In this work, we consider the influence of surfactants on the flow pattern transitions of air-water flow in vertical pipes. Surfactants cause the formation of foam, which suppresses the irregularities in the flow. Thereby, the foam significantly decreases the gas flow rate associated with the transition between annular and churn flow. Furthermore, this transition is no longer independent of the liquid flow rate, as the foam can more easily suppress the churning at low liquid flow rates. At sufficiently large surfactant concentrations, the foam suppresses all churning, leading to a direct transition between annular and slug flow. Using results from flow visualisation, the effect of the surfactants on the morphology of the different flow patterns is analysed. The results provide important subsidies for a mechanistic model of air-water-foam flow.

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