Laser Doppler Anemometer Measurements in Bubble Column: Effect of Sparger

Measurements of flow pattern in a 150 mm i.d. bubble column were carried out using a laser doppler anemometer (LDA) in a forward scatter mode. A superficial gas velocity of 20 mm/s was used in all the experiments. Two different spargers (perforated plate and porous plate) were employed. The liquid flow starts developing from the sparger where non-uniformities in the gas sparging exist. From z/D >=2 (z = axial location; D = column diameter), the fully developed axial liquid velocity profiles are seen. The radial variation of mean axial velocity shows gross liquid circulation in the column. Circulation velocities are smaller for the bubble column with porous plate sparger mainly due to the fact that the mean bubble size is small and the gas distribution is relatively uniform over the column cross-section. The difference in the hydrodynamic behavior with two different spargers is also apparent from the profiles of the turbulent kinetic energy and Reynolds stress measured from the velocity-time series.