Laser-Doppler measurements of bubble dynamics

Abstract

Laser-Doppler anemometry is applied to study the dynamics of small-diameter carbon dioxide and air bubbles rising in a quiescent liquid. Primary data include bubble velocity and diameter as functions of position above a generating nozzle. Direct observation of decreasing bubble diameter provides instantaneous mass transfer results for carbon dioxide bubbles, presented in terms of Sherwood versus Reynolds and Peclet numbers. The range of velocity measured is 1.0 to 35.0 cm/sec, and the range of diameter measured is 0.07 to 1.20 mm, corresponding to a Reynolds number range from less than unity to over 500. Bubble oscillations, observed through direct diameter measurements, are recorded for bubbles with initial diameter greater than 0.80 mm. Fourier transforms of the instantaneous diameter profiles result in a frequency of oscillation of about 10 Hz for most test runs. Drag studies indicate that bubbles with Reynolds number greater than 40 follow correlations developed for fluid spheres with attached boundary layers and wakes. Transition from solid-sphere to fluid-sphere drag behavior is observed for increasing Reynolds number in the range 10 to 40.