Analysis of Turbulence Modelling Approaches to Simulate Single-phase Buoyancy Driven Counter-current Flow in a Tilted Tube

In the present paper, both Large Eddy Simulation (LES) and unsteady-RANS (uRANS) CFD studies of single-phase buoyancy-driven counter-current flow in a pipe are presented for an Atwood number of 1.15×10−2 and an inclination angle of 15 degrees from the vertical. The basic flow phenomena involved are fundamentally the same as those encountered in various industrial applications including passive coolant flow in the loops of a nuclear reactor. Earlier work on this type of flow was focused on its physical aspects using both experimental measurements and Direct Numerical Simulation (DNS). The present work investigates the performance of several commonly used LES eddy viscosity subgrid-scale models. The results show that LES is able to reproduce accurately the experimental results and that the dynamic Smagorinsky subgrid-scale model gives the best predictions. The results of those calculations were then used to obtain more information on the physics of such flow. As regards uRANS simulations, a range of classic two-equation linear eddy viscosity models, based on low-y + formulation at the wall and the single gradient diffusion hypothesis for the turbulent density fluxes were compared against experimental data available in the literature. An elliptic blending Reynolds stresses model (EBRSM) with generalised gradient diffusion hypothesis (GGDH) for the scalar fluxes was then implemented and compared with experiments. The latter showed very good agreement for the first order moment with the experimental data whereas all the two-equation eddy viscosity based models showed rather high levels of discrepancy. The main cause of discrepancy was found to be the underprediction of the axial turbulent buoyancy production effect, which has a more detrimental impact on the eddy viscosity models than the second moment closure one.