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

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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