Abstract In this paper, the time-resolved particle image velocimetry (TR-PIV) and match index refractive (MIR) techniques were used to study the flow field in a large range (0 – 22 Dh) downstream a spacer grid (SG) in a 5 × 5 rod bundle channel at different Reynolds number. The sodium chloride solution (1%) is used as the working fluid to reduce the refractive index error of fluorinated ethylene propylene (FEP) and water. The proper orthogonal decomposition (POD) background removal technique was used to minimize the FEP reflection. These methods greatly reduced the interference of background noise and improved the accuracy of cross-correlation calculation. For TR-PIV velocity fields downstream of the mixing vanes, time-averaged, statistical, spectral, and cross-correlation analysis were performed for the instantaneous full-field experimental data. The transport characteristics of coherent structures in different subchannels of rod bundles are calculated and discussed. The results show that the SG caused a relatively large transverse velocity and reduces the axial velocity. With the increase of the Reynolds number, the SG promotes the generation of transverse flow and has a great resistance to the axial flow. There is relatively large turbulence intensity downstream of the SG due to the mixing effect. The attenuation of transverse turbulence intensity component is slower than the axial component. Moreover, spectrum analysis shows that cross-arranged mixing vanes will generate periodic vortices but single mixing vane will not. These periodic vortices gradually propagate downstream along the inner subchannel and dissipate in the gap subchannel due to the effect of viscosity. The cross-correlation analysis shows that the mixing effect of the SG will reduce the scale of the coherent structure, and increase the convection velocity. The results of current research are helpful for understanding the strong anisotropic turbulence in the rod bundle channel with SG. Finally, the experimental results can be utilized to benchmark the applicability of turbulence models under different Reynolds number and the performance of partially averaged Naiver–Stokes or multiple RANS algorithms downstream of the SG.

中文翻译：

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使用 TR-PIV 的 5 × 5 棒束中间隔网格下游大范围内流动结构的实验研究

摘要 在本文中，时间分辨粒子图像测速 (TR-PIV) 和匹配折射率 (MIR) 技术被用于研究大范围 (0 – 22 Dh) 下游间隔网格 (SG) 的流场。不同雷诺数下的 5 × 5 棒束通道。以氯化钠溶液（1%）为工作液，降低氟化乙丙烯（FEP）与水的折射率误差。适当的正交分解 (POD) 背景去除技术用于最小化 FEP 反射。这些方法大大降低了背景噪声的干扰，提高了互相关计算的准确性。对于混合叶片下游的 TR-PIV 速度场，对瞬时全场实验数据进行了时间平均、统计、光谱和互相关分析。计算并讨论了棒束不同子通道内相干结构的输运特性。结果表明，SG引起了较大的横向速度并降低了轴向速度。随着雷诺数的增加，SG促进了横向流的产生，对轴向流有很大的阻力。由于混合效应，SG 下游存在相对较大的湍流强度。横向湍流强度分量的衰减比轴向分量慢。此外，频谱分析表明，交叉排列的混合叶片会产生周期性涡流，而单个混合叶片不会。由于粘性的影响，这些周期性涡流沿着内部子通道逐渐向下游传播，并在间隙子通道中消散。互相关分析表明，SG的混合效应会降低相干结构的尺度，增加对流速度。目前的研究结果有助于理解具有SG的棒束通道中的强各向异性湍流。最后，实验结果可用于对湍流模型在不同雷诺数下的适用性以及部分平均 Naiver-Stokes 或 SG 下游多个 RANS 算法的性能进行基准测试。