CFD Study On Flowing Characteristics Of 5×5 Fuel Assembly Based On The Structured Grids

The reactor core is the key equipment of the first loop in PWR nuclear power plant, which provides energy for the whole reactor system. Fuel assembly is the main component of the core, flowing field characteristics of fuel assembly is very important for its structure design. Due to the complexity of test technology measuring the flow field of fuel assembly, the CFD method has now been widely used to study its flow field characteristics.In this paper, steady state thermo-hydraulic process of fuel assembly in nuclear reactor is the research object. The structured grid meshing method has been applied to generated mesh in computation domain of a fuel assembly, and a series of simulations are made to get the three-dimensional flow field and heat transfer process by CFD code. A fuel assembly with a grid including 3×3 rod bundle, a fuel assembly with a grid including 5×5 rod bundle and a 5×5 rod bundle assembly with three grids has been investigated. Effect of influence factors including inlet velocity and temperature of fluid, the heating mode of bundle and the influence of spacer grids along the height of fuel assembly on flow field of the fuel assembly has been studied. The local flow field surrounding the structure as spacer grid, steel convex, mixing vane has been analyzed in detail. Also the turbulence and heat transfer characteristics between and after the grids have been discussed. As an example case of flow field analysis of complete fuel assembly, a 5×5 bundle assembly with three grids studied consists of double spacer grids located at start and end of fuel assembly and a middle span mixing grid has been taken for study. Four different sets of bending angle and the five groups of different blade length of mixing vane are adopted to carry on the CFD numerical simulation. Effects of blade geometric parameters on turbulence characteristics and the pressure drop of the fuel assembly have been investigated.The results of this paper show that the spacer grid makes the fluid produce noticeable transverse flow, the biggest velocity of the cross flow has received sixty percent of the entrance’s velocity after the spacer grids, and increases the cross flow and heat transfer between the fluid and fuel bundle. The middle span mixing grid has the same function as the spacer grid to flow and heat transfer. Comparing cross flow and heat transfer of different flowing channels including the central channel, the middle channel, the side channel and the corner channel of 5×5 fuel assembly shows that, the central channel has the maximum velocity and highest temperature, the middle channel has a medium velocity and lower fluid temperature, the corner channel has minimum velocity and the lowest temperature. Geometry components on the grid have important effect on the lateral flow and pressure drop, that of the mixing vane is most remarkable. Increasing the deflection angle of mixing vane will enhance the mixing coefficient of spacer grid to the flow field, but also pressure drop of fuel assembly will increase. Increasing the length of mixing vanes in an extent will also increase mixing coefficients of the grid, but there is an optimist length of mixing vanes which is corresponding to the minimum pressure drop, the length is the standard length of mixing vane.The structured grid method proposed in this paper, can not only be used in the fuel assembly with local structure and whole structure, but also can be applied to tube channel and rod channel to greatly improve CFD solution’s speed and precision of flowing in the complicated fuel bundle. The research results of the three types of fuel assembly including 3-D flow field and turbulence characteristics can be used as guide for the structure design of the grid spacer, and provide some basic data for the thermal-hydraulic design and safe and steady operation of the reactor.