Study on the Effect of Heating Surface Structure on Subcooled Flow Boiling Heat Transfer in Horizontal Narrow Channel
Purpose
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Higher heat flux cooling and higher density of heat transportation are required for increasing heat generation rate of electronic equipment.
Two-phase flow thermal control system has attracted the attention for the requirement.
Higher heat transfer coefficient can be obtained by using the latent heat for vaporization. For the further boiling heat transfer enhancement, increase in nucleation site density is efficient. On the other hand, it is necessary to consider CHF due to the transition from nucleate boiling to film boiling during high heat flux cooling.
It is known that the CHF increases with increasing the inlet subcooling degree and mass flux.
Therefore, it is necessary for the cold plate not only to enhance the heat transfer coefficient but also to clarify the mechanism of the DNB.
In this study, thermal spray coated surface is investigated as boiling heat transfer surface. From our previous study, it was shown that the CHF became higher for the thermal spray coated surface than the smooth surface in subcooled boiling.
The reason might be on the condensation of vapor bubbles in subcooled liquid because detached bubbles were smaller than those for the smooth surface.
Now, the effects of physical properties of the working fluid and the heating surface structure on boiling heat transfer characteristics are investigated for subcooled boiling.
Research content
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This study deals with subcooled flow boiling in a horizontal narrow rectangular channel with bottom heating.
The channel width and height are 20 mm and 4 mm, respectively. The copper heating surface with the same width as the channel and heated length of 50mm was placed at the center of the channel.
FC-72 and HFC-245fa were used as the working fluid. The experimental set up is a two-phase fluid loop with an accumulator, and the inlet pressure was maintained by accumulator.
The inlet subcooling degree was varied from 10 to 30 K. Heat transfer coefficient and cross-sectional average void fraction were measured at the test section.
The figure shows boiling curves of HFC-245fa for each heating surface.
Since the gradients of the boiling curves were constant, so it can be said
that the flow was developed nucleate boiling. The thermal spray coated
surface produced about 2 times higher heat transfer coefficient and higher
CHF than the smooth surface. The porous coating might promote the liquid
spreading to prevent dryout of the heating surface. Currently, we research
on the test sections with the grooved surface with and without thermal
spray coating.
Publications
H. Asano, T. Nakamura
Effect of surface structure on void fraction of subcooled forced boiling
in rectangular narrow channel, Proc. of The 16th International Heat Transfer
Conference (2018) pp.1349-1356. https://doi.org/10.1615/IHTC16.bae.023156
H. Asano, J. Yoshidome, T. Nakamura, T. Gomyo
Effect of Heat Transfer Surface Structure on Wall Temperature and Void
Fraction Characteristics in Boiling Transition, Multiphase Science and
Technology 27 (2015), pp.133-146. https://doi.org/10.1615/MultScienTechn.v27.i2-4.30