Date of Award

6-28-2013

Document Type

Thesis

Publisher

Santa Clara : Santa Clara University, 2013.

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Dr. Hohyun Lee

Abstract

The thermal properties of paraffin-based nanofluids have been examined to investigate the use of enhanced phase change materials (PCMs) for thermal energy storage (TES). PCMs are promising for TES applications, but low thermal conductivity limits their rate of heat exchange with a working fluid. The nanofluid approach has been established as a method of thermal conductivity enhancement, but effects of particle addition on other thermal properties affecting TES are relatively ignored. Significant reduction in latent heat of fusion below traditional effective medium theory has been observed in nanofluids. An experimental study of paraffin nanofluids, containing various diameter multi-walled carbon nanotubes, has been conducted to investigate these findings. Results demonstrate that the magnitude of nanofluid latent heat reduction increases for smaller diameter particles in suspension. A method to approximate nanofluid latent heat of fusion is presented, considering the diameter-dependent reduction observed. Three possible mechanisms – interfacial liquid layering, Brownian movement, and particle clustering – are examined to explain further reduction in latent heat, through weakening of molecular bond structure. Although interfacial layering effects and Brownian motion contribute some reduction, experimental evidence suggests that particle clustering is the only mechanism capable of explaining the degree of latent heat reduction observed. Additional research is needed to explore these proposed mechanisms. Nanofluid latent heat and effective thermal conductivity were analyzed collectively to investigate the effects of particle size on PCM energy storage performance. It is shown that while particle diameter significantly impacts nanofluid latent heat, thermal conductivity exhibits a negligible size dependency. Governing equations for a finite element model of nanofluid phase change is presented, as a method of quantifying PCM energy storage performance. Measured and approximated thermal properties from this study can be applied as model parameters to size an appropriate storage container for TES applications. The future model will serve as a predictive tool for determining optimum particle diameter and volume fraction to maximize energy stored and extracted over a given period of time.

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