Geometric optimization of fin structures for accelerated melting of phase change material in a triplex tube heat exchanger

Abstract

This study aims to optimize the thermal energy storage performance of a novel Triplex Tube Heat Exchanger (TTHX) embedded with Phase Change Material (PCM). The PCM is placed in the inner tube and the outer annulus, while the heat transfer fluid (HTF) flows through the middle annulus section. This TTHX configuration outperforms conventional designs by effectively dividing the PCM volume into two distinct sections, consequently enhancing the interaction between HTF and PCM. The experimental results show that the PCM placed in the inner tube melts faster than the PCM in the outer annulus, indicating a need for fins to enhance heat transfer in the outer annulus. In this regard, several designs of the branched fin structures are proposed for the outer annulus region. Taguchi optimization method is used to identify the optimum fin design configuration by considering branched fin designs (F), their angular placement around the periphery of the middle tube (α), and the branch angle (β) as the independent design factors. The objective is to optimize the fin configuration so as to decrease the melt time of the PCM. All the cases of the L16 Taguchi design array are simulated numerically using the enthalpy-porosity model in ANSYS Fluent 2021 R2. The target parameters are the melt time, average temperature, and the energy storage features which are continuously recorded. The main effects plots of the Taguchi analysis suggest an optimum geometry as the one having fin design of F4 (containing angled and straight branches), α as 60° and β equal to 50°. The optimum fin configuration reduces the melt time by 35 % in comparison to the reference base case design. The energy storage characteristics are also improved with the optimum design storing 267 kJ/kg energy at 71.92 min, while the base case stores 208 kJ/kg at the same time. This gain has been achieved due to faster energy transfer rate which is increased to a value of 0.062 kW/kg for the optimum design as compared to 0.042 kW/kg for the base case, reporting an increase of 47.6 % in the energy transfer rate.