Multi-phase-fluid discrimination with local fibre-optical probes: II. Gas/liquid flows

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Abstract
We demonstrate fibre-optical sensors (or `local probes') for gas/liquid discrimination in multi-phase flows. These probes distinguish gas bubbles from liquid on the basis of refractive-index contrast. Oblique-tip designs use total internal reflection; a simple cleaved-fibre end uses the smaller Fresnel reflectivity of the fibre end. This paper mainly reports data on the discrimination of gas from liquid in air/water two-phase flows, but we include also some examples of discrimination in kerosene/air flows and in crude oil/nitrogen. Quantitative accuracy is again demonstrated by comparison of profiles, across a pipe diameter, of local, time-averaged gas volume fractions (`void fractions'), with pipe-averaged void fractions determined from a carefully calibrated gradiomanometer. Companion papers deal with the sensor designs used and results achieved in liquid/liquid flows and three-phase flows.