Multi-phase-fluid discrimination with local fibre-optical probes: I. Liquid/liquid flows

We demonstrate the use of a novel design of fibre-optical sensor (or 'local probe') for immiscible-fluid discrimination in multi-phase flows. These pr obes are made from standard silica fibres with plane oblique facets polishe d at the fibre tip, with various surface treatments, including a crucial on e for wettability control. Total internal reflection is used to distinguish drops, bubbles or other regions of fluid in multi-phase flows, on the basi s of refractive-index contrast. Such probes have quasi-binary outputs; we d emonstrate in this paper their use in distinguishing water from oil (kerose ne) in oil/water two-phase flows and compare the results with those obtaine d from a simple cleaved fibre relying on the (small) difference in Fresnel reflectivity for discrimination. Quantitative accuracy is demonstrated by c omparison of profiles, across a pipe diameter, of local, time-averaged volu me fractions ('hold-ups'), with pipe-averaged hold-ups determined from a ca refully calibrated gradio-manometer in a fully developed region of the flow . Companion papers deal with the sensors used and results achieved in gas/l iquid flows and three-phase flows.