A new intensity modulation based fiber optic probe for bubble shape detection, velocity and diameter measurements

In this article a new noncontact fiber optic probe, previously developed for bubble velocity measurements, is described. The sensing element is based on the optical interaction between the bubble and two focused laser beams. Not only the reflected but also the refracted beams are detected by the probe to perform bubble parameter measurements. The measurement principle is based on a transit-time technique, used, for example, in laser dual focus velocimetry or cross-correlation based measuring systems. The measurement system makes use of optical fiber components. The probe is designed for measurements on two-phase bubble flows without contact between the bubble and the probe tip and with minimal disturbance to the flow in the measurement region. The probe tip diameter is only 4 mm. No optical access to the flow is necessary because the probe can be installed inside the flow. Performances of the probe are theoretically and experimentally analyzed. Bubble-laser beam interaction has been analyzed by developing a special purpose simulation software. The probe was then tested in a hydraulic test rig circulated by a vertical air-water bubble flow at velocities up to 2 m/s and bubble diameter from 1 to 4 mm. The developed probe was calibrated by a laser Doppler velocimeter and classical photographic techniques. First results have shown uncertainty values of about 2% (one standard deviation) for velocity measurements. © 1996 American Institute of Physics.