Dynamic behavior of the flow field of a bubble column at low to moderate gas fractions

Time series of the vertical component of the liquid velocity in a bubble co lumn of 15 cm diameter have been obtained using LDA. The sets were measured close to the wall at h/D = 3.3. The ungassed height was H-0/D = 8; the gas fraction ranged from 0.4 to 18%. Each time series had a length of 900 s an d a number of data points between 220 and 300 k. The series were analyzed u sing a multiresolution analysis based on the Haar-wavelet. The standard dev iation of the low-frequency parts of the multiresolution decomposition was found to tend towards a non-zero value that is independent of the gas fract ion if the decomposition level increases. This suggests that the low-freque ncy oscillations are universal. They are probably caused by the meandering of the bubble clusters and the motion of vortical structures. The analysis shows that the changes in the liquid flow field are rather smooth when incr easing the gas fraction. If a transition from one regime to another is to b e identified, it is located around gas fractions as low as 1%. This conclus ion is supported by analysis of the same signals based on auto-regressive m odeling. (C) 1999 Elsevier Science Ltd. All rights reserved.