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We performed laboratory experiments on bubbly channel flows using silicone oil, which has a low surface tension and clean interface to bubbles, as a test fluid to evaluate the wall shear stress modification for different regimes of bubble migration status. The channel Reynolds numbers of the flow ranged from 1000 to 5000, covering laminar, transition and turbulent flow regimes. The bubble deformation and swarms were classified as packing, film, foam, dispersed, and stretched states based on visualization of bubbles as a bulk void fraction changed. In the dispersed and film states, the wall shear stress reduced by 9% from that in the single-phase condition; by contrast, the wall shear stress increased in the stretched, packing, and foam states. We carried out statistical analysis of the time-series of the wall shear stress in the transition and turbulent-flow regimes. Variations of the PDF of the shear stress and the higher order moments in the statistic indicated that the injection of bubbles generated pseudo-turbulence in the transition regime and suppressed drag-inducing events in the turbulent regime. Bubble images and measurements of shear stress revealed a correlated wave with a time lag, for which we discuss associated to the bubble dynamics and effective viscosity of the bubble mixture in wall proximity.

The flow of a liquid-gas foam may be represented by a modified Bingham plastic model, with the addition of a liquid-rich slip layer caused by bubble migration away from a solid-surface. The model was applied to experimental results for cone and plate rheometer flow and pipe flow. It was found that the consistency and flow behaviour index are to a first approximation independent of expansion ratio and bubble size. The yield stress is dependent on bubble size distribution. The slip layer thickness depends on bubble size distribution and flow geometry, and is the most important parameter in pipe flow. There are changes in foam properties at an expansion ratio around 5 due to the change from gas emulsion to froth configuration.