Comparison of Two CFD Approaches Using Constant and Temperature Dependent Heat Capacities during the Phase Transition in PCMs with Experimental and Analytical Results

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Modeling phase change materials (PCMs) has been a topic of research interest in the past, carried out experimentally and by means of computational fluid dynamics (CFD). The implemented solidification and melting (SM) model in Ansys Fluent-based on the enthalpy-porosity formulation is widely used in the literature. To the authors’ knowledge, few publications apply the apparent heat capacity (AHC) method in Ansys Fluent and even fewer have discussed both. The SM approach applies a linear relationship of the liquid fraction between solidus and liquidus temperature although it is known that the phase transition follows a non-linear behavior, which can be captured using the AHC method as a curve shape and location of the specific heat capacity containing information about the nature of phase transition behavior. Important factors in modeling are the temperature dependent thermophysical material properties density, viscosity, and thermal conductivity. They are often considered constant in the respective phase (solid or liquid) with a (linear) transition over the melting range. Temperature-dependent density is taken into account by using the Boussinesq approximation to model convective heat transfer. SM and AHC are compared to the analytical solution of the two-phase Stefan problem. As this does not include gravity and thus natural convection behavior, an additional comparison to two different PCMs, one from literature and a second data set gained in a new experiment is provided. The present work helps to evaluate the differences between the SM and AHC approach and to decide which is better suited for intended studies.