demand and decrease pollution. This thermal energy can be stored and delivered back when needed.Latent heat thermal energy storage (LHTES) has become interesting to store thermal energy with a high storage density in a small temperature range. However, the low charging and discharging power of these units is due to the poor thermal conductivity of the phase change material (PCM) [2]. Macro-encapsulation of PCM may enhance the thermal distribution in thermal storage units by increasing the heat transfer area between the heat transfer fluid (HTF) and PCM. Macro-encapsulated PCM are already installed in different applications like ventilated façade [3], thermal storage units [4] etc. But different experimental set-ups are required to investigate and improve the efficiency of a thermal storage system. Therefore numerical modelling and simulation is employed to improve the efficiency and to reduce experimental efforts. Different simplified approaches exist to analyse macro-encapsulated PCM but they are limited to specific applications and are also less accurate in predicting the performance of thermal storage units. Computational fluid dynamics (CFD) simulations allow a more detailed view on the melting within macro-capsules. Therefore, Asako et al. [5], Assis et al. [6] and Rösler [7] developed different CFD models to study the melting of a PCM with settling in a capsule. In general, modelling the moving inter-facial boundary in melting and solidification has been of greater importance in many industrial and research applications. The Eulerian fixed grid approach is most common to model moving boundary interfaces for a solid-liquid phase change. Crank [8] delivered an initial idea of modelling moving boundary problems by considering just the diffusion in a phase change problem. Later different researchers have developed efficient methods of computing phase change problems including the convection in the liquid phase [9][10][11]. Solid-liquid Abstract Thermal energy storage units using macroencapsulated PCM in industrial and residential applications are contemporary due to better efficiency during charging and discharging. This article focuses on numerical modelling of the melting process in a macro-encapsulated PCM. Accounting the non-linear enthalpy-temperature relation and ramping down the velocity in solid phase is therefore fundamental. In the present article the variable viscosity method is implemented to ramp down the solid velocity and allow settling of the solid phase. This complete numerical model of melting and settling of PCM in a capsule is implemented in OpenFOAM. The numerical results for different solid viscosities are validated with experiments and show good agreement. The influence of the solid viscosity value and the pressure-velocity convergence is studied. It is observed that the pressure-velocity convergence only plays a greater role in the case where the computation of the exact solid velocity is required.

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