The effect of swirl on transition from churn flow to annular flow in an intermediate diameter pipe

Swirling annular flow has been widely used in industries. The transition boundary of swirling annular flow which is important for its application in practice is rarely studied in published works. In this work, the effect of swirl on transition from churn flow to annular flow in an intermediate diameter pipe has been investigated by experimentation and modelling. The experiments were conducted with air–water two-phase flow in an 11-m vertical pipe (D = 62 mm) at the range of the test conditions: the liquid superficial velocity ranges from 0.0039 to 0.95 m/s and the gas superficial velocity ranges from 1.22 to 32.49 m/s. The experimental results showed that the churn flow can be transformed to swirling annular flow under the effect of swirl, and the gas velocity required for the transition from churn flow to swirling annular flow in a swirling flow is lower compared with that for the transition from churn flow to annular flow in a non-swirling flow. Then a theoretical model based on separated flow model was established to predict the transition boundary between churn flow and swirling annular flow in a vertical pipe. Calculated results were in good agreements with experimental results.