5×5花瓣形燃料棒束组件内单相流动与换热特性数值模拟研究

相较于传统圆柱形燃料棒，花瓣形燃料棒具有安全裕量高等优点，研究其在压水堆运行工况下的热工水力特性具有重要意义。本文通过STAR-CCM+对5×5花瓣形燃料棒束组件进行数值模拟研究，计算并分析了组件内二次流速度、温度、换热系数等关键热工参数，获得了入口流速、螺旋节距对组件内部流动与换热特性的影响规律。计算结果表明：花瓣形燃料棒的螺旋结构可增强冷却剂的横向流动，同一高度上燃料棒表面温度分布具有周期性，增大入口流速可增强燃料棒的表面换热，消除温度分布的不均匀性。此外，螺旋节距大于750 mm，燃料棒换热性能与无扭转的燃料棒相差不大，甚至更低。

Numerical Study of Single phase Flow and Heat Transfer Characteristics in 5×5 Petal-shape Fuel Rod Bundle Assembly

Compared with the traditional cylindrical fuel rod, petal-shape fuel rod has higher security, so it is of great significance to study thermal-hydraulic characteristics of petal-shape fuel rod utilized in the pressurized water reactor. In this paper, the 5×5 petal-shape fuel rod bundle assembly was numerically simulated by STAR-CCM+. The sensitivity of turbulence model was analyzed. In this study, the relative errors between the SST k-ω model and the experimental data were within 5%, so the SST k-ω model was selected herein. According to AP1000 PWR, boundary conditions were set as the outer wall with adiabatic non-slip wall, and the reference pressure of fluid domain, inlet velocity, inlet temperature and the outlet pressure were set as 15.5 MPa, 1.25-3 m/s, 565.55 K and 0 MPa, respectively. The heating mode of solid domain was set as uniform heating, and the volumetric heat release rate was set as 1.17 times that of AP1000 PWR. The interface between fluid domain and solid domain was connected by INTERFACE. Five different inlet velocities (v_in=1.25 m/s, 1.5 m/s, 2 m/s, 2.5 m/s, 3 m/s) and four different spiral pitches (H=250 mm, 500 mm, 750 mm, 1 000 mm) were selected. The key thermal-hydraulic parameters such as secondary flow velocity, temperature and heat transfer coefficient were calculated and analyzed. The influence of inlet velocity and spiral pitch on flow and heat transfer characteristics of the assembly was obtained. The results show that the spiral of petal-shape fuel rod can enhance the lateral flow of the coolant, so the secondary flow velocity near the fuel rod wall is related to the structure of fuel rod, and compared to the outer convex arc region, it is higher in the inner concave arc region. And also, it is very low in the central area of the subchannel because it is far away from the fuel rod wall. Besides, the surface temperature distribution of the fuel rod at the same height is periodic. The heat transfer coefficient firstly decreases along the flow direction, and then gradually stabilizes. And as inlet velocity increases, the mixing in the rod bundle is gradually enhanced, the non-uniformity of temperature distribution on the fuel rod surface is gradually reduced, and the heat transfer coefficient is increased. The heat transfer coefficient along the fuel rod bundle decreases with the increasing pitch. When the pitch is larger than 750 mm, the heat transfer coefficient of fuel rod is the same as that of non-twisted one, or even lower.