The flow distribution at core inlet plays a vital part on hydraulic design of pressurized water reactor. Nonuniform coolant flow distribution at core inlet is caused by many factors, among which the behavior of flow in the lower plenum is the most direct cause. Therefore, a further research on the flow behavior of coolant in the lower plenum is very important and necessary. However, the flow behavior is dominated by the variation of the flow environment related to the complex structure and the condition of upstream uniformity. Using CFD methods, the flow field in the pressure vessel under the uniform flux operation condition of three-loops is simulated in this paper. The standard k–ε turbulence model and upwind solver scheme are selected. Pressure and velocity along with the flow direction are investigated. The variation trend of flow characteristics is discussed by analysis on streamlines at different locations ranging from inlet pipe to lower core plate, which provides evidence for the formation of swirling eddies in the lower plenum and uneven flow flux at lower core plate. In order to understand the formation process of eddies of different sizes in the lower plenum, the velocity fields in the downcomer and lower plenum under the conditions of different inlet velocity are analyzed. Furthermore, the effects of key structures on the formation of swirl are also presented. The results show that the value of velocity flowing into the lower plenum is an important factor on the range of vertical reflux effect, also slowing down because of the resistance of the vortex suppression plate. And the value of angular momentum of swirls in the lower plenum is mainly determined by the inlet angle of converging streams flowing from the downcomer, which is caused by dispersed flow along the cylinder wall flowing from two cold legs, related with the inlet conditions.

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