Experimental Investigations on Bubble Detection in Water–Air Two-Phase Vertical Columns

Two-phase flow is a well-known phenomenon occurring in most of the liquid-carrying pipes and facilities. In specific to molten-metal applications for nuclear fusion blankets and power plants, consequences of trapped gases and two-phase flow include improper shielding, reduction in tritium breeding ratio and local hot-spots. To address some of the challenges in two-phase flow detection, a preliminary experimental study has been performed in static water columns with forced air bubbles injection at different frequencies. These experiments have been designed in view of further adaptations towards molten-metal applications. A prototype sensor probe based on electrical conductivity principle is fabricated and calibrated to detect the presence of vertically ascending array of bubbles. As a specific feature of this study, high frequency sampling of the order of MHz coupled with simultaneous imaging is performed to acquire detailed insights in probe interactions with injected bubbles. This study includes trajectory and shape-distortion analysis for bubbles during rise in a two-phase column. Experimental tools are further utilized for thickness estimations of distorted air-pockets along probe sensing length. This paper presents test facility design, probe fabrication, calibration details and test procedures along with preliminary attempts to estimate local void fractions.