OPTICAL PROBES FOR LOCAL VOID FRACTION MEASUREMENTS - CHARACTERIZATION OF PERFORMANCE

Optical probes are widely used in two phase gas-liquid flows, especially in laboratories for experimental research. However, despite all the work previously done on the subject, the performance of these sensors is not yet firmly established. To clarify this point, well controlled experiments dealing with the response of various optical probes during the piercing of an interface are presented. The latency length concept L*, which is the spatial resolution of the interface detection by a given probe, is introduced. The minimum value recorded is about 130 μm. Moreover, this parameter is closely related to the hydrodynamic response time of the sensor, and as such, to the geometric displacement velocity of each interface detected. Hence, the latency length is proposed as an objective basis for the comparison of the different probes. Finally, the relation between the response time and the signal processing techniques is discussed, and it is shown that the shortest length L* must be sought in order to ensure a better accuracy of the local void fraction.