Review on the temperature-dependent thermophysical properties of liquid paraffins and composite phase change materials with metallic porous structures

Pure or commercial paraffins are widely applied as phase change materials (PCMs) for specific thermal storage or thermal management devices. The proper design of these devices requires, in addition to the selection of a material with a target melting temperature, the knowledge of thermophysical properties of PCMs. This is also true for composite PCMs wherein the paraffins are combined to porous media having high melting temperature and high thermal conductivity to enhance the overall conductivity of the composite. The article reviews literature data on the composition, melting temperature, and temperature dependence of thermophysical properties (density, specific heat, thermal conductivity, viscosity) of pure or commercial paraffins above their melting temperature, ranging from 280 Kto 370 K. The specific focus is on the liquid phase of paraffins, rarely considered in articles related to composite PCMs, which is important for the onset of convective flow. Simple fitting models proposed in the literature for the temperature and composition dependence of the aforementioned properties are reviewed and evaluated on the basis of the available literature data. In most cases, improved fitting equations are proposed. Once the proper models for paraffin properties are established, the effective thermophysical properties of composite PCMs with porous structures in Al or Cu are calculated. Besides, for these materials, a literature review of available analytical models including the volume fraction of the paraffinic phase is carried out. The effectiveness of different models is obtained by comparing numerical results from direct simulation considering the metallic phase as a body-centered cubic lattice. The Progelhof model with a new fitting index predicts particularly good effective thermal conductivity data for paraffin-Al (or Cu) composite PCMs, with different volume fractions of its phases.