| Summary: | 碩士 === 國立臺北科技大學 === 能源與冷凍空調工程系 === 106 === In this thesis, different from the operation principle of traditional evaporators, we mainly observe the flow characteristics of falling water on the horizontal tube buddle with very low flow rate, and then to investigate its heat transfer performance. A stainless steel cylinder with hidden nozzles was installed above the test section which was machined from three red copper cylinders and was mounted vertically on a stainless steel plate. Experimental system, including a data acquisition, a thermostatic water bath, a high speed camara, was setup to examine the flow characteristics and heat transfer on the tube bundle. The heat flux was varied from 5 to 20 kW/m2, flow rate was from 50 to 150 mLPM, and the spray hole pitch was kept 12 and 24 mm, respectively. The measured results show that if the liquid falling water from each nozzle is not affected with each other, the thinnest part occurs at 105°~120°, and then it becomes thickened rapidly. When the falling liquid film is affected with each other after stretching, the measured results are reversed. When the nozzle pitch W=12 mm, the liquid film is easy to merge at the bottom of the tube, and then it drips to the next tube. When the nozzle pitch W is 24 mm, the phenomenon of aggregation at the bottom of the tube is not as obvious as that of W=12 mm, and often part of the water flow does not gather. Besides, It is found that the heat transfer coefficient of the upper tube is the highest, and the heat transfer coefficient increases as the flow rate and heat flux increase. In this thesis, at the flow rate of 150 mLPM and heat flux of 20 kW/m2, there is the highest heat transfer coefficient. The heat transfer coefficient is better for higher nozzle pitch. The heat transfer coefficient of the upper tube can reach 3883 W/m2·℃. In the case of low flow and low heat flux, nozzle pitch on the heat transfer coefficient is not significant.
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