Bubbles' slip velocity reduction in the near wall region of turbulent bubbly pipe flows under different gravity conditions

Bubbles' dynamics in the near-wall region of turbulent bubbly flows can significantly influence transfer mechanisms. The present paper is devoted to an analysis of the interfacial momentum transfer in the turbulent bubbly pipe flows under different gravity conditions, in particular in the near-wall region where the void fraction and turbulent shear stress gradients are important. A complete formulation of the turbulent contribution of the added mass force is proposed and applied to axisymmetric parallel bubbly flows. The resulting closure is analyzed using the vertical pipe bubbly flows experimental data carried out by Colin et al. (2012) under different gravity conditions, upward flow (1 g), downward flow (-1 g) and bubbly flow under microgravity condition (0 g). Sensitive analysis of the momentum transfer closure show that in the near-wall region of upward and downward bubbly flows, the turbulent contributions of the added mass force may become large enough to oppose the gravity action and to produce significant reduction of the bubbles' slip velocities as observed in experiments. In micro-gravity conditions, the turbulent contributions of the added force are of the same order of magnitude than the residual gravity. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.