Ivan Fořt

Macro-instabilities of velocity field in stirred vessel: : detection and analysis

Results of the analysis of velocity data obtained by the laser Doppler velocimetry (LDV) in a flat-bottomed cylindrical stirred tank (300 mm diameter, filled with the liquid to the height equal to its diameter) equipped with four radial baffles and stirred with a pitch-blade turbine impeller are presented. Water and 85% aqueous glycerine solution were used as working fluids at three values of Reynolds number of the impeller: 75000, 1200 and 750. The fluid velocity was recorded in a rectangular region of evenly spaced points close to the stirrer region. The macro-instability (MI) of the velocity field was extracted from the experimental data using the proper orthogonal decomposition (POD) technique and spectral analysis. This approach enabled not only detection of the macro-instability in measured velocity data but also to quantify its relative magnitude (a ratio of kinetic energy captured by the macro-instability to total kinetic energy of the flow). The dependence of chaotic invariants of the MI on the Reynolds number and position of the measuring point in the vessel is constructed. The time evolution of the MI is reconstructed from experimental records.

Velocity Field Macro-instabilities in an Axially Agitated Mixing Vessel

Abstract The flow pattern produced by a 45° pitched-blade turbine in a fully baffled cylindrical flat-bottomed tank is investigated by a qualitative approach using a laser sheet flow visualisation and a quantitative approach using laser Doppler velocimetry measurements in the turbulent and/or transition regime, with water and glycerine solution as the working liquids. The flow visualisation shows the turbulent macroinstabilities as vortices structures appearing between the turbine and the liquid surface. These macro-structures characterise an organized modification of the flow pattern, creating a transient violent flow activity in the upper part of the vessel which is in contrast with the concept of turbulence as being completely random. The time series and the spectral analysis show a non-stationary, pseudo-periodic phenomenon at a frequency much lower than the blade passage frequency, about 6%, and linearly coupled with the frequency of impeller revolution. This phenomenon is analysed in a volume surrounding the impeller and corresponds to a sharp increase in the intensity of the fluctuating motions when the Reynolds number is increased around 600. Information on the time organization is obtained by a time frequency analysis. We show that the phenomenon is present about 20% of the total time in water and more than 40% in a glycerine solution, for a same impeller rotation speed.

Investigation by laser Doppler velocimetry of the effects of liquid flow rates and feed positions on the flow patterns induced in a stirred tank by an axial-flow impeller

The (ow patterns established in a continuously-fed stirred tank, equipped with a Mixel TT axial-(ow impeller, have been investigated bylaser Doppler velocimetry , for a high and a low value of mean residence time—mixing time ratio. The pseudo-two-dimensional axial– radial-velocityvector plots, as well as the spatial distributions of the tangential velocitycomponent and the velocitypro;les around the impeller, show that the interaction between the incoming liquid and the liquid entrained bythe agitator rotation cause the (ow pattern in the vessel to become stronglythree-dimensional, especiallyin the region between the plane, where the feeding tube lies, and the 180 ◦ -downstream plane. The increase in the liquid (ow rate and the location of the feed entryboth a

Fermentation ofCandida utilis for uricase production

ect the (ow pattern, with the latter having a more pronounced e

Macro-instability: a chaotic flow component in stirred tanks

ect. The overall process, in this mode of operation, depends upon the appropriate con;guration and choice of parameters: for conditions corresponding to high liquid (ow rates, the (ow patterns indicate the possibilityof short-circuiting, when the liquid is fed into the stream being drawn bythe agitator and when the outlet is located at the bottom of the vessel. ? 2002 Elsevier Science Ltd. All rights reserved.

DISTRIBUTION OF DYNAMIC PRESSURE ALONG A RADIAL BAFFLE IN AN AGITATED SYSTEM WITH STANDARD RUSHTON TURBINE IMPELLER

SummaryConditions for the production of microbial uricase byCandida utilis were studied. For the selected strain, hypoxanthine proved to be the most effective inducer of uricase formation. The highest values of biomass as well as uricase activity in the mechanically agitated fermentor were obtained under the following conditions: 50 h, rotation impeller speed 7 s−1, air flow rate 1.25×10−5 m3s−1, concentration of inducer 0.1%.

Heat transfer similarities between impinging jets and axial-flow impellers

Chaotic features of the macro-instability (MI) of flow patterns in stirred tanks are studied in this paper. Datasets obtained by measuring the axial component of the fluid velocity and the tangential force affecting the baffles are used. Two geometrically identical, flat-bottomed cylindrical mixing tanks (diameter of 0.3 m) stirred with either pitched blade turbine impellers or Rushton turbine impeller are used in the experiments, and water and aqueous glycerol solutions are used as the working liquids. First, the presence of the MI component in the data is examined by spectral analysis. Then, the MI components are identified in the data using the proper orthogonal decomposition (POD) technique. The attractors of the macro-instability are reconstructed using either the POD eigenmodes or a method of delays and finally the attractor invariants are evaluated. The dependence of the correlation dimension and maximum Lyapunov exponent on the vessel operational conditions is determined together with their distribution within the tank. No significant spatial variability of the correlation dimension value is observed. Its value is strongly influenced by impeller speed and by the vessel–impeller geometry. More profound spatial distribution is displayed by the maximum Lyapunov exponent taking distinctly positive values. These two invariants, therefore, can be used to locate distinctive regions with qualitatively different MI dynamics within the stirred tank.

CFD Simulation of the Discharge Flow from Standard Rushton Impeller

The paper presents experimental results of axial distribution of tangential components of dynamic pressure along the height of the radial baffle in pilot plant cylindrical equipment with a standard six-blade Rushton disk turbine impeller and four baffles. Measurements were obtained under a turbulent regime of flow using the trailing target with balancing springs at eleven axial distances above the flat bottom. Experiments were carried for one impeller tank ratio D/T = 1/3 at flve levels of impeller speed and three levels of agitated liquid viscosity. The dimensionless mean dynamic pressure affecting the baffle exhibits maximum value at the levels of both investigated impeller off-bottom clearances C/T = 0.33 and 0.48 and then decreases both towards the bottom and towards the liquid surface. Transfer of the impeller torque by radial baffles in an agitated system with a standard Rushton turbine impeller exceeds 3/4 of the impeller torque for both investigated impeller offbottom clearances when more than 2/3 of the transferred torsional moment of the baffles is located in the area of interference between the impeller discharge flow and baffles.

Local dynamic effect of mechanically agitated liquid on a radial baffle

Similarities in the flow and heat transfer characteristics between simulation results of a round impinging jet and experimental data obtained on an agitated vessel with an axial flow impeller are presented in this paper. The electrodiffusion method was used in measuring the local heat transfer coefficients on a flat bottom of an agitated vessel. A small axial impeller has been built to provide a clearly defined flow imitating a submerged confined jet impinging the vessel bottom. Our simulation and experimental results show that the flow pattern in the impinging jet region below the axial flow impeller can be compared to the stagnation region of a round impinging jet with a corresponding tangential velocity component. CFD simulations of an impinging jet showed also the importance of different boundary conditions on small electrodes used with the electrodiffusion method and give an approximation of necessary correction factor.

Local velocity scaling in an impeller discharge flow in T400 vessel agitated by tooth impeller in a fully turbulent region

The radial discharge jet from the standard Rushton turbine was investigated by the CFD calculations and compared with results from the Laser Doppler Anemometry (LDA) measurements. The Large Eddy Simulation (LES) approach was employed with Sliding Mesh (SM) model of the impeller motion. The obtained velocity profiles of the mean ensemble-averaged velocity and r.m.s. values of the fluctuating velocity were compared in several distances from the impeller blades. The calculated values of mean ensemble-averaged velocities are rather in good agreement with the measured ones as well as the derived power number from calculations. However, the values of fluctuating velocities are obviously lower from LES calculations than from LDA measurements.