Experimental Study of Evaporation Heat Transfer and Pressure Drop Characteristics of R-134a and R-407C in Horizontal Small Tubes

• Main Authors: Feng-Qing Su, 蘇峰慶
• Other Authors: Tsing-Fa Lin
• Format: Others
• Language: en_US
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/pgxxy5

碩士 === 國立交通大學 === 機械工程系所 === 92 === An experiment is carried out in the present study to investigate the characteristics of the evaporation heat transfer and frictional pressure drop for refrigerants R-134a and R-407C flowing in horizontal small tubes having the same inside diameter of 0.83 mm or 2.0 mm. In the experiment for the 2.0-mm small tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m2s, imposed heat flux q from 5 to 15 kW/m2, inlet vapor quality xin from 0.2 to 0.8 and refrigerant saturation temperature Tsat from 5 to 15℃. While for the 0.83-mm small tubes, G is varied from 800 to 1500 kg/m2s with the other parameters varied in the same ranges as those for Di=2.0 mm. In the study the effects of the refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficients and frictional pressure drops are examined in detail. For R-134a and R-407C in the 2.0-mm or 0.83-mm small tubes, the experimental data clearly show that both the R-134a and R-407C evaporation heat transfer coefficients and frictional pressure drops increase almost linearly and significantly with the vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the evaporation heat transfer coefficients also increase substantially with the rise in the imposed heat flux. But the effect of q on the frictional pressure drop is rather weak. Moreover, a significant increase in the evaporation heat transfer coefficients results for a rise in Tsat, but an opposite trend is noted for the frictional pressure drop. Furthermore, both the R-134a and R-407C evaporation heat transfer coefficients and frictional pressure drops increase substantially with the refrigerant mass flux. At low R-134a mass flux and high imposed heat flux the evaporation heat transfer coefficient in the small tubes (Di=0.83 mm) may decline at increasing vapor quality when the quality is high. This decline of hr at rising x is attributed to the partial dryout of the refrigerant flow in the small tubes at these conditions. We also note that under the same xin, Tsat, G, q and Di, refrigerant R-407C has a higher evaporation heat transfer coefficient and a lower frictional pressure drop when compared with that for R-134a. Finally, the empirical correlations for the R-134a and R-407C evaporation heat transfer coefficient and friction factor in 0.83-mm and 2.0-mm small tubes are proposed.