Pump manufacturers do not normally provide the characteristic curves of their pumps working as turbines. Therefore, establishing a correlation between the performances of direct (pump) and reverse (turbine) modes is essential in selecting the proper machi

Two-phase gas/liquid flows in vertical pipes have been systematically investigated. Water and SDS surfactant solutions at various concentrations were used as the working fluids. In particular, we focus our work on the influence of surfactant addition on the flow regimes, the corresponding pressure gradients, and the bubble sizes and velocity. Adding the surfactant lowers the air critical Reynolds numbers for the bubble–slug flow and the slug flow transitions. The pressure gradients of SDS solutions are lower than those of pure water especially in the slug flow and the slug–churn flow regimes, implying turbulent drag reduction. At low Reair, the bubble sizes of the surfactant solution are lower than those of pure water due to the increase in viscosity. With increasing and at high Reair, the bubble sizes of the SDS solution become greater than those of pure water which is attributed to the effect of surface tension.