| Summary: | 碩士 === 國立臺北科技大學 === 能源與冷凍空調工程系碩士班 === 103 === This study investigates the evaporation heat transfer performance of R-134a falling film on three horizontal tubes in a vertical column. This study tested two kinds of tubes, a smooth tube, and a new enhanced tube which is called “Jigsaw Fin Tube”. The jigsaw fin tube was made by 0.4mm fin height, 60FPI (Fins Per Inch) and the tube surfaces had 40 axial grooves surrounding the tube. Experiments were performed at saturation temperatures of 10, 20 and 26.7℃. The liquid flowed through a liquid feeder with a row of circular holes at a rate of 0.0075~0.04376kg/ms, and heat fluxes varied from 4.4 to 49.6kW/m2.
The experimental results show that the heat transfer coefficient of the smooth tube increases with increasing heat flux, and it increases slightly with increasing flow rate before dry-out occurs. Falling film flow rate has minor influence of heat transfer coefficient for smooth tubes. At low film Reynolds number (Ref=415~592), the jigsaw fin tube is in thin film evaporation mode, which is revealed by its high heat transfer coefficient, and at high film Reynolds number (Ref=740~~888) the falling film evaporation heat transfer coefficients are lower than those in pool boiling. The jigsaw fin tube has the best heat transfer performance. It enhanced the heat transfer coefficient by 2.68~5.87 folds as compared with the smooth surface at 26.7℃ saturation temperature. As the saturation temperature decreases the enhancement ratio increases from 2.73~4.09 folds at 20℃ to 7.08~10.18 folds at 10℃. As compared with the fin tube without axial grooves, the present jigsaw fin tube enhances the falling film evaporation heat transfer in a wider range of flow rates and heat fluxes.
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