Investigating the Influence of Mutual Orientation Between Heat Flux and Gravity on the Melting Dynamics of Phase Change Material

The present work employs numerical simulations, using Rayleigh-Bénard (RB) convection as a platform, to investigate the influence of different orientations of the gravity vector and heat flux on the melting dynamics of a Phase Change Material (PCM). A paraffin-based PCM with Prandtl number Pr ≈ 71 and Stefan number Ste ≈ 0.33 is utilized to analyze the melting process over a wide range of Rayleigh numbers (102 ≤ Ra ≤ 107). The simulations are conducted in a differentially heated square enclosure, employing the control volume-based enthalpy porosity approach to examine the melting process at various angles between the gravity vector and the incoming heat flux. The results demonstrate that the mutual orientation of the incoming heat flux and gravity significantly affects features of convective motion in the liquid phase and the overall thermally driven heat transfer within the PCM. Specifically, when gravity and heat flux are oppositely directed, the melted fluid exhibits characteristics resembling RB convection, including flow instabilities, bifurcations, structured flow patterns, and chaotic flow behavior. However, a critical orientation angle exists above which melting occurs with a rotational convective motion of the liquid PCM, and the flow remains laminar throughout. In the case of co-directed gravity and heat flux, melting occurs solely through diffusion without the formation of convection currents. This work aims to explore potential changes in flow phenomena, establish criteria for the onset of instabilities, map changes in interface topology, elucidate complexities of nonlinear flow dynamics, and draw comparisons with classical RB systems. In summary, the present study provides comprehensive insights into the dynamics and stability criteria of a phase-change RB system across a broad range of Rayleigh numbers at different mutual orientations between heat flux direction and gravity vector.