Experimental study on Velocity Distribution in Rod Bundle Channels under Flow Fluctuation

Thermal-hydraulic behaviors within a PWR fuel assembly is very complex under accident conditions and marine conditions, which has a significant impact on reactor safety. It is crucial to carry out studies on thermal-hydraulic properties under transient conditions. A telecentric lens and a pulse signal generator are applied to improve the performance of PIV system, a high temporal and spatial resolution measurement for flow fields in rod bundle channels is achieved, the velocity distribution in rod bundles are obtained under accelerating flows and decelerating flows. The steady-state flow field is measured to use as a reference, the effect of flow fluctuations on the flow fields in the rod bundle is analyzed. The results present, the accelerating flows improve the velocity gradient between adjacent flow layers in rod bundles and weaken the lateral velocity. In the contrary, the decelerating flows weaken the velocity gradient between flow layers in rod bundles and improve lateral velocity. Conversely, the velocity gradient attenuates in decelerating flows between the rod beam layers and increases the lateral velocity. During the accident, the thermal-hydraulic behavior in the reactor has the characteristics of transient complexity and uncertainty. Studying the flow and heat transfer characteristics of the reactor core fuel assembly under accident conditions can improve a understanding for the accident development process.