Turbulent bubbly flow in pipe under gravity and microgravity conditions

Experiments on vertical turbulent flow with millimetric bubbles, under three gravity conditions, upward, downward and microgravity flows ( $1g$ , $\ensuremath{-} 1g$ and $0g$ ), have been performed to understand the influence of gravity upon the flow structure and the phase distribution. The mean and fluctuating phase velocities, shear stress, turbulence production, gas fraction and bubble size have been measured or determined. The results for $0g$ flow obtained during parabolic flights are taken as reference for buoyant $1g$ and $\ensuremath{-} 1g$ flows. Three buoyancy numbers are introduced to understand and quantify the effects of gravity with respect to friction. We show that the kinematic structure of the liquid is similar to single-phase flow for $0g$ flow whereas it deviates in $1g$ and $\ensuremath{-} 1g$ buoyant flows. The present results confirm the existence of a two-layer structure for buoyant flows with a nearly homogeneous core and a wall layer similar to the single-phase inertial layer whose thickness seems to result from a friction–gravity balance. The distributions of phase velocity, shear stress and turbulence are discussed in the light of various existing physical models. This leads to a dimensionless correlation that quantifies the wall shear stress increase due to buoyancy. The turbulent dispersion, the lift and the nonlinear effects of added mass are taken into account in a simplified model for the phase distribution. Its analytical solution gives a qualitative description of the gas fraction distribution in the wall layer.