Turbulent thermal mixing of hot and cold flows and flow structure in mixing T-junction

核电站中的管道系统有许多T形接头，其中冷热流体以不同的速度比相遇并混合在一起。现已指出，冷热水混合引起的空间和时间温度波动会导致T形接头周围管道结构的高周热疲劳。这种现象被称为热剥落，是核电站安全设计的一个严重问题。虽然已经从热流体工程的角度对此问题进行了一些研究，但对T形接头周围详细的非定常流动结构和混合特性还没有很好的了解。特别是，与交叉型混合流相比，关于逆流型混合流的详细数据非常稀少。针对上述情况，我们对逆T形结中的非定常流动和热混合特性进行了实验研究。用…对管道系统中的T形接头进行建模混合流道与主、反流道成直角连接，主流道与反流道流动迎头相撞的混合三通截面较多。主槽流雷诺数固定为3500，逆槽与主槽流速比由1.0变为5.0。为了更好地了解非定常流动特性，我们使用了PIV，结合高速摄像机和连续波长的YVO_4激光器，使我们能够以1 kHz的时间分辨率测量速度分布。温度测量被基于热质传递类比的浓度测量所取代。将罗丹明6G作为荧光示踪剂溶解在主流道中进行浓度测量，发现混合流道中的流动具有复杂的三维非定常结构，并伴随着分离、再附着和纵向涡流。浓度(温度)分布与这些流动特性密切相关。为了识别粘性流动结构，对混合通道内的脉动速度场进行了本征正交分解(POD)分析。可以提取沿剪切层的非定常涡旋运动和主通道与反通道流动界面的摆动运动，两者都会产生较高的浓度波动，并在热剥离过程中发挥重要作用。较少

A piping system in a nuclear power plant has many T-junctions, in which hot and cold fluids meet with various velocity ratios and are mixed together. It is now pointed out that spatial and temporal temperature fluctuations due to the mixing of hot and cold water induce high-cycle thermal fatigue of the piping structure around a T-junction. This phenomenon is called thermal striping and is a serious problem for the safety design of nuclear power plants. Although several researches have been conducted on this issue from a viewpoint of thermo-fluids engineering, detailed unsteady flow structure and mixing characteristics around the T-junction are not well understood yet. In particular, detailed data on the counter-type mixing flow are quite scarce in comparison with the cross-type mixing flow. In view of the above situation, we investigated experimentally the unsteady flow and thermal mixing characteristics in a counter-type T-junction. The T-junction in the piping system was modeled by a … More mixing tee with square cross sections, in which the mixing channel was connected with the main and counter channels at right angles and the flow in the main channel collided head-on with that in the counter channel. Reynolds number of the main-channel flow was fixed at 3500, and the velocity ratio of the counter-channel flow to main one was changed from 1.0 to 5.0. For a better understanding of the unsteady flow characteristics, we used PIV with the combination of a high-speed camera and a continuous wavelength YVO_4 laser, which enabled us to measure the velocity distribution with a time resolution of 1 kHz. The temperature measurement was replaced by the concentration measurement based on the heat and mass transfer analogy. PLIF was used for the concentration measurement, in which Rhodamine 6G was dissolved in the main channel flow as a fluorescence tracer.It was found that the flow in the mixing channel had a complex three-dimensional and unsteady structure accompanied by the flow separation, reattachment, and longitudinal vortices. The concentration (temperature) distribution was closely related to those flow characteristics. To identify the coherent flow structures, the proper orthogonal decomposition (POD) analysis was applied to the fluctuating velocity field in the mixing channel. The unsteady vortex motion along the shear layer and the wobbling motion of the interface between the main-channel and counter-channel flows could be extracted, both of which produced high concentration fluctuation and could play an important role in the thermal striping process. Less