Etude numérique de l'écoulement et de la consommation énergétique dans une cuve agitée par un mélangeur SCABA 6SRGT

Dans ce papier, on propose une étude numérique tridimensionnelle de l'écoulement d'un fluide non -Newtonien pseudoplastique (solutions de la gomme d'Xanthan), agité par un mobile de type SCABA 6SRGT, dans une cuve de forme cylindrique. L'approche de validation avec la littérature ainsi que les simulations numériques sont faites à l'aide du code de calcul CFX.13 qui offre une bonne visualisation tridimensionnelle des résultats. L'étude porte donc sur l'influence des paramètres géométriques du mobile ainsi que son excentricité avec l'axe de la cuve sur la structure de l'écoulement généré, l'homogénéité du mélange ainsi que la puissance consommé (énergie dissipée) par le ce système d'agitation. Le travail effectué a donné une très bonne concordance avec ceux de la littérature et permet de développer un modèle géométrique de mobile plus performant dans des tels processus d'agitation mécanique

Hydrodynamique de l'écoulement dans une cuve agitée par un agitateur hélicoidal

Ce travail est une investigation numérique des écoulements générés par un agitateur hélicoïdal, est une simulation numérique de l'écoulement d'un fluide Newtonien et un autre non Newtonien, dans une cuve mécaniquement agitée par un double ruban hélicoïdal. L'étude de l'hydrodynamique du fluide a été faite à l'aide du code de calcul (CFX 12.0), qui offre une visualisation tridimensionnelle des résultats sous différentes formes et configurations. Les résultats obtenus, donnent une très bonne corrélation avec le travail numérique de [Anne. Archard et al 2006], et mettrent en évidence l'influence du comportement rhéologique sur le milieu agité, ainsi que l'influence du régime hydrodynamique sur le processus. La puissance d'agitation a été étudiée aussi dans ce travail du fait qu'elle représente un paramètre clé pour un système d'agitation

Optimization of the Operating and Design Conditions to Reduce the Power Consumption in a Vessel Stirred by a Paddle Impeller

Design of the impeller blade is a determining factor in power consumption and mixing quality, which determines consequently the cost of the mixing operation. This study explores the flow patterns and the power required for stirring a Newtonian fluid by paddle impellers. Investigations are carried out via three dimensional (3D) numerical simulations. Effects of the blade curvature, blade diameter, blade number and Reynolds number are analyzed. The curved blade is found to be more efficient to reduce the power consumption, compared with the straight blade. A new correlation is proposed for predicting the power required with two-curved-bladed impellers. The straight and very large blade creates a dead zone in the space between the blade tip and the vertical wall of vessel. This issue may be overcome by the curved blade, which increases consequently the well-mixed region size. A wider well-mixed region may be obtained with the larger curved blade, but with an additional energy cost

Caractérisation de L'Ecoulement de Fluides Oswaldiens dans un Faisceau à Cylindres Rotatifs (S17)

La caractérisation des écoulements entre deux cylindres a été entreprise par voie de simulation numérique. Le cylindre interne tourne avec une vitesse de rotation constante, tandis que l'externe reste fixe. Le fluide mis en procédé présente un comportement pseudoplastique, modélisé par la loi d'Oswald De Waele. L'influence de la vitesse d'entrée d'écoulement et la rhéologie évolutive du fluide sur la structure hydrodynamique a été étudiée. Le processus est supposé isotherme dans un régime laminaire

Data on the agitation of a viscous Newtonian fluid by radial impellers in a cylindrical tank

In this paper, the data assembled concerning the agitation of a Newtonian fluid in a cylindrical vessel is disclosed. The stirred vessel is not provided with baffles and has a flat-bottom. The data presents some information on the characteristics of two impellers: a six-blade Rushton turbine and a six-blade paddle impeller. The flow patterns generated by both impellers are depicted and compared. Also, the power required when changing the impeller rotational speed is given. The data summarized here via three-dimensional calculations of velocities and viscous dissipation in the whole volume of the tank provides additional knowledge for the best choice of impellers for each industrial process. Keywords: Stirred tank, Rushton turbine, Paddle impeller, Power input, Flow pattern

Performance of Helical Ribbon and Screw Impellers for Mixing Viscous Fluids in Cylindrical Reactors

The present paper deals with the mixing of a highly viscous fluid by close-clearance impellers in cylindrical vessels. The study is performed via numerical simulations. Calculations are achieved by the discretization of continuity and momentum equations with the finite volume method. The effect of blade diameter and its shape on the well-stirred region size and the power consumption is investigated. For highly viscous fluids, the obtained results suggest the use of impellers rotating at low Reynolds number, and having a blade with the same shape of the tank to ensure mixing near the vessel base. A comparison is made between the performance of a simple helical ribbon (HR), a simple small screw (SS), helical ribbon-small screw (HR-SS) and a large screw (LS) impeller. The predicted results allow the following classification of impellers studied, based on less power requirements and small size of well-agitated region: SS < HR < HR-SS < LS

A numerical study of fluid flow and heat transfer over a fin and flat tube heat exchangers with complex vortex generators

A numerical work is carried out to investigate the heat transfer and fluid flow behaviors in a fin-and-flat-tube heat exchanger provided with complex vortex generators (CVGs). A new design of CVGs is proposed in the present paper, it consists of CVGs formed by two portions: a flat portion with various attack angles (β = 0°, 20°, 40° and 60°) and a curved portion with various curvature angles (α = 30°, 45° and 60°). Changes in CVGs position ratio (R*) inside the tube are also investigated and three values of R* are considered, namely: R* = 1.375, 1.750 and 2.125. Computations based on the finite volume method with the SIMPLE algorithm are conducted for the air flow. The Reynolds number is ranging from 25 to 400. The obtained results show that the vortex formed near the tubes is intensified by the flat potion of CVGs, and the curved tube guide the fluid flow towards the region behind the tubes, resulting thus in improved heat transfer rates. In a comparison with tubes without CVG, the new design suggested and especially the case with β = 60°, α = 60° and R* = 2.125 improve significantly the heat transfer (an increase by about 76%) with a moderate pressure loss penalty

Numerical Investigation of the Cooling of Shear Thinning Fluids in Cylindrical Horizontal Ducts

The present paper is an investigation of the cooling of hot shear thinning fluids flowing through cylindrical pipes. The study is achieved via numerical simulations with the help of the computer code CFX, which is based on the finite volume method to solve the governing equations. The efficiency of two techniques for achieving the cooling process is investigated, namely: the counter flow and the baffling techniques. In the first part and for the first strategy, the hot fluids are cooled by an external turbulent counter flow of a Newtonian liquid. In the second part and in an attempt to enhance the energy efficiency of the heat exchanger system, semi-circular baffles are inserted. We note that two strategies are used in combination in the second part of study. Effects of the flow rates and the pitch ratio of the inserted baffles on the flow and thermal fields are explored. The obtained results show a great enhancement of heat transfer rates when using both strategies in combination

A new geometrical model for mixing of highly viscous fluids by combining two-blade and helical screw agitators

Mixing processes are becoming today a huge concern for industrialists in various domains like the pharmaceutical production, oil refining, food industry and manufacture of cosmetic products especially when the processes are related to the mixing of highly viscous products. So the choice of a stirring system for this category of products or fluids must be rigorously examined before use because of the flows which are laminar in the most cases, something that is not good to obtain homogeneous particles or suspensions after the mixing operation. This CFD study allows developing a new geometrical model of mechanical agitator with high performance for mixing of highly viscous fluids. It consists of a combination of two bladed and helical screw agitators. The investigations of the flow structure generated in the vessel are made by using the computer code ANSYS CFX (version 13.0), which allows us to realize and test the effectiveness of the new stirrer on the resulting mixture and power consumption