Application of high-frame-rate neutron radiography with a steady thermal neutron beam to two-phase flow measurements in a metallic rectangular duct

To apply the neutron radiography (NRG) technique to fluid research, high-frame-rate NRG with a steady thermal neutron beam was developed by gathering up-to-date technologies for neutron sources, scintillators, high-speed videos, and image intensifiers. This imaging system has many advantages such as a long recording time, high-frame-rate (up to 1000 frame/s) imaging, and no need for a triggering signal. Visualization of air-water two-phase flow in a metallic rectangular duct was achieved at the recording speeds of 250, 500, and 1000 frame/s. The qualities of those consecutive images were good enough to observe the flow mechanism and to measure the flow characteristics. It was demonstrated that some characteristics of two-phase flow could be measured by using the current imaging system. To quantify geometric information from NRG images, measurements of flow regime, rising velocity of bubbles and wave height, interfacial length, and interfacial area in annular flow were performed by using the image processing technique. To quantify attenuation characteristics of neutrons in materials, measurements of average void fraction and void profile were conducted. It was confirmed that this new technique may have significant advantages in both visualizing and measuring high-speed fluid phenomena when the ordinary methods such as the optical method and X-ray radiography cannot be applied.