Numerical analysis of air layer effects on PCM melting in a horizontal double concentric tube

Latent heat thermal energy storage systems using phase change materials face key challenges, particularly the low thermal conductivity of phase change materials and the limited investigation of insulating effects like air layers within storage geometries. This study numerically examines the impact of air layers on the melting behavior of paraffin wax (RT42) inside a horizontal double concentric tube using a two-dimensional computational fluid dynamics model in ANSYS/FLUENT 16. Three configurations were analyzed: no air layer, a 1 mm air layer, and a 2 mm air layer surrounding the inner heated tube. The enthalpy-porosity method was applied to simulate the melting process, accounting for heat transfer by conduction and natural convection. Results show that introducing a 1 mm air gap extended the melting time by 28.57%, while a 2 mm air gap led to a 57.14% increase, primarily due to the added thermal resistance. This is the first study to quantitatively assess air layer effects in this geometry, offering new insights into optimizing phase change material-based storage systems by considering internal insulation phenomena.