Experimental study of bubble injection in a turbulent boundary layer

A bubbly flow experiment has been performed in a horizontal channel in order to simulate the dynamical effects of the nucleation of bubbles and their departure from the wall in boiling flows. Bubbles were injected through a porous plate located on the lower wall. The void fraction, bubble velocity and diameter were measured with a fibre-optic probe and the liquid flow in the bubble layer was studied with a hot film anemometer. The void fraction profiles are nearly self-similar. The expansion of the bubble layer is quasi-linear with the distance downstream, with a rate of expansion depending on the bubble diameter. Comparison with a simple model of bubble trajectories highlights the role of the lift force in the development of the bubble layer. The mean velocity in the bubble layer does not differ greatly from that measured in single-phase flow, except near the wall. The velocity profiles follow a logarithmic law similar to that for turbulent flow over a rough surface suggesting that bubbles attached to the wall act as roughness elements on the liquid flow. The turbulent kinetic energy is greater than in single-phase flow. The additional turbulence is analysed and attributed partly to the relative motion of the bubbles and partly to the augmentation of the turbulent shear stress in the bubble layer.