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Introduction 
 Stirred tanks with gas liquid two-phase flow are very widely 
used in chemical and biochemical engineering...
Introduction (Contd.) 
 3D CFD model of a gas-liquid two 
phase stirred tank with 2 six blade 
turbines and 4 baffles wer...
Hydrodynamic Characteristics 
 Gas holdup 
 A dimensionless key parameter for design purposes that 
characterizes transp...
Computational Fluid Dynamics 
 Branch of fluid dynamics, 
used to solve nonlinear 
differential equations 
involving flui...
Why CFD ???? 
 Less time consuming & less expensive compared to 
experiments 
 Powerful visualization capabilities 
 Pr...
Model 
 Eulerian approach is adopted 
 Stationary frame of reference 
 Unstructured grid 
 Two phases 
 Dispersed and...
Model (Contd.) 
 The momentum balance equation for each phase 
 Drag force exerted by dispersed phase on continuous phas...
Numerical Modeling 
 Finite Element Method with a multi-grid solver was 
adopted (CFX 10.0) 
 Computational domain of th...
Mesh partitions
Test Setup 
 5L distilled water is filled into the tank (fig shows the case 
when static water height was 20 cm while hei...
Test Setup (Contd.) 
Physical dimensions of the stirred tank reactor, side view and top view (unit: mm)
Results and discussions 
 Simulations of the stirred tank were carried out for five 
different operating conditions 
 Ga...
Gas holdup 
 The volume fraction of dispersed gas phase is referred to as 
the gas hold-up 
 Volume averaged overall gas...
Volume averaged overall gas holdups 
Along time courses under different operating conditions 
A: Qg = 6 L/min, Rs = 200, 4...
Model simulated time-averaged local gas 
holdups along transversal course 
(X = 0 mm, Y = 15–75 mm) at different vertical ...
Model simulated time-averaged local gas 
holdups along transversal course 
(X = 0 mm, Y = 15–75 mm) at different vertical ...
Model predictions of transient gas holdup 
distributions at the vertical sections 
(X = 0 mm) and under different operatin...
Model predictions of transient gas holdup 
distributions at the horizontal sections 
( Z = 5,30,65,100,125,160 mm) 
under ...
Liquid velocity 
 Axial liquid velocity is selected and simulated and 
experimentally measured to characterize liquid flo...
Volume averaged axial liquid velocities 
Along time course under different operating conditions 
A: Qg = 6 L/min, Rs = 200...
Model simulated and experimental measured Time 
averaged axial liquid velocities along transversal course 
(X = 0 mm, Y = ...
Model simulated and experimental measured Time 
averaged axial liquid velocities along transversal course 
(X = 0 mm, Y = ...
Model predictions of transient liquid velocity 
distributions at the vertical sections 
(X = 0 mm)and under different oper...
Model predictions of transient liquid velocity 
distributions at the horizontal sections 
(Z = 5,30,65,100,125,160 mm) and...
Bubble size distribution 
 The behavior of bubbles in gas–liquid stirred tanks are very 
important especially in supplyin...
Volume averaged bubble size fractions 
 Stirred tank was mainly occupied by small bubble groups 
(dia less than 4 mm) 
 ...
Volume averaged bubble diameters 
 Bubble Sauter mean diameter ranged from 1 to 2.5 mm 
 Increase in rotation speed obvi...
Model predictions of transient bubble diameter 
distributions at the vertical sections 
(X = 0 mm) and under different ope...
Model predictions of transient bubble diameter 
distributions at the horizontal sections 
(Z = 5,30, 65,100,125,160 mm) an...
Conclusion 
 A full-flow field, 3D transient CFD model based on 
Eulerian approach was developed for a gas-liquid two pha...
Conclusion (Contd.) 
 Increase in rotation speed made a more dispersed gas 
distribution all over the whole tank 
 Vorti...
References 
 Wang, H., Jia, X., Wang, X., Zhou, Z., Wen, J., Zhang, J., 
CFD modeling of hydrodynamic characteristics of ...
Questions
CFD modeling of hydrodynamic characteristics of a two phase gas–liquid stirred tank
CFD modeling of hydrodynamic characteristics of a two phase gas–liquid stirred tank
CFD modeling of hydrodynamic characteristics of a two phase gas–liquid stirred tank
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CFD modeling of hydrodynamic characteristics of a two phase gas–liquid stirred tank

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CFD modeling of hydrodynamic characteristics of a two phase gas–liquid stirred tank

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CFD modeling of hydrodynamic characteristics of a two phase gas–liquid stirred tank

  1. 1. Introduction  Stirred tanks with gas liquid two-phase flow are very widely used in chemical and biochemical engineering process  Stirred tanks are commonly used in reactors of  Detergent plants  Paint mixing units  Food processing plants  Many researchers carried out numerical simulations on this gas –liquid flow in a stirred tank, with many assumptions and predictions  Ranade and Khopkar, Pinelli  Zhang
  2. 2. Introduction (Contd.)  3D CFD model of a gas-liquid two phase stirred tank with 2 six blade turbines and 4 baffles were developed  Impeller rotation speeds and inlet gas flow rates are varied  Simulation, Analysis and Model predictions about gas holdup and liquid velocity distributions are carried out generally
  3. 3. Hydrodynamic Characteristics  Gas holdup  A dimensionless key parameter for design purposes that characterizes transport phenomena of bubble columns  Liquid velocity  Bubble size fractions  Transient bubble diameter distributions  Inlet air flow rate  Impeller rotation speed
  4. 4. Computational Fluid Dynamics  Branch of fluid dynamics, used to solve nonlinear differential equations involving fluid flow using numerical methods and algorithms  Extensive applications  Aerospace  Turbo machinery  Nuclear thermal hydraulics  Automotive etc Numerical Analysis Fluid Dynamics Compute r Science
  5. 5. Why CFD ????  Less time consuming & less expensive compared to experiments  Powerful visualization capabilities  Predicts performance before modifying or installing actual systems or a prototype  Predict which design changes are most crucial to enhance performance
  6. 6. Model  Eulerian approach is adopted  Stationary frame of reference  Unstructured grid  Two phases  Dispersed and continuous phases  Satisfies compatibility condition    1 g l  Continuity balance equation for each phase    u  0 g         g g g g t    u  0 l         l l l l t
  7. 7. Model (Contd.)  The momentum balance equation for each phase  Drag force exerted by dispersed phase on continuous phase  Lift force acting perpendicular to the direction of relative motion of two phases
  8. 8. Numerical Modeling  Finite Element Method with a multi-grid solver was adopted (CFX 10.0)  Computational domain of the stirred tank divided into  Rotating impeller domain  Stationary tank domain  Solver run over 70 s of computed time  Unstructured grids with a number of 1944 for impellers and 97104 for the tank were implemented
  9. 9. Mesh partitions
  10. 10. Test Setup  5L distilled water is filled into the tank (fig shows the case when static water height was 20 cm while height of tank as 30 cm)  Operating conditions:  Rotation speed : 200, 400, 600 rev/min  Inlet air flow rate : 4, 6, 8L/min  Each measurement repeated 3 times  Overall gas holdups measured from height fluctuations of the water after gas injection for specified operating conditions
  11. 11. Test Setup (Contd.) Physical dimensions of the stirred tank reactor, side view and top view (unit: mm)
  12. 12. Results and discussions  Simulations of the stirred tank were carried out for five different operating conditions  Gas flow number and Froude number are dimensionless  Rotation speed is fixed (Rs = 400 rev/min) an inlet air flow rate is varied and vice versa (Qg = 6L/min)  Summary of operating conditions in this study Case No. 1 2 3 4 5 Impeller speed (rpm) 400 400 400 200 600 Gas flow rate (L/min) 4 6 8 6 6 Gas flow number 0.060 0.090 0.120 0.180 .060 Froude number 0.249 0.249 0.249 0.062 0561
  13. 13. Gas holdup  The volume fraction of dispersed gas phase is referred to as the gas hold-up  Volume averaged overall gas holdup along time  Time averaged local gas holdups along transversal courses  Transient gas holdup distributions at horizontal and vertical positions were simulated and analyzed
  14. 14. Volume averaged overall gas holdups Along time courses under different operating conditions A: Qg = 6 L/min, Rs = 200, 400, 600 rev/min B: Rs = 400 rev/min, Qg = 4, 6, 8 L/min.
  15. 15. Model simulated time-averaged local gas holdups along transversal course (X = 0 mm, Y = 15–75 mm) at different vertical positions (Z = 5,30,65,100,125,160 mm) under different operating conditions(Qg = 6 L/min, Rs = 200, 400, 600 rev/min).
  16. 16. Model simulated time-averaged local gas holdups along transversal course (X = 0 mm, Y = 15–75 mm) at different vertical positions (Z = 5, 30,65,100,125,160 mm) under different operating conditions (Rs = 400 rev/min, Qg = 4,6,8 L/min).
  17. 17. Model predictions of transient gas holdup distributions at the vertical sections (X = 0 mm) and under different operating conditions (Rs = 400 rev/min, Qg = 4, 6, 8 L/min) with t = 10, 40, 70 s. (X = 0 mm) and under different operating conditions (Qg = 6 L/min, Rs = 200, 400, 600 rev/min) with t = 10, 40, 70 s.
  18. 18. Model predictions of transient gas holdup distributions at the horizontal sections ( Z = 5,30,65,100,125,160 mm) under different operating conditions (Rs = 00 rev/min, Qg = 4,6,8 L/min) with t = 40 s. (Z = 5, 30,65,100,125,160 mm) and under different operating conditions (Qg = 6 L/min, Rs = 200, 400, 600 rev/min) with t = 40 s.
  19. 19. Liquid velocity  Axial liquid velocity is selected and simulated and experimentally measured to characterize liquid flow fluctuations  Volume averaged overall liquid velocity along time  Time averaged liquid velocity along transversal courses  Transient liquid velocity distributions along horizontal and vertical positions
  20. 20. Volume averaged axial liquid velocities Along time course under different operating conditions A: Qg = 6 L/min, Rs = 200,400,600 rev/min B : Rs = 400 rev/min, Qg = 4,6,8 L/min
  21. 21. Model simulated and experimental measured Time averaged axial liquid velocities along transversal course (X = 0 mm, Y = 15–75 mm) at different vertical positions (Z = 5, 30,65,100,125,160 mm) and under different operating conditions (Qg = 6 L/min, Rs = 200,400,600 rev/min)
  22. 22. Model simulated and experimental measured Time averaged axial liquid velocities along transversal course (X = 0 mm, Y = 15–75 mm) at different vertical positions (Z = 5,30, 65,100,125,160 mm) and under different operating conditions (Rs = 400 rev/min, Qg = 4,6,8 L/min)
  23. 23. Model predictions of transient liquid velocity distributions at the vertical sections (X = 0 mm)and under different operating conditions (Rs = 400 rev/min, Qg = 4,6,8 L/min) with t = 10,40,70 s. X = 0 mm)and under different operating conditions(Qg = 6 L/min, Rs = 200,400,600 rev/min)with t = 10, 40,70s
  24. 24. Model predictions of transient liquid velocity distributions at the horizontal sections (Z = 5,30,65,100,125,160 mm) and under different operating conditions (Rs = 400 rev/min, Qg = 4,6,8 L/min) with t = 40 s. (Z = 5,30,65,100,125,160 mm) and under different operating conditions (Qg = 6 L/min, Rs = 200,400,600 rev/min) with t = 40 s.
  25. 25. Bubble size distribution  The behavior of bubbles in gas–liquid stirred tanks are very important especially in supplying oxygen from gas phase into liquid phase  The CFD model developed in the current work was coupled by the MUSIG model  This model considered several bubble group diameters which can be represented with Sauter mean diameter  Bubbles are divided into twenty groups and then predicted the bubble Sauter mean diameter.  Diameters of 20 bubble group ranged from 0.375 to 14.625mm
  26. 26. Volume averaged bubble size fractions  Stirred tank was mainly occupied by small bubble groups (dia less than 4 mm)  Advantageous for fermentation process  Improved mass and heat transfer  Increase in specific area with small bubble diameter
  27. 27. Volume averaged bubble diameters  Bubble Sauter mean diameter ranged from 1 to 2.5 mm  Increase in rotation speed obviously made a decrease in bubble diameter  Change in inlet air flow rate under investigated range had little effect on bubble diameter
  28. 28. Model predictions of transient bubble diameter distributions at the vertical sections (X = 0 mm) and under different operating conditions(Rs = 400 rev/min, Qg = 4,6,8 L/min)with t = 10, 40,70 s. (X = 0 mm) and under different operating conditions (Qg = 6 L/min, Rs = 200,400,600rev/min) with t = 10,40,70 s.
  29. 29. Model predictions of transient bubble diameter distributions at the horizontal sections (Z = 5,30, 65,100,125,160 mm) and under different operating conditions (Rs = 400 rev/min, Qg = 4,6,8 L/min) with t = 40 s. (Z = 5,30, 65,100,125,160 mm) and under different operating conditions (Qg = 6 L/min, Rs = 200, 400,600 rev/min) with t = 40 s.
  30. 30. Conclusion  A full-flow field, 3D transient CFD model based on Eulerian approach was developed for a gas-liquid two phase stirred tank with 2 six-blade turbines and 4 baffles  MUSIG model for bubble size distribution considering coalescence and breakup  Increase in inlet air flow rate and rotation speed  increase in overall gas holdup  Increase in inlet air flow rate or decrease in rotation speed  increase in volume-averaged axial liquid velocity  Gas accumulated mainly in regions between the two impellers, as well as between the upper impeller and the top surface when inlet air flow rate was large
  31. 31. Conclusion (Contd.)  Increase in rotation speed made a more dispersed gas distribution all over the whole tank  Vortices were also generated in regions of bottom of the tank  The tank was mainly occupied by small bubbles with diameters smaller than 4 mm  Larger bubbles accumulated in regions  near the lower impeller  between the two impellers  between the upper impeller and the top surface  Smaller bubbles accumulated in regions near wall  Increase in rotation speed made a decrease in bubble diameter
  32. 32. References  Wang, H., Jia, X., Wang, X., Zhou, Z., Wen, J., Zhang, J., CFD modeling of hydrodynamic characteristics of a gas– liquid two-phase stirred tank, 2014, J. Appl. Math. Mod., Vol. 38, No. 1, pp. 63-92  Kantarci, N., Borak, F., Ulgen, K.O., Bubble column reactors, 2005, J. Process Chemistry, Vol. 40, No. 7, pp. 2263-2283  André Bakker, Modeling Flow Fields in Stirred Tanks, Reacting Flows - Lecture 7, http://www.bakker.org
  33. 33. Questions

CFD modeling of hydrodynamic characteristics of a two phase gas–liquid stirred tank

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