Investigation of using multi-layer PCMs in the tubular heat exchanger with periodic heat transfer boundary condition

Storing and releasing thermal energy at an almost constant temperature is the benefit of employing phase change materials, which makes them a useful material to reduce energy usage in industrial applications. In the present study, the effect of charging and discharging of multi-layers of Phase Change Materials (PCMs) in coaxial cylinders with a time-periodic boundary condition is numerically investigated. The 2D problem is solved with the finite-element method, and convection modeling is also included. The Carman Kozeny model is utilized to control the natural convection velocities in solid and liquid regions. Fifteen cases are introduced so as to capture the effect of arrangements and thicknesses of the PCMs. First simulations were targeted to understand different arrangements effect on melting and transferring heat from the inner tube to the outer wall. Then, different layers' number and thickness of each section were examined. Results are shown in terms of liquid fractions, temperatures, and velocity streamlines. The results assert that unlike the outer section, the inner one is more susceptible to the change of hot heat transfer fluid (HHTF) temperature. Moreover, the rate of saved energy within PCMs to the inlet energy is calculated for all cases. In the case of one-layer of RT65, the system can only save 23.28% of inlet energy. While in the specific thickness and arrangement of three-layer PCM, the amount of saved energy within the PCMs reach to 41.67%. (C) 2019 Elsevier Ltd. All rights reserved.