Assessment of a Twisted Double-Tube Latent Heat Storage System from the Perspectives of the First and Second Laws of Thermodynamics Throughout the Melting and Solidification Processes

Abstract

The current study numerically investigates the impact of twist angle on the performance of a twisted double-tube latent heat storage (LHS) system, evaluating first- and second-law efficiencies during melting and solidification. Water serves as the heat transfer fluid (HTF) circulating within the inner tube, while the phase change material (PCM) RT35 is situated in the annular space. Performance is evaluated by examining the PCM temperature, liquid fraction, and the rates of thermal and frictional entropy generation over time. The findings indicate that lower twist angles result in reduced melting and solidification durations. A notable exception is the 360◦ case during melting, which achieves the shortest melting time among all configurations due to the dominance of vortex-enhanced convective heat transfer over the minor stagnation-zone penalty at this twist angle. Frictional entropy generation is consistently negligible compared to thermal entropy generation. Thermal entropy generation exhibits an increasing-then-decreasing trend during solidification, with peak values of 7.8 W/m3 /K and 1.8 W/m3 /K recorded during solidification and melting, respectively. At higher twist angles, both phase-change durations are extended.