Two-phase flow pressure drops in U-tubes: Towards more accurate measurement methods and prediction models

In this study, flow pattern observations and pressure drop measurements in a U-bend and contiguous straight tubes are presented. The flow pattern observations were made with R134a at an inlet saturation temperature of 5 °C flowing at a mass flux of 300 kg s−1 m−2 inside a horizontal test section with an internal and bend diameters of 13 and 66 mm, respectively. The experimental data were obtained for the same experimental conditions, but with two test sections with one internal diameter of 11.7 mm and two bend diameters (31.7 and 54.8 mm) for different U-bend orientations. The experimental data was compared to the predictions of a new multi-orientation flow pattern based frictional pressure drop method for U-bends of Silva Lima and Thome (2012c) showing that the model predicts 97% of the database within an error window of ±30%. An extensive discussion was made on the benefits of using a data reduction method that allows obtaining the pressure drops effectively occurring in the U-bend and reducing the errors associated with that measure.