Abstract
A comprehensive CFD study was conducted to understand the hydrodynamics characteristics in a top-covered unbaffled stirred tank. The turbulent flow in the tank was described by the Reynolds stress turbulence (RST) model, and the impeller rotation was simulated using a single reference frame (SRF) approach. The flow field and power number in the tank stirred by a Rushton turbine were investigated firstly. It is found that circumferential flow is dominant in the unabffled tank, and the flow field can be divided into forced and free vortex zones. A weak two-loop flow pattern forms in the vertical plane, and would evolve into one single-loop when impeller was equipped very close to the bottom of the tank. In general, impeller off-bottom clearance and blade thickness have small effects on flow field in the unbaffled tank. With increasing of rotational speed, the dimensionless liquid velocity rises, especially near the critical radius of two zones. Critical radius is larger for larger impeller, while the variation range of critical radius to impeller diameter ratio is narrow. The effects of operational and geometric parameters on impeller power number (Np) in unbafffled tank are far different from those in baffled tank. In the unbaffled stirred tank, rotational speed, blade thickness and impeller off-bottom clearance have small effects on power number, while impeller diameter has a significant effect on power number. Under the same operating condition, the larger the impeller diameter, the smaller the power number is. Finally, the hydrodynamics characteristics in the tank stirred by four common impellers were investigated and compared.
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Abbreviations
- A :
-
Surface area of impeller and shaft m2
- C :
-
Impeller off-bottom clearance m
- D :
-
Impeller diameter m
- D c :
-
Disc diameter of impeller m
- Ds :
-
Diameter of shaft m
- D L,ij :
-
Molecular diffusion in Eq. (1) kg m−1 s−3
- D T,ij :
-
Turbulent diffusion in Eq. (1) kg m−1 s−3
- F ij :
-
Production by system rotation in Eq. (1) kg m−1 s−3
- G ij :
-
Buoyancy production kg m−1 s−3
- H :
-
Liquid level m
- k :
-
Turbulent kinetic energy m2 s−2
- l :
-
Blade length m
- M :
-
Impeller torque in rotational direction N m
- N :
-
Impeller rotation speed r min−1
- Np :
-
Impeller power number
- P ij :
-
Stress production in Eq. (1) kg m−1 s−3
- r c :
-
Critical radius of two zones m
- r c * :
-
Dimensionless critical radius of two zones
- r :
-
Radial location m
- R :
-
Tank radius m
- Re:
-
Reynolds number
- t b :
-
Blade thickness m
- T :
-
Tank diameter m
- u :
-
Mean velocity m/s
- u a :
-
Axial velocity m/s
- u r :
-
Radial velocity m/s
- u t :
-
Tangential velocity m/s
- u tip :
-
Velocity on blade tip m/s
- z :
-
Axial location m
- ε :
-
Turbulent energy dissipation m2 s−3
- μ :
-
Viscosity kg m−1 s−1
- ρ :
-
Density kg m−3
- τ :
-
Stress tensor kg m−1 s−2
- ϕ ij :
-
Pressure strain correlation kg m−1 s−3
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The authors would like to acknowledge the support by Key Scientific Research Project of Sichuan Provincial Education Department (15ZA0107).
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Li, L., Xu, B. Numerical analysis of hydrodynamics characteristics in a top-covered unbaffled stirred tank. Chem. Pap. 75, 5873–5884 (2021). https://doi.org/10.1007/s11696-021-01767-9
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DOI: https://doi.org/10.1007/s11696-021-01767-9