Localized heating element distribution in composite metal foam-phase change material: Fourier's law and creeping flow effects

Abstract

Copyright © 2021 The Authors. A numerical parametric study is presented of a domestic thermal storage heat exchanger to explore the effect of highly localized positive temperature coefficient cylindrical heating elements in a phase change material (PCM) with conductive enhancement by open-pore metal foam. By using 90 L of commercially available Rubitherm RT70HC wax, 5.7 kWh of thermal energy is captured by the unit. The discharge is via a central convective air channel. The constant low-temperature heating elements are inherently safe for combustible PCM. The heat distribution by Fourier's law and the creeping flow is investigated using the local thermal equilibrium assumption between the PCM and metal foam. Heating element position, diameter, and temperature are varied to optimize charge time and exit air temperature. Two heating elements of 1 cm diameter and constant temperature of 90°C produce a suitable performance for overnight store charging of 7.23 hours. Discharge via the air channel provides an average temperature of the output air over 30°C. The results indicated that the PCM inside metal foam almost follows Fourier's law. The creeping flow of molten PCM inside the pores of the porous medium (free convection heat effect) has an inconsiderable influence on heat transfer in the domain.