Inception and Termination of the Core-Annular Flow Pattern for Oil-Water Downflow Through a Vertical Pipe Sumana Ghosh and Gargi Das Dept. of Chemical Engineering, IIT Kharagpur 721302, India Prasanta Kumar Das Dept. of Mechanical Engineering, IIT Kharagpur 721302, India DOI 10.1002/aic.12741 Published online in Wiley Online Library (wileyonlinelibrary.com). Different flow patterns for lube oil–water and for kerosene-water downflow through a vertical glass tube have been analyzed with the help of flow visualization. Core-annular flow is the dominant flow regime, with oil forming the core, and water is forming the wall film. When the velocities are increased, transition to slug flow and transition to dispersed flow are found. The waves found during the transition to slug flow depend on oil viscosity: axisymmetric bamboo waves are seen in kerosene-water downflow and the waves are asymmetric in case of lube oil–water flow where they have a cork-screw shape. Based on the experimental observations, simple mathematical models have been proposed for predicting the flow pattern transition curves. V V C 2011 American Institute of Chemical Engineers AIChE J, 00: 000–000, 2011 Keywords: multiphase-flow, oil–water downflow, transition criteria, flow regimes, core-, annular flow Introduction Liquid-liquid two-phase flow is found in many industrial applications, like chemical processes, multiphase reactors and during the production and transport of oil. In all these applications, the distribution of the two liquids inside the pipeline within a specific flow regime and transition between various flow regimes determine the transport properties, such as the pressure drop and the holdup of the phases. Therefore, knowledge of the flow patterns and an understanding of the transition mechanisms between the flow regimes are neces- sary for a proper design and operation of liquid-liquid flow in pipelines. In the past few decades a number of studies for oil–water transport in pipelines were published. The studies on experimental estimation of flow patterns and theoretical prediction of transition criteria reveals that the researchers have used oils with different properties and have observed the flow patterns to vary with the oil–water throughput, the physical properties (density, viscosity and interfacial ten- sion), the pipe diameter, the pipe wall material (hydrophilic or hydrophobic) and the pipe orientation. For example, strati- fied flow can be observed in case of horizontal flow of low- viscosity oil and water, while it is hardly observed for high- viscosity oil–water systems. The core-annular flow pattern is a common occurrence for high-viscosity oil, but only a few researchers have reported this pattern for low-viscosity oil– water flows. Similarly the dispersed flow pattern comprising of oil dispersed and water dispersed distributions are more common for low-viscous oil–water flow due to their low- interfacial tension. Nevertheless, almost every past study has considered a specific oil–water pair, instead of explicitly analyzing the effect of a range of oil properties on the oil– water transport. In addition, theoretical studies on modeling the transition criteria are primarily confined to the onset and termination of the stratified flow regime. One of the earliest theoretical studies on liquid-liquid flow pattern transition was reported by Brauner and Moalem Maron 1 for horizontal flow. They have predicted the transi- tion from the stratified and annular flow regimes based on linear stability analysis performed on transient two-fluid model. The annular flow to slug flow transition is proposed to occur at a critical holdup of 0.5. They have considered experimental data of Russell et al. 2 (l ¼ 0.018 Pa-s, and q ¼ 834 kg/m 3 , pipe dia. D ¼ 0.0205 m) and Guevara et al. 3 (l ¼ 10 Pa-s, and q ¼ 995 kg/m 3 , pipe dia. D ¼ 0.203 m) for validating their model. Recently Grassi et al. 4 have noticed that the proposition by Brauner and Maron 1 to apply a critical holdup fraction of 0.5 gave a poor estimate for the prediction of the transition from core annular flow to slug flow. Trallero 5 observed that the transition criteria developed for gas-liquid systems were inadequate for horizontal oil– water system. Accordingly, he proposed new transition crite- ria for the stratified and three-layer flow pattern based on Kelvin-Helmoholtz instability mechanism for oil (l ¼ 0.0296 Pa-s, and q ¼ 850 kg/m 3 )—water horizontal flow through an acrylic resin pipe of dia. 0.05 m. He reported that due to Kelvin-Helmoholtz instability interfacial waves increased in amplitude during stratified flow and the wave- length decrease with increase in phase velocities. Brauner 6 has modified the theory of Hinze 7 to predict the transition to gas-liquid and liquid-liquid dispersed flows. The model pro- poses dispersion to exist if the stable bubble diameter is less than the critical diameter. She has noted that phase Correspondence concerning this article should be addressed to G. Das at gargi@che.iitkgp.ernet.in. V V C 2011 American Institute of Chemical Engineers AIChE Journal 1 2011 Vol. 00, No. 0