Numerical analysis of transient vortex formation at the outlet of a tank containing gas-liquid phases

Abstract

One of the basic phenomena when a liquid leaves a tank, is the formation of vortices. This phenomenon can significantly affect the remaining liquid mass in the tank and the penetration of air and bubbles into the system. As a result, system performance may be disrupted. This study numerically investigates the influence of parameters such as discharge time, gas pressure, liquid depth, and angular velocity of the reference frame on the vortex characteristics. In addition, the near-wall behavior of the analytical relations proposed for the tangential velocity has been revised based on the boundary layer theory. The "volume of fluid" model (VOF) and the SST k-ω turbulence model have been used to solve the two-phase turbulent flow. The results show that increasing the gas pressure from 1 to 5 bar and its direct impact on the liquid surface significantly accelerates the formation of the vortex and the critical height. This phenomenon causes, after the starting of liquid discharge, the air core to reach the inlet port of the outlet pipe about 7 seconds earlier. As a result, much more liquid mass remains in the tank. Also, the increase in the angular velocity of the reference frame from 0.1 to 1 rad/s causes the critical height to be formed much sooner and the remaining liquid mass to increase by 32 kg. Furthermore, the amount and the variations of turbulent viscosity significantly differ from the semi-empirical constant values, limiting their use to particular streams.