Deteraiination of the Hydrodynamics in a Bubble Column by means of LDA

The hydrodynamic behaviour ofthe liquid flow in a 3D 'air-water ' bubble column 15 cm in diameter and a gas-fraction of 10 % was investigated. The study focussed on the low-frequency part (0-2 Hz) of liquid velocity time series measured close to the wall in order to get an indication as to how slow oscillations of the flow evolve in the column. Laser Doppler Anemometry (LDA) was used to measure liquid velocities. When measuring with LDA in a two-phase flow, air bubbles were found to disturb the LDA measurements in several ways. Therefore, part of the investigation was fo-cussed on validating the velocity time series and optimising the different param-eters of the LDA-system. Multiple validation of Doppler signals was one of the main problems when measuring in a two phase flow. Multiple validation could be reduced by optimising the LDA parameters. Furthermore, an averaging procedure was used to compensate for the multiple validation. Frequency spectra of the velocity time series were determined with the Lomb method, which can deal with unevenly sampled time series (measured with LDA). Joint time frequency spectra were calculated with the Short Time Fourier Trans-form. Dominant frequencies ranging from 0.1 to 0.2 Hz occurred. The low frequen-cies were stronger in the bottom part of the bubble column and gradually became weaker when going up in the column. This means that vortices are stronger in the bottom part of the column and become weaker higher in the column. The joint time frequency analysis revealed that there were separate frequency peaks (0.1 to 0.2 Hz) spaced at irregular time intervals (30-50 s). The frequencies found can be linked to vortices moving through the column and slow oscillation waves. It was found that, in contrast to 2D bubble columns, the oscillatory motion in 3D bubble columns is irregular. An average frequency could be defined for the slow oscillations