| Summary: | 碩士 === 淡江大學 === 化學工程與材料工程學系碩士班 === 93 === The study presents a theoretical prediction of the device performance of a sheet-and-tube solar water heater with external recycle, and a comparison of the device performance was made between these two device with and without external recycle. The device with internal fins attached in each tube is also discussed. Analytical results show that the recycle effect can effectively enhance the collector efficiency compared with that in a single-pass device with the same flow rate. The desirable effect of increasing convective transfer rate and the undesirable effect of reducing heat transfer driving force are the two conflict effects produced by the recycle operation. It was found that the convective transfer-rate increment by increasing the recycle ratio could generally compensate for the temperature driving force decrement, leading to improved performance, especially for low flow rate. Also, the collector efficiency increases with increasing collector aspect ratio (i.e., either increasing the tube length or decreasing tube number) for a specified collector area and the distance between tubes.
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