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.

Surface Deformation and Thermal Convection in Electrostatically-Positioned Droplets Under Microgravity

1999 ◽  
Author(s):  
Suping Song ◽  
Ben Q. Li
Keyword(s):  
Surface Tension ◽  
Temperature Gradient ◽  
Laser Heating ◽  
Surface Deformation ◽  
Marangoni Convection ◽  
Stokes Equations ◽  
Energy Balance Equation ◽  
Droplet Surface ◽  
Fundamental Study ◽  
Recirculating Flow

Abstract Electrostatically positioned droplets are very useful for the fundamental study of solidification phenomena and the measurement of thermal physical properties. This paper descries a numerical analysis of surface deformation and surface tension driven flows in electrostatically positioned droplets in microgravity. The analysis is based on a fully coupled boundary element and finite element solution of the Maxwell equations, the Navier-Stokes equations and the energy balance equation. Results show that an applied electrostatic field results in a nonuniform electric stress distribution along the droplet surface, which, combined with surface tension, causes the droplet to deform into an ellipsoidal shape in microgravity. Laser heating induces a non-uniform temperature distribution in the droplet, which in turn produces Marangoni convection in the droplet. It is found that the viscous stress contribution to the deformation is small for a majority of cases. Also, a higher temperature gradient produces a stronger Marangoni convection in droplets with higher melting points that require more laser power. The internal recirculating flow may be reduced by more uniform laser heating. During the undercooling of the droplet, both temperature and fluid flow fields evolve in time such that the temperature gradient and the tangential velocities along the droplet surface subside in magnitude and reverse their directions.

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.

scholarly journals Integral Solution of Two-Region Solid–Liquid Phase Change in Annular Geometries and Application to Phase Change Materials–Air Heat Exchangers

Energies ◽  
2019 ◽  
Vol 12 (23) ◽  
pp. 4474 ◽  
Author(s):  
Hamidreza Shabgard ◽  
Weiwei Zhu ◽  
Amir Faghri
Keyword(s):  
Liquid Phase ◽  
Boundary Conditions ◽  
Phase Change ◽  
Phase Change Materials ◽  
Volume Fraction ◽  
Control Volume ◽  
Solid Volume Fraction ◽  
Solidification Time ◽  
Design And Optimization ◽  
Solid Liquid

A mathematical model based on the integral method is developed to solve the problem of conduction-controlled solid–liquid phase change in annular geometries with temperature gradients in both phases. The inner and outer boundaries of the annulus were subject to convective, constant temperature or adiabatic boundary conditions. The developed model was validated by comparison with control volume-based computational results using the temperature-transforming phase change model, and an excellent agreement was achieved. The model was used to conduct parametric studies on the effect of annuli geometry, thermophysical properties of the phase change materials (PCM), and thermal boundary conditions on the dynamics of phase change. For an initially liquid PCM, it was found that increasing the radii ratio increased the total solidification time. Also, increasing the Biot number at the cooled (heated) boundary and Stefan number of the solid (liquid) PCM, decreased (increased) the solidification time and resulted in a greater (smaller) solid volume fraction at steady state. The application of the developed method was demonstrated by design and analysis of a PCM–air heat exchanger for HVAC systems. The model can also be easily employed for design and optimization of annular PCM systems for all associated applications in a fraction of time needed for computational simulations.

scholarly journals Thermal and Physical Characterization of PEG Phase Change Materials Enhanced by Carbon-Based Nanoparticles

Nanomaterials ◽  
2020 ◽  
Vol 10 (6) ◽  
pp. 1168 ◽  
Author(s):  
David Cabaleiro ◽  
Samah Hamze ◽  
Jacek Fal ◽  
Marco A. Marcos ◽  
Patrice Estellé ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Surface Tension ◽  
Carbon Black ◽  
Phase Change ◽  
Phase Change Materials ◽  
Physical Characterization ◽  
Plastic Behavior ◽  
Diamond Nanopowders ◽  
Carbon Based

This paper presents the preparation and thermal/physical characterization of phase change materials (PCMs) based on poly(ethylene glycol) 400 g·mol−1 and nano-enhanced by either carbon black (CB), a raw graphite/diamond nanomixture (G/D-r), a purified graphite/diamond nanomixture (G/D-p) or nano-Diamond nanopowders with purity grades of 87% or 97% (nD87 and nD97, respectively). Differential scanning calorimetry and oscillatory rheology experiments were used to provide an insight into the thermal and mechanical changes taking place during solid-liquid phase transitions of the carbon-based suspensions. PEG400-based samples loaded with 1.0 wt.% of raw graphite/diamond nanomixture (G/D-r) exhibited the lowest sub-cooling effect (with a reduction of ~2 K regarding neat PEG400). The influences that the type of carbon-based nanoadditive and nanoparticle loading (0.50 and 1.0 wt.%) have on dynamic viscosity, thermal conductivity, density and surface tension were also investigated in the temperature range from 288 to 318 K. Non-linear rheological experiments showed that all dispersions exhibited a non-Newtonian pseudo-plastic behavior, which was more noticeable in the case of carbon black nanofluids at low shear rates. The highest enhancements in thermal conductivity were observed for graphite/diamond nanomixtures (3.3–3.6%), while nano-diamond suspensions showed the largest modifications in density (0.64–0.66%). Reductions in surface tension were measured for the two nano-diamond nanopowders (nD87 and nD97), while slight increases (within experimental uncertainties) were observed for dispersions prepared using the other three carbon-based nanopowders. Finally, a good agreement was observed between the experimental surface tension measurements performed using a Du Noüy ring tensiometer and a drop-shape analyzer.

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.

scholarly journals Temperature Control in (Translucent) Phase Change Materials Applied in Facades: A Numerical Study

Energies ◽  
2019 ◽  
Vol 12 (17) ◽  
pp. 3286 ◽  
Author(s):  
Tenpierik ◽  
Wattez ◽  
Turrin ◽  
Cosmatu ◽  
Tsafou
Keyword(s):  
Temperature Gradient ◽  
Phase Change ◽  
Specific Heat ◽  
Melting Temperature ◽  
Phase Change Materials ◽  
Numerical Study ◽  
Melting Process ◽  
Large Temperature ◽  
Energy Efficient Buildings ◽  
High Solar Radiation

Phase change materials (PCMs) are materials that can store large amounts of heat during their phase transition from solid to liquid without a significant increase in temperature. While going from liquid to solid this heat is again released. As such, these materials can play an important role in future energy-efficient buildings. If applied in facades as part of a thermal buffer strategy, e.g., capturing and temporarily storing solar energy in so-called Trombe walls, the PCMs are exposed to high solar radiation intensities, which may easily lead to issues of overheating. This paper therefore investigates the melting process of PCM and arrives at potential solutions for countering this overheating phenomenon. This study uses the simulation program Comsol to investigate the heat transfer through, melting of and fluid flow inside a block of PCM (3 × 20 cm2) with a melting temperature of around 25 °C. The density, specific heat and dynamic viscosity of the PCM are modeled as a temperature dependent variable. The latent heat of the PCM is modeled as part of the specific heat. One side of the block of PCM is exposed to a heat flux of 300 W/m2. The simulations show that once part of the PCM has melted convection arises transporting heat from the bottom of the block to its top. As a result, the top heats up faster than the bottom speeding up the melting process there. Furthermore, in high columns of PCM a large temperature gradient may arise due to this phenomenon. Segmenting a large volume of PCM into smaller volumes in height limits this convection thereby reducing the temperature gradient along the height of the block. Moreover, using PCMs with different melting temperature along the height of a block of PCM allows for controlling the speed with which a certain part of the PCM block starts melting. Segmenting the block of PCM using PCMs with different melting temperature along its height was found to give the most promising results for minimizing this overheating effect. Selecting the optimal phase change temperatures however is critical in that case.

Experimental Analysis of the Heat Transfer Induced by Thermocapillary Convection Around a Bubble

1999 ◽  
Vol 122 (1) ◽  
pp. 66-73 ◽  
Author(s):  
P. Arlabosse ◽  
L. Tadrist ◽  
H. Tadrist ◽  
J. Pantaloni
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Heat Flux ◽  
Temperature Gradient ◽  
Marangoni Convection ◽  
Heated Wall ◽  
Meridian Plane ◽  
Oil Viscosity ◽  
Dimensionless Numbers ◽  
Wide Range

The surface tension driven flow in the liquid vicinity of gas bubbles on a heated wall and its contribution to the heat transfer are investigated experimentally in a configuration where surface tension force and buoyancy forces oppose one another. This liquid flow caused by the temperature gradient along the interface is called thermocapillary or thermal Marangoni convection. The studies were made with silicone oils of different viscosities so that a wide range of dimensionless numbers were encountered. The velocity fields are determined from the motion of carbon particles in the meridian plane of the bubble. The influence of the temperature gradient, the oil viscosity, and the bubble shape on the profiles along the interface and in the direction normal to the interface is analyzed. The temperature field is determined by holographic interferometry. For the axisymmetric problem, the interferograms are evaluated by solving the Abel-integral equation. From the isotherms, the temperature distribution along the bubble surface and in the liquid beneath the bubble is measured. To quantify the contribution of thermocapillarity to the heat transfer, the heat flux transferred by thermocapillarity is measured. A heat exchange law giving the increase in heat flux due to Marangoni convection in comparison to the conductive regime is proposed. [S0022-1481(00)70501-9]

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