Numerical Investigation and FEM Validation of Flow Dynamics and Heat Transfer in a Paraffin-Encapsulated Packed Bed Thermal Energy Storage System

Abstract

Packed bed latent heat storage systems using phase change materials enable efficient thermal energy storage and recovery, motivating investigation of their coupled flow and heat transfer behavior during charging. This study develops a theoretical and numerical model of an axisymmetric storage tank filled with paraffin and traversed by downward hot-water flow to analyze system dynamics under realistic conditions. Transient momentum and heat transport were solved using a finite element approach with separate energy equations for the heat transfer fluid and storage medium, and the model was validated against experimental data with good agreement. Simulations conducted at a mass flow rate of 0.2 L/min, inlet temperature of 90 °C, and porosity of 0.49 show that fluid velocity decreases markedly inside the porous region by 93.75% (compared to the clear region) due to pressure losses and viscous resistance, resulting in the successful maintenance of a sharp and highly stratified thermocline front along the bed. Thermal results indicate faster temperature diffusion in the fluid than in the phase change material, producing distinct thermocline thicknesses. Melting onset and duration vary along the tank height, occurring earlier near the inlet and later downstream. The local liquid fraction follows the temperature distribution of the storage medium, rising significantly at each level until it reaches 52%, after which it increases slowly by about 2% before the tank reaches full saturation ( = 100%) within a reasonable time. These findings clarify the interplay between flow resistance, heat transfer, and phase change, providing quantitative insight for design and optimization of packed bed thermal storage systems.