Flow phenomena in stirred tanks. Part I. The impeller stream

Abstract

Flow parameters were measured in a baffled, turbulent, stirred tank agitated by a six-blade, disk style turbine. The working fluid was air. Mean and fluctuating velocities were measured between the blades of the impeller with a probe mounted on the spinning impeller. Mean velocities, turbulent velocities, one-dimensional energy spectra, and Eulerian autocorrelation functions were measured in the impeller stream by using the shielded hot-wire anemometer of Gunkel et al. (1971) which permits the measurement of turbulence parameters in flows of very high turbulence intensity. Reliable impeller pumping capacities were obtained yielding Q r a d / N D 3 = 1.0 at the impeller periphery. An energy balance on a control volume containing the impeller and the impeller stream showed that the energy put into the tank via the impeller appeared as a net efflux of kinetic energy leaving the control volume. Therefore, most of the energy input to the tank is dissipated in the bulk of the tank. SCOPENo progress in the understanding of mixing operations in stirred tanks can be achieved without a better picture of the flow phenomena in stirred tanks. The main objectives of this study were: to measure flow parameters in a turbulent stirred tank, from them to determine how the energy put into the tank was dissipated, and then to derive theoretically supported scale-up procedures for operations in stirred tanks. The achievement of this last objective is of most immediate importance for practical applications.Previous studies on the flow phenomena in the impeller stream of a stirred tank have reported mean velocities, turbulent velocities, one-dimensional energy spectra, and correlation functions (Kim and Manning, 1964;Bowers, 1965;Cutter, 1966; Sat0 et al., 1967;Mujumdar et al., 1970;Rao and Brodkey, 1972;Cho et al., 1971). The main shortcoming of these studies is the poor precision of the experimental techniques which were not suitable for the high turbulence intensities (up to Svo)found in stirred tanks. Nevertheless, Cutter (1966) concluded that 70% of the energy put into the tank was dissipated within the impeller and the impeller stream, and only 30y0 was dissipated in the bulk of the tank. In the first part of the present paper, accurate flow parameters are reported in the impeller stream from a six-blade, dipk style turbine. An energy balance is then used to determine the amount of energy dissipated in the impeller and the impeller stream. CONCLUSIONS A N D SIGNIFICANCEA very complete picture of the flow pattern in the neighborhood of a six-blade, disk style turbine impeller has been obtained. For the first time velocities between the blades of a rotating impeller have been measured. These measurements indicated that four vortices are present between each pair of blades, two above and two below the disk.Mean velocity profiles in the impeller stream showed a jet like behavior, with the impeller stream entraining fluid as it traveled from the periphery of the impeller to the tank wall. Integration of the velocity profiles yielded ...

Angle-resolved stereo-PIV measurements close to a down-pumping pitched-blade turbine

“…He concluded that around 20% of the total kinetic energy is dissipated in the impeller region, 50% in the impeller stream and 30% in the bulk. Although Gunkel and Weber (1975) used a similar approach to Cutter (1966), they found that most of energy was dissipated in the bulk of the tank and not in the impeller region or impeller discharge. The main reason for this disagreement may be the difference in the D/T ratio.…”

“…The main reason for this disagreement may be the difference in the D/T ratio. Cutter (1966) used an impeller of D = 0.3 T whereas Gunkel and Weber (1975) used an impeller diameter of 0.5 T. Furthermore, from 2-D PIV measurements of the flow field in RDT stirred flows (Khan, 2005) it has been found that the axial velocity component is not small enough to be neglected. The flows generated by the PBT have strong axial components and so Cutter's Eq.…”

Laser Doppler Measurements of Turbulent Parameters in Different Multiple-Propeller Systems

“…3. [14] and the pitched blade turbines [13]. This difference is due to the elimination of periodic oscillations from the autocorrelation coefficient.…”

“…The calculation of the turbulence characteristics from the energy spectrum requires a large definition of the one-dimensional energy spectrum. Gunkel and Weber [13] estimate the maximum dissipation of the kinetic energy in the range of 10 Hz±1 kHz, a sampling frequency equal to 2 kHz appears sufficient to calculate the turbulent characteristics. The problem is that this value of the sampling frequency cannot be obtained in the whole tank.…”

“…Much more detailed information about mixing quality results from spectral analysis: the one-dimensional energy spectrum [12,13], the dissipation rate of kinetic energy e [14], the autocorrelation coefficient, the integral scales [15], the Taylor and Kolmogorov microscales [16]. The knowledge of these turbulence characteristics can help us in choosing the appropriate impeller for a given application.…”