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. Les comportements locaux des bulles ont ete etudies dans trois colonnes a bulles de diametres differents, soient 200, 400 et 800 mm. Au moyen d'une nouvelle sonde a fibre optique a embout unique employant la technique laser Doppler, des mesures locales de la retention de gaz, de la frequence des bulles, de la taille des bulles et de la vitesse ont ete effectuees simultanement a differents endroits dans les colonnes. Ces mesures ont ete effectuees pour un systeme air-eau a des vitesses de gaz superficielles atteignant 90 mm/s. Les profils moyennes et les mesures instantanees ont ete analyses puis compares pour differentes colonnes. La presence d'une structure de circulation unique coherente occupant tout le diametre de la colonne dans la colonne la plus large plutot qu'une paire de cellules de circulation symetriques observees dans les colonnes plus petites est confirmee.