Local bubble dynamics and macroscopic flow structure in bubble columns with different scales

Local bubble behaviours were investigated in three bubble columns with different diameters of 200, 400 and 800 mm. By means of a novel single-tip optical fibre probe employing laser Doppler technique, the local gas holdup, bubble frequency, bubble size and velocity were measured simultaneously at different locations of the columns. Measurements were performed in air-water system at superficial gas velocities up to 90 mm/s. The averaged profiles and instantaneous measurements were analyzed and compared for different columns. The presence of a coherent gross circulation structure spanning the entire column diameter in the larger column rather than a pair of symmetrical circulation cells observed in the smaller columns has been confirmed.