Modelling the conditions for natural convection onset in open-cell porous Al/paraffin composite phase change materials: Effects of temperature, paraffin type and metallic structure geometry

Composite Phase Change Materials (PCMs) can be made combining a PCM, i.e., a material that is able store/release heat by its melting/solidification, and a low-amount of well distributed high-melting and high-thermal conductivity phase with the aim of improving the overall conductivity of the material and keeping its high heat storage capability. The composite made by a paraffin and a porous structure of aluminium (Al) has been considered as the representative of this material class. The design of these materials should not only take into account the melting temperature (Tm) and the volume fraction of the paraffin, but also the geometrical distribution and coarseness of the Al phase, which relate to the effective thermal conductivity of the composite as well as the occurrence of natural convection once the PCM is in the molten state. In the present paper, the inverse Body Centred Cubic (BCC) structure has been confirmed to be the most suitable to model high porosity Al foams. For their BCC modelled structure, an analytical equation is proposed for the evaluation of the overall thermal conductivity of the composite PCMs. Also, new best fit equations for predicting permeability of BBC structure are proposed. Analytical description is also given for the Rayleigh-Darcy number obtained as a product of material-dependent term (related to Tm and volume fraction of PCM) and the geometry dependent term (related to volume fraction of PCM, permeability as well as to material coarseness alternatively given in terms of pores per inch, pore size or unit cell length). The model has been validated by means of literature available experimental data. The proposed simplified model can further be adjusted to correlate the onset of natural convection through the local temperature gradient for the composite PCMs.