Convection heat transfer of NaK-78 liquid metal in a circular tube and a tri-lobe channel

Abstract

Computational Fluid Dynamics (CFD) analysis of turbulent convection heat transfer of NaK-78, liquid metal alloy in a uniformly heated circular tube are performed and the results are compared to reported experimental measurements. Also presented are the results for a tri-lobe channel, of the same heated length, equivalent hydraulic diameter, wall heat flux, liquid mass flux, setup and the inlet and exit bulk temperatures as in the circular tube experiments. The tri-lobe channel has 28% larger cross-sectional and surface areas than the circular tube. Examined are the effect of using the two-layer realizable k–ε, the Shear Stress Transport (SST) k–ω, and the SST Detached Eddy Simulation (DES) turbulence models on the computation time and the accuracy of the results. The calculated axial distribution of the wall temperature for the circular tube compares favorably to reported measurements at Pe=163 and 796. This agreement confirms the soundness of the implemented methodology and the numerical meshing grid. The calculated and experimental Nu values for fully developed flows in the uniformly heated circular tube, covering a wide range of Pe values, are correlated to within ±10%, as:Nu=5.6+0.013Pe0.863Although the calculated Nu values using the different turbulence models are well within the ±10% of the experimental data, the computation time using the DES turbulence model is 13 and 14 time that using the k–ω and the k–ε model, respectively. Unlike for the circular tube, the local Nu in the tri-lobe channel varies azimuthally with the values in the corners are much lower than in the concavities of the tri-lobe channel. The calculated average values of Nusselt number, N¯u for the fully developed flows of liquid NaK-78 in the tri-lobe channel are correlated, to within ±5% as:N¯u=6.6+0.007Pe0.94For Pe<100, the N¯u values for the tri-lobe channel are ∼15% higher than for the uniformly heated circular tube, but the difference gradually decreases with increasing Pe.