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Research Papers: Evaporation, Boiling, and Condensation

R134a/Al2O3 Nanolubricant Mixture Pool Boiling on a Rectangular Finned Surface

[+] Author and Article Information
M. A. Kedzierski

Fellow ASME
National Institute of Standards and Technology,
Building 226, Room B114,
Gaithersburg, MD 20899
e-mail: Mark.Kedzierski@NIST.gov

Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.

Kedzierski [15] provides functional forms of the Laplace equation that were used in this study in the same way as was done in Kedzierski [17] and in similar studies by this author.

This was done by adding pure lubricant to the R134a/3AlO mixture to create 2AlO and also by adding pure refrigerant so that the R134a/2AlO mixture remained at the (99/1) composition.

The surface area ratios (As/Ap) were calculated from scaled photographs of fin cross sections. The As/Ap of the rectangular finned surface of this study was calculated to be approximately equal to 1.9.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received December 1, 2011; final manuscript received May 29, 2012; published online October 5, 2012. Assoc. Editor: Louis C. Chow.

J. Heat Transfer 134(12), 121501 (Oct 05, 2012) (8 pages) doi:10.1115/1.4007137 History: Received December 01, 2011; Revised May 29, 2012

This paper quantifies the influence of Al2O3 nanoparticles on the pool boiling performance of R134a/polyolester mixtures on a rectangular finned surface. Nanolubricants having 10 nm diameter Al2O3 nanoparticles of various volume fractions (1.0%, 2.3%, and 3.6%, a.k.a., 1AlO, 2AlO, and 3AlO) in the base polyolester lubricant were mixed with R134a at two different mass fractions (0.5% and 1%). The study showed that nanolubricants can significantly improve R134a/lubricant boiling on a rectangular finned surface. For example, the R134a/1AlO (99/1), R134a/3AlO (99/1), and the R134a/2AlO (99/1) mixtures exhibited average enhancement of approximately 18%, 102%, and 113%, respectively. The nanoparticles had practically no effect on the heat transfer relative to that for R134a/polyolester mixtures without nanoparticles for R134a boiling with the 1AlO nanolubricant at a 0.5% mass fraction with the refrigerant. This confirms, what was shown in a previous publication for a smooth surface, that for a particular system, a critical loading of nanoparticles must be exceeded before an enhancement can be achieved. The present study suggests that passively enhanced surfaces are likely to require more nanoparticle loading than a smooth surface to achieve similar heat transfer enhancement. This is based on the finding that the boiling heat transfer enhancement was shown to be a strong function of the absolute nanoparticle surface density that resides on the heat transfer surface and not the nanoparticle concentration in the nanolubricant as previously believed. The enhancement was shown to increase for three different boiling surfaces (from three different studies) as more nanoparticles accumulate on the boiling surface. Accordingly, a previously developed model for predicting refrigerant/nanolubricant boiling on a smooth surface was corrected so as to be dependent on the nanoparticle surface density rather that the nanoparticle concentration. In addition, the model was modified in order to predict the refrigerant/nanolubricant boiling on the rectangular finned surface. The model and the measurements agreed to within 10% for all of the data with heat fluxes less than 100 kW m−2.

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References

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Figures

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Fig. 1

Schematic of test apparatus

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Fig. 2

OFHC copper flat test plate with rectangular finned surface and thermocouple coordinate system

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Fig. 3

Photograph of rectangular finned surface

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Fig. 4

TEM of Al2O3 nanolubricant [16]

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Fig. 5

Pure R134a boiling curve for rectangular finned surface

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Fig. 6

R134a/RL68H mixtures boiling curves for rectangular finned surface

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Fig. 7

Boiling heat flux of R134a/RL68H mixture relative to that of pure R134a for rectangular finned surface

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Fig. 8

R134a/RL68AlO mixtures boiling curves for rectangular finned surface

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Fig. 9

Boiling heat flux of R134a/nanolubricant mixtures relative to that of R134a/RL68H without nanoparticles for rectangular finned surface (arrows show heat flux ratio)

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Fig. 10

Absolute number of nanoparticles determines average heat transfer enhancement for three different surfaces

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Fig. 11

Comparison of predicted to measured heat flux ratio for four different refrigerant/nanolubricant mixtures boiling on a rectangular finned surface

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