Study on Phase Difference and Transient Flow Field in Rod Bundle under Pulsating Flow

The thermal-hydraulic behaviors in fuel assembly of reactor core have transient coupling characteristics under unsteady conditions including possible accident scenarios and ocean conditions, thus the study of transient characteristics in fuel assembly is necessary to improve the reactor safety. In this paper, the response time between frictional pressure drop and flow rate in a rod bundle was measured to perform the study of phase differences under the pulsating flow. A comparison of phase differences under different amplitudes and periods was conducted and the function of spacer grid for phase difference was also analyzed. In addition, the distribution characteristics of the flow field in the rod bundle under pulsating flow were studied based on the particle image velocimetry (PIV) technique. The differences of flow fields under steady conditions and transient conditions were compared. The flow fields of rod bundle under different accelerations were analyzed. The experimental results show that the spacer grid can reduce the phase difference of pulsating flow in the rod bundle and the flow field evolution in the rod bundle lags behind the variation of main flow. The experimental results are helpful to reveal the transient characteristics of the fuel assembly under unsteady conditions and provide a foundation for the design and optimization of the fuel assembly.