Electric Field-Dependent Charge Injection In Thermal Storage Systems

Abstract

The present study aims to investigate the effect of different charge injection functions on electrohydrodynamic-driven flows within a latent heat thermal storage system (LHTSS), focusing on three specific injection models: the Heaviside step function, Schottky, and autonomous injection. Previous research predominantly relied on the autonomous charge injection, which oversimplifies the boundary condition and neglects the influence of the electric field, resulting in space charge density distribution on the boundary and within the medium that does not accurately mimic the realistic charge injection phenomena. This study utilizes the lattice Boltzmann method (LBM) to simulate the behavior of paraffin wax, a phase change material (PCM), in an LHTSS under both autonomous and non-autonomous charge injection. Initially, the solid PCM begins melting due to thermal conduction from the hot top wall, after which electro-convection generated by charge injection from a central circular electrode in the LHTSS enhances the heat transfer rate. An LBM solver was employed to solve governing equations, which was verified against multiple experimental and numerical benchmarks. Current-voltage curves presented by Hassan and Cotton [1] were used in this study. The electrode in the simulation injects charges under 6 kV, and charges are collected by two flat electrodes at the top and bottom of the computational domain. The heat transfer coefficient, liquid fraction over time, and space charge density pattern at the boundary and within the domain were plotted for each charge injection function. The results for Schottky and Heaviside step functions of injection are very close but differ from autonomous injection with the maximum deviation of 30%, despite all cases having the same current. Moreover, higher material permittivity results in a significant divergence between autonomous and non-autonomous injection behaviors.