Fig. 8-1 Measurement system for boiling flow using a neutron beam Boiling flow in a test section was exposed to the neutron beam of JRR-3M. Neutrons penetrate metals easily while they are absorbed in water. By measuring the intensity of a neutron beam through the test section, we can measure the vapor fraction in boiling flow.

Fig. 8-2 Flow visualization of subcooled boiling flow Because no sensor is installed in the flow channel, this measuring system causes no disturbance to the flow. We observed vapor bubbles generated in a narrow test channel under thermal nonequilibrium conditions. This example shows that the small vapor bubbles coalesced and formed a big bubble.

A measurement system for boiling flow was developed using the neutron beam of the JAERI Research Reactor-3M (JRR-3M), to understand phenomena in subcooled boiling flow (Fig. 8-1). Neutrons penetrate easily in metals while they are absorbed in water. We apply the neutron beam to the measurement of the vapor fraction in boiling flow. By measuring the intensity of the neutron beam through the test section, we can measure the vapor fraction in boiling flow. Because no sensor is installed in the flow channel, there is no disturbance to the flow caused by this measuring system. We observed vapor bubbles generated in a narrow test channel under thermal non-equilibrium conditions (Fig. 8-2). We confirm that the time and spatial resolutions of this measuring system are about 1 ms and 0.5 mm, respectively. The spatial distribution of vapor fraction was measured easily within an error of 10%. The measured data are very useful to develop heat removal techniques for many applications such as nuclear reactors and accelerators.

Reference M. Kureta et al., Visualization and Void Fraction Measurement of Subcooled Boiling Water Flow in a Narrow Rectangular Channel Using High-Frame-Rate Neutron Radiography, TPI45: Proc. of 2nd Int. Symp. on Two-Phase Flow Modeling and Experimentation, May 23-26, 1999, Pisa, Italy, 1509 (1999).

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