596 research outputs found
Modeling Turbulent Flow in Stirred Tanks with CFD: The Influence of the Modeling Approach, Turbulence Model and Numerical Scheme
Single phase turbulent flow in a tank stirred by a down- and an up-pumping pitched blade turbine has been
simulated using CFD. The effect of the modeling approach, discretization scheme and turbulence model on mean
velocities, turbulent kinetic energy and global quantities, such as the power and circulation numbers, has been
investigated. The results have been validated by LDV data. The stationary and time-dependent modeling
approaches were found to have little effect on the turbulent flow, however the choice of the numerical scheme
was found to be important, especially for the predicted turbulent kinetic energy. A first order method was found
to highly underestimate LDV data compared with higher order methods. The type of the turbulence model was
limited to the k-e and RNG models due to convergence difficulties encountered with a Reynolds Stress Model
(RSM) and there was found to be little effect of these models on the mean flow and turbulent kinetic energy.
This latter quantity was found to be largely under predicted in the discharge region of the down-pumping
impeller in comparison with LDV data. Better agreement was found for the up-pumping pitched blade turbine.
Estimated power numbers were found generally to be in good agreement for the down- and up-pumping data.
However, the circulation number tended to be over predicted by about 30% and 40% for the down- and uppumping
agitators, respectively
Gas-liquid mass transfer : influence of sparger location
The performance of three sparger diameters (DS = 0.6D, DS = D, DS = 1.6D) in combination with three positions
(below, above or level with the impeller) for gas-liquid dispersion and mass transfer were evaluated in the case
of the Rushton turbine and the A315 propeller in up- or down-pumping mode. The results show that the best
results in terms of gas handling and mass transfer capacities are obtained for all impellers with the sparger placed
below it and with a diameter at least equal to the impeller diameter. For the sparger position below the agitator,
the kLa values of the Rushton turbine are greater than those of the A315 propeller, whatever the pumping mode.
The A315 propeller in up-pumping mode is, however, more economically efficient in terms of mass transfer. In
all cases, the up-pumping mode gives better results than the down-pumping one
Effect of microchannel aspect ratio on residence time distributions and the axial dispersion coefficient
The effect of microchannel aspect ratio (channel depth/channel width) on residence time distributions and the axial dispersion coefficient have been investigated for Newtonian and shear thinning non-Newtonian flow using computational fluid dynamics. The results reveal that for a fixed cross sectional area and throughput, there is a narrowing of the residence time distribution as the aspect ratio decreases. This is quantified by an axial dispersion coefficient that increases rapidly for aspect ratios less than 0.3 and then tends towards an asymptote as the aspect ratio goes to 1. The results also show that the axial dispersion coefficient is related linearly to the Reynolds number when either the aspect ratio or the mean fluid velocity is varied. However, the fluid Péclet number is a linear function of the Reynolds number only when the aspect ratio (and therefore hydraulic diameter) is varied. Globally, the results indicate that microchannels should be designed with low aspect ratios (†0.3) for reduced axial dispersion
Alternate operating methods for improving the performance of a continuous stirred tank reactor
The effect of the pumping direction of an axial flow impeller, the feeding rate and the number of feed inlets on
the operation of a continuously-fed stirred tank has been studied using CFD. The flow patterns generated by the
up-pumping and down-pumping impeller, under both âtypicalâ and âintensifiedâ operating conditions, are
compared. The effect of various tank configurations on the performance of the vessel is assessed by analysing
the flow and power numbers, as well as the concentration field of a non-reactive tracer. Furthermore, the inlet
feed jets are reduced using traditional jet similarity analysis and are compared with that of a typical round jet.
The results show that up-pumping impellers improve circulation in the upper part of the tank and reduce shortcircuiting
of the feed stream with only a small increase in power consumption. Furthermore, by using multiple
feed inlets to increase the total throughput capacity, the amplitude of torque fluctuations is decreased and
impeller bypassing is also decreased. The ensemble of conclusions suggest that the throughput capacity and
mixing quality of a CSTR can be improved, without problems of short-circuiting, by employing up-pumping
impellers coupled with multiple surface feed points
Design of Multiple Impeller Stirred Tanks for the Mixing of Highly Viscous Fluids Using CFD
The effect of multiple Intermig impeller configuration on hydrodynamics and mixing performance in a
stirred tank has been investigated using computational fluid dynamics. Connection between impeller stages and
compartmentalisation has been assessed using Lagrangian particle tracking. The results show that by a rotating
Intermig impeller by 45° respect to its neighbours, instead of a 90° rotation as recommended by manufacturers,
enables a larger range of operating conditions, i.e. lower Reynolds number flows, to be handled. Furthermore by
slightly decreasing the distance between the lower two impellers, fluid exchange between the impellers is
ensured down to Re = 27
Characteristics of liquids lugs in gasâliquid Taylor flow in microchannels
The hydrodynamics of liquid slugs in gasâliquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (Ls/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity UTP. The product of the liquid slug residence time and the recirculation rate is independent of UTP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug
PIV measurements in an aerated tank stirred by a down- and up-pumping axial flow impeller
Liquid phase hydrodynamics in an aerated tank stirred by a down- and an up-pumping pitched blade turbine have been investigated using Particle Image Velocimetry. The effect of agitator configuration and the gas phase on the mean velocity fields and turbulent quantities in the vessel have been investigated. The global mean gas holdup has also been evaluated for the two pumping conditions. For the gas flow rate used, the presence of gas only slightly alters the liquid flow patterns produced by both the down- and up-pumping configurations and causes a general decrease in the mean liquid velocities. The turbulent kinetic energy in the impeller discharge region was not affected by the presence of gas, but in the bulk of the tank, aeration caused a decrease in this value. Global gas holdup was found to be ~36% greater for the up-pumping impeller and a large amount of gas was found to be entrained by the primary circulation loop
Amélioration par micro-ordinateur des acquisitions de données en spectrométrie Auger
La rapidité de mémorisation des techniques numériques permet d'accélérer les mesures et de suivre les évolutions des phénomÚnes de surfaces
Hydrodynamics of gas-liquid Taylor flow in rectangular microchannels
The effect of fluid properties and operating conditions on the generation of gasâliquid Taylor flow in microchannels has been investigated experimentally and numerically. Visualisation experiments and 2D numerical simulations have been performed to study bubble and slug lengths, liquid film hold-up and bubble velocities. The results show that the bubble and slug lengths increase as a function of the gas and liquid flow rate ratios. The bubble and slug lengths follow the model developed by Garstecki et al. (Lab chip 6:437-446, 2006) and van Steijn et al. (Chem Eng Sci 62:7505-7514, 2007), however, the model coefficients appear to be dependent on the liquid properties and flow conditions in some cases. The ratio of the bubble velocity to superficial two-phase velocity is close to unity, which confirms a thin liquid film under the assumption of a stagnant liquid film. Numerical simulations confirm the hypothesis of a stagnant liquid film and provide information
on the thickness of the liquid film
Blending of Newtonian and Shear-Thinning Fluids in a Tank Stirred with a Helical Screw Agitator
Newtonian and non-Newtonian laminar fluid flow has been simulated using Computational Fluid Dynamics for a cylindrical vessel stirred by a helical screw agitator. Simulations have been performed for a vessel geometry with and without a draft tube. Simulated flow patterns in the vessel have been examined and compared with the experimental work of previous authors. The power number and the circulation number have been evaluated, and interpreted in a similar manner to other works. The PO.Re constant, A, has been determined to be 295 for the geometry with the draft tube and 150 for that without the draft tube. These results are in the same range as previously reported values. The Metzner and Otto constant, k, has been evaluated to be 16.23 which is in excellent agreement with experimental results reported in the literature
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