Numerical Study on Gas Entrainment by Vortex in T-Junctions

Abstract

Abstract Gas entrainment by free surface vortices in the Residual Heat Removal System (RHRS) would affect the safety of the reactor at the mid-loop operation for nuclear power plants. Thus, it is important to estimate the occurrence of this entrainment accurately. In this work, two-phase Computational Fluid Dynamics (CFD) simulations of a T-junction are performed. In the T-junction, the diameters of the branch/main pipes are 40/142.5 mm, and the lengths of the branch/main pipes are 3Db/6Dm. The Volume of Fluid (VOF) method and RNG k-ε model are employed in this simulation to get a clear gas-liquid interface. And, several cases of different flow rates at the mid-loop operation are studied. The simulation results could show the phenomena of free surface waves, the free surface vortex formation and evolution, gas entrainment by free surface vortex. At a low flow rate, the free surface in the main pipe is relatively stable with no wave, there is no occurrence of vortex core or vortex in the T-junction. With the flow rate increasing, the free surface in the main pipe begins to fluctuate, vortex core occurs below the free surface. It evolves downstream and disappears in the main pipe soon. The free surface waves in the main pipe become more intense when the flow rate continues to increase, and the free surface vortex begins to entrain air into the branch as the vortex core evolving downstream in the main pipe in a very short time. The phenomena of numerical simulation show a good agreement with experimental results. In this paper, physical fields are also analyzed to determine the effect and mechanism of the gas entrainment by free surface vortices.