to extend the heat charging duration until boiling initiation.