1,721,062 research outputs found
Experiments and predictions of the transition of the flow pattern with impeller clearance in stirred tanks
No full textIn the present work, the double- to single-loop flow pattern transition in a stirred vessel equipped with a Rushton turbine is investigated by Laser Doppler anemometry (LDA). In particular, the clearance at which such transition occurs is assessed by comparing axial velocity profiles underneath the impeller. Computational fluid dynamics (CFD) simulations of the same system are carried out, by employing the 'inner-outer' fully predictive computation strategy. The comparison of predicted results with the experimental data collected shows that the transition is well reproduced by simulations. A good agreement on the mean velocities is also observed but for the impeller discharge stream angle to the horizontal in the single-loop flow pattern configuration. Finally the predicted turbulent fluctuations are underestimated, a finding in agreement with those of most earlier CFD predictions. © 2001 Elsevier Science Ltd
Numerical simulations of the dependency of flow pattern on impeller clearance in stirred vessels
No full textThe flow and turbulence fields in a fully baffled vessel stirred by a Rushton turbine have been simulated by means of computational fluid dynamics (CFD) techniques. The simulation techniques adopted (the "Sliding-grid" and "Inner-outer" methods) are fully predictive and require no experimental data as boundary or initial conditions. The effect of clearance (C) on the flow patterns in stirred vessels was simulated as the flow field transition observed can be considered a particularly stringent benchmark for the testing of CFD techniques. The results are compared with previously obtained LDA data and show that the double- to single-loop transition experimentally observed when impeller clearance from the vessel bottom is suitably reduced, can be well reproduced by the CFD simulations. The single-loop flow structure present for C/T = 0.15 is compared in detail with the experimental data and good overall agreement is shown between the experiment and simulation. The periodic component of the kinetic energy is well predicted, but the random component is underestimated, a finding also observed in earlier investigations. The mean flow in most of the vessel is also well predicted but the angle to the horizontal of the impeller discharge flow is overestimated. Predictions are also reported with grid refinements and different turbulence models, in an effort to identify means of improving agreement with the measurements. The findings indicate that further improvements in turbulence modelling might be necessary. © 2001 Elsevier Science Ltd. All rights reserved
Spectral and wavelet analysis of the flow pattern transition with impeller clearance variations in a stirred vessel
No full textThe double- to single-loop pattern transition in stirred vessels stemming from a change in the off-bottom clearance of a Rushton turbine has been investigated by laser doppler anemometry. Time-resolved data showed the transition occurring within a range of clearance values and allowed the distinction of three types of flow: the double-loop regime, the single-loop regime and an unstable one termed "transitional state". Experiments of up to 3-4 h duration showed that both the onset and the lifetimes of these types of flow were random; however, in the transitional state, the flow varied between the two circulation patterns in a periodic manner, with a frequency linearly related to the impeller rotational speed. The results have important implications for mixing process and vessel design as well as CFD predictions of the flows which are discussed. © 2003 Elsevier Ltd. All rights reserved
Flow instabilities associated with impeller clearance changes in stirred vessels
No full textFlow instabilities associated with changes in the clearance (C) of a Rushton impeller from the bottom of a stirred vessel of diameter T=294 mm were studied experimentally with laser Doppler anemometry. It is shown that for C/T=0.17-0.18 flow changes from a double- to a single-loop pattern occur randomly. The transition from one regime to the other is characterized by periodic oscillations at a frequency (f) related to the impeller speed (N) by f/N=0.135. The lifetimes of the stable and unstable patterns were found to be affected by fluid density and viscosity as well as clearance. The implications of the results for both mixing processes in stirred vessels and related predictions of the flows are discussed
Observations on the significance of instabilities, turbulence and intermittent motions on fluid mixing processes in stirred reactors
No full textFlow instabilities of different nature have been observed inside stirred vessels and the motions associated with them may enhance mixing in a number of ways. In the present work the different kinds of flow instabilities which occur continuously for most configurations (precessional macro-instabilities, (Chem. Eng. Sci. 58 (2003) 2937) and constantly/intermittently for particular vessel/impeller configurations (Chem. Eng. Sci. 58 (2003) 2297, Chem. Eng. Sci. 58 (2003) 3859), were investigated simultaneously through two-point Laser Doppler Anemometry (LDA) measurements and their effect on mixing operations is discussed. A multi-channel LDA was used to investigate the flow instabilities in different regions of a stirred vessel for both radial and axial impellers, and velocity data were analysed with wavelet and spectral/FFT analysis. The energy associated with the different kinds of flow instabilities was estimated in many locations of the vessel. The amount of broadening of the turbulence levels that such instabilities produce through the superimposition of the instability flow variation on the mean flow pattern was quantified. The relative strength of the different instabilities in different parts of the vessel as well as the presence of intermediate frequencies that may arise from the combined effect of different instability frequencies are also discussed
An experimental study of double-to-single-loop transition in stirred vessels
No full textThe velocity characteristics of the flows in a fully baffled vessel of diameter T = 290 mm stirred by a Rushton impeller of diameter D = T/3 were investigated by means of laser-Doppler anemometry measurements. The effects of clearance and rotational speed on the flow patterns in the vessel were studied. It was found that at impeller clearances from the bottom of the vessel (C) around 0.2 T the characteristic double-loop flow pattern undergoes a transition to a single-loop one with the impeller stream direction becoming partly axial and being inclined at around 25 to 30° to the horizontal. The impeller stream inclination varied with radial distance from the impeller, as well as with angular position between blades (blade angle). Impeller speed was found to have no effect on the flow pattern or the mean velocities and turbulence levels normalized by Vtip for C/T > 0.20 or C/T < 0.15. The flow structure measured with C = 0.15T is described in detail and the implications of the data for fluid mixing in stirred vessels are discussed
Numerical simulations of the dependency of flow pattern on impeller clearance in stirred vessels
No full textThe flow and turbulence fields in a fully baffled vessel stirred by a Rushton turbine have been simulated by means of computational fluid dynamics (CFD) techniques. The simulation techniques adopted (the "Sliding-grid" and "Inner-outer" methods) are fully predictive and require no experimental data as boundary or initial conditions. The effect of clearance (C) on the flow patterns in stirred vessels was simulated as the flow field transition observed can be considered a particularly stringent benchmark for the testing of CFD techniques. The results are compared with previously obtained LDA data and show that the double- to single-loop transition experimentally observed when impeller clearance from the vessel bottom is suitably reduced, can be well reproduced by the CFD simulations. The single-loop flow structure present for C/T = 0.15 is compared in detail with the experimental data and good overall agreement is shown between the experiment and simulation. The periodic component of the kinetic energy is well predicted, but the random component is underestimated, a finding also observed in earlier investigations. The mean flow in most of the vessel is also well predicted but the angle to the horizontal of the impeller discharge flow is overestimated. Predictions are also reported with grid refinements and different turbulence models, in an effort to identify means of improving agreement with the measurements. The findings indicate that further improvements in turbulence modelling might be necessary. © 2001 Elsevier Science Ltd. All rights reserved
Macro-instability phenomena in stirred vessels in the laminar, transitional and turbulent flow regimes
No full textExperiments and predictions of the transition of the flow pattern with impeller clearance in stirred tanks
No full textIn the present work, the double- to single-loop flow pattern transition in a stirred vessel equipped with a Rushton turbine is investigated by Laser Doppler anemometry (LDA). In particular, the clearance at which such transition occurs is assessed by comparing axial velocity profiles underneath the impeller. Computational fluid dynamics (CFD) simulations of the same system are carried out, by employing the 'inner-outer' fully predictive computation strategy. The comparison of predicted results with the experimental data collected shows that the transition is well reproduced by simulations. A good agreement on the mean velocities is also observed but for the impeller discharge stream angle to the horizontal in the single-loop flow pattern configuration. Finally the predicted turbulent fluctuations are underestimated, a finding in agreement with those of most earlier CFD predictions. © 2001 Elsevier Science Ltd
On the origin, frequency and magnitude of macro-instabilities of the flows in stirred vessels
No full textThe mean flow and turbulence fields in a fully baffled vessel stirred by a Rushton impeller at three clearances and a pitched-blade turbine at one clearance have been investigated with laser-Doppler anemometry (LDA) to characterise the macro-instabilities (MIs) present in such flows. Time-resolved velocity measurements were made and the frequency content of the velocity recordings was analysed with FFT techniques. The study aims to throw light into the frequency, magnitude, nature and origin of such flow variations, especially in view of the different findings that have been reported in the published literature. The frequency of the MIs was found to be linearly related to the rotational speed of the impeller and to be essentially independent of impeller design. A single fundamental frequency, around 0.015-0.02N (Hz), where N is the impeller rotational speed, was present for all configurations, together with harmonic frequencies, the prominence of which depended on impeller geometry and/or clearance. The LDA data and direct observations made with laser-sheet flow visualisation indicated clearly that the macro-instability stems from a precessional motion about the vessel axis, similar to the precession encountered in most swirling flows. The results show that MIs, as a mean flow motion superimposed on the flow pattern in the vessel, can result in a broadening of the measured turbulence levels by up to 25%, and a mean velocity variation of up to 0.3Vtip. The findings indicate therefore that it might be necessary and indeed it could be advantageous for improved process prediction to take MI into account in models of the flows in stirred vessels. © 2003 Elsevier Science Ltd. All rights reserved
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