A profound knowledge of huge wave and droplet entrainment mechanism is crucial for the thorough study on the gas-liquid churn flow. Although studies have shown that the entrained fraction is high in churn flow and reaches the minimum around the churn-annular transition, the underlying mechanism of the drop entrainment in churn flow is still not well explored. To address this, we investigated the properties of the huge waves and the droplet entrainment in two vertical pipes with the inner diameter of 19 mm and 34 mm under churn flow conditions. We found that the flooding of the film was a characteristic of the churn flow throughout the regime. In addition, increasing the gas or liquid flow rate could lead to the transition from churn flow to annular flow or reverse to slug flow, providing the insight into the differences among slug, churn and annular flow. We also discussed the film instability under different flow conditions and tried to reveal the physical mechanism based on the instability analysis. In our study, the bag breakup and the ligament breakup were observed to coexist The analysis of the liquid distribution in the cross-section of the pipes not only revealed the variations of the entrained fraction of churn flow from that of annular flow, but also indirectly illustrated the differences between their breakup mechanisms. Moreover, the wave properties (amplitude and frequency) were also analyzed in detail. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.