PBT Mixing H

Mixing time simulation using CFD

This CFD represents a mixing time experiment using a 45o pitched bladed turbine (PBT). The fluid to mix is spread out upon the base of the tank. Notice that the last zones of mixing are behind the baffles.

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Computational Fluid Dynamics, or in this specific case, computational fluid mixing is a very powerful tool when experimentation is either not possible, dangerous, or too expensive.

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Description of this CFD

Basic Summary of the Physical Conditions:

• Standard turbulent tank 0.24M Dia, 0.24M Deep, with 4 baffles and 45o pitched bladed turbine (PBT)
• Impeller diameter D = 1/3 * Tank diameter T , and bottom clearance = 1/3 * Z
  Impeller speed, N = 250 RPM, counterclockwise rotation
• Fluid is water, density (Rho) = 1000 kg/m3, viscosity (Mu) = .001 kg/ms
• Species Dispersion:
  Initial Conditions - 6% by Volume at bottom of tank, Concentration = 1.0
  Final Conditions - dispersed 6% concentration throughout the volume
  Fluid characteristics - identical to water - viscosity and density
• Procedure / Assumptions:
  Fluid Flow solution to steady state, then study transient dispersion
  This differs from the assumption of totally transient dispersion i.e.
  Fluid initial conditions at rest rather than initially at steady state flow.
• Note for this case, steady flow is reached in approx. 5 seconds.
• Computational Model:
  • 1.28M tetrahedra, using minimal definition Rotating Reference Frame
  • Turbulence Model = Spalart-Allmaras
  • Delta t for transient species = 1.0 sec
• Computational Solution:
  • 1.8 Ghz Intel Laptop - 512MB memory
  • Flow Solution: 16 Non-linear Iterations to convergence (.001 tolerance)
  • CPU Time (elapsed): 2h, 12min
• Transient Species:
  • 25 steps (.001 tolerance) - using up to 10 nonlinear steps per iteration to reach convergence
    CPU Time (elapsed): 2h, 6min
• Results Animation:
  • Tracks the 5% Isosurface as a function of time.
  • Interpretation of which is that below and subsequently within the volume is concentration that is greater than 5%.
  • There are 26 animation frames consisting of initial condition and 1 second intervals for 25 seconds. 
• Notes:
  • toward the end, concentrations in trailing corners of baffles and walls.
  • programs are available to describe transient volume associated with concentration levels i.e. relative variance .0023 normalized by volume at 25 seconds, or volume fraction at 6% concentration = 0.99, the bulk of remaining 1% within range of .057 to .062 .

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Last modified: February, 2013
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