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Laminar Mixing in Stirred Tank Agitated by an Impeller Inclined  [PDF]
Koji Takahashi,Yoshiharu Sugo,Yasuyuki Takahata,Hitoshi Sekine,Masayuki Nakamura
International Journal of Chemical Engineering , 2012, DOI: 10.1155/2012/858329
Abstract: The mixing performance in a vessel agitated by an impeller that inclined itself, which is considered one of the typical ways to promote mixing performance by the spatial chaotic mixing, has been investigated experimentally and numerically. The mixing time was measured by the decolorization method and it was found that the inclined impeller could reduce mixing time compared to that obtained by the vertically located impeller in laminar flow region. The effect of eccentric position of inclined impeller on mixing time was also studied and a significant reduction of mixing time was observed. To confirm the experimental results, the velocity profiles were calculated numerically and two novel numerical simulation methods were proposed. 1. Introduction In recent years, many theoretical and experimental studies have been carried out for laminar chaotic mixing and have provided much beneficial information on how laminar mixing can be enhanced. However, it has also been recognized that the mixing in periodic flows is not necessarily complete, because such systems often display coexisting chaotic and nonchaotic regions. Fluids can neither penetrate nor leave these islands of unmixed fluids by regular motion. Therefore, the isolated mixing regions may become barriers to mixing. In the pioneering studies in this field, Lamberto et al. [1] first attempted to solve this problem using time-dependent rotational velocity to enhance the mixing in a stirred tank equipped with ordinary small impellers. After that, several experimental studies have been also undertaken [2, 3] to demonstrate that the mixing performance can be improved markedly by increasing the chaotic degree in temporal terms. While the chaotic degree can be increased effectively by temporal terms, the approach is restricted to practical applications because of the restriction of the motor and the speed reducer machine to drive the impeller. In practice, the temporal approach has rarely been employed in the mixing industry except for washing machines. In contrast, the chaotic degree can also be increased by spatial terms, for example, by reducing the circumferential symmetry and shifting the complexity in normal mixing equipment. While this spatial method does not improve the mixing performance as impressively as the temporal method, it places less demanding requirements on the machinery. Consequently, this approach has been widely used in industry. The special measures employed in an agitated vessel related to spatial chaotic mixing include baffles, off-center impeller mounting, and uncircumferentially
Numerical Simulation of Macroscopic Mixing in a Rushton Impeller Stirred Tank
WANG Zheng,MAO Zai-sha,SHEN Xian-qian,

过程工程学报 , 2006,
Abstract: The macroscopic mixing in a stirred tank with different tracer injection locations, impeller speeds and impeller positions is simulated numerically by solving the transport equation of the tracer based on the whole flow field in the baffled tank with a Rushton disk turbine numerically resolved using the improved inner-outer iterative procedure. Predicted mixing time is compared well with the literature correlations. The predicted residence time distribution of the stirred tank is very close to the present experimental results. The effect of the installation of a draft tube on the mixing time and residence time distributions is addressed.
CFD simulation of solids suspension in stirred tanks: Review
Ochieng Aoyi,Onyango Mrice S.
Hemijska Industrija , 2010, DOI: 10.2298/hemind100714051o
Abstract: Many chemical reactions are carried out using stirred tanks, and the efficiency of such systems depends on the quality of mixing, which has been a subject of research for many years. For solid-liquid mixing, traditionally the research efforts were geared towards determining mixing features such as off-bottom solid suspension using experimental techniques. In a few studies that focused on the determination of solids concentration distribution, some methods that have been used have not been accurate enough to account for some small scale flow mal-distribution such as the existence of dead zones. The present review shows that computational fluid dynamic (CFD) techniques can be used to simulate mixing features such as solids off-bottom suspension, solids concentration and particle size distribution and cloud height. Information on the effects of particle size and particle size distribution on the solids concentration distribution is still scarce. Advancement of the CFD modeling is towards coupling the physical and kinetic data to capture mixing and reaction at meso- and micro-scales. Solids residence time distribution is important for the design; however, the current CFD models do not predict this parameter. Some advances have been made in recent years to apply CFD simulation to systems that involve fermentation and anaerobic processes. In these systems, complex interaction between the biochemical process and the hydrodynamics is still not well understood. This is one of the areas that still need more attention.
Determining Critical Submergence in Tanks by Means of Reynolds & Weber Numbers  [PDF]
Carlos Julián Gavilán Moreno
World Journal of Engineering and Technology (WJET) , 2014, DOI: 10.4236/wjet.2014.23024
Abstract: Critical submergence in pumping systems can be determined using a number of calculations, all of which result from heterogeneous geometries based on water. The most widely spread critical submergence formula is that of the Hydraulic Institute. A study, carried out in Germany, looked at eight different formulations used to calculate critical submergence, comparing their results with those of a hydraulic model test. The conclusion is that the simplest models, overestimate the critical submergence. Similarly, a study of submergence in water intake structures concluded that predicted values were much higher than real values. A detailed analysis has been done to detect the origin of the off-set between the measured submergence and the calculated value. The main aspects selected from the analysis were the fluid properties involved in the surface deformation and the dynamic behavior outlet flow, so two a-dimensional numbers have been selected, Weber and Reynolds. To build an equation, to calculate the critical submergence, based on the mentioned a-dimensional numbers, a mixed technique (numerical and testing) has been used. The first step was driving a test in a hydraulic model to verify the critical submergence level. Then, a numerical model was built to simulate the same phenomenon and calibrate it, to be used in the future. After that, the second step is to simulate and calculate the critical submergence with other boundary condition (fluid, flow rate, pipe diameter). Once the critical submergence is calculated, a non-linear least squared approach has been developed to build the equation to calculate the critical submergence based on the Reynolds and Weber number. The numerical method used in this paper is a finite element model with a fluid volume scheme, used normally in the fluid simulation activities.
Solid-Liquid Suspension in a Vertical Three-impeller Stirred Tank

LI Yong-gang,HUANG Xiong-bin,

过程工程学报 , 2012,
Abstract: In order to improve the related efficiency in a storage tank, the solid-liquid suspension by three CBY III impellers 0.174 m diameter (D)] was studied. The three CBY III impellers were symmetrically fixed in a stirred tank with the diameter (T) of 2 m and height of 0.8 m. The optimum installing location and height of agitators were determined by analyzing the deposition area and just-suspension blade tip speed for glass beads of average 100 μm. The results show that the minimum just-suspension speed appears at 0.285R (R, the radius of tank, m) away from the tank wall, and the liquid level of 0.3T is superior to the situations of 0.2T and 0.25T, meanwhile the liquid level between 0.2T and 0.3T has only weak influence on power consumption. The power consumption of vertical three-impeller stirred tank is 2.8 times as that of standard stirred tank (single vertical agitator), but a vast of manufacturing cost can be saved. Compared with the three side-entering agitators, the vertical three-impeller agitation can save 20% of the energy when solids have 95% suspension, and installation and maintenance are very convenient and concise. It can be concluded that the vertical three-impeller agitation has an extensive application prospect in industrial storage tanks.
CFD simulation of flow patterns in unbaffled stirred tank with CD-6 impeller  [PDF]
Devi Tamphasana Thiyam,Kumar Bimlesh
Chemical Industry and Chemical Engineering Quarterly , 2012, DOI: 10.2298/ciceq111130029d
Abstract: Understanding the flow in stirred vessels can be useful for a wide number of industrial applications. There is a wealth of numerical simulations of stirring vessels with standard impeller such as Rushton turbine and pitch blade turbine. Here, a CFD study has been performed to observe the spatial variations (angular, axial and radial) of hydrodynamics (velocity and turbulence field) in unbaffled stirred tank with Concave-bladed Disc turbine (CD-6) impeller. Three speeds (N=296, 638 & 844.6 rpm) have been considered for this study. The angular variations of hydrodynamics of stirred tank were found very less as compared to axial and radial variations.
Oil filaments produced by an impeller in a water stirred thank  [PDF]
Rene Sanjuan-Galindo,Enrique Soto,Gabriel Ascanio,Roberto Zenit
Physics , 2010,
Abstract: In this video, the mechanism followed to disperse an oil phase in water using a Scaba impeller in a cylindrical tank is presented. Castor oil (viscosity = 500 mPas) is used and the Reynolds number was fixed to 24,000. The process was recorded with a high-speed camera. Initially, the oil is at the air water interface. At the beginning of the stirring, the oil is dragged into the liquid bulk and rotates around the impeller shaft, then is pushed radially into the flow ejected by the impeller. In this region, the flow is turbulent and exhibits velocity gradients that contribute to elongate the oil phase. Viscous thin filaments are generated and expelled from the impeller. Thereafter, the filaments are elongated and break to form drops. This process is repeated in all the oil phase and drops are incorporated into the dispersion. Two main zones can be identified in the tank: the impeller discharge characterized by high turbulence and the rest of the flow where low velocity gradients appear. In this region surface forces dominate the inertial ones, and drops became spheroidal.
Chemical Industry and Chemical Engineering Quarterly , 2010,
Abstract: Hydrodynamics and mixing efficiency in stirred tanks influence power draw and are therefore important for the design of many industrial processes. In the present study, both experimental and simulation methods were employed to determine the flow fields in different mixing tank configurations in a single phase system. Laser Doppler velocimetry (LDV) and computational fluid dynamics (CFD) techniques were used to determine the flow fields in systems with and without a draft tube. There was reasonable agreement between the simulation and experimental results. It was shown that the use of a draft tube with a Rushton turbine and hydrofoil impeller resulted in a reduction in the homogenization energy by 19.2 and 17.7%, respectively. This indicates that a reduction in the operating cost can be achieved with the use of a draft tube in a stirred tank and there would be a greater cost reduction in a system stirred by the Rushton turbine compared to that stirred by a propeller.
Oxygen mass transfer in a stirred tank bioreactor using different impeller configurations for environmental purposes  [cached]
Karimi Ali,Golbabaei Farideh,Mehrnia Momammad,Neghab Masoud
Iranian Journal of Environmental Health Science & Engineering , 2013, DOI: 10.1186/1735-2746-10-6
Abstract: In this study, a miniature stirred tank bioreactor was designed for treatment of waste gas containing benzene, toluene and xylene. Oxygen mass transfer characteristics for various twin and single-impeller systems were investigated for 6 configurations in a vessel with 10 cm of inner diameter and working volume of 1.77L. Three types of impellers, namely, Rushton turbine, Pitched 4blades and Pitched 2blades impellers with downward pumping have been used. Deionized water was used as a liquid phase. With respect to other independent variables such as agitation speed, aeration rate, type of sparger, number of impellers, the relative performance of these impellers was assessed by comparing the values of (KLa) as a key parameter. Based on the experimental data, empirical correlations as a function of the operational conditions have been proposed, to study the oxygen transfer rates from air bubbles generated in the bioreactor. It was shown that twin Rushton turbine configuration demonstrates superior performance (23% to 77% enhancement in KLa) compared with other impeller compositions and that sparger type has negligible effect on oxygen mass transfer rate. Agitation speeds of 400 to 800 rpm were the most efficient speeds for oxygen mass transfer in the stirred bioreactor.
Ozone absorption in a mechanically stirred reactor
Journal of the Serbian Chemical Society , 2007,
Abstract: Ozone absorption in water was investigated in a mechanically stirred reactor, using both the semi-batch and continuous mode of operation. A model for the precise determination of the volumetric mass transfer coefficient in open tanks without the necessity of the measurement the ozone concentration in the outlet gas was developed. It was found that slow ozone reactions in the liquid phase, including the decomposition of ozone, can be regarded as one pseudo-first order reaction. Under the examined operating conditions, the liquid phase was completely mixed, while mixing in a gas phase can be described as plug flow. The volumetric mass transfer coefficient was found to vary with the square of the impeller speed.
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