Experimental Study on Air-Water Two-Phase Bubbly Flow Across a Vertical U-Bend

Flow restrictions, such as U-bends, are commonly used in nuclear reactor systems. The geometric effects stemming from the flow channel curvature could be of interest for analyzing hydrodynamic phenomena in these systems. However, existing U-bend studies are limited to the small radius of curvature and low-flow conditions. This study experimentally investigates air-water two-phase flows through a vertical U-bend with a curvature-to-diameter ratio of 9 over a wide range of flow rates. A detailed database for global and local two-phase flow parameters was established using four-sensor conductivity probes, a pressure transmitter, and a high-speed video camera.The obtained data were used to investigate U-bend effects on two-phase flow parameters, including pressure drop, void fraction, interfacial area concentration, and bubble velocity. An analysis of bubbly flows showed bubble accumulation near the inner side of the pipe at the U-bend’s apex and exit, influenced by secondary flow and liquid inertia. Around three diameters downstream, the bubbles dispersed, indicating the dissipation of the U-bend effects, while a dual-peaked void fraction profile emerged at around eight diameters downstream, corresponding to the core regions of the secondary-flow vortices.Farther downstream, the profile gradually became center peaked. This study found that the pressure change due to acceleration or deceleration across the U-bend can contribute up to 18% of the total pressure loss under high void fraction conditions. The Lockhart-Martinelli correlations with C=34 and C=68 predicted two-phase pressure drops within ±10% for the vertical upward and downward sections, respectively, while a modified correlation by Kim et al. with C=40,k=0.10 and C=85,k=0.20, effectively modeled frictional losses in the vertical upward elbow and U-bend.The area-averaged void fraction α changed significantly across the U-bend due to pressure and bubble velocity variations. The void-weighted bubble velocity ⟨⟨vg⟩⟩ varied primarily with bubble distribution, exhibiting an opposite trend to α, while the interfacial area concentration ai followed a similar trend pattern to α. The data suggest that additional mechanisms may influence flow between L/DVD = 0 to 3, offering insights for interfacial area transport modeling across the U-bend.