| Summary: | 碩士 === 國立交通大學 === 機械工程學系 === 99 === An experiment is carried out in the present study to investigate the characteristics of the evaporation heat transfer for refrigerant R-410A flowing in a horizontal narrow annular duct. The gap of the duct is fixed at 2.0 mm. In the experiment, the effects of the refrigerant mass flux, saturation temperature, and vapor quality and the imposed heat flux on the measured evaporation heat transfer coefficient and the photos of the evaporating flow are explored. For the duct gap of 2 mm, the refrigerant mass flux G is varied from 300 to 500 kg/m2s, imposed heat flux q from 5 to 15 kW/m2, mean vapor quality xm at the middle axial location of the duct from 0.05 to 0.95 and refrigerant saturation temperature Tsat from 5 to 15℃.
The experimental data for δ=2.0 mm clearly show that the evaporation heat transfer coefficient increases almost linearly with the vapor quality of the refrigerant and the increase is more significant at a higher q. Besides, the evaporation heat transfer coefficient also rises substantially at increasing G. moreover, a significant increase in the evaporation heat transfer coefficient results for a rise in Tsat.
The photos of R-410A evaporating flow taken from the duct side over the entire duct for selected cases show that the bubble nucleation on the heating surface is found to be important at low vapor quality. Besides, at low vapor quality merging of small to from big bubbles and merging of bug bubbles into bubble slugs take place. At intermediate quality only in the entry portion of the duct the bubble nucleation dominates. Slightly downstream merge of big bubbles to form a few bubble slugs appears in the top portion of the annular duct. Near the exit of the duct the evaporation of liquid film flow dominates. For high vapor quality bubble nucleation and strong moving waves can still be seen in the small portion near the duct inlet at a high imposed heat flux, although the liquid film on the heating surface is very thin. Moreover the high-speed photos of the flow for selected small regions in the duct further indicate that at low vapor quality bubble nucleation density on the heating surface increases with the imposed heat flux. At the intermediate vapor quality some bubble nucleation on the heating surface can also be observed in the middle section of the duct at a high q and lower mass flux. And at the high vapor quality bubble nucleation can still be seen at q=15kW/m2 although the liquid film covering the heating surface is very thin.
Comparison of the evaporation heat transfer of refrigerants R-410A and R-134a in the same annular duct manifests that the heat transfer coefficients of R-410A are much higher than R-134a. Besides the annular flow prevails over a larger portion of the duct for the R-134a evaporation for the same set the experimental parameters.
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