Homogeneous swarm of high-Reynolds-number bubbles rising within a thin gap. Part 2. Liquid dynamics

Bouche, Emmanuella; Roig, Véronique; Risso, Frédéric; Billet, Anne-Marie

doi:10.1017/jfm.2014.544

Cited by 42 publications

(53 citation statements)

References 18 publications

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“…The shape of the spectrum of the spatial fluctuation is weakly dependent on the Reynolds number over all the range of Re investigated: it follows a −3 slope at large scales and a −5/3 slope at small scales. A k −3 spectral subrange has been observed in a bubble swarm confined in a Hele-Shaw cell where no turbulence can develop, and hence only the spatial fluctuation is present (Bouche et al 2014). Thus, it seems to be a general feature of localized flow disturbances randomly distributed over space.…”

Section: Wavenumber Spectra Of Velocity Fluctuationsmentioning

confidence: 99%

“…Unfortunately, there is no easy way to separate these two contributions in a configuration where the bubbles are moving. By considering a swarm of two-dimensional bubbles rising at high Reynolds number in a Hele-Shaw cell, Bouche et al (2012Bouche et al ( , 2014 managed to investigate experimentally spatial fluctuation in the absence of turbulence, thanks to the friction at the cell walls which prevents from any turbulence development. They found that the PDFs of the spatial fluctuation also showed the signature of the wakes and the spatial spectrum contained a k −3 subrange.…”

Section: Introductionmentioning

confidence: 99%

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Velocity fluctuations generated by the flow through a random array of spheres: a model of bubble-induced agitation

et al. 2017

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To cite this version:Zouhir Amoura, Cédric Besnaci, Frédéric Risso, Véronique Roig. Velocity fluctuations generated by the flow through a random array of spheres: a model of bubble-induced agitation. Journal of Fluid Mechanics, Cambridge University Press (CUP), 2017, 823, pp.592-616. <10.1017/jfm.2017 This work reports an experimental investigation of the flow through a random array of fixed solid spheres. The volume fraction of the spheres is 2 %, and the Reynolds number Re based on the sphere diameter and the average flow velocity is varied from 120 to 1040. Using time and spatial averaging, the fluctuations have been decomposed into two contributions of different natures: a spatial fluctuation that accounts for the strong inhomogeneity of the flow around each sphere, and a time fluctuation that comes from the instability of the flow at large enough Reynolds numbers. The evolutions of these two contributions with the Reynolds number are different, so that their relative importance varies. However, when each is normalized by using its own variance and the integral length scales of the fluctuations, their spectra and probability density functions (PDFs) are almost independent of Re. The spatial fluctuation mostly comes from the velocity deficit in the wakes of the spheres, and is thus dominated by scales larger than one or two sphere diameters. It is found to be responsible for the asymmetry of the PDFs of the vertical fluctuations and of the major part of the anisotropy level between the vertical and the horizontal components of the fluctuations. The time fluctuation dominates at scales smaller than the integral length scale. It is isotropic and its PDFs, well described by an exponential distribution, are non-Gaussian. The spectra of the spatial and the time fluctuations both show an evolution as the power −3 of the wavenumber, but not exactly in the same subrange. All these properties are found in remarkable agreement with the results of both experimental investigations and large eddy simulations (LES) of a homogeneous bubble swarm. This confirms that the main mechanism responsible for the production of bubble-induced fluctuations is the interaction of the velocity disturbances caused by obstacles immersed in a flow and that the structure of this agitation is weakly dependent on the precise nature of the obstacles. The understanding and the modelling of the agitation generated by the motion of a dispersed phase, such as the bubble-induced agitation, therefore require one to distinguish between the roles of these two contributions.

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Section: Wavenumber Spectra Of Velocity Fluctuationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Velocity fluctuations generated by the flow through a random array of spheres: a model of bubble-induced agitation

et al. 2017

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“…The liquid perturbations induced in the bubble wakes are localised just behind the bubbles and are strongly attenuated by the shear stress at sidewalls . In that case, collective instability and turbulence cannot develop due to the confinement ( Bouche et al, 2014 ).…”

Section: Experimental Set-upmentioning

confidence: 99%

“…In particular, the friction at the walls and the velocity gradient throughout the gap play a major role in the structure of the bubble wakes as shown by Roig et al (2012) , Bouche et al (2014) and Filella et al (2015) . Within a viscous time scale w 2 /(4 ν), the in-gap velocity profile generated by a bubble passage evolves from a plug flow profile to a Poiseuille flow profile with a maximum velocity decreasing exponentially with time.…”

Section: Experimental Set-upmentioning

confidence: 99%

“…Here we have checked that the present interpolation has not significant influence upon PDFs of concentration fluctuations. Concerning spectra, different other techniques have been used for their determination for velocity fluctuations from signals that are interrupted by the passages of the bubbles, such as smoothing the discontinuities by a Gauss function ( Lance and Bataille, 1991 ) or considering only intervals between bubbles where the signal is continu- ous ( Martínez-Mercado et al, 2010;Bouche et al, 2014 ). However, all techniques eventually lead to a similar spectrum in power −3 of the wavenumber.…”

Section: Validation Of the Concentration Measurements Provided By Spementioning

confidence: 99%

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

Alméras

Cazin

Roig

et al. 2016

International Journal of Multiphase Flow

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a b s t r a c tThe 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 mm 2 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.

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Experimental investigation of interfacial mass transfer mechanisms for a confined high‐reynolds‐number bubble rising in a thin gap

et al. 2016

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Interfacial mass transfer is known to be enhanced for confined bubbles due to the efficiency of the transfer in the thin liquid films between them and the wall. In the present experimental investigation, the mechanisms of gas–liquid mass transfer are studied for isolated bubbles rising at high Reynolds number in a thin gap. A planar laser induced fluorescence (PLIF) technique is applied with a dye the fluorescence of which is quenched by dissolved oxygen. The aim is to measure the interfacial mass fluxes for pure oxygen bubbles of various shapes and paths rising in water at rest. In the wakes of the bubbles, patterns due to the presence of dissolved oxygen are observed on PLIF images. They reveal the contrasted contributions to mass transfer of two different regions of the interface. The flow around a bubble consists of both two thin liquid films between the bubble and the walls of the cell and an external high‐Reynolds‐number in‐plane flow surrounding the bubble. Mass transfer mechanisms associated to both regions are discussed. Measurement of the concentration of dissolved oxygen is a difficult task due to the nonlinear relation between the fluorescence intensity and the concentration in the gap. It is however possible to accurately measure the global mass flux transferred through the bubble interface. It is determined from the fluorescence intensity recorded in the wakes when the oxygen distribution has been made homogeneous through the gap by diffusion. Assuming a reasonable distribution of oxygen concentration through the gap at short time also allows a measurement of the mass fluxes due to the liquid films. A discussion of the results points out the specific physics of mass transfer for bubbles confined between two plates as compared to bubbles free to move in unconfined flows. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2394–2408, 2017

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Homogeneous swarm of high-Reynolds-number bubbles rising within a thin gap. Part 2. Liquid dynamics

Cited by 42 publications

References 18 publications

Velocity fluctuations generated by the flow through a random array of spheres: a model of bubble-induced agitation

Velocity fluctuations generated by the flow through a random array of spheres: a model of bubble-induced agitation

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

Experimental investigation of interfacial mass transfer mechanisms for a confined high‐reynolds‐number bubble rising in a thin gap

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