| Summary: | 碩士 === 國立雲林科技大學 === 機械工程系碩士班 === 89 === All evaporator and condenser coils contain bends or other fittings to compact the heat exchanger size. The design of air-cooled heat exchangers requires the knowledge of heat transfer and frictional loss. For typical air-cooled coils, use of hairpin is very common. As expected, the hairpin that contains many 180° return bends (U bend) will cause higher pressure drop than the corresponding straight tube. As a consequence, the single-phase and two-phase frictional performance of a return bend is very important for accurate estimation of the performance of an air-cooled heat exchanger. 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 two-phase flow in 180° return bends.
The objective of this study is to conduct experimental investigations of two-phase flow air-water mixture and single-phase flows of air and water flowing in 180° horizontal return bends at room temperature. A total of nine 180° return bends has been tested for the present study. The test tubes are made of glass tube having inner diameters (D) of 3.0, 4.95, and 6.9 mm with dimensionless tube curvature ratio (2R/D) of 3, 5 and 7, where R is the radius of centerline of bend. Test range of Reynolds numbers for water and air single-phase tests is about 400< ReD <10000. The range of mss flux for two-phase mixture is between 50 ~ 400 kg/m2s.
Two-phase flow patterns were taken by a combination of visual observations and high-speed camera photos to form the flow regime maps for each test tube. The data for the transition between flow patterns were recorded. Most of the observed flow pattern data in 180° return bends are similar to that in straight tubes with the same tube diameter, except for the transition data from wavy stratified to annular. For smaller curvature ratio of 3, one can see the flow pattern recovery region is temporally turned from stratified flow into annular flow. The liquid was observed to switch to the outer tube wall by centrifugal force, the liquid at out wall was soon forced to move toward to the upper side of tube wall and then switch back to the inside of the tube wall by secondary flow. However, this phenomenon is not observed at higher vapor quality due to the lack of liquid and increase of gas-phase inertia.
Two-Phase frictional pressure drop data for 180° return bend including the additional loss in the downstream straight tube were obtained. In general, the frictional pressure drop across a horizontal return bend includes the effects of friction, curvature and pressure recovery at bend exit to the downstream straight tube. The effects of total mass flux, vapor quality, curvature ratio and tube diameter were examined. The resulted two-phase pressure drops were observed to algebraically increase with increasing total mass flux and vapor quality, and decreasing bend radius and tube diameter.
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