Local measurements are of primary importance for the characterization of gas-liquid two-phase flows. Generally, the twophase flows are described by the local void fraction, the gas and liquid velocity fields and the interfacial area concentration. The measurement of these parameters may be necessary both for processes control and numerical modeling validation. It is a very active research field due to the increasing number of applications in the thermohydraulics of heat exchangers, nuclear plants, chemical processes and oil industries. In this paper different measurement techniques of local void fraction in gas-liquid flows are reviewed. The techniques based on optical, impedance and thermal probes are detailed. Both probe practical designs and signal processing are presented. The measurement techniques based on a dual probe are also reviewed herein. The various theoretical treatments used to bubble velocity determination are analyzed. Finally, the performance of the different techniques are compared on the basis of calibration, signal processing, accuracy and potential applications.
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