Session: 05-02-02 CFD Methods

Paper Number: 86863

86863 - Validation of Low and High-Fidelity Turbulence Models for Prediction of Turbulent Heat Transfer in Low Prandtl Number Flows Under Buoyant and Separated Flow Conditions 

Liquid metals such Sodium (Na), Sodium-Potassium (Na-K) and Lead (Pb) are characterized by their high thermal conductivity and low heat capacity compared to other fluids such as water and air (Pr ~1), which makes them excellent coolant that are used in various engineering applications, (e.g., solar power technologies, material processing and advanced nuclear reactors). The turbulent heat transfer modeling of low-Pr flows remains an open challenge as it involves a scale gap in momentum and thermal turbulent diffusion and dissipation.

The objective of this study is to assess the predictive capabilities of low and high-fidelity turbulence models for low-Pr flows. For this purpose, predictions by two-equation (k-ε) Reynolds-Averaged Navier Stokes (RANS), partially-averaged Navier Stokes (PANS) hybrid RANS/Large Eddy Simulation (LES), and DSM, WALE, filtered LES models are compared for four different test cases.

First, simulations are performed for flows in vertical and horizontal channel for Re_τ = 150 and 640 for Pr = 0.025 and 0.71 including both forced and mixed force/natural convection conditions (Ri = 0.1 to 0.2). The results are compared with available DNS database and complimentary DNS datasets generated by the authors. For this case, the accuracy of the models are assessed for their ability in predicting the effect of aiding and opposing buoyant flow conditions on the near wall turbulence structures and thermal diffusion from the aiding side to the opposing side.

Flow through reactors can include flow separation and reattachment. To investigate the predictive capabilities of the different turbulence models for such complex geometries a forced and mixed convective turbulent liquid flow over a vertical backward-facing step with a constant heat flux applied on the wall for Re = 4800, Pr = 0.71 and 0.0088, and Ri = 0.338. The results are compared with available DNS data. This test case involves complex thermal transport and flow behavior such as augmentation of heat transfer in the recirculation and reattachment regions, and those of the recirculation bubble due to favorable buoyant flow conditions.

Next, flow over a bare rod bundle with pitch to diameter ratio (P/D) of 1.3 is performed for Re_h = 22600 and Pr ranging from 0.002 – 2. The results are compared with complimentary DNS datasets generated in the project. This test case involves temperature accumulation in the narrow portion of sub-channel because of turbulent heat diffusion.

Finally, the models are compared for the prediction of heat transfer by ascending and descending flow through a pipe with a constant wall heat flux for different Reynolds (Re ~ 19000 to 29000, Pr = 0.00585 and Ri = 0.17 to 0.65). The results are compared with experimental data available in the literature.

Presenting Author: Shanti Bhushan Mississippi State University