On the role of droplets in cocurrent annular and churn-annular pipe flow

Decreasing the gas flow-rate in an initially vertical upward annular dispersed pipe-flow, will eventually lead to a down-flow of liquid. The onset of this down-flow has been related in the literature to the presence of the dispersed phase and the instability of the liquid film. Here we investigate how the dispersed-phase may influence the down-flow, performing detailed PDA-measurements in a 5 cm vertical air-water annular-flow. It is shown that the dispersed-phase does not cause the liquid down-flow, but that it delays the onset of liquid down-flow. In cocurrent annular flow the dispersed phase seems to stabilise the film flow, whereas in churn-annular flow the opposite seems to be true.