Advertisement

Sadhana

, 23:505 | Cite as

LDA measurements and CFD simulations of flow generated by impellers in mechanically agitated reactors

  • J B Joshi
  • A K Sahu
  • P Kumar
Surveys in Fluid Mechanics-IV

Abstract

The turbulent flow produced by various designs of axial flow impellers in a stirred vessel was measured using a laser Doppler anemometer (LDA). Flat-bottomed cylindrical vessels of diameters 0.3 m and 0.5 m provided with 4 baffles ofT/10 width were used as the reactors. The standard two-equation (k-ε) turbulence model was used to numerically simulate the flow (both 2D and 3D). Three numerical schemes, namely upwind scheme, hybrid scheme and power-law scheme, were used to evaluate the competitiveness of the various schemes. The effects of the initial guess values of the flow variables, the under-relaxation parameters and internal iterations etc. on the rate of convergence were analysed for both 2D and 3D models. The effect of grid size was also studied in both the cases. The 2D and 3D predictions were compared and it was observed that the 2D predictions were not good enough to give the details of the flow. So, the sensitivity of the model parameters on the flow characteristics was investigated thoroughly in the case of 3D models. It was observed that no single set of model parameters could yield even reasonable agreement between the model predictions and the experimental observations throughout the vessel. Therefore, the concept of zonal modelling was introduced for ther-z plane. It was observed that with the introduction of zonal modelling the predicted values of flow variables were in good agreement with experimental data even close to the top surface of the tank. The same could not be obtained with a standard set of parameters and single zone.

Keywords

LDA measurements CFD turbulence parameters zonal modelling mechanically agitated reactors pitched blade turbine 

References

  1. Abujelala M T, Lilley D G 1984 Limitations and empirical extensions of thek-ε model as applied to turbulent confined swirling flows.Chem. Eng. Commun. 31: 223–236CrossRefGoogle Scholar
  2. Armenante P M, Chou C-C 1996 Velocity profiles in a baffled vessel with single or double pitched blade turbines.AIChE J. 42: 42–54CrossRefGoogle Scholar
  3. Bakker A, Van den Akker H E A 1994 Single-phase flow in stirred reactors.Trans. Inst. Chem. Eng. 72: 583–593Google Scholar
  4. Brucato A, Ciofalo M, Grisafi F, Micale G 1994 Complete numerical solution of flow fields in baffled stirred vessels: The inner-outer approach.Proceedings Inst. Chem. Eng. Symposium series (136): 155–162Google Scholar
  5. Desouza A, Pike R W 1972 Fluid dynamics and flow patterns in stirred tanks with a turbine impeller.Can. J. Chem. Eng. 50: 15–23CrossRefGoogle Scholar
  6. Drbohlav J, Fort I, Maca K, Placek J 1978 Turbulent characteristics of discharge flow from turbine impeller.Collect. Czech. Chem. Commun. 43: 3148–3162Google Scholar
  7. Dutta S, Acharya S 1993 Heat transfer and flow past a backstep with the nonlineark-ε turbulence model and the modifiedk-ε turbulence model.Numer. Heat Transfer 23: 281–301CrossRefGoogle Scholar
  8. Elkaim D, Reggio M, Camarero R 1993 Control volume finite element solution of a confined turbulent diffusion flame.Numer. Heat Transfer 23: 259–279CrossRefGoogle Scholar
  9. Ferziger J H, Kline S J, Avva R K, Bordalo S N, Tzuoo K L 1988 Zonal modelling of turbulent flows — Philosophy and accomplishments: Near-wall turbulence.Zoran Zaric Memorial Conference (eds) S J Kline, N H Afgan (Hemisphere) pp 801–817Google Scholar
  10. Fokema M D, Kresta S M, Wood P E 1994 Importance of using the correct impeller boundary conditions for CFD simulations of stirred tanks.Can. J. Chem. Eng. 72: 177–183Google Scholar
  11. Fort I 1967 Studies on mixing XIX — pumping capacity of propeller mixer.Collect. Czech. Chem. Commun. 32: 3663–3678Google Scholar
  12. Fort I 1986 Flow and turbulence in vessels with axial impeller.Mixing theory and practice (eds), V W Uhl, J B Gray (New York: Academic Press) vol. 3Google Scholar
  13. Fort I, Neugebauer R, Pastyrikova M 1971 Studies on mixing. XXIX-Spatial distribution of mechanical energy dissipated by axial impeller in a system with radial baffles.Collect. Czech. Chem. Commun. 36: 1769–1793Google Scholar
  14. Fort I, Vonzy M, Forstova B 1991 Distribution of turbulence characteristics in agitated systems with axial high speed impeller and baffles.7th European Conference on Mixing 1: 33–41Google Scholar
  15. Harris C K, Roekaerts D, Rosendal F J J 1996 Computational fluid dynamics for chemical reactor engineering.Chem. Eng. Sci. 51: 1569–1594CrossRefGoogle Scholar
  16. Harvey P S, Greaves M G 1982a Turbulent flow in an agitated vessel. Part I: a predictive model.Trans. Inst. Chem. Eng. 60: 195–200Google Scholar
  17. Harvey P S, Greaves M G 1982b Turbulent flow in an agitated vessel. Part II: Numerical solution and model predictions.Trans. Inst. Chem. Eng. 60: 201–210Google Scholar
  18. Hockey R M, Nouri J M 1996 Turbulent flow in a baffled vessel stirred by a 60° pitched blade impeller.Chem. Eng. Sci. 51: 4405–4421CrossRefGoogle Scholar
  19. Jaworski Z, Fort I 1991 Energy dissipation rate in a baffled vessel with pitched blade turbine impeller.Collect. Czech. Chem. Commun. 56: 1856–1867CrossRefGoogle Scholar
  20. Jaworski Z, Nienow A W, Koutsakos E, Dyster K, Bujalski W 1991 An LDA study of the turbulent flow in a baffled vessel agitated by a pitched blade turbine.Chem. Eng. Res. Des. 69: 313–320Google Scholar
  21. Kresta S M, Wood P E 1993a The mean flow field produced by a 45 degree pitched blade turbine: Changes in the circulation pattern due to off bottom clearance.Can. J. Chem. Eng. 71: 42–53CrossRefGoogle Scholar
  22. Kresta S M, Wood P E 1993b The flow field produced by a pitched blade turbine: Characterization of the turbulence and estimation of the dissipation rate.Chem. Eng. Sci. 48: 1761–1774CrossRefGoogle Scholar
  23. Launder B E, Spalding D B 1974 The numerical computation of turbulent flows.Comput. Methods Appl. Mech. Eng. 3: 269–289CrossRefMATHGoogle Scholar
  24. Obi S, Peric M 1991 Second moment calculation procedure for turbulent flows with collocated variable arrangement.AIAAJ. 29: 585–590Google Scholar
  25. Patankar S V 1980Numerical heat transfer and fluid flow (New York: Hemisphere)MATHGoogle Scholar
  26. Patel V C, Rodi W, Scheuerer G 1986 Turbulence models for near wall and low Reynolds number flows: A review.AIAA J. 23: 1308–1319MathSciNetCrossRefGoogle Scholar
  27. Peaceman D, Rachford H H 1955 The solution of parabolic and elliptic differential equations.J. Soc. Ind. Appl. Math. 3: 28–41MATHCrossRefMathSciNetGoogle Scholar
  28. Peric M, Kessler R, Schellerer G 1987 Comparison of finite volume numerical methods with staggered and non-staggered grids. Report no. 163/T/87, Lehrst. für. Stromungsmechanik, Univ. Erlangen-NbgGoogle Scholar
  29. Pericleous K A, Patel M K 1987 The modelling of tangential and axial agitators in chemical reactors.Phys.-Chem. Hydrodyn. 8: 105–123Google Scholar
  30. Placek J, Fort I, Strek F, Jaworski Z, Karcz Z 1978 Velocity field at the wall of fully baffled vessel with turbine impeller.Proceedings of the 5th Congress CHISA (Prague: Czechslovak Soc. Chem. Eng.) pp 272–285Google Scholar
  31. Platzer B 1981 A contribution to the evaluation of turbulent flow in baffled tanks with radial outflow impellers.Chem. Tech. (Liepzig) 33: 16–19Google Scholar
  32. Platzer B, Noll G 1981 An analytical solution for the flow in baffled vessel with radial outflow impellers.Chem. Tech. (Liepzig) 33: 648–655Google Scholar
  33. Ranade V V, Dommeti S M S 1996 Computational snapshot of flow generated by axial impellers in baffled stirred vessels.Trans. Inst. Chem. Eng. 74: 476–484Google Scholar
  34. Ranade V V, Joshi J B 1989 Flow generated by pitched blade turbines I: Measurements using laser Doppler anemometer.Chem. Eng. Commun. 81: 197–224CrossRefGoogle Scholar
  35. Ranade V V, Joshi J B 1990 Flow generated by a disc turbine: Part I. Experimental.Chem. Eng. Res. Des. 68: 19–33Google Scholar
  36. Ranade V V, Joshi J B, Marathe A G 1989 Flow generated by pitched blade turbines. II: Simulation usingk-ε model.Chem. Eng. Commun. 81: 225–248CrossRefGoogle Scholar
  37. Ranade V V, Bourne J R, Joshi J B 1991 Fluid mechanics and blending in agitated tanks.Chem. Eng. Sci. 46: 1883–1893CrossRefGoogle Scholar
  38. Ranade V V, Mishra V P, Saraph V S, Deshpande G B, Joshi J B 1992 Comparison of the axial flow impellers using laser Doppler anemometer.Ind. Eng. Chem. Res. 31: 2370–2379CrossRefGoogle Scholar
  39. Rodi W 1993Turbulence models and their application in hydraulics (Monograph) (Rotterdam: A A Balkema)Google Scholar
  40. Sahu A K, Joshi J B 1995 Simulation of flow in stirred vessels with axial flow impellers: Effects of various numerical schemes and turbulence model parameters.Ind. Eng. Chem. Res. 34: 626–639CrossRefGoogle Scholar
  41. Sahu A K, Kumar P, Joshi J B 1998 Simulation of flow in stirred vessel with axial flow impeller: Zonal modelling and optimization of parameters.Ind. Eng. Chem. Res. 37: 2116–2130CrossRefGoogle Scholar
  42. Tatterson G B, Yuan H S, Brodkey R S 1980 Stereoscopic visualisation of the flows for pitched blade turbines.Chem. Eng. Sci. 35: 1369–1375CrossRefGoogle Scholar
  43. Xu Y, Mcgrath G 1996 CFD predictions in stirred tank flows.Trans. Inst. Chem. Eng. 74: 471–475CrossRefGoogle Scholar

Copyright information

© Indian Academy of Sciences 1998

Authors and Affiliations

  1. 1.Department of Chemical Technology, Chemical Engineering DivisionUniversity of MumbaiMatunga, MumbaiIndia

Personalised recommendations