Solidification of Phase Change Materials Infiltrated in Porous Media in Presence of Voids

2014 ◽  
Vol 136 (11) ◽  
Author(s):  
Mahmoud Moeini Sedeh ◽  
J. M. Khodadadi
Keyword(s):  
Phase Change ◽  
Porous Structure ◽  
Phase Change Materials ◽  
Solidification Process ◽  
Volume Shrinkage ◽  
Freezing Process ◽  
Infiltration Process ◽  
Noticeable Increase ◽  
Melting Characteristics ◽  
Micron Size

Infiltration of phase change materials (PCM) into highly conductive porous structures effectively enhances the thermal conductivity and phase change (solidification and melting) characteristics of the resulting thermal energy storage (TES) composites. However, the infiltration process contributes to formation of voids as micron-size air bubbles within the pores of the porous structure. The presence of voids negatively affects the thermal and phase change performance of TES composites due to the thermophysical properties of air in comparison with PCM and porous structure. This paper investigates the effect of voids on solidification of PCM, infiltrated into the pores of graphite foam as a highly conductive porous medium with interconnected pores. A combination of the volume-of-fluid (VOF) and enthalpy-porosity methods was employed for numerical investigation of solidification. The proposed method takes into account the variation of density with temperature during phase change and is able to predict the volume shrinkage (volume contraction) during the solidification of liquids. Furthermore, the presence of void and the temperature gradient along the liquid–gas interface (the interface between void and PCM) can trigger thermocapillary effects. Thus, Marangoni convection was included during the solidification process and its importance was elucidated by comparing the results among cases with and without thermocapillary effects. The results indicated that the presence of voids within the pores causes a noticeable increase in solidification time, with a sharper increase for cases without thermocapillary convection. For verification purposes, the amount of volume shrinkage during the solidification obtained from numerical simulations was compared against the theoretical volume change due to the variation of density for several liquids with contraction and expansion during the freezing process. The two sets of results exhibited good agreement.

Effect of Voids on Solidification of Phase Change Materials Infiltrated in Graphite Foams

2012 ◽  
Author(s):  
Mahmoud Moeini Sedeh ◽  
J. M. Khodadadi
Keyword(s):  
Energy Storage ◽  
Phase Change ◽  
Porous Structure ◽  
Phase Change Materials ◽  
Solidification Rate ◽  
Fluid Phase ◽  
Void Space ◽  
Infiltration Process ◽  
Change Behavior ◽  
Air Pockets

As a fundamental process during production of composite thermal energy storage systems, infiltration of phase change materials (PCM) leads to formation of voids (air pockets) inside the pores of graphite foams. The presence of voids inside graphite cells (i.e. the presence of air pockets next to the conductive walls of the porous structure) markedly affects the thermal and phase change behavior of the composite. Therefore, it is vitally important to investigate the effect of voids on phase change behavior of latent heat energy storage composites. In complementing recent work devoted to modeling of the infiltration of PCM into graphite foams, a numerical approach was employed to study the solidification of PCM infiltrated into a graphite pore in the presence of a void. For this purpose, a two-dimensional model of the porous structure was developed based on the typical geometrical features of the pores. Grid independence study was performed on different unstructured grid systems. Since more than one fluid phase is present in this problem (PCM being the liquid phase and air pocket or void as the gas phase), the volume-of-fluid (VOF) method was utilized for investigation of solidification problem and tracking the interface. Considering various forces operating at the scale of the pore (i.e. 500 microns in diameter), this problem is under the influence of surface tension, gravity, and pressure gradient. The simulation was transient and continued until the entire liquid PCM inside the pore freezes. The volume of final void space will represent a combination of infiltration and shrinkage voids. Results of the simulation indicate the presence of 9.8% void (from the infiltration process) that can greatly alter the solidification rate of the PCM inside the pore. It is concluded that formation of shrinkage void during solidification can be predicted using this multi-phase model. For verification purposes, the volume of the predicted infiltration void was compared to reported experimental measurements and the volume of shrinkage void was compared to theoretical volume change. Good agreements were found in both cases.

Phase Change Process Inside a Small-radii Cylinder Subjected to Cyclic Convective Boundary Conditions: a Numerical Study

2021 ◽  
Author(s):  
Yousef Kanani ◽  
Avijit Karmakar ◽  
Sumanta Acharya
Keyword(s):  
Heat Transfer ◽  
Surface Temperature ◽  
Phase Change ◽  
Phase Change Materials ◽  
Numerical Study ◽  
Convective Boundary Condition ◽  
Freezing Process ◽  
Fourier Number ◽  
Two Phase ◽  
Convective Boundary

Abstract We numerically investigate the melting and solidi?cation behavior of phase change materials encapsulated in a small-radii cylinder subjected to a cyclic convective boundary condition (square wave). Initially, we explore the effect of the Stefan and Biot numbers on the non-dimensionalized time required (i.e. reference Fourier number Tref ) for a PCM initially held at Tcold to melt and reach the cross?ow temperature Thot. The increase in either Stefan or Biot number decreases Tref and can be predicted accurately using a correlation developed in this work. The variations of the PCM melt fraction, surface temperature, and heat transfer rate as a function of Fourier number are reported and analyzed for the above process. We further study the effect of the cyclic Fourier number on the periodic melting and freezing process. The melting or freezing front initiates at the outer periphery of the PCM and propagates towards the center. At higher frequencies, multiple two-phase interfaces are generated (propagating inward), and higher overall heat transfer is achieved as the surface temperature oscillates in the vicinity of the melting temperature, which increases the effective temperature difference driving the convective heat transfer.

Preparation and Thermal Propertites of Polyethylene Glycol/ Silica Hollow Nanospheres Composite as Shape-Stabilized Phase Change Materials

2013 ◽  
Vol 773 ◽  
pp. 534-537 ◽  
Author(s):  
Li Li Feng ◽  
Jing Jing Tong ◽  
Chong Yun Wang
Keyword(s):  
Thermal Properties ◽  
Polyethylene Glycol ◽  
Phase Change ◽  
Porous Structure ◽  
Phase Change Materials ◽  
Hollow Nanospheres ◽  
Lower Phase ◽  
Phase Change Temperature ◽  
Change Temperature ◽  
Change Material

Shape-stabilized phase change material (PCM) composed of polyethylene glycol and silica hollow nanospheres was prepared by a vacuum impregnating method. Thermal properties of the composite PCM were investigated by various techniques. Lower phase change temperature and enthalpy of the composite PCM were observed. It is concluded that the phase change properties of the composite PCM are influenced by the adsorption confinement of the PEG segments from the porous structure of the silica hollow nanospheres.

Detailed Numerical Modeling of a Hybrid PCM-Air Heat Sink

2013 ◽  
Author(s):  
Y. Kozak ◽  
G. Ziskind
Keyword(s):  
Numerical Modeling ◽  
Phase Change ◽  
Heat Sink ◽  
Phase Change Materials ◽  
Void Formation ◽  
Solidification Process ◽  
Melting Process ◽  
Significant Rise ◽  
Air Interface ◽  
Vof Model

The ability of phase-change materials (PCMs) to absorb large amounts of heat without significant rise of their temperature during the melting process may be utilized in thermal energy storage and passive thermal management. This paper deals with numerical modeling of a hybrid PCM-air heat sink, in which heat may be either absorbed by the PCM stored in compartments with conducting walls, or dissipated to the air using fins, or both. Under the assumptions of perfect insulation (except for the air fins), identity and symmetry between all PCM channels, and negligible 3-D boundary effects, a 2-D model of the problem for half a PCM compartment of the heat sink is solved, saving calculation time and yet taking into account the essential physical phenomena. A commercial program, ANSYS Fluent, is used in order to solve the governing conservation equations. Phase-change is solved using the enthalpy-porosity method. PCM-air interface is modeled using the volume-of-fluid (VOF) approach. The model takes into account natural convection in the liquid PCM and air, volume change, phase- and temperature-dependence of thermal properties, and PCM-air interface interaction. Various scenarios for the hybrid heat sink operation are simulated and compared. The difference in the melting patterns is analyzed for the cases of heating with and without the fan operating. The solidification process with the fan operating is also simulated. It is shown that the VOF model enables simulating realistic void formation in the solidification process.

Melting of Phase Change Materials With Volume Change in Metal Foams

2010 ◽  
Vol 132 (6) ◽  
Author(s):  
Zhen Yang ◽  
Suresh V. Garimella
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Phase Change ◽  
Volume Change ◽  
Phase Change Materials ◽  
Metal Foam ◽  
Melting Process ◽  
Metal Foams ◽  
Volume Shrinkage ◽  
Two Temperature

Melting of phase change materials (PCMs) embedded in metal foams is investigated. The two-temperature model developed accounts for volume change in the PCM upon melting. Volume-averaged mass and momentum equations are solved, with the Brinkman–Forchheimer extension to Darcy’s law employed to model the porous-medium resistance. Local thermal equilibrium does not hold due to the large difference in thermal diffusivity between the metal foam and the PCM. Therefore, a two-temperature approach is adopted, with the heat transfer between the metal foam and the PCM being coupled by means of an interstitial Nusselt number. The enthalpy method is applied to account for phase change. The governing equations are solved using a finite-volume approach. Effects of volume shrinkage/expansion are considered for different interstitial heat transfer rates between the foam and PCM. The detailed behavior of the melting region as a function of buoyancy-driven convection and interstitial Nusselt number is analyzed. For strong interstitial heat transfer, the melting region is significantly reduced in extent and the melting process is greatly enhanced as is heat transfer from the wall; the converse applies for weak interstitial heat transfer. The melting process at a low interstitial Nusselt number is significantly influenced by melt convection, while the behavior is dominated by conduction at high interstitial Nusselt numbers. Volume shrinkage/expansion due to phase change induces an added flow, which affects the PCM melting rate.

Numerical Simulation of the Effect of the Size of Nanoparticles on the Solidification Process of Nanoparticle-Enhanced Phase Change Materials

2012 ◽  
Author(s):  
Yousef M. F. El Hasadi ◽  
J. M. Khodadadi
Keyword(s):  
Particle Size ◽  
Phase Change ◽  
Phase Change Materials ◽  
Concentration Field ◽  
Solidification Process ◽  
Liquid Interface ◽  
Constitutional Supercooling ◽  
Fluid Mixture ◽  
Solid Liquid Interface ◽  
Solid Liquid

Nanoparticle-enhanced phase change materials (NEPCM) were proposed recently as alternatives to conventional phase change materials due to their enhanced thermophysical properties. In this study, the effect of the size of the nanoparticles on the morphology of the solid-liquid interface and evolving concentration field, during solidification had been reported. The numerical method that was used is based on the one-fluid-mixture model. The model takes into account the thermal as well as the solutal convection effects. A square cavity model was used in the simulation. The NEPCM that was composed of a suspension of copper nanoparticles in water was solidified from the bottom. The nanoparticles size used were 5 nm and 2 nm. The temperature difference between the hot and cold sides was 5 degrees centigrade and the loading of the nanoparticles that have been used in the simulation was 10% by mass. The results obtained from the model were compared with those existing in the literature, and the comparison was satisfactory. The solid-liquid interface for the case of NEPCM with 5 nm particle size was almost planar throughout the solidification process. However, for the case of the NEPCM with particle size of 2 nm, the solid-liquid interface evolved from a planar stable shape to an unstable dendritic shape, as the solidification process proceeded with time. This was attributed to the constitutional supercooling effect. It has been observed that the constitutional supercooling effect is more pronounced as the particle size decreases. Furthermore, the freezing time increases as the particle size decreases.

Effect of Marangoni Convection on Solidification of Phase Change Materials Infiltrated in Porous Media in Presence of Voids

2013 ◽  
Author(s):  
Mahmoud Moeini Sedeh ◽  
J. M. Khodadadi
Keyword(s):  
Surface Tension ◽  
Porous Media ◽  
Temperature Gradient ◽  
Phase Change ◽  
Porous Structure ◽  
Phase Change Materials ◽  
Marangoni Convection ◽  
Solidification Time ◽  
Interfacial Forces ◽  
Pore Level

Void formation is encountered in the form of air pockets during preparation of composite thermal energy storage systems, consisting of phase change materials (PCM) infiltrated into a high-conductivity porous structure. The presence of voids within the pores of a porous structure degrades the thermal and phase change behavior of such composites. Recent work devoted to multiphase modeling of the infiltration of PCM in liquid state into porous media and formation of voids showed that among the various contributing driving forces (i.e. gravity, pressure gradient and interfacial forces), the interfacial forces (resulting from surface tension and contact angle) play a significant role at the pore level. Additionally, modeling the solidification and melting of PCM within the pores in presence of a void revealed that there is a temperature gradient along the interface between the PCM and void. Considering the surface tension as the major driving force at the pore level, this temperature gradient is large enough to give rise to a gradient in surface tension that then triggers the Marangoni convection at the interface. Thus, as a convection mechanism, it affects the phase change process as well as the interface shape. Therefore, in this paper, the effects of the Marangoni convection on PCM solidification time and shape of the interface was investigated at the pore level. A numerical approach was employed for solidification of a PCM based on the combination of the Volume-of-fluid (VOF) and enthalpy-porosity methods, including the variation of the surface tension with temperature, i.e. Marangoni effects. A two-dimensional model of a pore was developed based on the average geometric features of the pores in a porous structure with interconnecting pores. Following the grid independence study, the transient simulation of solidification was performed, whereas the PCM within the pore and the air within the void were treated as incompressible liquid and compressible gas, respectively. The liquid density change during the solidification was included to explicate the formation of shrinkage void and its distribution within the pores. The PCM solidification time and shape of the final interface between the PCM and air pocket (representing the amount and distribution of the shrinkage void evolving during the solidification) were extracted and compared between the cases with and without Marangoni convection. For verification purposes, the volume of the predicted infiltration void is in agreement with experimental measurements and the volume of the shrinkage void shows a good agreement with theoretical volume change. The final shape of the interface was justified and turned out to be in agreement with the prevailing Marangoni convection pattern.

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