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Research Papers: Melting and Solidification

Unconstrained Melting Heat Transfer of Nano-Enhanced Phase-Change Materials in a Spherical Capsule for Latent Heat Storage: Effects of the Capsule Size

[+] Author and Article Information
Nan Hu, Zi-Rui Li, Jing Tu

School of Energy Engineering,
Institute of Thermal Science and
Power Systems,
Zhejiang University,
Hangzhou 310027, China

Zi-Qin Zhu

School of Energy Engineering,
Institute of Thermal Science and Power Systems,
Zhejiang University,
Hangzhou 310027, China
e-mail: 11327015@zju.edu.cn

Li-Wu Fan

School of Energy Engineering,
Institute of Thermal Science and Power Systems,
Zhejiang University,
Hangzhou 310027, China;
State Key Laboratory of Clean Energy Utilization,
Zhejiang University,
Hangzhou 310027, China
e-mail: liwufan@zju.edu.cn

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received October 6, 2018; final manuscript received April 17, 2019; published online May 14, 2019. Assoc. Editor: Gennady Ziskind.

J. Heat Transfer 141(7), 072301 (May 14, 2019) (8 pages) Paper No: HT-18-1650; doi: 10.1115/1.4043621 History: Received October 06, 2018; Revised April 17, 2019

Toward accelerated latent heat storage, the unconstrained melting heat transfer in spherical capsules was revisited experimentally in the presence of nano-enhanced phase-change materials (NePCMs), with an emphasis on the influence of capsule size on the rates of melting, heat transfer, and latent heat storage. It was shown that increasing the size of the spherical capsule leads to two competing effects, i.e., thicker molten layer in the close-contact melting (CCM) region and stronger natural convection. However, the NePCM with a high loading (3 wt % graphite nanoplatelets (GNPs)) is not preferred for all capsule sizes as a result of the significantly deteriorated heat transfer in both CCM and natural convection, because the dramatic viscosity growth at such a high loading leads to increased thermal resistance across the molten layer and loss of natural convection that overweigh the increased thermal conductivity. The 1 wt % NePCM sample was exhibited to be able to facilitate latent heat storage for two cases, i.e., in the smallest capsule having a radius of 14.92 mm at a higher wall superheat of 30 °C and in the intermedium 24.85 mm capsule at a lower wall superheat of only 10 °C. It was suggested that a relatively low loading of a specific NePCM can cause a faster rate of latent heat storage over the baseline case of the matrix phase-change material (PCM), if the capsule size (and the wall superheat) can be adjusted properly to regulate the molten layer thickness and the intensity of natural convection.

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Figures

Grahic Jump Location
Fig. 1

(a) Experimental setup for unconstrained melting in a spherical capsule based on the volume expansion method, (b) photography and schematic diagram of the main test section, and (c) stainless steel spherical capsules with various sizes used in this work

Grahic Jump Location
Fig. 4

Measured and predicted transient variations of the total heat stored during unconstrained melting of the NePCM in various spherical capsules at the wall superheat of (a) 10 °C and (b) 30 °C

Grahic Jump Location
Fig. 5

Time-averaged heat flux over the entire course of unconstrained melting of the NePCM in various spherical capsules at the wall superheat of (a) 10 °C and (b) 30 °C

Grahic Jump Location
Fig. 2

Measured and predicted transient variations of the melt fraction during unconstrained melting of the NePCM in various spherical capsules at the wall superheat of (a) 10 °C and (b) 30 °C

Grahic Jump Location
Fig. 3

Measured and predicted transient variations of the heat transfer coefficient during unconstrained melting of the NePCM in various spherical capsules at the wall superheat of (a) 10 °C and (b) 30 °C

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