Experimental research on gas-liquid two-phase flow in inclined U-tubes

In contrast to the extensive research on gas-liquid two-phase flow within horizontally oriented U-tubes, investigations into the flow characteristics within inclined U-tubes remain relatively limited. The experimental design encompassed the acquisition of flow pattern data and pressure measurements across the entire spectrum of upward and downward inclined pipe configurations. Flow patterns were systematically documented at varying inclination angles, with subsequent analysis focusing on the impact of different angles on pressure loss, considering diverse flow patterns, Reynolds numbers, and pipe diameters at the same time. Experimental visualization through flow pattern photography revealed that three distinct flow patterns—stratified-plug flow, plug-bubble flow, and slug-wavy flow—exhibit significant sensitivity to variations in inclination angle. Conversely, annular-wavy flow and annular-dispersed flow demonstrate comparatively less susceptibility to such angular changes. The flow pattern maps illustrate that regions corresponding to gravity-driven flow patterns and their transitional boundaries undergo notable modifications with alterations in the inclination angle. In contrast, flow patterns governed primarily by inertial forces show minor variation under similar conditions. The effect of inclination angle on pressure loss varies under different flow patterns. Specifically, the influence of inclination angle on pressure loss becomes more pronounced with increasing gas-liquid superficial Reynolds numbers. Furthermore, the pressure loss in smaller-diameter pipelines exhibits higher sensitivity to inclination angle variations than their larger-diameter counterparts.