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Description
The dynamics of co-current liquid-gas flow controls the flow patterns and phase distribution inside the submerged entry nozzle (SEN) in the continuous casting. According to the flow parameters during the argon gas injection, the unsteady two-phase flow in the SEN experiences different regimes from bubbly flow to annular flow. This regime transition is usually associated with a hysteresis effect that is not fully understood yet. In this study, we investigate the regime transition in an analogous downward water-air system using the volume of fluid (VOF) method. A vertical pipe with a liquid inlet is considered as the computational domain at the top of which the gas is injected at different flow rates. Initially, the gas is injected at lower rates to establish a bubbly flow. By a temporally-linear increase of the gas volume rate (ramp-up), the bubbles become larger and the flow approaches the transition point where a huge amount of gas forms an annular flow. Once the annular flow is established, the gas injection rate is reduced with the same slope (ramp-down) to its initial value. This results in a transition from annular flow to bubbly flow, but at a different operation point. This hysteresis phenomenon is pictured by the numerical simulation. We studied the turbulence-interface interactions during the regime transition and analyzed the production and dissipation mechanisms in the transport equations of turbulent kinetic energy and enstrophy in such inhomogeneous flow with density and viscosity contrasts. The analysis unveils the contribution of interfacial topological changes in the flow to the hysteresis occurrence. The findings provide physical insights into the formation of interfacial structures in turbulent multiphase flows and could have a direct implication in the quality control of the continuous casting process.