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

Impact of geometric field of view on speed perception

This paper deals with changes of the geometric field of view on speed perception. This study has been carried out using the SAAM dynamic driving simulator (Arts et Métiers ParisTech). SAAM provides motion cues thanks to a 6 DOF electromechanical platform and is equipped with a cylindrical screen of 150°. 20 subjects have reproduced 2 speeds (50 km/h and 90 km/h) without knowing the numerical values of these consigns, and with 5 different visual scale factors: 0.70, 0.85, 1.00, 1.15 and 1.30. This visual scale factor correspond to the ratio between the driver’s field of view covered by the screen (constant) and the geometric field of view. This study shows that this visual scale factor has a significant impact on the speed reached by the subjects and thus shows that perceived speed increases with this visual scale factor. A 0.15 modification of this factor is enough to obtain a significant effect. The modification of the geometric field of view remained unnoticed by all the subjects, which implies that this technique can be easily used to make drivers reduce their speed in driving simulation conditions. However, this technique may also modify perception of distances.Cet article présente l’effet du changement du champ de vision géométrique sur la perception de la vitesse. Cette étude a été réalisée sur le simulateur de conduite dynamique SAAM (Arts et Métiers ParisTech). SAAM utilise une plate-forme électromécanique à 6 DDL et est équipé d’un écran cylindrique de 150° pour restituer la sensation de mouvement. 20 sujets ont reproduit 2 vitesses (50 km/h et 90 km/h), sans connaître les valeurs de ces vitesses, et avec 5 facteurs d’échelle visuelle différents : 0.70, 0.85, 1.00, 1.15 et 1.30. Ces facteurs d’échelle correspondent aux rapports entre le champ de vision du conducteur couvert par l’image (constant) et le champ de vision géométrique. Cette étude montre que ce changement visuel a un impact significatif sur la vitesse qu’atteignent les sujets et montre donc que la vitesse perçue augmente avec ce facteur d’échelle visuelle. Un changement de 0.15 de ce facteur suffit pour obtenir un effet significatif. Les changements de champ de vision géométrique n’ont été détectés par aucun des sujets, ce qui implique que cette technique peut facilement être utilisée pour amener les conducteurs à réduire leur vitesse en conditions de simulation de conduite. Cependant, cette technique pourrait aussi modifier la perception des distances.Le Grand Chalo

On numerical simulation of cavitating flows under thermal regime

International audienceIn this work, we investigate closure laws for the description of interfacial mass transfer in cavitating flowsunder thermal regime. In a first part, we show that, if bubble resident time in the low pressure area of theflow is larger than the inertial/thermal regime transition time, bubble expansion are no longer monitoredby Rayleigh equation, but by heat transfer in the liquid phase at bubbles surfaces. The modelling of inter-facial heat transfer depends thus on a Nusselt number that is a function of the Jakob number and of thebubble thermal Péclet number. This original approach has the advantage to include the kinetic of phasechange in the description of cavitating flow and thus to link interfacial heat flux to interfacial mass fluxduring vapour production. The behaviour of such a model is evaluated for the case of inviscid cavitatingflow in expansion tubes for water and refrigerant R114 using a four equations mixture model. Comparedwith inertial regime (Rayleigh equation), results obtained considering thermal regime seem to predictlower local gas volume fraction maxima as well as lower gradients of velocity and gas volume fraction.It is observed that global vapour production is closely monitored by volumetric interfacial area (bubblesize and gas volume fraction) and mainly by the Jakob number variations. It is found that, in contrast withphase change occurring in common boiling flow, Jakob number variation is influenced by phasic temper-ature difference but also by density ratio variation with pressure and temperature

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

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

International audienc

Study of latency gap corrections in a dynamic driving simulator

Simulator sickness is a well-known side effect of driving simulation which may reduce the passenger well-being and performance due to its various symptoms. The present study investigated the possible effect of the latency gap correction on simulator sickness. Indeed, dynamic simulators can have different delays in their motion and visual systems (the latency gap), which may cause motion sickness. For now twenty-four participants have done two automated driving sessions in a driving simulator, with and without the correction. Simulator sickness has been estimated thanks to the Simulator Sickness Questionnaire (SSQ)

A simulation sickness study on a driving simulator equipped with a vibration platform

Simulator sickness is a well-known side effect of driving simulation which may reduce the passenger well-being and performance due to its various symptoms, from pallor to vomiting. Numerous reducing countermeasures have been previously tested; however, they often have undesirable side effects. The present study investigated the possible effect of seat vibrations on simulator sickness. Three configurations were tested: no vibrations, realistic ones and some that might affect the proprioception. Twenty-nine participants were exposed to the three configurations on a four-minute long automated driving in a simulator equipped with a vibration platform. Simulator sickness was estimated thanks to the Simulator Sickness Questionnaire (SSQ) and to a postural instability measure. Results showed that vibrations help to reduce the sickness. Our findings demonstrate that some specific vibration configurations may have a positive impact on the sickness, thus confirming the usefulness of devices reproducing the road vibrations in addition to creating more immersion for the driver