Daniel D. Joseph
Water-lubricated transport of heavy viscous oils is a technology based on a gift of nature in which the water migrates into the region of high shear at the wall of the pipe where it lubricates the flow. Since the pumping pressures are balanced by wall-shear stresses in the water, the lubricated flows require pressures comparable to pumping water alone at the same throughout, independent of the viscosity of the oil (if it is large enough). Hence savings of the order of the oil-to-water viscosity ratio can be achieved in lubricated flows. Lubricated flow in an oil core is called core-annular flow, or CAF.
Typically, waves appear on the surface of the oil core and they appear to be necessary for levitation of the core off the wall when the densities are different and for centering the core-annular flows (WCAF). Perfect core-annular flows (PCAF) of density-matched fluids in horizontal pipes, and generally in vertical pipes, are possible but are rarely stable (Joseph & Renardy 1993, Preziosi et al 1989, Chen et al 1990).
The science behind the technology of CAF has given rise to a large literature which has been reviewed by Oliemans & Ooms (1986) and more recently by Joseph & Renardy (1993). This literature has many facets, including models for levitation, empirical studies of energy efficiency of different flow types, empirical correlations giving the pressure drop versus mass flux, stability studies, and reports of industrial experience.
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