Characterization of turbulence in rectangular bubble column

Turbulence in bubbly flows is of utmost practical and theoretical interest. In the current work, particle image velocimetry (PIV) measurements have been carried out in a rectangular bubble column. The superficial gas velocity was varied from 2 mm/s to 20 mm/s, with the gas hold-up values up to 4.75%. Both the homogeneous and heterogeneous regimes of operation were covered for the bubble column. Prior to turbulence measurement with PIV the bubble size distribution, time averaged velocity profile, and gas hold-up profile were measured with high speed camera and modified Pitot tube probe. In order to resolve low speed (large scale) and high speed (small scale) flow structures in a column properly, measurements were carried out with a low speed (7 Hz) and high speed (900 Hz) PIV system. Image processing program to identify bubbles from PIV image and mask the PIV vector field was developed. The energy spectrum of the liquid phase turbulence was calculated from PIV velocity field with FFT. Wavelet transform was used to separate small scale fluctuations from large scale fluctuations in liquid velocity field. Energy spectrum of large scale fluctuations was found to be dependent on the sparger type, while energy spectrum of small scale fluctuations was found to be similar for all spargers. The spectral slope of inertial subrange was found to be close to −5/3. The spectral slope for wavenumber higher than inertial subrange was found to be −3. Reasoning for this behavior based on energy transfer and energy dissipation rate has been provided.

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► 2D velocity field has been measured in a rectangular bubble column.
► Bubble size, mean velocity profile and gas hold-up profile have been measured.
► Large and small scale velocity fluctuations are separated with wavelet transform.
► Energy spectra of small scale are found to be invariant with sparger type.
► Spectral slope of inertial subrange was -5/3 with -3 slope at higher wavenumbers