| Summary: | 碩士 === 國立交通大學 === 機械工程系所 === 94 === Experiments were conducted to investigate the fully developed flow boiling heat transfer of dielectric fluid FC-72 in a horizontal small-gap rectangular channel under 1 atmosphere. The heated surfaces were smooth and fin structure type. The channel was constructed with the height of 2 mm and the width of 10 mm, to attain a hydraulic diameter of 3.33 mm. The testing parameters are: Reynolds number in the range of 500 to 2000 and subcooling temperature 10℃ to 30℃ as well.
Results showed that flow velocity and the heated surface temperature are the major factors that would influence the onset of nuclear boiling, and the narrowness of the narrow gap would attenuate the boiling hysteresis. As the boiling mode reached partially developed nuclear boiling, subcooling temperature and flow velocity started to lose their influence on the wall temperature and finally disappeared on CHF (critical heat flux). The CHF always increased as the values of subcooling and flow velocity increased. The wall temperature of the fin heated surface before CHF could sustain a higher temperature than that of smooth heated surface, therefore the CHF could also be promoted. Different gap size of rectangular channel would lead to different on set boiling mode, in which boiling in small gap channel could be formed gradually while boiling in large gap channel would break out suddenly.
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