Piv study of mixing characteristics in a stirred vessel with a non-Newtonian fluid

PIV is used to analyze the flow induced by a Rushton turbine in a shear-thinning fluid, at constant input power, constant impeller velocity but different concentrations. The rheology of each shear-thinning fluid is first addressed. The mean velocity fields are compared. POD methodology is applied to estimate coherent structures and turbulence levels. Finally, the heterogeneity of shear rate is estimated and the spatial distribution of dissipation rate of total kinetic energy is addressed

Liquid spreading in trickle-bed reactors: Experiments and numerical simulations using Eulerian--Eulerian two-fluid approach

Liquid spreading in gas-liquid concurrent trickle-bed reactors is simulated using an Eulerian twofluid CFD approach. In order to propose a model that describes exhaustively all interaction forces acting on each fluid phase with an emphasis on dispersion mechanisms, a discussion of closure laws available in the literature is proposed. Liquid dispersion is recognized to result from two main mechanisms: capillary and mechanical (Attou and Ferschneider, 2000; Lappalainen et al., 2009- The proposed model is then implemented in two trickle-bed configurations matching with two experimental set ups: In the first configuration, simulations on a 2D axisymmetric geometry are considered and the model is validated upon a new set of experimental data. Overall pressure drop and liquid distribution obtained from $\gamma$-ray tomography are provided for different geometrical and operating conditions. In the second configuration, a 3D simulation is considered and the model is compared to experimental liquid flux patterns at the bed outlet. A sensitivity analysis of liquid spreading to bed geometrical characteristics (void-fraction and particles diameter) as well as to gas and liquid flow rates is proposed. The model is shown to achieve very good agreement with experimental data and to predict, accurately, tendencies of liquid spreading for various geometrical bed characteristics and/or phases flow-rates

Déploiement d'ordonnanceurs de processus spécifiques dans un système d'exploitation généraliste

L'environnement Bossa permet la conception d'ordonnanceurs spécifiques offrant ainsi une gestion précise de la ressource processeur au sein d'un système d'exploitation généraliste. Toutefois, le déploiement conjoint d'une application et de son ordonnanceur suscite des problèmes liés à la réservation de la ressource processeur. Dans cet article, nous étudions les problèmes dus à ce type de déploiement et proposons quelques solutions. En particulier, nous proposons d'établir des contrats de qualité de service et des mécanismes de reconfiguration de la hiérarchie d'ordonnanceurs. –––––Bossa is a framework to develop new processes schedulers in commodity operating systems. Although Bossa enables fine-grained management of the processor through new scheduling policies, deploying an application with its own scheduler raises some problems. In this paper we study the problems caused when deploying an application and its scheduler and to adresse these, we propose to establish Quality of Service contracts and mechanisms to reconfigure the scheduler hierarchy

Matériaux pérovskites complexes pour applications transducteurs acoustiques de puissance

Le cœur des transducteurs électro-acoustiques en détection sous-marine est une céramique piézoélectrique de structure Pérovskite ABO3. Dans le cas des dispositifs d émission, le matériau piézoélectrique est soumis d une part à un fort champ électrique alternatif et à des déformations élastiques élevées entraînant une non linéarité de la réponse, et d autre part, à des pertes mécaniques et diélectriques entraînant un échauffement. L objectif de cette thèse est de développer des matériaux piézoélectriques stables en température et sous fort champ électrique alternatif. Nous avons, dans un premier temps, synthétisé des monocristaux de 0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3 dopés et non dopés au manganèse. Nous avons montré que le manganèse entraînait une diminution des pertes diélectriques et mécaniques et avait une contribution sur les propriétés électromécaniques intrinsèques responsable en partie du durcissement du matériau. Un nouveau matériau : le 0.63Pb(In1/2Nb1/2)O3-0.37PbTiO3 a été ensuite élaboré sous forme de céramiques. Les caractérisations effectuées ont montré que ce matériau possédait une température de Curie de 290C et un champ coercitif de 1660V/mm, entraînant une excellente stabilité en température et en champ, ainsi que des propriétés électromécaniques supérieures à celles des céramiques couramment utilisées. Des céramiques orientées de composition ternaire Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ont été finalement élaborées. En effet, la synthèse de telles céramiques est une solution pour obtenir à bas coût des échantillons de tailles importantes présentant des propriétés électromécaniques intermédiaires entre celles des céramiques classiques et celles des monocristaux.The core of electro-acoustic transducers for underwater systems is a piezoelectric ceramic with a ABO3 Perovskite structure. For emission devices, the piezoelectric material is submitted on one hand, to high alternative electric fields and to high stresses resulting in a non linear response and on the other hand, to mechanical tgom and dielectric tgoe losses resulting in an increase of the material temperature.The aim of this study is to develop temperature and alternative electric field stable materials. Manganese doped and undoped 0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3 single crystals have been first synthesized. Manganese has been proved to decrease the mechanical and dielectric losses and to have a contribution on the intrisic electromechanical properties partially responsible for the stiffening of the material. 0.63Pb(In1/2Nb1/2)O3-0.37PbTiO3 new ceramics have been then synthesized. The characterisations have shown a Curie temperature of 290C and coercive field of 1660 V/mm resulting in excellent temperature and alternative electric field stabilities of the high electromechanical properties. Orientated Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics have been finally synthesized. The synthesis of such materials is clearly an option to achieve low cost and high sized materials with electromechanical properties between those of classical ceramics and those of single crystals.CHATENAY MALABRY-Ecole centrale (920192301) / SudocSudocFranceF

Bubble size and liquid-side mass transfer coefficient measurements in aerated stirred tank reactors with non-Newtonian liquids

International audienceOxygen transfer is a key element in aerobic fermentations, especially if the culture broth's rheology is non-Newtonian, as in the case of cultures of fil-amentous fungi. It is well known that viscosity negatively affects the volu-metric mass transfer coefficient (k L a), but mechanisms involved in terms of change of interfacial area (a) and liquid-side mass transfer coefficient (k L) have still not been clearly identified. This lack of knowledge is in part due to the difficulty in measuring bubble size in aerated media, especially in vis-cous fluids. In the present study, a recently developed dual-probe method was validated and then used to measure bubble Sauter mean diameter (d 32) in water and in xanthan gum solutions, which exhibit rheological behaviors similar to filamentous fungi's broths. Ethanol was used to modify the coales-cence of the dispersed phase and the results were compared to data obtained in a filtered fermentation culture of Trichoderma reesei. Additional experiments were carried out to obtain the volumetric mass transfer coefficient and the global gas holdup (α G). It was then possible to obtain the liquid-side mass transfer coefficient and to compare it with existing models. Finally, empirical correlations were proposed to estimate d 32 , α G , k L a and k L in a mechanically stirred tank. It was observed that k L depended only on the apparent viscosity of the system, unlike all the other measured properties (k L a, d 32 , α G) which were very sensitive to power consumption, superficial gas velocity, viscosity and the presence of additives

Influence of the sampling area of the stem on the mechanical properties of hemp fibers

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COVID-19 and wastewater: a crisis management tool?

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COVID-19 and wastewater: a crisis management tool?

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