Characteristics of laser-doppler signals from bubbles

Abstract The present paper concerns the application of laser-Doppler anemometry to two-phase bubbly flows. A summary of available LDA-measurements in particulate, droplet and bubble flow systems is given which shows detailed measurements of signal properties and their dependence on optical parameters were not previously available. However, such information is necessary for optimizing the optical and electronic schemes for size measurements. Consequently, the signal characteristics of light scattered from single bubbles rising in quiescent water were obtained for a forward-scatter arrangement and for three different beam-intersection angles. In addition to the dependence of the signal properties of the bubble path through the beam-intersection region, the effects of aperture diameter and aperture position in the optical system are discussed. The resultant data were used to determine an optimum forward-scatter LDA-system which was then employed to measure the dependence of bubble velocity on bubble diameter over the range 0.2–1.0 mm. Finally, a theoretical model proposed to describe the dependence of the signal properties on the location of the centreline bubble path through the beam-intersection region is shown to be in agreement with the measurements. The treble amplitude peaks observed in the signals from bubbles in this size range are readily explained by applying the laws of geometrical optics.