Nano-Scale Phase Change Materials (Pcm’s) and Their Effect on Heat Transfer Fluids

Topics: Heat transfer, Thermodynamics, Thermal conductivity Pages: 6 (1733 words) Published: June 17, 2013
Nano-scale Phase Change Materials (PCM’s) and Their Effect on Heat Transfer Fluids Sean Schulte, Chaoming Wang, Ming Su
1NanoScience Technology Center, 2Department of Mechanical, Materials, and Aerospace Engineering, University of Central Florida, Orlando, Florida 32826. mingsu@mail.ucf.edu
Abstract:
Introduction:
Heat transfer fluids (HTF’s) are often used as carriers in heat transfer equipment. It therefor comes as no surprise that finding a way to make HTF’s more efficient is desirable in the scientific community. Research is being done to attempt to rectify this issue by adding materials with a high thermal conductivity (metal’s and metal oxides) to HTF’s in accordance with Maxwell’s heat transfer equations [1]. Until recently this problem was addressed by using micro scale solid particles with millimeter sizes which were blended into the HTF’s. Unfortunately problems which include abrasion, clogging, phase separation etc. occurred which limits the practical application of this technique [2].

To attempt to rectify this Choi introduced what he referred to as nanofluids [3]. Nano particles were suspended in heat transfer fluids to attempt to increase the thermal conductivity of the heat transfer fluids. The effectiveness of this approach has been widely debated amongst the scientific community.

In 2009 a study consisting of over 30 organizations was published in the Journal of Applied Physics [5]. The aim of this study was to test the thermal conductivity of nanofluids using various methods which include the transient hot wire method, steady-state methods, and optical methods. The results seemed to suggest no noticeable enhancement of thermal conductivity of the nanofluids.

The purpose of this research is to create phase-change nano-particles (nano PCM’s) to increase the heat capacity of heat transfer fluids (HTF’s). Fluids have low heat conductivity and limited heat capacity and the phase change nano-particles are added due to their large latent heat of fusion. They have a larger heat capacity then the HTF’s, which is due to them absorbing heat during the solid-liquid phase change as well as the superior conductivity of metals compared to that of liquids. [2] In order for the nano-particles to stay suspended in the fluid they need to be encapsulated with a shell so they don’t bond to each other and precipitate out of solution. If the nano-particles do precipitate out they can stick on fluid channels and cause corrosion, and decrease the dielectric properties of the fluids.

The encapsulated alloy should have a melting point less than the boiling point of the base fluid. The shell should have a melting point higher than that of the alloy and higher than the boiling point (BP) of the base fluid. The idea is to have the alloy undergo a complete phase change while keeping the shell in a solid form to prevent agglomeration of the nano- particles.

In our case the base fluid has a BP of 200 °C and the alloy (Fields metal) has a MP of 60 °C, 156 ℃ for Indium . Instead of using the encapsulated silica shell like done in previous research [2] we are testing out surface modification with 1H,1H,2H,2H-Perfluorooctyltriethoxysilane. Using this opposed to the shell method should increase the enthalpy of fusion obtained from the nano-PCM’s due to a reduced diameter of the shell.

Previous research involving encapsulating the nano-PCM’s used silica and found the enthalpy of fusion decreased by 31.2% when using the silica shells opposed to (in their case) pure Indium. The difference is due to the fact that the MP of the silica shell is 1600 °C. In their case the base fluid had a BP of 200 °C while the Indium has a MP of 156 °C. The reason for the larger difference opposed to using surface modification is due to the extra energy required to get past the silica shell, silica has a melting point of 1600 ℃ [2]. By decreasing the shell’s diameter you should be able to maximize the enthalpy of fusion from the nano-PCM’s...

References: 1. Maxwell, C. J., Electricity and Magnetism. Oxford : Clarendon Press: 1873.
2. Hong, Yan University of Central Florida (dissertation) 2011
3. Choi, S. U. S.; eastman, J. A. In Enhancing thermal conductivity of fluids with nanoparticles,
Int. Mech. Eng. Cong. Exh., San Francisco, CA.
4. Nan, C. W.; Birringer, R.; Clarke, D. R.; Gleiter, H., Effective thermal conductivity of
particulate composites with interfacial thermal resistance. J. Appl. Phys. 1997, 81, 6692.
5. Buongiorno, J.; Venerus, D.; Prabhat, N.; et al, A benchmark study on the thermal
conductivity of nanofluids. J. Appl. Phys. 2009, 106.
6. T. Hawa, M.R. Zachariah; Coalescence kinetics of unequal sized nanoparticles. J. Aerosol Science, 2005.
7.
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