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 fluorescent 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 distinguish 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 concentration 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.