| Summary: | 碩士 === 國立中央大學 === 機械工程學系 === 106 === Due to the rapid development of electronic equipment, dissipating heat from
high heat flux sources becomes more important. For the past few decades, excellent
thermal management solutions have been attained using well-established
technologies such as air cooling and liquid cooling system. However, they are no
longer viable solutions due to low heat transfer coefficient. Therefore, there is an
increasing need for compact and high heat-flux thermal management system. Two
phase cooling system offers a potential solution for higher heat transfer.
Considering the same heat transfer coefficient, two-phase flow required much
lower flow rate and pumping power compare to single-phase flow, as result of
higher heat transfer coefficient of two phase flow.
Surface structure or porous surface due to increasing the cavity and nucleation
site, can enhance the pool boiling heat transfer coefficient. Porous surface is made
of different or single-size metal particles, and the stack of particles will produce
cavities and interconnected channels. Cavities increase the nucleation sites which
generate more bubbles, while the interconnected flow paths provide fluid flow and
bubble escape path. The traditional porous material coating is thick, which cause a
large thermal resistance between the heating surface and the evaporation surface,
therefore heat transfer performance of the porous surface decreases. Micro-porous
surface can improve this problem, because micro-porous surface is all in the
superheat region, it is easier to produce bubbles while the porous layer is thin,
thermal resistance is also small.
In this study, we designed different size of straight fin type heat exchanger
and use Aluminum powder with average particle size of 20 µm, micro porous used to coating those heat exchanger to compare the heat transfer performance of coating
and non-coating heat exchanger.
The experimental results show that the microporous surface has a fin spacing
of 2 mm. The heat transfer enhancement effect is better at high heat flux, which is
17%~62% higher than that of the smooth surface, and the particle accumulation on
the microporous surface forms a channel that is mutually entangled. It helps to
replenish the liquid to the heated surface, and the dry out of the microporous surface
is delayed compared to the smooth surface. Microporous surface with fin spacing
1mm, because the fin spacing is small, after the bubble is generated, it is easy to
combine with the surrounding bubbles. Therefore, the heat transfer enhancement effect is lower than when the fin pitch is 2 mm.
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