Determination of Time-Dependent Void Fraction Distribution in Bubbly Two-Phase Flow by A Real-Time Neutron Radiography Technique

Radiographic images generated by the real-time neutron radiography system (RTNR) are analyzed by customized image processing software for the determination of instantaneous void fraction distribution. The cross-sectional averaged axial void fraction profiles and the two-dimensional void fraction profiles are determined simultaneously for each image. Various flow regimes are generated to determine if the RTNR system can accurately predict the void distribution in the radial, axial, and temporal coordinates. The results show the RTNR system correctly determines the void fraction distribution for each flow regime; however, accuracy decreases with decreasing void fraction. For net water thicknesses >1.0 cm, the accuracy decreases with increasing water thickness due to the extreme sensitivity of thermal neutron interactions with light water. Heavy water is a more suitable fluid than light water for void fraction measurements in large-diameter flow systems.