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 straigh t forward also experiments on a single bubble train are reported. It i s discussed that in the latter case when using seeding the backscatter LDA measures predominantly the liquid velocity. No improvement from t hresholding on the discrimination between gas and liquid was found. Th e bubble column experiments show that the radial averaged liquid veloc ity profile represents the well known gross scale circulation present in the column. More interesting, it is also seen that the fluctuating velocity held can be studied in great detail. The velocity probability density functions directly indicate high turbulence intensity. Low fr equency fluctuations are observed in agreement with visual observation s. The data rate is an exponential function of the distance from the c olumn wall. This limits the possibilities of spectral analysis in the central part of the flow. However, close to the wall the mean data rat e is sufficient to study the frequency contents of the signal. It is s hown 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. (C) 1998 Elsevier Science B.V. All righ ts reserved.