Numerical modelling of melting and settling of an encapsulated PCM using variable viscosity

Abstract

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.

Spatially varying heat flux driven close-contact melting – A Lagrangian approach

“…Correspondingly, A has the dimension m 2 for the cylindrical heat source (n = 1) or m for the rectangular heat source (n = 0). Nondimensionalizing the contact pressure (34) according to (14), and multiplying with a reference areaà = [n (π − 2) + 2] yields the non-dimensional contact forceF…”

“…Due to its own weight the PCM tends to sink down to the bottom wall at which close-contact melting can be observed. This can be exploited to increase the rate of heat transfer into the storage and hence to optimize the charging time [14,10]. Other applications for close-contact melting are hot wire cutting manufacturing technology [18], nuclear technology [7,6], planetary protection [17] as well as robotic ice exploration technologies based on melting probes for polar research and solar system research, e.g.…”

Solid-Liquid Phase Change in the Presence of Gas Phase: Numerical Modeling and Validation

“…Miehe [5] implemented and compared the viscosity and Darcy model to investigate the twin-roll casting process of the magnesium alloy. Kasibhatla et al [6] adopted the variable viscosity model to study the melting and settling of PCM in a capsule. More recently König-Haagen et al [7] performed a comprehensive benchmarking of fixed-grid methods and presented a qualitative and quantitative analysis of most common energy formulations for the modeling of melting.…”

Improving the performance of finned latent heat thermal storage devices using a Cartesian grid solver and machine-learning optimization techniques

“…Since specific heat is also held constant in these simulations, thermal diffusivity ∝ = k/(ρCp) varies predominately with thermal conductivity k (which is variable in the simulations) in both the physical experiments and the simulations. Second, while recent work that has shown that so-called close contact melting created by sinking solids within a PCM can lead to increases in melting rates [100,[121][122][123], the effects of close-contact melting are likely to be limited in this work. This is because the experiments depend on the use of 7-8 TCs within the salts, and these TCs would severely restrict the movement of a free-floating solid.…”

“…This might affect the physical results both in the expansion and contraction of the overall volume of the salts within the chamber and also in the possibility that solid salts might become unattached to the side of the LHTSD and sink within the liquid. The latter concern is particularly notable considering recent work that has shown that so-called close contact melting created by sinking solids within a PCM can lead to large increases in melting rates [100,[121][122][123].…”