Effect of microstructure on melting in metal-foam/paraffin composite phase change materials

Abstract

The performance of phase change materials (PCM) as energy storage units are often limited by their low thermal conductivities that constrain the rates of melting or solidification. Highly porous metal foam composite PCMs are increasingly being used to abate this limitation and enable greater control over the thermal and phase change characteristics of the system. In the present study, a pore-scale computational analysis is carried out to characterize the performance of an n-eicosane-aluminium-foam composite PCM with a porosity of 0.94, over varying microstructural properties including strut, pore and cell sizes, and specific surface area. The simulations are carried out using OpenFOAM by employing the enthalpy-porosity formulation for modeling phase change during melting. The foam geometries are generated computationally using tools developed by the authors, published recently (Abishek et al., 2017, Ref. [1]). The statistics of the pore-scale structures of the virtual foam geometries and the numerical methodology employed for the modeling were validated against theoretical and empirical data from the literature. The simulations reveal that the presence of metal foam significantly enhances the melting rate as compared to pure PCM. It is also found that the melting rates are strongly correlated to the specific surface of the foam – highlighting a vital parameter that can be used to optimize the performance of the composite PCM for a given application. An empirical relationship correlating the dimensionless melt fraction with the Fourier number, Stefan number and dimensionless specific surface area is also presented for the range of parameters considered in this study.