| Summary: | 碩士 === 國立交通大學 === 機械工程系所 === 94 === Experiments were conducted to investigate the pool boiling heat transfer of dielectric fluid FC-72 under 1 atmosphere with dissolved gas in a vertical narrow rectangular space. The heated surface is a smooth, 10 × 10 mm copper surface. Gap sizes (S) and gassy-subcooled are 3, 2, 1, 0.5 mm, and 0, 10, 20 and 30K respectively. Results showed that heat transfer coefficients were enhanced with increased dissolved gas under low heat flux, because the dissolved gas reduced the incipient surface superheat. Under middle and high heat flux, the liquid was degassed with the result that the boiling was pure liquid nucleate pool boiling, and, hereafter, the heat transfer coefficients could not be raised with increasing dissolved gas. The heat transfer coefficients increasing with decreasing gap sizes in low heat flux, because the smaller gap sizes, the greater the liquid superheat in the narrow space, and incipient boiling might occur more easily. On the contrary, under middle and high heat flux, the heat transfer coefficients decreased with decreasing gap sizes because vapor film might cover on heated surface more easily. The critical heat flux (CHF) also decreased with decreasing gap sizes. In the experiment, we also took photographs of the bubble growth from low to high heat flux in order to observe the bubble characteristics in the pool boiling phenomena. A correlation was proposed to predict the critical heat flux and was compared with measured results which showed that the correlation deviated quite obviously in small gap sizes.
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