In reactor cooling systems of nuclear power plants, the fluctuating stresses on a piping system are potential causes of thermal fatigue failures. These stresses are generated by temperature fluctuations in regions where hot and cold flows are intensively mixed together. The present paper describes BSL Reynolds Stress (BSL-RSM) and Shear Stress Transport (SST) simulation data of thermal mixing in a mixing piping compared with results from experimental data carried out by the Laboratory for Nuclear Energy Systems, Institute for Energy Technology, ETHZ, Zu¨rich, Switzerland. The experimental measurements (concentration of de-ionized water) have been carried out by the use of the 16×16 electrode wire-mesh sensors (WMS) on the basis of difference in liquid electrical conductivities. To verify the analogy between electrical conductivity of de-ionized water and temperature of water, the inlet temperatures of the hot and cold water are set to 80°C and 15 °C respectively in the simulation process. The analytic results are calculated by the commercial CFD code ANSYS CFX 11.0. It is known from literatures, that the different mesh types (Tetrahedral, Tetrahedral/Wedge, Hexahedral), different turbulence models (BSL-RSM, SST), and turbulent Prandtl numbers (0.9, 0.2, 0.1) will affect the simulation results of temperature fluctuations. Since the coefficient of determination, R2, obtained by using Prt = 0.1 for SST turbulence model is between 0.999 and 0.915, which is higher than that by using Prt = 0.2 and Prt = 0.9, the perditions for smaller turbulent Prandtl number are in better agreement with measurements. Moreover, predicted results for Prt = 0.1 obtained by using BSL-RSM turbulence model are almost the same as those by using SST turbulence model. The computational results are in qualitative good agreement with experimental data that the mixing phenomenon of two different temperature water can be explained by de-ionized water with two different electrical conductivities. Additionally, the simulation results with Tetrahedral/Wedge and Hexahedral mesh types are better than those with Tetrahedral mesh type.

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