含绕丝棒束内横向流动的可视化实验与数值模拟研究

Flow-visualization Experiment and Numerical Investigation of Cross Flow in Wire-wrapped Rod Bundle

螺旋绕丝是第4代先进核反应堆燃料组件的定位装置之一,冷却剂在绕丝作用下发生定向交混和周期性横流,是影响燃料元件包壳温度的重要因素,有必要进行深入研究。本文采用粒子图像测速法对含绕丝7棒束内水的横向流动进行实验测量,得到了横截面整体速度分布和局部流动特性,并利用STAR CCM+软件进行了稳态数值模拟,分析了无量纲横向流速的变化规律。结果表明,棒束内存在整体环流和若干局部涡流,大尺度涡出现在棒和内壁附近,小尺度涡出现在棒和绕丝附近。随着雷诺数的增大,横向流动规律基本一致,但速度大小呈线性增长趋势。湍流模型敏感性分析表明,雷诺应力-椭圆混合模型对流场的预测与实验数据最为接近。横向流动具有明显的周期性,对于所选定的棒束结构,最大横向流速约为轴向流速的25%,且在绕丝经过棒间隙时出现突变。

Wire-wrapped spacer is a type of locating device,which has been planned to be used in the design of fuel assembly of Gen-Ⅳadvanced nuclear reactors,e.g.[KG-*3],sodium-cooled fast reactor and supercritical water-cooled reactor.As the coolant sweeps the wrapped wires,directional mixing and periodic lateral flow can be encountered under the interaction of wires.The cross flow and mixing significantly affect the fuel cladding temperature,which is a concerned issue in the thermal-hydraulic design of the fuel assembly.In this paper,the cross flow of water in a 7-pin wire-wrapped rod bundle was studied by means of particle image velocimetry(PIV)and computational fluid dynamics(CFD).The diameter and wall thickness of each tube were 30 mm and 0.5 mm,respectively.The wrapped wires were made of fluorinated ethylene propylene(FEP)whose index of refraction is close to water at room temperature.This meets the requirement of MIR(matching index of refraction)in performing PIV experiment.The diameter,wall thickness and axial pitch of each wire were 5 mm,0.3 mm and 500 mm,respectively.A continuous-wave laser with a power of 8 W was utilized as the laser source.Hollow glass microspheres of 10-14μm in mean diameter and 1.04 g/cm 3 in density were used as the seeding particles.The movement of the particles was captured by a Fastec TS3 high-speed camera with a resolution of 1024 pixels×1024 pixels.The flow-visualization experiment was carried out under three Reynolds numbers,i.e.[KG-*3],3000,6000 and 9000.This corresponds to streamwise mean velocity of 0.2,0.4 and 0.6 m/s,respectively.The cross-section of the rod bundle was divided into 18 sub-areas which were photographed separately.Vectors in each sub-area were merged together eventually to obtain the cross flow on the entire cross-section.Based on the experimental measurements,it is found that small-scale turbulent vortex and large-scale circulation flow exist inside the rod bundle.The former usually appears at the windward of the wire,which is caused by the fluid-wire collision.The latter generally appears at the outer region of the bundle,which is mainly resulted from the blockage effect of the rod and consequent flow diversion.With the increase of Reynolds number,the distribution of cross flow stays universal,but the magnitude of velocity increases almost linearly.In addition to the experimental measurement,CFD software STAR-CCM+was utilized as well to study the characteristics of cross flow inside the wire-wrapped rod bundle.Sensitive study of turbulence models reveals that the prediction given by elliptical hybrid Reynolds stress model agrees well with experimental data.For the selected bundle geometry,the maximum cross flow is about 25%of the axial velocity,and changes abruptly when the wire passes the rod gap.