Wavelet transform of velocity-time data for the analysis of turbulent structures in a bubble column

Hydrodynamic and turbulent structures in bubble column reactors have been characterized from velocity-time series data obtained by laser Doppler anemometer. Measurements were made in two bubble columns of 100 and 150 mm inner diameter under various operating conditions. The superficial gas velocity was varied in the range of 13-36.5 mm/s and in order to investigate the role of sparger design, different types namely sieve plate, single hole and sintered plate were employed. Wavelet transform analysis was used for denoising the measured data and also for evaluating quantifiers of turbulence, viz., local intermittency measure, intermittency index and flatness factor. These quantifiers help in detecting the passage of bubbles and reveal the hidden structures and patterns in the data. The low-frequency structures that were detected for a wide range of experimental conditions showed the likely existence of circulation cells. Also, the wavelet scalewise analysis of the turbulence at various locations in the column and under different operating conditions could be used to build a direct relationship between fractional gas hold-up and flatness factor values. The methodology is therefore suitable for online evaluation of bubble column performance and shows promise for developing strategies for improving process performance. (C) 2001 Elsevier Science Ltd. All rights reserved.