CHARACTERIZATION OF TURBULENT REGIME BEHAVIOR IN THE DILUTE ZONE OF A CIRCULATING FLUIDIZED BED RISER

This study consists in characterizing the solid phase behavior in the dilute region of a circulating fluidized bed riser (CFB) for the turbulent regime (i.e. for superficial gas velocities U

Analyse de la structure de l’écoulement gaz-particules dans un lit fluidisé circulant par la PIV

Une étude expérimentale de l'écoulement gaz-particules dans une colonne à lit fluidisé circulant a été réalisé à l'aide de la technique de mesure PIV (Particle Imaging Velocimetry). Les vitesses axiales moyennes des particules et leurs écarts types ont été relevés sur trois hauteurs de la colonne de 0,5 m de diamètre et de 5 m de hauteur, et sont comparés aux résultats obtenus antérieurement à l'aide de la technique LDV (Laser Doppler Velocimetry) sur une colonne de même rapport d'aspect H/D. Ils sont en bon accord qualitatif. La technique de la PIV a mis en évidence l'existence de la structure coeur-anneau (flux de solide montant au centre et un autre descendant prés de la paroi) caractérisant l'écoulement dans le lit fluidisé circulant, avec une diminution de l'épaisseur de la zone annulaire en fonction de la hauteur. Ensuite, les vitesses axiales et transversales moyennes des particules et leurs écarts types ont été relevés et analysés pour une hauteur relative z/H = 0.35 et ceci pour trois masses chargées en particules, respectivement de 10, 25 et 40 kg. Ces résultats ont montré l'existence d'une zone de transfert de matière entre le coeur et l'anneau, où son épaisseur croît avec la masse chargée. Une augmentation en valeur absolue de vitesses axiales moyennes des particules est observée à la paroi en fonction de la masse chargée

Modélisation numérique de la réduction de traînée d'arrière-corps par injection diphasique additionnelle

Le système de réduction de traînée appelé système "base-bleed" consiste en l'injection d'un gaz dans la zone d'arrière-corps d'engins en vol qui va localement augmenter la pression et permettre un gain de portée. Ce manuscrit présente les travaux qui ont été effectués dans le domaine de la simulation numérique et de la modélisation des écoulements diphasiques réactifs compressibles pour développer un outil permettant la prédiction de la réduction de traînée de culot par effet "base-bleed". Les simulations permettent de décrire correctement les différents régimes d'écoulement qui se mettent en place lorsqu'un écoulement axisymétrique fortement décollé se trouve modifié par la présence d'une injection. Elles contribuent à améliorer la compréhension des mécanismes de transition entre ces régimes, notamment sous l'influence des paramètres débit et température. Enfin, les effets de particules d'aluminium réactives ont été étudiés, celles-ci peuvent améliorer l'efficacité du système.AIX-MARSEILLE1-Inst.Médit.tech (130552107) / SudocSudocFranceF

The effect of air velocity on slugs in a confined channel

International audienceThe present work focuses on the study of slugs occurring in a two-phase flow of a confined rectangular channel: conditions of appearance and effect on the flow behavior. Three-dimensional numerical simulations have been carried out to examine the effect of superficial air velocity on flow behavior. The Volume Of Fluid model (VOF) is used to track the air-water interface. Validation of the numerical model is obtained by comparing the results of the simulated axial velocity with experimental data determined using the Laser Doppler Anemometry (LDA) technique. The numerical results revealed that for a fixed water level and superficial water velocity, higher superficial air velocities generate a slug flow that causes channel blockage. The position of these slugs and the timing of their occurrence were correlated in terms of air and water superficial Reynolds number

Heat transfer characteristics induced by multiple tandem elevated inclined jets sources in cross flows

Consideration is given in this paper to the heat transfer induced in an elevated triplet-jet-group in cross flow numerical configuration. The jets are discharged from inclined, inline and elevated cylindrical ducts. We are particularly concerned with the generated aerothermic mixing, a critical and highly affecting parameter in the protection of turbine blade surfaces subject to high temperature levels, in the optimization of air-fuel mixing, and then combustion within scramjet combustors, in the reduction of noise by means of air curtains, etc. Since the generated heat fluxes are closely related to the interacting flows' dynamics and corresponding turbulent features, we intend to bring here some clearance on the development of the aerothermic mixing of an elevated cascaded jet model within a cross flow. To reach our goal, a finite volume method is adopted to reproduce experimental data relative to three isothermal and 60°-inclined jets, placed three diameters apart, sent at a given height from the wind tunnel ground and discharged from similar elliptical cross-sections, under different injection ratios. A mesh sensibility study and different turbulence modeling tests were carried out to obtain the consistent validation of the jets’ velocities and trajectories. The first-order kωSST and the second-order RSM models gave particularly interesting results. Once a satisfactory agreement was achieved, a temperature difference between the jets and the mainstream was adopted to upgrade the model. We could then account for the induced heat transfer through the analysis of the reduced static temperature, a particularly interesting parameter for its closeness (if not similarity under some circumstances) to the film cooling efficiency

A novel ventilation method to prevent obstruction phenomenon within sewer networks

The phenomenon of blockage is a widely encountered issue in industrial applications with multiphase flows such as sewer networks. This paper proposes a solution to eliminate the blocking phenomenon in such confined horizontal channels. The study presents alternative longitudinal ventilation using inclined jets. Numerical simulations were performed to evaluate several parameters such as the position of the jets, their orientation and the velocity ratio between the main air inlet and the jets. The Volume of Fluid model (VOF) is used to follow the evolution of the interface between the air and water phases. Numerical results of the axial velocity were confronted to the experimental data obtained by the Laser Doppler Anemometry (LDA) technique and gave good agreement. The parametric study shows that the lateral injection has a direct effect on the dynamic of the air-water flow inside channels. With the optimal depicted position and orientation of the injectors, the obstruction is avoided for low air velocities and is moved away from the entrance for the higher ones. Results show also that side-by-side lateral injection all over the channel with an optimal ventilation ratio fully avoid the occurrence of slug flow and thus the obstruction of the channel

Experimental and CFD analyses of pollutant dispersion around an isolated cylindrical building

International audienceAn investigation of the dispersion of pollutants ejected from a chim- ney around a three-dimensional cylindrical obstacle within a cross- flow air stream was conducted in this paper. The dynamic evolution of air seeded with glycerin particles ejected from an elevated jet around the isolated obstacle in a wind tunnel was experimentally studied using the particle image velocimetry technique and under different velocity ratios between the chimney ejection and the wind source. The dispersion of CO2 pollutant around the cylindrical build- ing was numerically predicted based on measured experimental data using the finite volume method and the Reynolds stress model. A good agreement between experimental and computational results was found. Velocity, temperature, and concentration results revealed that the velocity of the wind together with the presence of the isolated building obstacle influenced the flow structure

Wind Tunnel Investigation and Numerical Analysis of Fume Behavior in the Vicinity of Rectangular Building Under Moderate Velocity Wind

International audienceAbstract A research was conducted to examine the dispersion of pollutants ejected from a chimney around three-dimensional rectangular building. Regarding the experimental study, the wind tunnel experiment comprises data acquired through the dispersion of continuous source tracer discharges from a punctual source situated in a regular network of building-like obstacle, and these data include measurements of mean velocity and turbulence parameters. The relevant data are followed using Particle Image Velocimetry to track various instantaneous and mean dynamic characteristics. Concerning the numerical study, the suggested model simulates both the dynamics and the heat transfer flow field using the overall mean three-dimensional Navier-Stokes equations with an RSM turbulence closure model. The findings of a deep comparison of turbulent flow and dispersion between a full wind tunnel experiment and the model predictions are reported. A high degree of concordance was obtained with the experimental flow and numerical simulation data. The detailed investigation, in which numerical and wind tunnel studies, was performed to evaluate the impact of wind velocity on the pollutant dispersion issued from a chimney around the building in their vicinity. the results clearly showed how wind velocity influenced the environmental air flows and pollutant dispersal pathways. The results of this study show that the shape of the building and the resulting interaction between the wind structure play a determining factor in the distribution of pollutants around a building, thereby affecting the air quality in the various parts of the building