Development of a Fast X-ray Line Detector System for Two-Phase Flow Measurement

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Virginia Tech

Measuring void fraction distribution in two-phase flow has been a challenging task for many decades because of its complex and fast-changing interfacial structure. In this study, a non-intrusive X-ray measurement system is developed and calibrated to mitigate this challenge. This approach has several advantages over the conventional methods such as the multi-sensor conductivity probe, wire-mesh sensor, impedance void meter, or direct optical imaging. The X-ray densitometry technique is non-intrusive, insensitive to flow regime changes, capable of measuring high temperature or high-pressure flows, and has reasonable penetration depth. With the advancement of detector technology, the system developed in this work can further achieve high spatial resolution (100 micron per pixel) and high temporal resolution (1000 frames per second). This work mainly focuses on the following aspects of the system development: establishing a geometrical model for the line detector system, conducting spectral analysis for X-ray attenuation in two-phase flow, and performing calibration tests. The geometrical model has considered the measurement plane, geometry of the test-section wall and flow channel, relative position of the X-ray source and detector pixels. By assuming axisymmetry, an algorithm has been developed to convert void fraction distribution along the detector pixels to the radial void profile in a circular pipe. The X-ray spectral analysis yielded a novel prediction model for non-chromatic X-rays and non-uniform structure materials such as the internal two-phase flow which contains gas, liquid and solid wall materials. A calibration experiment has been carried out to optimize the detector conversion factor for each detector pixels. Finally, the data measured by the developed X-ray system are compared with the double-sensor conductivity probe and gas flow meter for sample bubbly flow and slug flow conditions. The results show reasonable agreement between these different measuring techniques.

Attenuation Coefficient, X-ray, Void Fraction, Two-Phase Flow, Polychromatic Energy, Beam-hardening