Abstract
This study explores numerically the solidification of a spherical composite phase change capsule (PCC) with high thermal conductivity, in which expanded graphite (EG) is added into the high-density polyethylene (HDPE)/paraffin shape-stabilized phase change material (SSPCM). The mathematical model is solved numerically by the apparent heat capacity method. In the simulations, the volume change of paraffin during phase transformation and the distribution of cavities arising from the manufacturing process are considered. As an important thermophysical property of the HDPE/paraffin/EG material, the effective thermal conductivity is determined based on fractal theory, the laws of minimal thermal resistance and specific equivalent thermal conductivity. The model is validated by comparison with similar available models and the agreement is found to be satisfactory. The influence of several significant parameters on the heat transfer process is analyzed, such as the content of EG, volume change of the PCM, etc. The results show that the volume change of paraffin has a great impact on the heat transfer process during the later stages. A certain amount of EG addition can effectively improve the heat transfer characteristics of the phase change capsule, including the moving rate of the phase interface and instantaneous release rate of heat flux.
Highlights
Of the various methods of heat storage, latent heat storage is the most attractive due to its high storage density
One way to alleviate these problems is to prepare a so-called composite shape-stabilized phase change capsule (SSPCC), in which a shape-stabilized phase change material (SSPCM) is firstly obtained by dispersing paraffin into high-density polyethylene (HDPE) acting as a supporting material, the above composites are encapsulated into granules by hydrophilic materials
The main assumptions used for the simulations are as follows:21 1) For smaller capsules, there is no convective heat transfer in the liquid phase of the phase change materials (PCMs); 2) The thermophysical properties of the PCM are constant in the solid and liquid zone but not the mushy zone; 3) In the phase change process, the volume change of paraffin has no mechanical effect on the capsule; 4) The cavities are well distributed as porosities in the capsule; 5) The PCM is in the liquid state initially; 6) The expanded graphite (EG) is distributed uniformly in the HDPE/paraffin SSPCM
Summary
Of the various methods of heat storage, latent heat storage is the most attractive due to its high storage density. Paraffin waxes are good phase change materials (PCMs) because of characteristics such as a high latent heat of fusion, negligible super-cooling, chemical inertness and stability They have been investigated for use in many building materials, including concrete and gypsum, etc. The analysis of phase change heat transfer problems, termed moving boundary problems in the literature, is especially complicated due to the non-linearity of the solid–liquid front This dynamic boundary problem is usually numerically solved by the enthalpy method or apparent heat capacity method.[8,9] N.Shamsundar and E.M.Sparrow[10] demonstrated the equivalence between the energy conversation equation applied in the three zones (solid, liquid, and solid/liquid) and the enthalpy model. The influence of several significant parameters on the heat transfer process are analyzed, such as the content of EG, volume change (or density change) of the PCM, cavity ratio, etc
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