CFD Study of Power and Mixing Time for Paddle Mixing in Unbaffled Vessels
01 Jul 2002-Chemical Engineering Research & Design (Elsevier)-Vol. 80, Iss: 5, pp 482-498
TL;DR: In this article, a mechanically stirred eight-blade paddle impeller in an unbaffled vessel over a range of Reynolds numbers covering laminar, transitional and turbulent flow regimes is presented.
Abstract: CFD-based computations of the flow field, power consumption and mixing time are presented for a mechanically stirred eight-blade paddle impeller in an unbaffled vessel over a range of Reynolds numbers covering laminar, transitional and turbulent flow regimes. The flow field calculations were performed using the sliding mesh technique to account for the motion of the impeller, and mixing time studies were done using a simulated tracer injection experiment. The effect of grid density and the choice of the turbulence model were investigated. The results are compared with flow field data from Dong et al . 1 , and power and mixing time correlations from the literature and show satisfactory agreement. It is shown that the product of mixing time and rotational speed remains constant for paddle impellers for laminar flow and that the use of a low Reynolds number turbulence model is necessary for good prediction of mixing time in the transitional flow.
TL;DR: In this paper, the mixing of an inert scalar in a vessel stirred by a Rushton impeller using the large eddy simulation technique was investigated, showing that the largest scalar fluctuations were detected in the injection plane, in the area close to the impeller where turbulence levels are highest.
Abstract: Although reactive mixing is of utmost interest for most industrial mixing-sensitive processes, a thorough understanding of passive scalar behaviour and mixing provides an important step in understanding of the relevant processes, especially when turbulent flow conditions are present over much of the vessel volume. The objective of the present work is to characterise the mixing of an inert scalar in a vessel stirred by a Rushton impeller using the large eddy simulation technique. Recent studies have demonstrated the capability of this approach for the accurate modelling of the turbulent flow field in a stirred vessel. However, to the best of the authors’ knowledge, no comprehensive investigation of mixing characteristics and assessment of mixing time against experimental data has been performed so far. The current simulations provides a very detailed picture of the spatial and temporal evolution of the scalar concentration that cannot be obtained with the standard Reynold's averaged Navier–Stokes approach. The largest scalar fluctuations were detected in the injection plane, in the area close to the impeller where turbulence levels are highest. Scalar concentrations recorded at several other points inside the domain revealed different mixing patterns across the vessel. The predicted mixing time compared well, on average within 18%, with values determined from correlations reported in the literature. This agreement demonstrates the capability of this modelling approach to simulate accurately the mixing characteristics in a stirred vessel and therefore to be employed for the optimisation of industrial vessel designs.
TL;DR: In this work, laser-Doppler velocimetry (LDV) and computational fluid dynamics (CFD) were used to experimentally map and computationally predict the velocity distribution inside a standard USP Apparatus II under the typical operating conditions mandated by the dissolution test procedure.
Abstract: The USP Apparatus II is the device commonly used to conduct dissolution testing in the pharmaceutical industry. Despite its widespread use, dissolution testing remains susceptible to significant error and test failures, and limited information is available on the hydrodynamics of this apparatus. In this work, laser-Doppler velocimetry (LDV) and computational fluid dynamics (CFD) were used, respectively, to experimentally map and computationally predict the velocity distribution inside a standard USP Apparatus II under the typical operating conditions mandated by the dissolution test procedure. The flow in the apparatus is strongly dominated by the tangential component of the velocity. Secondary flows consist of an upper and lower recirculation loop in the vertical plane, above and below the impeller, respectively. A low recirculation zone was observed in the lower part of the hemispherical vessel bottom where the tablet dissolution process takes place. The radial and axial velocities in the region just below the impeller were found to be very small. This is the most critical region of the apparatus since the dissolving tablet will likely be at this location during the dissolution test. The velocities in this region change significantly over short distances along the vessel bottom. This implies that small variations in the location of the tablet on the vessel bottom caused by the randomness of the tablet descent through the liquid are likely to result in significantly different velocities and velocity gradients near the tablet. This is likely to introduce variability in the test.
TL;DR: In this paper, the flow pattern and the distribution of energy dissipation rate in a batch rotor-stator mixer were investigated using a sliding mesh and standard k-ɛ turbulence model.
Abstract: The flow pattern and the distribution of energy dissipation rate in a batch rotor–stator mixer have been investigated. Sliding mesh and standard k–ɛ turbulence model were employed to predict velocity and energy dissipation rate distributions verified experimentally by the Laser Doppler Anemometry measurements. The agreement between predicted and measured bulk flow field as well as the flow pattern of jets emerging from the stator holes was very good. Results showed that the periodicity of the jet can be related to the rotor's velocity and number of blades. The energy balance based on measured velocity distribution indicated that about 70% of energy is dissipated in close proximity to the mixing head. Both simulation and measurement showed that the jet velocity and flowrate through the holes were proportional to N while the energy dissipation rate scaled with N3.
TL;DR: In this article, a simulation of a fully baffled vessel with two six-blade Rushton turbines was used to investigate the flow field, power and mixing time in a stirred vessel.
Abstract: Computational and experimental methods have been used to investigate the flow field, power and mixing time in a fully baffled stirred vessel with two six-blade Rushton turbines. Flow in a stirred tank involves interactions between flow around rotating impeller blades and stationary baffles. In computational fluid dynamics (CFD), the flow field was developed using the sliding mesh (SM) approach. The large eddy simulation (LES) was used to model the turbulence. For validation of simulation results two series of experiments were performed: (i) velocity measurements of the liquid phase using particle image velocimetry (PIV) and (ii) concentration measurements of the determining tracer in the liquid phase using the planar laser-induced fluorescence (PLIF) technique. In each series three different rotational speeds of impellers: 225, 300 and 400 rpm were employed. The stirring power input was also calculated based on the PIV results. A considerable reduction in mixing time was achieved and stirring power input was increased by increasing the impeller speed. The satisfactory comparisons indicate the potential usefulness of this CFD approach as a computational tool for designing stirred reactors.
01 Jan 2008
TL;DR: Comparisons indicate the potential usefulness of this CFD approach as a computational tool for designing stirred reactors and a considerable reduction in mixing time was achieved and stirring power input was increased by increasing the impeller speed.
Cites background from "CFD Study of Power and Mixing Time ..."
...where D is the outer diameter of the impellers ....
TL;DR: In this paper, a methode numerique par volume fini pour the resolution des equations de Navier-Stokes bidimensionnelles, incompressible, and stationnaires, en coordonnees generales curvilignes, is presented.
Abstract: Presentation d'une methode numerique par volume fini pour la resolution des equations de Navier-Stokes bidimensionnelles, incompressibles, et stationnaires, en coordonnees generales curvilignes Application de la methode aux ecoulements turbulents sur des profils avec et sans separation au bord de sortie posterieur Comparaison des calculs avec des donnees experimentales
TL;DR: In this article, the local turbulent viscosity is determined from the solution of transport equations for the turbulence kinetic energy and the energy dissipation rate, and the predicted hydrodynamic and heat-transfer development of the boundary layers is in close agreement with the measured behaviour.
Abstract: The paper presents a new model of turbulence in which the local turbulent viscosity is determined from the solution of transport equations for the turbulence kinetic energy and the energy dissipation rate. The major component of this work has been the provision of a suitable form of the model for regions where the turbulence Reynolds number is low. The model has been applied to the prediction of wall boundary-layer flows in which streamwise accelerations are so severe that the boundary layer reverts partially towards laminar. In all cases, the predicted hydrodynamic and heat-transfer development of the boundary layers is in close agreement with the measured behaviour.
TL;DR: The performances of SIMPLE, SIMPLER, and SIMPLEC are compared for two recirculating flow problems and several modifications to the method are shown which both simplify its implementation and reduce solution costs.
Abstract: Variations of the SIMPLE method of Patankar and Spalding have been widely used over the past decade to obtain numerical solutions to problems involving incompressible flows. The present paper shows several modifications to the method which both simplify its implementation and reduce solution costs. The performances of SIMPLE, SIMPLER, and SIMPLEC (the present method) are compared for two recirculating flow problems. The paper is addressed to readers who already have experience with SIMPLE or its variants.
01 Jan 1975
01 Jan 1985
TL;DR: In this paper, a review of liquid mixing equipment is presented, including powder mixers, mixing in fluidized beds, mixing of cohesive powders, dispersion of fine particles in liquid media, and mixing of liquids in stirred tanks.
Abstract: CONTENTS INCLUDE: Introduction to mixing problems, Characterization of powder mixtures, The selection of powder mixers, Mixing in fluidized beds, The mixing of cohesive powders, The dispersion of fine particles in liquid media, A review of liquid mixing equipment, Mixing of liquids in stirred tanks, Jet mixing, Mixing in single-phase chemical reactors, Laminar flow and distributive mixing, Static mixers, Mechanical aspects of mixing, Dynamics of emulsification, Gas-liquid dispersion and mixing, The suspension of solid particles, The mixer as a reactor: liquid/solid systems
Related Papers (5)
01 Jan 1975