| Summary: | 碩士 === 國立雲林科技大學 === 機械工程系碩士班 === 90 === The calculation of pressure drop in any two-phase flow system is very important in the design of steam-power and petrochemical plants, refrigeration and air-conditioning system. The pressure drop in tubes will influence the saturation temperature in heat exchangers, which will significantly affects the efficiency of refrigeration and two-phase thermal system. All evaporator and condenser coils contain bends or other fittings to compact the heat exchanger size. Flow disturbance is created for fluid flow through bends and additional pressure drop will be induced. Therefore, the pressure loss in bends is very significant to the overall efficiency of refrigeration and other two-phase system. It has been estimated that as much as 30﹪of the two-phase pressure drop in the residential air condition would have been induced in the return bends and other fittings. While there are a large number of investigations focusing on two-phase pressure drop in a straight tube, however, very limited data, models and correlations are currently available for the evaluation of two-phase pressure drop in bend.
Recently, the design of high-efficiency residential air condition has employed smaller diameter tube(d<5mm) in order to improve the airside performance and to reduce the refrigerant change into the system. Knowledge of the two-phase frictional characteristics in small tubes is essential since it would certainly improve the accuracy of a thermal system. Unfortunately, the existing predictive correlation for two-phase pressure drop in curved tubes are based on the experimental data with tube diameter greater than 10mm. Extrapolations of these correlation to applications utilizing the small diameter tubes are uncertain. It seems despite the important application of two-phase flow for bends with small diameter tube, the adequate frictional two-phase pressure drop correlations and published experimental data are very rare. Most of the residential air condition include many U-bends to compact their sizes. Recently, Refrigerant R-407C and R-410A are the most likely potential substitutes for R-22 to reduce the impact to ozone layer. So far , two-phase flow of R-407C and R-410A in bends has not been reported yet. R-407C is composition HFC-32/ HFC-125/ HFC-134a (23/25/52 wt. %) Nonazeotrope refrigerant,R-410A is composition HFC-32/ HFC-125 (50/50 wt. %) azeotrope refrigerant.
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