Single-phase and two-phase pressure drops in 90° bends

For typical air-cooled coils, bend is very common used. As expected, the elbows with many 90° bends will cause higher pressure drop than the corresponding straight tube. While there are many investigations focusing on two-phase pressure drop in a straight tube. However, very limited data and correlations are available for two-phase flow in 90° bends.The objective of this study is to conduct experimental investigations of single-phase and two-phase flows in 90° horizontal bends. Tw o 90°bends had been tested for the present study. The two bends have inner diameters (D) of 5.5 and 9.5 mm with tube curvature ratio (2R/D) of 5.4 and 4.2, respectively. The bend friction factor (f_B) is always greater than that of the straight tubes at the same liquid Reynolds number. Also, the f_B values of D = 9.5 mm with smaller curvature ratio are greater than theses of D = 5.5 mm. The results may imply higher centrifugal force caused by the smaller curvature ratio in bend. The comparison between the data of 90° bend single-phase pressure drops to the Chisholm's predictions [12] has 23.5% mean deviation. The resulted two-phase pressure drops of 90° bend are observed to be greater than straight tubes, and algebraically increase with increasing total mass flux and vapor quality. Sánchez et al. [14] recently proposed an accurate correlation from 90° bends with large diameters. Their predictions show very good accuracy to the present two-phase data with smaller diameter. The mean deviations for D = 5.5 mm and 9.5 mm are 6.78% and 5.13%, respectively.