Advanced Experimental Characterization of Bubbly Flows in Rectangular and Rod-Bundle Geometries

Abstract

Bubbly flow is a fundamental flow regime in gas-liquid two-phase systems and plays an important role in nuclear reactor thermal-hydraulics. Accurate prediction of bubbly-flow behavior is essential for evaluating phase distribution, interfacial transport, heat transfer, and reactor safety. However, the predictive capability of current Multiphase Computational Fluid Dynamics (MCFD) models remains limited due to uncertainties in interfacial force formulations and the lack of high-resolution experimental data for rigorous validation. This dissertation addresses these challenges through advanced experimental characterization of bubbly flows in rectangular and rod-bundle geometries. A comprehensive experimental investigation was conducted in a 30 mm × 10 mm rectangular channel to establish a high-resolution, validation-grade bubbly-flow database. Gas-phase and liquid-phase measurements were obtained using conductivity probes, high-speed imaging, and Particle Image Velocimetry–Planar Laser-Induced Fluorescence (PIV-PLIF). The resulting dataset provides detailed measurements of void fraction, interfacial area concentration, bubble size distribution, bubble number density, liquid velocity, and turbulence statistics for MCFD model validation. Using this database, interfacial force models commonly employed in two-fluid formulations were systematically evaluated. While existing models showed reasonable agreement in the streamwise direction, significant discrepancies were observed in the transverse momentum balance, particularly near wall regions. The results demonstrated that near-wall void-fraction gradients are strongly influenced by geometric exclusion effects imposed by solid boundaries, leading to overestimation of gradient-based interfacial forces. To address this limitation, a near-wall geometric correction method and a Monte Carlo-based bubble population reconstruction framework were developed to separate hydrodynamic and geometric contributions to the void-fraction gradient. To extend the investigation from a controlled rectangular channel to a reactor-relevant geometry, flow boiling experiments were performed in a 3 × 3 heated rod-bundle test facility designed and constructed in this work. A key contribution of this effort is the development of an advanced imaging and analysis framework that integrates high-speed imaging, machine-learning-based bubble detection, multi-scale segmentation, and stereoscopic three-dimensional reconstruction. This framework was developed to address the challenges of dense bubbly-flow visualization in rod-bundle geometry, including bubble overlap, wall effects, and geometric confinement. The developed framework enabled quantitative reconstruction of the shapes, sizes, and spatial locations of individual bubbles within the rod bundle. The resulting database provides detailed information for understanding bubble transport behavior and supporting the validation of MCFD models in reactor-relevant boiling-flow geometries.