Density-Stable Displacement Flow of Immiscible Fluids in Incline Pipes

We experimentally study the iso-viscous displacement flow of two immiscible Newtonian fluids in an inclined pipe and the displacement flow of a viscoplastic fluid by a Newtonian fluid in an inclined pipe. The less dense displacing fluid is placed above the more dense displaced fluid in a density-stable configuration. In the Newtonian flow, the displacing and displaced solutions are oil and water-based respectively. The former exhibits non-wetting behavior in the vicinity of the pipe wall whereas the latter is wetting. The pipe has a small diameter-to-length ratio. The mixing and interpenetration of two fluids is observed over a wide range of controlling parameters, revealing remarkable results. A major contribution of the work is that compared to the previously studied miscible limit, there is novel behavior observed at the interface between the two fluids: the displaced fluid stays "pinned" to the lower wall of the pipe upon pumping the displacing one. This phenomenon, which is observed over all ranges of flow rate, inclination angle, and density difference, is associated with the wetting characteristic of the displacing liquid and is also present when light and heavy viscosity mineral oils are used as the displacing fluid. Ultrasonic Doppler Velocimetry (UDV) revealed a segmented velocity profile at the interface of the immiscible fluids. Due to pinning in the Newtonian case, the efficiency of the removal of displaced fluid in the immiscible limit can be lowered by 14% compared to the miscible case. Within the family of Newtonian immiscible fluids, the maximum efficiency is achieved at close-to-vertical inclination angle, large density difference, and counter-intuitively low imposed flow rate, which is of great importance in industrial design. In the immiscible viscoplastic flow, the displacing solution is a light oil while the displaced solution is a carbopol gel which is densified by glycerol. "Difficult" displacements (Bingham number >> 1), are studied, revealing center-type displacements similar to those in previous studies, in which the displacing oil leaves a ring of carbopol gel behind in the pipe. Displacement efficiency as measured by the dimensionless displaced fluid area α is discussed, as is fracturing of the gel layer due to high shear.