Analysis of Hysteresis in the Regime Transition of Cocurrent Liquid-Gas Flow

The dynamics of cocurrent liquid-gas flow control the flow patterns and phase distribution inside the submerged entry nozzle (SEN) in the continuous casting. This regime transition from bubbly flow to annular flow is usually associated with a hysteresis effect that is not fully understood yet. Herein, the regime transition in an analogous liquid-gas flow is investigated using the volume of fluid (VOF) method. A downward turbulent water flow in a vertical pipe is considered as the computational domain at the top of which the gas is injected as a volumetric source in the VOF equation. By temporal variation of the gas volume rate following a linear ramp-up and then ramp-down, a transition from bubbly to annular flow and vice versa is observed. However, the transition occurs at different operation points and the numerical simulation pictures this hysteresis phenomenon. The regime transition is connected to the evolution of interfacial turbulence in each phase represented by the amount of vortical energy, that is, enstrophy. In addition, the temporal variation of different enstrophy generation/destruction mechanisms is evaluated. The hysteresis phenomenon is explained by the differences in the history of these mechanisms and the difference in the enstrophy generation level upon transition.