Laser-induced fluorescence (LIF) studies of liquid–liquid flows. Part I: Flow structures and phase inversion

A non-intrusive dye tracing technique, laser-induced fluorescence (LIF), has been applied to investigate the co-current flow of two immiscible organic–aqueous liquid flows in a vertical pipe. This technique allowed detailed visualization of the dynamic evolution of the flows. Flow structures in liquid–liquid flows at high dispersed phase fraction were revealed which had not been seen before. In pipe flow, an unstable range was found in the flow pattern map in which oil-in-water (o/w) and water-in-oil (w/o) dispersions could co-exist. Secondary dispersions (o/w/o and w/o/w) were observed for most volume fractions and velocities, especially in the unstable range. It was observed that, when the flow is in the unstable region, both w/o/w and o/w/o secondary dispersions could appear in the same set of experiments. It was found that this unstable range in the pipe flow, in spite of the similar appearance, was different to the ambivalent range seen in agitated systems. The one-dimensional drift flux model of Wallis (1969) for dispersed flow and a laminar model for co-current downward annular flow, were also applied to predict the in situ oil holdup; good agreement was obtained.