AbstractA modeling and experimental study of the core‐annular flow (CAF) was carried out with a viscous liquid membrane emulsion as the core and an aqueous solution as the annulus. A hydrodynamic model has been developed for stable CAF with a centered, nondeformable core surrounded by a turbulent aqueous annulus with or without a drag reducer. This model has been verified with data from flow tests with viscous emulsions in a horizontal flow loop of 3/4‐in. (19‐mm) tubing and a fullscale, down‐hole well tubing of 2‐7/8 in. (73 mm). By the use of this model along with the data, the effects of annular size on total and core flow rates and pressure drop are investigated for annuli with and without a drag reducer.