Simultaneous void fraction measurement, bubble velocity, and size estimate using a single optical probe in gas–liquid two-phase flows

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Abstract
Optical probes are now commonly used in industrial conditions as well as in laboratory experiments. Although they have been primarily devoted to void fraction measurements, additional information could be extracted from the raw signals they deliver. For a stretched optical probe, it is shown that the modulus of the ensemble velocity of a bubble ‖V0‖ could be inferred from the rise time Tu associated with the liquid/gas transition. These two parameters are correlated thanks to piercing experiments in which the interface curvature R and the angle β between the probe and the normal to the interface are controlled. While the influence of R is negligible, the rise time is very sensitive to β. A one‐to‐one relation between Tu and ‖V0‖ is ensured for quasiperpendicular impactions which must be accordingly distinguished among the bubble signatures. The software which includes the adequate signal processing criteria is described. It gives access to void fraction and to the chord‐velocity joint distribution which provides a guide to determine the flow regime. The void fraction is shown to be weakly sensitive to the criteria. Besides, velocity and chords estimates are in rather good agreement with those deduced from direct visualization, both in a bubble and a slug flow. Such extensions of the measuring capabilities of optical probes are expected to be also applicable to other phase detection probes.