A liquid plug moving in an annular pipe—Flow analysis

The flow in a liquid plug moving in an annular pipe is analytically solved. The interaction with the two concentric walls of the annular pipe results in two toroidal vortexes within the concentric plug. Focus is put on long plugs with aspect ratio beta > 2, which have vortex circulation flow rates and volume ratio independent of the plug length. Based on the analytical results, correlations are derived for the circulation flow rates of the plug and each vortex and for the volume ratio of the two vortexes. Correlations are also developed for evaluating the radial transport of the plug flow. The friction factor for concentric plugs is a function of the aspect ratio and the radius ratio. For very long plugs with beta >> 1, the friction factor approaches that of the fully developed continuous flow in the annular pipe. Published by AIP Publishing.