Turbulent bubbly flow in tube under gravity and microgravity conditions
Résumé
Experiments in vertical turbulent flow with millimetric bubbles, under three gravity conditions, upward, downward and microgravity flows (1g, -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 velocity, shear stress, turbulence production, gas fraction and bubble size have been measured or determined. The results of 0g flow obtained during parabolic flights are taken as reference for buoyant 1g and -1g flows. Two buoyancy numbers, the inverse frictional Froude number and the dimensionless slip velocity, 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 when =0 (0g flow) whereas it deviates when >0 (1g and -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. It leads to a dimensionless correlation to quantify the wall shear stress increase by 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.
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