Earthquake is one of the most serious phenomena for safety of a nuclear reactor, especially in Japan. Therefore, nuclear reactors were contracted considering structural safety for a big earthquake. On the other hand, thermal-fluid behavior of the nuclear reactor is not fully confirmed under the earthquake. The behavior of nuclear reactor may be affected by the fluctuation of gas-liquid two-phase flow caused by the earthquake but not studied enough. For example, the bubble behavior under the fluctuation caused by the earthquake is not clear. In order to clear the two-phase flow behavior under the earthquake, it is studied systematically in the project of “Development of Prediction Technology of Two-Phase Flow Dynamics under Earthquake Acceleration”. In the project, bubbly or plug flow with flow rate fluctuation in a fixed horizontal pipe is studied experimentally. The bubbly or plug flow under the structural vibration but without the flow rate fluctuation is also studied. Here, the structural vibration done by using the pipe on the oscillation table. Moreover, the numerical code which can simulate the flows measured in the experiments is under development. In this paper, the result on the bubbly flow under the flow rate fluctuation is shown. The flow was bubbly flow and/or plug flow in a horizontal circular pipe. The working fluids were water and nitrogen gas. The water was driven by a pump and the flow rate fluctuation was given by a reciprocating piston attached to the main flow loop. In the study, the behavior of horizontal gas-liquid two-phase flow under the flow rate fluctuation is investigated with image processing and particle image velocimetry (PIV). As the result, the bubble’s deformation under the flow rate fluctuation was investigated by the image processing. The velocity profile around the bubble under flow rate fluctuation was measured with PIV. The velocity field around the deformed bubble showed the characteristic profile. Therefore, the velocity field changing with the flow rate fluctuation affected the bubble deformation.

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