What is entrainment in vertical two-phase churn flow?

Churn flow, the least understood of the flow patterns in vertical upflow in pipes, has been receiving attention recently. One particular aspect addressed is the fraction of liquid entrained in the gas phase. This paper reports on how data on the phase split at T-junctions has been used to determine this fraction at liquid flow rates much higher than hitherto. The results and other available data are examined to question whether the entrained liquid is being carried as drops or travelling in huge waves which have been identified as being present at the flow rates corresponding to churn flow.