Transition from stratified to non-stratified oil-water flows using a bluff body

In this paper the effect of a transverse cylindrical rod immersed in water on the flow patterns and interfacial characteristics of an oil-water pipe flow is investigated experimentally. The cylinder is used to passively actuate the transition from stratified to non-stratified flows and to localise the formation of waves and the detachment of drops. The studies are carried out in an acrylic test section with 37 mm ID using as test fluids tap water and Exxsol D140 (density 830 kg m-3 and viscosity 5.5 cP). The rod has 5 mm diameter and is located at 460 mm from the test section inlet. Flow patterns and interface characteristics were studied with high speed imaging. It was found that the presence of the rod generates waves shortly downstream, from which drops detach, and reduces the mixture velocity for the transition from stratified to non-stratified flows. The average interface height and wave amplitude increase with distance from the rod, while the average wave length and frequency remain almost constant. The Strouhal number is found to be equal to 0.24, while the wave velocities are slightly higher than the mixture velocities