Close-contact melting and natural convection in unconstrained melting: A parametric study

Close-contact melting (CCM) is a phenomenon that can be achieved in latent thermal energy storage (LTES) units where the falling of solid PCM is realized. On the other hand, natural convection (NC) plays an important role in LTES units due to circulations based on the buoyant flow of liquid phase change material (PCM). However, the relative contribution of CCM and NC on the melting heat transfer remains unclear when they coexist. This study considers a set of parametric cases that numerically investigate the effects of CCM and NC on an LTES unit with annular fins where CCM is enabled with a shell made of aluminum heated by the heat transfer fluid (HTF). The enthalpy-porosity method is used to model the melting in the LTES unit, and it is established that CCM can be realized using this method with a small value of mushy zone constant. However, with this small value, the mushy zone turns into a highly viscous liquid. The parametric study considers the number of fins and their clearance and concludes that with a decrease in fin clearance and an increase in fins, the effects of CCM are boosted, resulting in faster melting. The behavior of CCM and NC are characterized using the transient behavior of heat transfer rate for different surfaces and defining a factor that calculates the contribution of each surface on the melting. The role of CCM in the melting is up to 50%, while NC plays a less significant role. This study helps with the design of LTES units where CCM is enabled.