Condensation Heat Transfer Enhancement by Porous Microchannel
Date Issued
2012
Date
2012
Author(s)
Li, Po-Yen
Abstract
The microchannel evaporator with two phase heat transfer is considered to be one of the most potential cooling techniques because of its high heat flux, good temperature uniformity and the lesser requirement for coolant flow rate.
In recent years, the heat dissipation rate of the high tech products has increased day by day. The traditional single-phase heat exchanger could not efficiently cool down in a limited area, so the microchannel condenser with two phase heat transfer is regarded as a high potential cooling component in the future.
The central purpose of the present research is to enhance the condensation heat transfer by utilizing the two pore size distributions of a biporous surface structure. This surface is sintered from the mixture of dendritic copper powders and the pore former, Na2CO3, which formed the different size pores in the microchannel. By changing the volumetric ratio of pore former, it was able to alter the porosity and the numbers of larger pores, further increasing the heat transfer coefficient. During condensation, vapor could go through the larger pores. The smaller pores could absorb the liquid and help to reduce the liquid film thickness. It decreased the heat resistant and increased the heat transfer coefficient.
First, a plane surface microchannel system was built as a compared base. The test section of the 30 channels where the width and the depth is 500μm and 155μm, respectively. The test section was made by oxygen-free copper. Water steam is using as working fluid.
In the experiment of the plane surface microchannel, the heat transfer coefficient and the pressure drop werw positively related to the increasing mass flux. When increasing the mass flux, the velocity of working fluid becomes faster due to the increasement of wall shear stress. Therefore, it caused the thickness of the liquid film much thinner, decreased the heat resistance and also increased the heat transfer coefficient. Compared with the heat transfer correlation of the conventional channel, the result showed the MAE is quite large. That means there is much room to make progress on the heat transfer correlation of the microchannel. With regard to the pressure drop, comparing with the correlation of microchannel in recent years, it considerably correlated with the results. That shows the result is reliable.
For experiment of the biporous surface microchannel, the parameter with copper powder is 61~70 μm diameter and volumetric ratio of Na2CO3 is 30%. Comparing with the plane surface microchannel, the results showed that the heat transfer coefficient is enhanced to 72.9% on average when increased the pressure drop to 28.6% on average. The main reasons of enhancing the heat transfer are high water absorbing capacity and good ability for reducing liquid thickness.
Subjects
microchannel
condensation heat transfer enhancement
biporous surface
Type
thesis
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