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A new measuring technique dedicated to bubble velocity and size measurementsin complex bubbly flows such as those occurring in bubble columns is proposed.This sensor combines the phase detection capability of a conical optical fiber,with velocity measurements from the Doppler signal induced by an interfaceapproaching the extremity of a single-mode fiber. The analysis of the probefunctioning and of its response in controlled situations, have shown that theDoppler probe provides the translation velocity of bubbles projected along theprobe axis. A reliable signal processing routine has been developed thatexploits the Doppler signal arising at the gas-to-liquid transition: theresulting uncertainty on velocity is at most 14\%. Such a Doppler probeprovides statistics on velocity and on size of gas inclusions, as well as localvariables including void fraction, gas volumetric flux, number density and itsflux. That sensor has been successfully exploited in an air-tap water bubblecolumn 0.4m in diameter for global gas hold-up from 2.5 to 30\%. In theheterogeneous regime, the transverse profiles of the mean bubble velocityscaled by the value on the axis happen to be self-similar in the quasi fullydeveloped region of the column. A fit is proposed for these profiles. Inaddition, on the axis, the standard deviation of bubble velocity scaled by themean velocity increases with Vsg in the homogeneous regime, and it remainsstable, close to 0.55, in the heterogeneous regime.

A new, optimized Doppler optical probe for phase detection, bubble  velocity and size measurements: investigation of a bubble column operated in  the heterogeneous regime