Computational Investigation of the Influence of Novel Fins on Melting of Phase Change Materials within Shell-and-Tube Latent Heat Thermal Energy Storage Units

Abstract

Numerical investigations are carried out to analyze the melting characteristics of the phase change material (PCM, n-octadecane) in three shell-and-tube latent heat thermal energy storage (LHTES) systems. These systems are: (a) horizontal units featuring arrow-shape fins, (b) L-shaped system and (c) vertical units with split annular fins. Simulations of the arrow-fin system were limited to 2D analysis with constant thermophysical properties, however for the other two systems, 3D analyses with temperature-dependent thermophysical properties were performed. The Boussinesq approximation was utilized to handle buoyancy-driven convection, and the single-domain enthalpy-porosity formulation of phase change was adopted. The thermal performances of the units during melting were assessed by analysis of the trends of the liquid fraction, average temperature, progression of the melt front, temperature distribution and kinematics of the molten fluid, etc. In the horizontal shell-and-tube LHTES units featuring arrow-shape longitudinal fins, 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 the fin length ratio representing the gap distance between the bottom of the heated tube surface and the fin branch. Common to all cases, persistence of a time-dependent upwelling thermal plume (UTP) 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. Results showed that increasing the fin angle while using a fixed fin length ratio of ϒ = 0 improved the melting rate. When the fin ratio 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. For the L-shaped system shell-and-tube LHTES units composed of horizontal, elbow transition and vertical parts connected sequentially, the effects of the elbow radius, radial eccentricity of the inner tube, elbow’s bend angle, inclination angle, and length ratio of the horizontal and vertical parts were explored. HTF being water was circulated through the inner copper tube. All the units with horizontal HTF inlet streams featured high-speed melting above the bottom half of the hot inner tube, induced by intense natural convective UTP. It was observed that increasing the radial eccentricity led to higher heat transfer enhancement compared to other arrangements, due to greater amount of PCM being subjected to natural convection. As for other geometrical factors, the elbow radius made no apparent difference on the melting. Formation and evolution of UTPs rising from the HTF tube and sculpting wrinkled structures at the liquid-solid interface were observed with novel visualization of the interface. By increasing the radial eccentricity, the total stored thermal energy storage declined. Formation and strengthening of spiraling streamlines originating from the horizontal section toward the bend and the vertical segments is observed through innovative presentation of the kinematics of the liquid PCM. For the studies of the vertical shell-and-tube LHTES units with split annular fins featuring circumferential staggering for two axial configurations, i.e. uniformly-spaced (US) and vertically-shifted (VS) were studied. For each configuration, two numbers of split fins (4 and 8) on each of the 7 layers of annular fins were investigated. For each number of splits, three different split angles are chosen. Twenty-six cases were considered to examine the effects of pertinent geometrical parameters, i.e. radial length of the copper fins initiating from the inner tube, number of fin-splits, angle of the fin split, and circumferential staggering. The charging performance of the cases with 4 split annular fins were compared with the cases with continuous (un-split) annular fins. For the US alignment, the overall melting rate was reduced with excessively greater split angle. For the VS alignment, the melting improvement of adopting splits is more significant compared to the US arrangement. For the VS alignment with 8 splits in each layer, all three cases led to shorter complete melting time compared to case with full annular fins. The intensity of the UTP close to the inner HTF tube is observed to be greater for the case without splits. By adopting the split fins, the melting rate is enhanced in the top sections of the units, but suppressed in the lower sections, due to natural convection. For the VS configuration, the melting characteristics in the lowest section exhibited features of re-solidification near the end of melting process. The mixing of the liquid PCM flow between neighboring compartments was improved with the split angles. The locations of large heat flux on the upper surface of fins that facilitate the emergence of UTP and fast charging of heat within the region above fins are noticed.