In view of the practical importance of predicting void fraction in two-phase flow, a correlation based on the drift-flux model is often utilized in many industrial fields. To use the drift-flux type correlation, two drift-flux parameters such as distribution parameter and void fraction weighted mean drift velocity should be modeled accurately. Since the drift-flux parameters are susceptible to channel size, they are modeled differently depending on whether the channel size is characterized as small (or mini), medium or large. Although drift-flux type correlations with modeled distribution parameter and drift velocity for medium size channels with various geometries have been well-developed, limited drift-flux correlations are available for large size channels. Two-phase flow characteristics in a large size channel are quite different from those in a medium size channel at low flow conditions. Large slug bubbles observed in a medium size channel do not form in a large size channel due to the presence of surface instability. As large slug bubbles disintegrate into smaller cap bubbles, the drift velocity is increased. Complex two-phase flow characteristics in a large size channel also affect the distribution parameter. This study successfully models the distribution parameter and drift velocity for two-phase flow in a vertical concentric annulus. The drift-flux correlation with modeled distribution parameter and drift velocity is validated by existing data taken in large size annuli. The applicability of the drift-flux correlation to two-phase flow in a vertical eccentric annulus is also examined by existing data. (C) 2018 Elsevier Ltd. All rights reserved.