Modeling of Bubble-Induced Fluctuations in turbulent bubbly up-flow from Direct Numerical Simulations

International audienceVelocity fluctuations have very different physical origins in two-phase flows. The effect of bubbleson liquid velocity fluctuations varies widely depending on the relative importance of buoyancy, viscousforces and liquid inertia in the flow. In addition to single-phase turbulence (SPT), two-phase bubbly flowsdevelop a wake-induced turbulence (WIT) linked to the wake interactions and instabilities as well as othernon-turbulent fluctuations related to the averaged wake and to the potential flow around each bubbles(wake-induced fluctuations WIF) [1]. In classical Rij – ε modelling, these added phenomena are modeledas a source term of turbulent kinetic energy in the Rij transport equation. In order to take into account thedifferent nature of those fluctuations, an innovative Rij – ε closure is proposed by modelling separatelySPT, WIT and WIF. In-depth analyses of bubble swarms and turbulent bubbly flows in channels are performed relyingon Direct Numerical Simulations (DNS, i.e. Front-tracking algorithm, see figure 1). These configurationsare used in conjunction to better understand and quantify the relative effects of wake-induced fluctuationsand instabilities on the level of turbulence in the liquid. For turbulent bubbly flows, a method is proposedto distinguish the SPT and the bubble-induced fluctuations (WIF & WIT) from the total Reynolds stressescomputed from DNS solution. It enables the study of their interactions. Following the work of F.Risso [2],we also separate WIF and WIT in a bubble swarm. Statistical profiles of key quantities of turbulence areextracted. They are analyzed in order to improve our understanding of WIT and WIF. The insights fromthese are used to up-scale and build improved closures to the Euler-Euler two-fluid RANS (ReynoldsAverage Navier Stokes) model

Biological Activities of Ulva sp. Hydrolysates: Relation Between observed Activities and Composition of Extracts

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A criterion to classify void fraction profiles in bubbly flows based on averaged flow quantities for use in subchannel codes

ISBN: 978-0-89448-767-5International audienceHydrodynamics of multiphase flows can have a large influence on the overall design and operation of nuclear reactors. Interfacial forces drive bubbles towards or away from the wall, resulting in wall-peaked or core-peaked void fraction profiles. This change has a dramatic effect on the averaged flow quantities such as wall shear stress, velocity profiles and turbulence levels. Subchannel codes lack information of the underlying flow profiles, and rely on closure models to impose the correct averaged flow quantities. In this work, we propose a criterion to classify bubbly flows as wall-peaked or core-peaked, based on the turbulence level, and the ratio of surface tension to buoyancy forces. The criterion has been developed leveraging a large set of adiabatic air-water bubbly flow experiments, and can be used in subchannel codes to identify the underlying void fraction profile and apply the appropriate closure model. The criterion shows low sensitivity when applied to different operating fluids, and thus can be extended into a generalized formulation that can be applied to all adiabatic bubbly flows. An experiment has also been proposed to confirm the classification criterion at reactor conditions, using water with additives at atmospheric pressure to mitigate the high cost of high-pressure experiments. The new criterion proposed in this work, along with the improved understanding from new experiments, will allow us to develop more physically consistent closure models for subchannel codes

Validation of the hydrodynamics in a turbulent un-baffled vortex reactor at two different stirring Reynolds numbers

International audienceIn this paper, a turbulent flow is simulated in an un-baffled stirred tank reactor with vortex effect by means of large eddy simulations. This work aims at validating the influence of increasing the stirring frequency on the hydrodynamics and compares the mixing regions that form in such configurations due to the presence of two main vortex types. The open source TRUST-TrioCFD code is used with a discontinuous front-tracking algorithm to capture the vortex that develops at the free surface. A rotating magnetic stirrer is modelled by an immersed boundary method with a penalization concept. Analysis of the auto-correlation function shows a different range of time micro-scales which depends on the spatial location and stirring frequency. A very good agreement is reported between the numerical solution and the experimental measurements and thus reflects the good quality of our numerical study. As far as no similar detailed work is reported in the literature, the numerical data is considered as a reference and will serve in a next step in modelling the formation of solid particles due to a precipitation process that takes place in many applications of energy and chemistry

Preparative separation of marine bioactive compounds by centrifugal partition chromatography

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Biochemical and antiviral activities of enzymatic hydrolysates from different invasive French seaweeds

International audienceProliferations of green, brown and red algae appear in shallow sandy bays in North Brittany (France), and they represent a real economic constraint for the affected communities. In addition to the nuisance for residents and tourist activity, the communities must carry out systematic collection. The collected algae are spread on agricultural land spreading or composted, but these solutions reach their limits rapidly, bringing little added value to the collected algae. Seaweeds are potentially excellent sources of bioactive metabolites that could represent useful leads in the development of new functional ingredients in pharmaceutical and cosmetic industries. The aim of this study was to propose the use of an enzyme-assisted extraction as a tool to improve the extraction efficiency of antiviral compounds from three invasive French seaweeds. We selected the red Solieria chordalis, the green Ulva sp. and the brown Sargassum muticum as models for these experiments. In comparison with water extraction at 50 °C for the same time of treatment, enzymatic hydrolysis increased the yields. The data suggest the potential of enzymatic hydrolysis for producing active fractions in the function of the algal biomass, the behaviour of the cell wall, the selectivity and the action of the enzyme. Enzymatic hydrolysis appeared less effective for polyphenol recovery, but was a promising softer technique for recovering proteins, neutral sugars, uronic acids and sulphate groups. The solvent-free process, higher extraction rate and higher yields, coupled to time-saving and lower cost, make this method economical and sustainable. By using a cell viability assay, all hydrolysate fractions tested were shown to be non-toxic to Vero cells. After 3 days of treatment, no microscopically visible alteration of normal cell morphology was observed even at 500 μg mL−1. S. chordalis extracts have an effective antiviral activity with EC50 between 23.0 and 101.1 μg mL−1 at a multiplicity of infection of 0.001 ID50/cells; 100 % and 98 % cellular protection were obtained for 500 μg mL−1 of hydrolysate extracts carbohydrase C3 and blank, respectively. Other extracts from S. chordalis inhibited viral production less effectively

Study of a 900 MW PWR by a substructuring method - Spectral response to a seismic excitation & comparison with a beam model

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : 26165 A, issue : a.1997 n.89 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Antiherpetic (HSV-1) activity of carrageenans from the red seaweed Solieria chordalis (Rhodophyta, Gigartinales) extracted by microwave-assisted extraction (MAE)

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