Impact of convective flows in melting and thermal energy storage of Phase Change Materials*

Phase change materials (PCMs) use the latent heat of the solid/liquid phase change to store or release large amounts of thermal energy, allowing for compact thermal management and energy storage systems.

We  show the dynamic regimes occurring during the melting of an organic PCM  up to  Ra~10^9: (i)  base state, (ii)  plumes, (iii) plume coarsening, and (iv) turbulence.  Each one of these creates a distinctive melting front visible in experiments.  The transition to turbulence occurs through a secondary instability that forces the coarsening of the thermal plumes. By inclining the domain,   above a critical tilting only exists a laminar flow at the melted phase, irrespective of the Rayleigh number. The Nusselt, Reynolds numbers, and boundary layers follow power laws whose exponents agree with the high Rayleigh convection literature.  However, some striking differences appear as the stabilization of turbulent states further increasing the Rayleigh number.  

We show as well how thermocapillary enhances the heat transfer performance of  PCMs.  The interplay between conduction and convection-dominated regions in the liquid phase determines the melting dynamics.   Magnitudes as the position of the melting front, time to total melting, or stored energy follow power laws.