Study on Visualization and Void-fraction Measurement of Boiling Two-Phase Flow by Using Neutron Radiography

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The purpose of this study is to investigate flow characteristics (flow regime and void fraction) as a first step of thermal-hydraulic research on boiling two-phase flow in narrow channels. Using neutron radiography with a steady neutron beam, subcooled-boiling two-phase flow in a narrow rectangular channel is visualized and the void fraction is measured.In FY1998, a movable thermal-hydraulic test loop for neutron radiography experiment was set up and the observation of flow regimes and the void fraction was measured for subcooled-boiling in the flow channel. The experiment was performed using the JRR-3M reactor of Japan Atomic Energy Research Institute as a neutron source and the high-frame-rate neutron radiography system as a imaging system. Comparing the experimental results with existing correlations, it turned out that the evaluation of measurement error was very important. For this, in FY1999, the method for void fraction measurement was examined and the measurement error was eval … More uated by inserting a polystyrene sheet in the same flow channel as used in FY1998 to simulate the boiling two-phase flow. Images taken by the high-frame-rate imaging system were processed by an image processing system and the equivalent void fractions of the test section with the polystyrene sheet were obtained by the Σ-scaling method. Then the measurement errors were evaluated for instantaneous and time-averaged void fractions by comparing the measured and theoretical equivalent void fractions. In FY2000, instantaneous and time-averaged void fraction profiles were obtained for boiling two-phase flow in a narrow rectangular channel by using the same method as developed and verified in FY1999, and the flow characteristics were investigated. By analyzing the data, useful information was obtained on the flow regimes near critical heat flux conditions, detailed structure of void fraction distribution in the subcooled-boiling region, and so forth, for boiling two-phase flow in a narrow rectangular channel. Less