During the water-filling and air-exhaustion commissioning process of pipes systems in engineering fields such as nuclear power plants, gas trapped in the branch structures cannot be fully discharged, posing safety hazards to system operation. In this paper, experiments revealed that the gas-volume ratio retained in the branch structure exhibits an overall monotonic relationship with liquid flow velocity, decreasing as the flow velocity increases; when the flow velocity reaches a critical value under the operating conditions, the gas is completely expelled. The RNG k-ε turbulence model and the Mixture gas-liquid two-phase flow model were employed to numerically simulate gas entrainment in pipes with the branch structure The research shows that the Kelvin–Helmholtz vortices promote gas entrainment effectively.
 

gas-liquid two-phase flow; branch structure; gas entrainment; CFD simulation