Effects of arrow-shape fins on the melting performance of a horizontal shell-and-tube latent heat thermal energy storage unit

Abstract

A numerical investigation was carried out to analyze the melting characteristics of phase change materials (PCM, specifically n-octadecane) in a horizontal shell-and-tube latent heat thermal energy storage (LHTES) system that exhibit low-speed melting of the PCM within the lower half of the unit, thus limiting its utilization in practical applications. Novel arrow-shape longitudinal fins were added to improve the overall heat transfer, including both conduction and natural convection. A two-dimensional symmetric simulation model featuring the Boussinesq approximation and an enthalpy-porosity model was employed. The case with bare heat transfer fluid (HTF) tube was compared with finned tube cases of 6 configurations. Three different fin branch angles (θ = 40°, 50°, and 60°) and four fin length ratios (ϒ = 0, 0.222, 0.571, and 1.198) were studied, with ϒ related to the gap distance between the bottom of the heated tube surface and the fin branch. The trends of the liquid fraction, average temperature of the PCM, and total heat transfer rate through the outer surface of the inner tube with the fins featuring two distinct fluctuating stages were presented. Progression of the melt front, temperature distribution and velocity vector/streamline fields were also provided. Common to all cases, persistence of a time-dependent rising thermal plume originating from the top surface of the heated inner tube was observed. Depending on the orientation of the branched fin and its distance from the inner tube, similar thermal plumes rising from the top surface of the branched fin cooperated with the molten PCM in the top one half of the annulus in realizing expedited melting. These cases exhibited a variety of recirculating vortices and enhanced natural convection within both halves of the unit. The results showed that increasing the fin angle while using a fixed fin length ratio of ϒ = 0 improved the melting rate of the PCM. When the fin ratio was varied from 0 to 1.198 while the fin angle was invariant (θ = 60°), the melting time was observed to prolong. The case with θ = 60° and ϒ = 0 exhibited the highest heat transfer enhancement than other arrangements. In effect, when the branch fin angle is increased and the fin length ratio is reduced, greater amount of PCM is subjected to natural convection. At the same time, heat conduction is enhanced below the branched fin since less volume of the solid layer PCM existed there.