Experimental Study of Two-Phase Flow Pressure Drop Across a U-Bend and Evaluation of Existing Correlations

Nuclear power system operation and safety analysis require accurate prediction of the pressure drop to predict flow rates and pump head requirements. The complex phase interactions in two-phase flow confound measuring and modeling of flow parameters in these systems and is further complicated if flow restrictions are present. Restrictions such as elbows and U-bends cause secondary flow and minor loss that need to be considered, and currently there is no comprehensive mechanistic model for this minor loss. At the same time, two-phase flow through flow restrictions is crucial to understand due to its presence in helical coil steam generators as well as pressurized water reactor steam generators under accident scenarios. Therefore, two-phase flow in these systems, subject to pipe curvature and inclination effects, needs to be studied. The existing correlations for two-phase pressure drop through elbows or U-bends are limited to refrigerant flows, horizontal orientations, and very small pipe diameters. Additionally, the majority of these correlations are empirical without having any mechanistic treatments for the phenomena related to the additional loss stemming from the geometric configurations. Therefore, the applicability of these correlations needs to be evaluated for air-water two-phase flows across a larger U-bend in both vertical and horizontal orientations, which requires a detailed two-phase flow database. Experiments are performed on a U-bend with a radius of curvature–to-diameter ratio, Rc/D, of 9 in an inclinable air-water two-phase flow facility. In the vertical orientation, eight experimental conditions are used concentrating in the bubbly flow regime with some transitional to slug flow, while in the horizontal orientation, eight experimental conditions are used with all being bubbly flow. Five correlations from the literature were chosen for evaluation using the experimental data obtained. It was found that Kim et al.’s model, based on the original Lockhart-Martinelli model, predicts the best for both the horizontal and vertical U-bend data well with percentage differences of 0.50% and 6.57%, respectively. This model was further analyzed to physically determine the values of the Chisholm parameter and minor loss parameter by comparing available pressure drop databases and reasoning how the parameters should change based on orientation and geometry.