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Numerical simulation on solidification heat transfer of spherical phase change capsule
Abstract
The present study explored numerically the process of solidification of spherical phase change capsule, which spherical high-density polyethylene (HDPE)/paraffin shape stabilized phase change material (PCM) encapsulated by Calcium alginate. The mathematical model was solved numerically by using apparent heat capacity method. In simulation, it described the pore microstructure by the fractal geometry after the paraffin in capsule was extracted, and the volume change of paraffin in phase transformation and the cavity caused by manufacturing technology were also considered. Based on the fractal characterization, as one of the important physical properties of the HDPE/paraffin, the effective thermal conductivity was determined. The results show that The initial solidification process is almost unaffected when the volume change of the PCM is considered. However the heat transfer rate begins to shift to a significantly slower with the increment of solid rate. The initial cavity ratio influences the rate of the phase change heat transfer and reduces the phase change latent heat of each unit volume of capsule. The larger the size of spherical capsule, the greater the impact of Ste number on the complete solidification time. The conclusion would have important guidance significance to how to improve heat storage capacity and heat storage rate of the PCM.
... show that the melting rate of phase change materials can be increased by increasing Stephen number and expanding the range of phase-change temperature. Dai et al. 19 applied the apparent heat capacity method to investigate the microcapsules. The results show that considering the volume change of the phase change material has little effect on the initial solidification process, but the heat transfer rate decreases obviously with the increase of the solid phase rate. ...
... 19, it can be seen that the melting time of microcapsule with MgCl 2 ·6H 2 O as the phase change material is only 0.05 s. Besides, it is clear that the microcapsules withFIGURE 16 Temperature distribution of microcapsules with different wall materials at different time [Colour figure can be viewed at wileyonlinelibrary.com]FIGURE 15 The melting time of microcapsules with different wall materials different phase change materials have different melting time. ...
Phase change microcapsules have a wide application in the heat storage system. The medium temperature heat storage systems such as medium temperature solar thermal plants, waste heat recovery systems and wind power absorption systems. In order to analyse the effects of configuration parameters and materials on phase change heat transfer process in a single medium temperature microcapsule, an enthalpy‐transforming model was applied to trace the location of the solid‐liquid interface and obtain the liquid fraction at different time in the melting process. Based on this model, the effects of particle size, the effects of wall thickness, the effects of wall materials and different medium temperature phase change materials were analyzed. The numerical results show that the larger particle size has a longer melting time, the melting time of 50 μm particle size and 250 μm particle size is 0.036 s and 2.48 s, respectively. In addition, the melting time of microcapsules with different wall thicknesses from the 1μm to 9μm is the same i.e., 0.14 s. Therefore, the wall thickness has little effect on the melting time of microcapsules. Besides, the microcapsule with the erythritol as inner material and the polystyrene as wall material has the longest melting time. Furthermore, the thermal conductivity of the wall materials is the main factor affecting the melting time. Moreover, the product of latent heat and density of phase change material is the main factor of the melting time.
... 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 PCM; 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 EG is distributed uniformly in the HDPE/paraffin SSPCM. ...
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.
In order to better improve thermoregulation and energy-saving effect of the integrated phase change building system, a heat transfer theoretical model of building envelopes with shape-stabilized phase change capsule (SSPCC) and their room is presented, in which the indoor temperature response is considered. The apparent heat capacity method is used to simulate the heat transfer progress of the phase change wall and variations of the indoor temperature under cyclical outdoor temperature variations. The calculation results show that the mass fraction of SSPCC is one of the significant factors that influence the degree of decay of the indoor air temperature. It is better to make the best phase temperature as close to the core temperature of the phase change wall layer as possible. Furthermore, the phase change temperature fluctuation range only affects the fluctuation range of the indoor air temperature, its effect on the equilibrium temperature is weak.
This article has been retracted at the request of the Editor-in-Chief and Author. Please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy).
Reason: This article duplicates significant parts of a paper that had already appeared in Energ. Convers. Manage., 47 (2006) 2515–2522, doi:10.1016/j.enconman.2005.10.031. One of the conditions of submission of a paper for publication is that authors declare explicitly that the paper is not under consideration for publication elsewhere. Re-use of any data should be appropriately cited. As such this article represents a severe abuse of the scientific publishing system. The scientific community takes a very strong view on this matter and we apologize to readers of the journal that this was not detected during the submission process.
The problem of heat conduction in fibrous composites is analyzed, using
a technique of local fractal dimensions to reduce the geometric
complexity of the relative fiber arrangement in the composite cross
section. A generalized unit cell is constructed based on the
fiber-volume fraction and local fractal dimensions along directions
parallel and transverse to the heat-flow direction, and an approximate
thermal model based on the analysis of the unit cell is derived. The
model is shown to be very effective in predicting the conductivities of
composites with both ordered and disordered arrangement of fibers.
This paper investigates the phase change behavior of 65 mol% capric acid and 35 mol% lauric acid, calcium chloride hexahydrate, n-octadecane, n-hexadecane, and n-eicosane inside spherical enclosures to identify a suitable heat storage material. Analytical models are developed for solidification and melting of sphere with conduction, natural convection, and heat generation. Both the models are validated with previous experimental studies. Good agreement was found between the analytical predictions and experimental study and the deviations were lesser than 20%. Heat flux release at the wall, cumulative energy release to the external fluid, are revealed for the best PCM. The influence of the size of encapsulation, initial temperature of the PCM, the external fluid temperature on solidified and molten mass fraction, and the total phase change time are also investigated.