Time-resolved measurement of concentration fluctuations in a confined bubbly flow by LIFReport as inadecuate

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1 IMFT - Institut de mécanique des fluides de Toulouse 2 IFPEN - IFP Energies nouvelles

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.

Keywords : Confined bubbly flow Mixing LIF and spectrometry

Author: Elise Alméras - Sébastien Cazin - Véronique Roig - Frédéric Risso - Frédéric Augier - Cécile Plais -

Source: https://hal.archives-ouvertes.fr/


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