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 column 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 H₀/D = 8; the gas fraction ranged from 0.4 to 18%. Each time series had a length of 900 s and a number of data points between 220 and 300 k. The series were analyzed using a multiresolution analysis based on the Haar-wavelet. The standard deviation of the low-frequency parts of the multiresolution decomposition was found to tend towards a non-zero value that is independent of the gas fraction if the decomposition level increases. This suggests that the low-frequency 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 increasing the gas fraction. If a transition from one regime to another is to be identified, it is located around gas fractions as low as 1%. This conclusion is supported by analysis of the same signals based on auto-regressive modeling.