On the accuracy of the void fraction measurements using optical probes in bubbly flows

The accuracy of the measurement of the void fraction in bubbly flows using an optical probe is investigated. Experiments were performed in tap water with ellipsoidal-shaped air bubbles with equivalent diameters and velocities in the range of 2.8–5.2 mm and 0.22–0.28 m∕s. Comparison of charge coupled devices (CCD) images of dynamic bubble piercing events with optical probe signals shows that for piercing in the area around the bubble side, the so-called low-level criterion gives the best agreement with the actual gas-liquid transition for the undisturbed bubble. In addition, residence time underestimation due to a partial blinding effect is observed in the outer regions of the bubble. Residence times of the probe inside the bubble are obtained from the probe signal and from CCD images of the undisturbed bubble. These are compared to study the relevance of various probe-bubble interaction effects. The crawling effect is found to play an important role. For perpendicular piercing, the experiment shows that in the central area of the bubble deceleration effects induced by the probe lead to local overestimation of residence times. In the outer region of the bubble, large-scale deformation leads to local underestimation of residence times. The larger cross-sectional area associated with the underestimation leads to a net underestimation of the total bubble volume. For nonperpendicular piercing, the probe inclination is found to generate an additional drifting effect, creating an additional source of underestimation.