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Research Papers: Melting and Solidification

Experimental Characterization of Inward Freezing and Melting of Additive-Enhanced Phase-Change Materials Within Millimeter-Scale Cylindrical Enclosures

[+] Author and Article Information
Md Mahamudur Rahman

Mem. ASME
Department of Mechanical Engineering and
Mechanics,
Drexel University,
3141 Chestnut Street,
Philadelphia, PA 19104
e-mail: mr698@drexel.edu

Han Hu

Mem. ASME
Department of Mechanical Engineering and
Mechanics,
Drexel University,
3141 Chestnut Street,
Philadelphia, PA 19104
e-mail: hh398@drexel.edu

Hamidreza Shabgard

Mem. ASME
Department of Mechanical Engineering and
Mechanics,
Drexel University,
3141 Chestnut Street,
Philadelphia, PA 19104
e-mail: h.shabgard@drexel.edu

Philipp Boettcher

Department of Mechanical Engineering and
Mechanics,
Drexel University,
3141 Chestnut Street,
Philadelphia, PA 19104
e-mail: pab78@drexel.edu

Ying Sun

Mem. ASME
Department of Mechanical Engineering and
Mechanics,
Drexel University,
3141 Chestnut Street,
Philadelphia, PA 19104
e-mail: ysun@coe.drexel.edu

Matthew McCarthy

Mem. ASME
Department of Mechanical Engineering and
Mechanics,
Drexel University,
3141 Chestnut Street,
Philadelphia, PA 19104
e-mail: mccarthy@coe.drexel.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received August 3, 2015; final manuscript received March 9, 2016; published online April 19, 2016. Assoc. Editor: Gennady Ziskind.

J. Heat Transfer 138(7), 072301 (Apr 19, 2016) (13 pages) Paper No: HT-15-1515; doi: 10.1115/1.4033007 History: Received August 03, 2015; Revised March 09, 2016

The inward melting and solidification of phase-change materials (PCM) within millimeter-scale cylindrical enclosures have been experimentally characterized in this work. The effects of cylinder size, thermal loading, and concentration of high-conductivity additives were investigated under constant temperature boundary conditions. Using a custom-built apparatus with fast response, freezing and melting have been measured for time periods as short as 15 s and 33 s, respectively. The enhancement of PCM thermal conductivity using exfoliated graphene nanoplatelets (xGnPs) has also been measured, showing a greater than 3× increase for a concentration of 6 wt.%. Reductions in the total melting and freezing times of up to 66% and 55%, respectively, have been achieved using xGnP concentrations of only 4.5 wt.%. It is shown that the phase-change dynamics of pure and enhanced PCM are well predicted using a simple conduction-only model, demonstrating the validity of approximating enhanced PCM with low additive loadings as homogenous materials with isotropic properties. While general consistency between the measurements and model is seen, the effect of additives on heat transfer rate during the initial stages of freezing and melting is lower than expected, particularly for the smaller cylinder sizes of 6 mm. These results suggest that the thermal resistance of the PCM is not the limiting factor dictating the speed of the solid–liquid interface during these initial stages.

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References

Figures

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Fig. 1

Experimental apparatus used to characterize the inward freezing and melting dynamics of PCM within cylindrical enclosures, showing (a) a schematic of the test setup and (b) a photograph of an instrumented assembly, sealed in various places using room temperature vulcanization (RTV) silicone, with a PCM cylinder diameter of D = 14 mm

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Fig. 2

Experimental characterization of melting and freezing dynamics under constant wall temperature conditions, showing (a) the wall and PCM center temperatures as well as (b) the measured change of air pressure, and the resulting liquid volume of the PCM, as a function of time. The melting (interface) temperature, Ti, as well as superheating, ΔTsup, and subcooling, ΔTsub, values are labeled.

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Fig. 3

Thermal conductivity enhancement of n-eicosane with xGnP, showing (a) thermal conductivity as a function of xGnP concentration and isothermal PCM temperature and (b) enhancement factor relative to n-eicosane as a function of xGnP concentration, compared against the theoretical predictions from Nan et al. [41] using the thermal interface resistance between graphite nanoplatelet and paraffin as Rk ≈ 0.9 × 10−7 m2 K/W [32] and measurements from Warzoha and Fleischer [31]. In (b), the solid n-eicosane conductivity is measured at 23 °C and the liquid n-eicosane conductivity is taken as the average of the values at 40 °C and 55 °C, as shown in (a). The representative experimental uncertainties are estimated from the standard deviation of six conductivity tests.

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Fig. 4

Inward freezing and melting within a cylindrical geometry

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Fig. 5

Phase-change dynamics of pure n-eicosane under constant temperature boundary conditions, showing (a) the solid volume during freezing and the liquid volume during melting within a 6 mm diameter cylinder and (b) the solid volume during freezing and the liquid volume during melting within a 14 mm diameter cylinder. The representative experimental uncertainties of (a) 2.8 × 10−8 m3 and (b) 2.6 × 10−7 m3 are estimated from the propagation of measurement errors.

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Fig. 6

The effects of exfoliated graphene nanoplatelets additives (xGnP) on n-eicosane phase-change dynamics, showing solid volume during inward freezing within a (a) 6 mm and (b) 14 mm cylinder, as well as liquid volume during inward melting within a (c) 6 mm and (d) 14 mm cylinder

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Fig. 7

Nondimensional freezing and melting volume (ϕ) as a FoSte for all of the experiments conducted in this work. (a) The dynamic volume change of pure n-eicosane as compared to the analytical prediction, as well as the results of Larson and Sparrow [8]. (b) The dynamic volume change of additive-enhanced n-eicosane for various concentrations of xGnP, showing general consistency with the conduction-only model evaluated using the measured thermophysical properties of enhanced PCM in Table 1.

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Fig. 8

Effect of latent and sensible heat on total heat transfer rate during the freezing of 6 mm diameter pure n-eicosane cylinders under different superheating and subcooling conditions

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Fig. 9

Total heat transfer rate during the initial stages of (a) freezing and (b) melting within a 6 mm cylinder at varying thermal loadings, showing the effect of xGnP concentration to be minimal

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Fig. 10

Nusselt number as a FoSte for all of the experiments conducted in this work, showing both freezing and melting data for (a) 6 mm and (b) 14 mm diameter cylinders compared against the modeling predictions neglecting the effects of natural convection and sensible energy

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