Dynamics of bubble growth and detachment from rigid and flexible orifices

The objective of this paper is to understand how and why the orifice nature (rigid or flexible) governs the bubble generation. The differences in orifice nature and properties have strong consequences on the bubbles generated. Indeed, the dynamics of the formation and the nature of the detached bubbles are fundamentally different depending on whether the bubbles are generated from the rigid orifice or from the flexible orifice. Keywords. Gas-Liquid reactors, aeration, rigid and flexible orifices, bubble formation dynamics. Résumé. L’objectif de cette étude est de comprendre comment et pourquoi la nature de l’orifice (rigide ou flexible) contrôle la génération de bulles. Les différences de nature et de propriétés entre les deux orifices ont des conséquences notables sur les bulles générées. En effet, la dynamique de formation et la nature des bulles détachées sont fondamentalement différentes selon si elles sont générées par un orifice rigide ou par un orifice flexible

Mass or heat transfer inside a spherical gas bubble at low to moderate Reynolds number

Mass (or heat) transfer inside a spherical gas bubble rising through a stationary liquid is investigated by direct numerical simulation. Simulations were carried out for bubble Reynolds number ranging from 0.1 to 100 and for Péclet numbers ranging from 1 to 2000. The study focuses on the effect of the bubble Reynolds number on both the interfacial transfer and the saturation time of the concentration inside the bubble. We show that the maximum velocity Umax at the bubble interface is the pertinent velocity to describe both internal and external transfers. The corresponding Sherwood (or Nusselt) numbers and the saturation time can be described by a sigmoid function depending on the Péclet number Pemax = Umaxdb/D (db and D being the bubble diameter and the corresponding diffusion coefficient)

Experiments and modelling of a draft tube airlift reactor operated at high gas throughputs

One-dimensional modelling of global hydrodynamics and mass transfer is developed for an annulus sparged draft tube airlift reactor operating at high gas throughputs. In a first part, a specific closure law for the mean slip velocity of bubbles in the riser is proposed according for, in one hand, the collective effects on bubble rise velocity and, in the other hand, the size of the liquid recirculation in the airlift riser. This global hydrodynamics model is found towel explain the global gas volume fraction measurements in the airlift riser for a wide range of superficial gas velocity (0.6 ≤ Jg ≥10 cm sˉ¹). In a second part, mass transfer in the airlift has been studied by using the gassing-out method and a dual-tip optical probe to measure the bubble size distributions. As for bubble columns, in such airlift, the volumetric mass transfer coefficient appears to be quite proportional to the gas superficial velocity. Finally, as in Colombet et al. (2011), mass transfer at the bubble scale seems to be weakly influenced by an increase of gas volume fraction

Experiments and modelling of a draft tube airlift reactor operated at high gas throughputs

One-dimensional modelling of global hydrodynamics and mass transfer is developed for an annulus sparged draft tube airlift reactor operating at high gas throughputs. In a first part, a specific closure law for the mean slip velocity of bubbles in the riser is proposed according for, in one hand, the collective effects on bubble rise velocity and, in the other hand, the size of the liquid recirculation in the airlift riser. This global hydrodynamics model is found towel explain the global gas volume fraction measurements in the airlift riser for a wide range of superficial gas velocity (0.6 ≤ Jg ≥10 cm sˉ¹). In a second part, mass transfer in the airlift has been studied by using the gassing-out method and a dual-tip optical probe to measure the bubble size distributions. As for bubble columns, in such airlift, the volumetric mass transfer coefficient appears to be quite proportional to the gas superficial velocity. Finally, as in Colombet et al. (2011), mass transfer at the bubble scale seems to be weakly influenced by an increase of gas volume fraction

Nanoparticles in wastewaters: hazards, fate and remediation

The increasing use of nanoparticleswill inevitably result in their release into the aquatic environment and thereby cause the exposure of living organisms. Due to their larger surface area, high ratio of particle number tomass, enhanced chemical reactivity, and potential for easier penetration of cells, nanoparticles may be more toxic than larger particles of the same substance. Some researchers have been showing some relations between nanoparticles and certain diseases. However, the doses, surface shapes, material toxicity and persistence of nanoparticles may all be factors in determining harmful biological effects. In order to better evaluate their risks, potential exposure route of nanoparticles has to be taken into consideration aswell. Finally, a brief summary of techniques for nanoparticle removal inwaters andwastewaters is presented, but it seems that no treatment can absolutely protect the public from exposure to a large-scale dissemination of nanomaterials

Effect of interface contamination on particle–bubble collision

This study focuses on the impact of the interface contamination on the collision efficiency between bubbles and inertial particles. The bubble's surface mobility has been integrated into the collision modelling by using the hydrodynamics stagnant-cap model, in which the clean angle O clean is used to characterise the interface contamination level. Direct numerical simulations have been performed for various bubble's Reynolds numbers (1≤Reb≤100), particle to bubble size ratio (0:001≤rp/rb≤0:02) and particle's Stokes numbers (0:001 O crit, the contact point of the "grazing trajectory" can only be situated on the mobile interface, while for O clean < O crit, the contact point may be on both mobile and immobile part of the interface and only the positive inertial effect is observed. A simple model has been proposed that makes possible the description of collision efficiency for clean or contaminated bubbles

Dynamics and mass transfer of rising bubbles in a homogenous swarm at large gas volume fraction

The present work focuses on the collective effect on both bubble dynamics and mass transfer in a dense homogeneous bubble swarm for gas volume fractions ↵ up to 30%. The experimental investigation is carried out with air bubbles rising in a square column filled with water. Bubble size and shape are determined by means of a high-speed camera equipped with a telecentric lens. Gas volume fraction and bubble velocity are measured by using a dual-tip optical probe. The combination of these two techniques allows us to determine the interfacial area between the gas and the liquid. The transfer of oxygen from the bubbles to the water is measured from the time evolution of the concentration of oxygen dissolved in water, which is obtained by means of the gassing-out method. Concerning the bubble dynamics, the average vertical velocity is observed to decrease with α in agreement with previous experimental and numerical investigations, while the bubble agitation turns out to be weakly dependent on α. Concerning mass transfer, the Sherwood number is found to be very close to that of a single bubble rising at the same Reynolds number, provided the latter is based on the average vertical bubble velocity, which accounts for the effect of the gas volume fraction on the bubble rise velocity. This conclusion is valid for situations where the diffusion coefficient of the gas in the liquid is very low (high Péclet number) and the dissolved gas is well mixed at the scale of the bubble. It is understood by considering that the transfer occurs at the front part of the bubbles through a diffusion layer which is very thin compared with all flow length scales and where the flow remains similar to that of a single rising bubbl

Vagus nerve stimulation: State of the art of stimulation and recording strategies to address autonomic function neuromodulation

International audienceObjective. Neural signals along the vagus nerve (VN) drive many somatic and autonomic functions. The clinical interest of VN stimulation (VNS) is thus potentially huge and has already been demonstrated in epilepsy. However, side effects are often elicited, in addition to the targeted neuromodulation. Approach. This review examines the state of the art of VNS applied to two emerging modulations of autonomic function: heart failure and obesity, especially morbid obesity. Main results. We report that VNS may benefit from improved stimulation delivery using very advanced technologies. However, most of the results from fundamental animal studies still need to be demonstrated in humans

Modélisation de réacteurs Gaz-Liquide de type colonne à bulles en conditions industrielles

L oxydation du cyclohexane est l un des procédés les plus importants dans la chaîne de production du Nylon où l oxygène et le cyclohexane entrent en contact pour former le cyclohexanol, la cyclohexanone puis l acide adipique. Le rendement est influencé à la fois par le transfert de l oxygène et par le mélange des réactifs en phase liquide. Des réacteurs de type colonne à bulles sont généralement utilisés pour fournir une aire interfaciale importante et garantir une agitation efficace en phase liquide. Cependant, la complexité des mécanismes impliqués (hydrodynamique, transfert, réaction, fort taux de vide) rend difficile la prédiction des performances des réacteurs. Ce travail est consacré à l amélioration des lois de fermetures (quantité de mouvement et transferts) pour la modélisation Euler/Euler des réacteurs industriels utilisés pour le procédé d oxydation du cyclohexane. Dans un premier temps, des expériences de laboratoire avec le système eau/air ont été réalisées jusqu à de forts taux de vide (> 30%) pour mesurer les effets collectifs sur la force de traînée et le transfert de masse dans un essaim de bulles homogène. Les résultats ont confirmé que le coefficient de traînée des bulles augmente de manière significative avec le taux de vide alors que de manière surprenante l effet est très faible sur le transfert. Dans un second temps, des expériences ont été réalisées avec le système cyclohexane/diazote dans des conditions industrielles (P = 1 - 20 bar, T = 30 - 150C). L analyse des résultats de transfert en condition industrielle a nécessité la simulation numérique directe du transfert à l intérieur d une bulle sphériqueCyclohexane oxidation is one of the most important processes in the production line of Nylon, where oxygen and cyclohexane get in contact to produce cyclohexanol, cyclohexanone and then adipic acid. The production yield is influenced by both the oxygen transfer and the reactants mixing in liquid phase. Bubble column type reactors are usually used to provide a large interfacial area and efficient liquid phase agitation. However, the complexity of the mechanisms involved (hydrodynamic, transfer, reaction, high void fraction) makes it difficult to predict the performance of such reactors. This work is devoted to improve the associated closure laws of momentum and transfer equations used in Euler/Euler modelling of industrial reactors for cyclohexane oxidation. Bench-scale experiment for air-water system has been firstly carried out to measure the collective effects on the drag force and the mass transfer of a bubble in a homogenous bubble swarm with a high void fraction up to 30%. The results confirmed that bubble s drag coefficient increases significantly with the void fraction. Meanwhile surprisingly, weak effect has been observed on the transfer. Nextly, pilot experiments with nitrogen-cyclohexane system have been performed under industrial conditions (P = 1 - 20 bar, T = 30 - 150C). Analysis of the results of transfer under industrial conditions required finally direct numerical simulation of transfers inside a spherical bubble.TOULOUSE-INSA-Bib. electronique (315559905) / SudocSudocFranceF

Image processing for the experimental investigation of dense dispersed flows : Application to bubbly flows

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