Application of LDA to bubbly flows

The fluctuating velocity field in an air–water bubble column (i.d. 15.2 cm) at a gas fraction of 25% is investigated using backscatter LDA. Since the interpretation of LDA signals in bubbly flows is not straight forward also experiments on a single bubble train are reported. It is discussed that in the latter case when using seeding the backscatter LDA measures predominantly the liquid velocity. No improvement from thresholding on the discrimination between gas and liquid was found. The bubble column experiments show that the radial averaged liquid velocity profile represents the well known gross scale circulation present in the column. More interesting, it is also seen that the fluctuating velocity field can be studied in great detail. The velocity probability density functions directly indicate high turbulence intensity. Low frequency fluctuations are observed in agreement with visual observations. The data rate is an exponential function of the distance from the column wall. This limits the possibilities of spectral analysis in the central part of the flow. However, close to the wall the mean data rate is sufficient to study the frequency contents of the signal. It is shown that the power spectral density function obeys a −5/3 power law and that the autocorrelation function is of similar shape as reported in literature on bubbly flows. © 1998 Elsevier Science B.V. All rights reserved.