LIQUID VELOCITY-FIELD IN A BUBBLE-COLUMN - LDA EXPERIMENTS

This paper reports on LDA experiments in bubble columns 15.2, 23.4, an d 38.4 cm in diameter. The gas fraction ranges up to 25%, but the colu mns are still in the bubbly regime, i.e. coalescence of bubbles is min or. It is shown that both the axial and tangential liquid velocity com ponents can be measured with confidence, especially close to the wall. In that domain the data rates are sufficiently high to obtain time se ries that can be studied at high frequencies, i.e. above 1000 Hz. It i s reported that the fluctuations in the velocity field are of the same order as the mean velocity. Furthermore, results on the Reynolds stre sses are presented. The axial normal stress is higher than the tangent ial one indicating anisotropic turbulence. Furthermore, the axial norm al stress shows a minimum at a radial position of 0.8 of the column ra dius which has not been reported before. This minimum becomes more pro nounced with increasing gas fraction. The axial-tangential Reynolds sh ear stress is zero. A frequency analysis shows that for the higher fre quencies the -5/3 power law is obeyed. At low frequencies the presence of vortical structures is found. Short-time frequency analysis indica tes that these structures are well separated in space and arrive at ir regular intervals. (C) 1997 Elsevier Science Ltd.