The lift force on a bubble in a sheared suspension in a slightly inclined channel

The lattice Boltzmann method was applied to simulate the free rise of monodisperse non-coalescing spherical bubbles in slightly inclined channels bound by solid walls. The Reynolds number based on the relative velocity between the bubbles and the fluid ranged from 4 to 16, the volume fraction from 5% to 10% and the inclination angle from 2 degrees to 6 degrees. The simulations revealed that the weak buoyancy component normal to the walls led to a layer of bubbles near the upper wall and a depleted layer near the bottom wall. These thin layers drove a nearly viscometric shear flow within the bulk of the channel that allowed an unambiguous determination of the lift force in a sheared homogeneous and freely evolving bubble suspension. The lift force coefficients calculated from our simulations were always higher than those for isolated spherical bubbles, suggesting that the lift force is enhanced by hydrodynamic interactions among the bubbles. Experimental measurements of the velocity gradient in 10% volume fraction bubble suspensions in glycerine-water-electrolyte mixtures in slightly inclined channels yielded lift coefficients in excess of those predicted for isolated bubbles and confirmed the qualitative predictions of the simulations.