Slug flow was studied for air velocities less than 8 m/s with polyethylene pellets in a straight pipe 32 mm in diameter and 5.5 m in length, and was classified into 2 regimes: successive slug flow and solitary slug flow. A settled layer of particles did not form in the latter and high air pressure was required to push the slug as compared with the former. Successive slug flow was found in the low-velocity region where the power consumption reaches a minimum, a minimum that is lower than the minimum of the suspension flow region. (C) 2009 Elsevier B.V. All rights reserved.