| Summary: | 碩士 === 國立中央大學 === 機械工程學系 === 105 === In this study, we designed two-phase evaporative heat exchangers with: straight fin type, expansion type and chevron, and compared the heat transfer performance of each type. There are three geometry of straight fin type: 1. Fin height 2 mm with fin spacing 1 mm, 2. Fin height2 mm with fin spacing 2 mm, 3. Fin height 5 mm with fin spacing 1 mm. The fin space of the expansion type is 1 mm, and fin height reduced from 2 mm to 1 mm. The space and height of the chevron are 1, 2 mm and the inclination angle is 30 °.
The straight fin type with the fin height of 5 mm has the highest heat transfer rate, and the wall superheat temperature is the lowest, which means that the heat transfer performance is the best. The flow pattern inside the channel are wavy flow, which keep the channel surface wetted. So that the surface temperature become low. When fin height is 2 mm, the flow pattern become slug flow by small hydraulic diameter. Which cause the channel surface become partial dry out easier.
The expansion designed allows the bubble to cover the channel, which let vapor stock in the channel. The chevron does not cause dry out in the experiment, but it can’t be observed the liquid between the flow channel from the visualization.
The straight fin type with a fin height of 5 mm has a minimum thermal resistance of 0.067 °C W. Heat transfer rate is higher than other types and the flow pattern inside the channel is not easy to dry out, so that it is the best type in this study.
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