Experimental And Numerical Investigation On Unsteady Flow Behavior In Tight Lattice

Fuel rod is one of the essential components of the pressure water reactor, the majority of the fuel rods are arranged in rectangular or triangular array, which form the axial flow of coolant fluid in the subchannel constitute of rods, while the adjacent subchannels are connected by the gap, which is determined by the ratio of the pitch between the rods'center and the diameter of the rod(P/D). Adopting the tight lattice arrangement of the rods whose P/D is relatively small could cut down the water needed, reduce the mass ratio of water and uranium, harden the neutron power spectrum of the core in order to enhance the utilization of the fuel. However, many special flow behaviors also exist in the tight lattice, including the strong mixing effect of coolant between subchannels. In order to study the special structure of flow in tight lattice, experiment investigation and numerical simulation are performed in a 2x3 square channel with P/D= 1.1, and followings are some detailed contents.Visual measurement method and dye tracers are adopted in the experiment, while the Reynolds number is varied from 2000 to 40000. Large scale quasi-periodic oscillation is observed in the narrow gap between rod and wall, while some characteristic parameters of oscillation are obtained. The results show that the large scale oscillation occurs at Re≥5000 which appears periodic behavior strongly and is accompanied by the transition from laminar flow to turbulent flow. The wavelength of the oscillation is 65mm on average, which is also independent of Reynolds number, whereas the oscillation frequency increases proportionally with Re.On the aspect of numerical simulation, firstly, the simulation method of Unsteady Reynolds Averaged Navier-Stokes equations(URANS), combined with periodic boundary condition, are adopted and the results are compared to the existed experiment data in order to systematically assess the validation of two types of turbulent models, and the result shows that the SSG turbulent model is more accurate than any vortex viscous models, which ensure the significance of anisotropy hypothesis of turbulent model in simulation the turbulent flow in subchannel; secondly, the computing model which has the same cross section as the experiment is built to simulate the transient flow behavior in tight lattice, and Large scale quasi-periodic oscillation in the gap between rod and wall is successfully obtained, while the calculated characteristic parameters don't agree much well with the measured data. At the same time, the velocity vibration of other monitor points are also investigated, and the results show that both frequency and amplitude are maximum in the center point of the cross section, and are minimum in the center point of subchannel, moreover, some regular pattern of distortions are found in the velocity oscillation profile. Finally, the mechanism of large scale quasi-period oscillation is preliminarily studied, and the relationship between coherent structure and large scale quasi-period oscillation is verified through both static pressure and factor Q criteria, which confirms the threshold effect of Reynolds number in the occurrence of large scale periodic oscillation.