Time-resolved measurement of concentration fluctuations in a confined bubbly flow by LIF

Abstract : The present work investigates the mixing of a low-diffusivity dye in a swarm of bubbles at high Reynolds number confined in a Hele-Shaw cell for gas volume fractions ranging from 1.4 to 5.4%. A patch of a fluo- rescent dye is injected within the swarm and, during its mixing, its concentration is measured at a given location in an observation volume of 4.5 mm2 by means of Laser Induced Fluorescence at a frequency of 250 Hz. A spectrometer is used to analyse the light issued from the observation volume and to dis- tinguish the fluoresced light from other light sources. Simultaneously, the bubble distribution around the observation volume is imaged with a high speed camera synchronised with the spectrometer in order to assess the LIF technique in bubbly flow. Thanks to the good time resolution, rapid and intense concen- tration fluctuations corresponding to dye patches passing through the observation volume are recorded and are superimposed to a slow global evolution. This slow global evolution shows first an increase of the concentration and then an exponential decrease due to the mixing by bubble-induced agitation. This exponential decay, which is incompatible with a diffusion process, is consistent with the transport by dye capture in bubbles wakes that are quickly dampened by the shear-stress at the walls. The one-point statistics of the concentration fluctuations (probability density function and spectrum) also point out that mixing in a confined bubbly flow is intermittent and convective.
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Submitted on : Wednesday, June 15, 2016 - 2:50:13 PM
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Elise Alméras, Sébastien Cazin, Véronique Roig, Frédéric Risso, Frédéric Augier, et al.. Time-resolved measurement of concentration fluctuations in a confined bubbly flow by LIF. International Journal of Multiphase Flow, Elsevier, 2016, vol. 83, pp. 153-161. ⟨10.1016/j.ijmultiphaseflow.2016.03.011⟩. ⟨hal-01332255⟩

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