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Research Papers: Porous Media

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

[+] Author and Article Information
Mahmoud Moeini Sedeh

Department of Mechanical Engineering,
Auburn University,
1418 Wiggins Hall,
Auburn, AL 36849-5341
e-mail: moeini@auburn.edu

J. M. Khodadadi

Alumni Professor
Mem. ASME
Department of Mechanical Engineering,
Auburn University,
1418 Wiggins Hall,
Auburn, AL 36849-5341
e-mail: khodajm@auburn.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received December 24, 2013; final manuscript received August 18, 2014; published online September 16, 2014. Assoc. Editor: Sujoy Kumar Saha.

J. Heat Transfer 136(11), 112603 (Sep 16, 2014) (9 pages) Paper No: HT-13-1664; doi: 10.1115/1.4028354 History: Received December 24, 2013; Revised August 18, 2014

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.

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References

Figures

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Fig. 2

(a) Grid-independence study (spatial) and (b) detailed view of the grid system

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Fig. 1

Scanning electron microscope (SEM) image of the graphite foam (PocoFoam) showing its microstructure at (a) 100×, (b) 200× magnifications, and (c) the developed 2D model of the pore and its features

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Fig. 3

Evolving VOF volume fraction distribution (λ) during the infiltration of cyclohexane into the pore at time instants of (a) 0.5, (b) 1.2, (c) 1.56, (d) 1.75, (e) 2.1, (f) 2.5, (g) 3 ms, and (h) the final state of infiltration with 9.8% void content (evaluated based on area), selected as the initial condition for solidification [35]

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Fig. 4

Variations of density with temperature for cyclohexane as a typical hydrocarbon-based PCM

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Fig. 5

Time-evolving contours of liquid fraction during the solidification of cyclohexane in the absence of thermocapillary effects (i.e., constant surface tension of 0.025 N/m) at time instants of (a) 5, (b) 25, (c) 50, (d) 100, (e) 150, (f) 250, (g) 350, and (h) 450 ms

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Fig. 6

Time rate of solidification of cyclohexane (typical PCM) within the pore

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Fig. 7

Time-evolving contours of liquid fraction during the solidification of cyclohexane in the presence of thermocapillary effects (i.e., surface tension is a function of temperature) at time instants of (a) 5, (b) 25, (c) 50, (d) 100, (e) 150, (f) 250, (g) 350, and (h) 450 ms

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Fig. 8

The convection pattern within the pore at time instant of 25 ms for cases of (a) excluding and (b) including thermocapillary effects in solidification (arrows indicate velocity vectors in PCM)

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