Numerical Investigation of Melting in a Hemi‐Hexagonal Cavity Filled With Paraffin Wax: Effect of an Air Bubble at Different Locations on Heat Transfer and Energy Storage

ABSTRACT

Phase change materials (PCMs) have high thermal energy storage (TES) density but are limited by low thermal conductivity and slow melting rates, not allowing them to be practically used in a larger scope. This is a numerical experiment to examine how an air bubble at five different positions (no bubble, bottom, middle, right, and top) influences the melting process and heat transfer efficiency of paraffin wax (RT42) in a hemi‐hexagonal cell. The continuity, momentum, and energy equations are solved using a two‐dimensional, unsteady, laminar, incompressible flow model with an enthalpy‐porosity formulation (mushy zone constant C = 10 ⁵ ). The results indicate that the bottom bubble geometry has the largest melting fraction (around 0.35) at 10 min because of the improved natural convection and disruption of the boundary layers. Nevertheless, the best bubble case is most effective (melting fraction of approximately 0.85) by 30 min, since it provides free circulation of hot liquid PCM under the bubble without hindering the heated wall. The bubble‐free case has the slowest total melting (~0.65 at 30 min), which supports the idea that a strategically positioned air bubble can enhance phase change even though it has a low thermal conductivity. The central bubble location also gives the best homogeneous temperature distribution and has two recirculation regions, and the right bubble synthesizes asymmetric melting and a permanent cool region. Accordingly, the bubble position does not maximize all the melting phases; the bottom position maximizes early melting, the top position maximizes later melting, and the middle position provides a predictable intermediate behavior. The adaptive or multiple bubble strategies should be taken into account in future designs to ensure maximum energy storage efficiency during the entire melting period.