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Flows in Complex Systems

Effect of Impeller Speed Perturbation in a Rushton Impeller Stirred Tank

[+] Author and Article Information
Somnath Roy

Department of Mechanical Engineering,  Indian Institute of Technology Patna, Bihar, 800013, India

Sumanta Acharya

Department of Mechanical Engineering,  Louisiana State University, Baton Rouge, LA, 70803

J. Fluids Eng 134(6), 061104 (Jun 11, 2012) (15 pages) doi:10.1115/1.4006471 History: Received November 05, 2011; Revised March 17, 2012; Published June 11, 2012; Online June 11, 2012

Flow inside an unbaffled Rushton-impeller stirred tank reactor (STR) is perturbed using a time dependent impeller rotational speed. Large eddy simulation (LES) revealed that the perturbation increased the width of impeller jet compared to the constant rotational speed cases. The turbulent fluctuations were also observed to be enhanced in the perturbed flow and showed higher values of production and convection of turbulent kinetic energy. Changes in the mean flow-field during the perturbation cycle are investigated. The trailing edge vortices were observed to propagate farther both in the radial and azimuthal direction in the perturbed case. Production of turbulent kinetic energy is observed to be related to the breakup of the impeller jet in the perturbed case. Dissipation of turbulent kinetic energy is augmented due to the perturbation ensuring a better mixing at the molecular scale.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic of the Rushton impeller tank (all dimensions are in mm)

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Figure 2

Comparison between experimental and computational data along axial lines at (a) r/R = 1.1 and (b) r/R = 1.6 at 30 deg blade angle. Phase averaged radial velocity is compared in the left, right figures show comparison of the in-plane cross-term of Reynolds stress tensor.

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Figure 3

FFT of radial velocity signals showing macro-instability periods for 3 RPS fixed impeller rotational speed at three different locations: (a) below impeller (r/R = 1.2, z/T = −0.23), (b) above impeller (r/R = 2.4, z/T = 0.33)

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Figure 11

Production of turbulent kinetic energy for different RPM at two axial planes (a) on the impeller plane, (b) 30 deg angle with the impellers

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Figure 12

Convection of turbulent kinetic energy for different RPM at two axial planes (a) on the impeller plane, (b) 30 deg angle with the impellers

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Figure 13

Dissipation of subgrid scale turbulent kinetic energy for different RPM at two axial planes (a) on the impeller plane, (b) 30 deg angle with the impellers

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Figure 14

Contour of Negative second eigenvalue of S2+Ω2 identifying the time-averaged trailing edge vortex surfaces: (a) 3D view, (b) top view

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Figure 15

Phase averaged velocity vectors on the impeller plane for five different phases

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Figure 4

The perturbation cycle showing the phases on which flow-field has been analyzed

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Figure 5

Mean velocity vectors averaged over six angular locations showing the profile of the radial jet-stream

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Figure 6

Comparison of radial jet-width for different impeller speeds (experimental and computational results) (symbols are for experimental data, lines are for computational results: 3 RPS constant □, ··········; 4 RPS constant ○, – – –; and 2–4 RPS with 15 s perturbation frequency Δ, —————)

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Figure 7

Nondimensionalized radial velocity fluctuation RMS for different RPM

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Figure 8

Nondimensionalized RMS of radial velocity fluctuations at two different radial locations: (a) r/R = 1.1 and (b) r/R = 1.6 (Symbols are for experimental data, lines are for computational results: 3 RPS constant □_ _ _ _ _ _, and 2–4 RPS with 15 s perturbation frequency ○, ——————)

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Figure 9

Nondimensionalized axial velocity fluctuation RMS for different RPM

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Figure 10

Nondimensionalized RMS of axial velocity fluctuations at three different radial locations: (a) r/R = 0.85, (b) r/R = 1.1 and (c) r/R = 1.6, and (d) at an axial location of 2z/W = −0.33 (Symbols are for experimental data, lines are for computational results: 3 RPS constant □, _ _ _ _ and 2–4 RPS with 15 s perturbation frequency ○ —————)

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Figure 16

Interaction between impeller jet-stream and trailing edge vortex for (a) phase-2 and (b) phase-3

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Figure 17

FFT spectrum for radial velocity along the impeller jet-stream at (a) r/R = 1.1, (b) r/R = 1.5, and (c) r/R = 2.9

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Figure 18

Phase averaged in-plane turbulent kinetic energy on the impeller plane for five different phases

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