The effect of surfactants on air-water annular and churn flow in vertical pipes. Part 2: Liquid holdup and pressure gradient dynamics

In this paper, we study the effect of surfactants on both the liquid holdup and the dynamics of the pressure gradient in annular and churn flow in vertical pipes. This effect is linked to the influence of the surfactants on the morphology of the air-water interface, which is studied in a related paper (van Nimwegen et al., 2014). The experimental results, obtained using a vertical flow loop with a 5 cm internal diameter at ambient pressure, show three different effects of the surfactants on the measured quantities, depending on the air and water flow rates. (i) At large air flow rates, in the annular flow regime for air-water flow, the surfactants increase the pressure gradient; this is solely due to the increase of the frictional pressure gradient, caused by the larger interfacial stress between the foamy waves overlaying the foam substrate along the wall and the gas core. (ii) At low air flow rates and low water flow rates, in the churn flow regime for air-water flow, the surfactants decrease significantly both the average pressure gradient and the pressure gradient fluctuations. While the frictional pressure gradient increases, the liquid holdup decreases by more than a factor of two; the foam suppresses the flooding waves, making the flow much more regular, leading to the small pressure gradient fluctuations. Furthermore, there exists an optimum surfactant concentration for decreasing the average pressure gradient and the pressure gradient fluctuations. (iii) At low air and high water flow rates, in the churn flow regime for air-water flow, the average pressure gradient and the pressure gradient fluctuations are both somewhat decreased by the surfactants.