Wettability Effect on Microchannel Condenser Heat Transfer Enhancement
Date Issued
2012
Date
2012
Author(s)
Chen, Kuei-Yen
Abstract
In recent years, the microchannel evaporator with two phase heat transfer, due to its highly heat flux and little requirement for coolant flow rate, is considered as one of potential cooling techniques. When the traditional single phase heat exchanger cannot efficiently cool in a limited area, collocating the microchannel condenser with two phase heat transfer is regarded as a potential cooling component. Because of the highly developed technology, the heat dissipation rate raise in many products. Therefore the microchannel condenser with enhanced heat transfer is more applicale. The hydrophobic surface has wide contact angle, and worse wettbility. In the process of condensation it will form dropwise condensation. The heat transfer coefficient is increased dramatically in the large scale condenser within hydrophobic surface. We assume that it will show the same result in the microchannel condenser. Thus, this research design and manufacture the hydrophobic and hydrophilic microchannel condenser, compare to the heat transfer coefficient and pressure drop with uncoated microchannels condenser.
The test section has 30 channels 500μm in width and 155μm in depth using water as working fluid. Using layer-by-layer (LbL) assembly method manufacture the hydrophobic and hydrophilic structure the same geometric dimensions microchannel condenser.
In the experiment of the uncoated microchannel, the heat transfer coefficient and pressure drop is positive correlative with the increasing mass flux. Because increasing the mass flux, the velocity becomes faster, along with increasing the wall shear stress. This will make the thickness of the liquid film become much thinner, and reduce the heat resistant, further increase the heat transfer coefficient. Compared with the heat transfer correlation of the conventional channel, the result shows the MAE is still large. Currently, there is still much room to make progress on the heat transfer correlation of the microchannels. With regard to the pressure drop, compare with the correlation of micrchannel developed recently, it correlated with our result and shows that our result is reliable.
In coated surface, we use layer-by-layer (LbL) assembly method to change the contact angle between water and copper, to manufacture the hydrophobic and hydrophilic structure. The contact angle of the hydrophilic structure change from 87°to 43°. Also the contact angle of the hydrophobic structure rise up to 135°.
Compared with the heat transfer coefficient of the uncoated surface, the hydrophobic microchannels can increase roughly 100% on average, with remarkable difference. The droplet cannot adhere on the hydrophobic surface to cause the dropwise condensation. The mechanism is different from the filmwise condensation. Therefore, the heat transfer coefficient is much higher.
When the mass flux is small, the heat transfer coefficient doesn’t increase remarkably. Because the velocity is slower, the liquid film is easy to form. Therefore, the mechanism that dropwise condensation could increase the heat transfer coefficient cannot be observed within the small mass flux in this experiment. As increasing the mass flux, the velocity is so fast that is not easy to form the liquid film. This will make the heat transfer coefficient increase remarkably.
For the pressure drop, the contact angle of the hydrophilic structure is smaller, and it can extend liquid, making the flow easier. In this experiment the pressure drop decrease about 40% on average. It shows that the hydrophilic structure can greatly improve the pressure drop for the microchannel condenser.
Subjects
microchannel
condensation heat transfer enhancement
hydrophobic and hydrophilic surface
Type
thesis
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