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Pump CFD - performance prediction: a tutorial . Niels P. Kruyt Engineering Fluid Dynamics, Department of Mechanical Engineering, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands n.p.kruyt@utwente.nl www.ts.wb.utwente.nl/kruyt/.

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pump cfd performance prediction a tutorial

Pump CFD - performance prediction: a tutorial

Niels P. KruytEngineering Fluid Dynamics, Department of Mechanical Engineering, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlandsn.p.kruyt@utwente.nl

www.ts.wb.utwente.nl/kruyt/

5th International Symposium on Pumping Machinery, 2005 ASME Fluids Engineering Division Summer Meeting and Exhibition, 19-23 June, Houston, TX, USA

cfd for pump design pitfalls and opportunities

CFD for pump design: pitfalls and opportunities

CFD for pump design: a magic bullet?

Pump CFD - performance prediction

overview of tutorial
Overview of tutorial
  • Why is fluid dynamics important for pump design?
  • What is Computational Fluid Dynamics (CFD)?
  • Opportunities provided by CFD
  • Components of CFD
  • Essential fluid dynamics
  • Examples of performance prediction
  • Trends
  • “Do’s” and “don’t’s” of CFD
basics of pump design analysis
Basics of pump design/analysis
  • One-dimensional flow model
  • Euler pump relation
  • Slip factor is empirical
  • Hydraulic efficiency is empirical
what is computational fluid dynamics cfd
What is Computational Fluid Dynamics (CFD)?

Determination of flow:

  • Analytical ® impossible
  • Experiments ® expensive
  • Numerical ® CFD(“computer test-rig”)
benefits of cfd for pump design
Benefits of CFD for pump design
  • Improved designs
  • More reliable design methods
  • Cheaper design process
design phases
Design phases
  • Conceptual design
  • Preliminary design
  • Detailed design
  • Use different CFD-methods for different design phases!
components of cfd
Components of CFD
  • Model formulation
    • geometry
    • flow model
    • boundary conditions
  • Grid/mesh generation
  • Discretisation of governing equations
  • Solution of discretised equations
  • Interpretation
selection of modelled geometry
Selection of modelled geometry
  • Single channel of impeller
  • Full pump: impeller & volute/diffusor
    • steady
    • unsteady
  • Leakage-flow region
  • Piping system / pump intake
  • Single stage vs. multi-stage
closure problem
Closure problem
  • Averaging over time ® Reynolds-averaged Navier-Stokes equations (RANS)
  • Contains ‘Reynolds stresses’
  • Extra quantities in equations ® ‘closure’ problem
  • Model required for Reynolds stresses in terms of time-averaged velocities
turbulence models
Turbulence models

“Turbulent viscosity”

  • Mixing-length model
  • k-e models (k-w)
  • Reynolds-stress models
  • ¼

Increasing complexity

Pope (2000); Bradshaw (1996)

flow models
Flow models
  • Stream-surface methods
  • Potential-flow model
  • Euler flow model
  • RANS-based models
  • Large-eddy simulations (LES)
  • Direct Navier-Stokes simulations (DNS)

Increasing complexity

boundary layers
Boundary layers
  • High Reynolds numbers
  • Main flow is inviscid
  • Boundary-layer flow is viscous
  • Boundary-layer is thin
  • Large variation of velocity in direction normal to wall

Re = 107:

L = 25 cm; d = 0.4 cmL = 9.8 in; d = 0.1 in

logarithmic layer
Logarithmic layer
  • Large variation of velocity perpendicular to wall ® many grid points
  • ‘Universal’ behaviour near wall ® “logarithmic layer”
  • “Wall functions” in RANS-based CFD-methods ® boundary conditions

Craft et al. (2002); Pope (2000)

separation
Separation

Attached boundary-layer

Separated boundary-layer

grid mesh 1
Grid/mesh (1)

Structured

Unstructured

grid mesh 2

Structured

multi-block

Viscous accuracy

Unstructured

Easeof use

Grid/mesh (2)

Baker (2005)

discretisation
Discretisation
  • Replace partial differential equations by a finite set of equations
    • Finite difference method
    • Finite volume method
    • Finite element method
  • Discretisation error solution depends on grid/mesh size!
sources of errors in cfd predictions
Sources of errors in CFD-predictions
  • Modelling errors
    • Geometrical uncertainties
    • Limited validity of adopted flow model
    • Uncertain boundary conditions
  • Numerical errors
    • Discretisation error due to finite grid-size
    • Lack of convergence in iterative solution process
    • Insufficient mesh/grid quality
  • User/programmer errors
choice of flow model

DNS

Cost

RSM

k, e

Potential& B.L.

Potential

1D

Accuracy

Choice of flow model

‘Around’ design point

inviscid viscous interaction methods

RAE101 wing

Inviscid-viscous interaction methods
  • Outer flow ® inviscid flow equations
  • Boundary-layer flow ® boundary-layer equations
  • Coupled solution ® mildy separated flows

Milewski (1997)

comparison of rans predictions
Comparison of RANS-predictions
  • Different machines
  • Many contributors
  • Draft tube
  • Wing/body
turbine draft tube
Turbine draft tube

Turbine draft tube flow; Engström et al. (2001)

Experimental

wing body
Wing/body

DLR F6 wing/body study

Baker (2005)

differences cfd pump aerospace applications
Differences CFDpump «aerospace applications
  • Compressibility effects are absent; no shock waves
  • Cavitation is important
  • Rotating/stationary parts
  • More boundary layers need to be resolved
  • Flow separation more important for off-design conditions
  • Effect rotation and curvature on turbulence
implementation of cfd in pump design process
Implementation of CFD in pump-design process
  • Integrate CFD in all design phases
  • Different CFD-models for each design phase
  • Simple models give more insight
  • Tune model parameters from database
  • RANS-methods require intense use
  • Set accuracy targets clearly
  • Be cautious of designs from CFD that deviate strongly from experience
trends
Trends
  • Maturing of commercial/general-purpose CFD-packages
  • Main problem remains turbulence modelling
  • Multi-phase CFD-methods
  • Adaptive mesh refinement
  • Open-source CFD-methods (“GNU-CFD”)
  • Verification of CFD-methods ® “blind” tests
  • Design-oriented CFD-methods
    • Optimisation methods
    • Inverse-design methods
inverse design method
Inverse-design method
  • Specified
    • meridional plane
    • duty
    • “blade loading”
  • Obtained
    • Blade angles

Westra et al. (2005)

[Click on figure to start movie]

don t s of cfd
“Don’t’s” of CFD
  • Use CFD-package as a black-box tool
  • Forget that turbulence needs to be modelled
  • Use RANS-methods for all design phases
do s of cfd
“Do’s” of CFD
  • Choose right tool for the task
  • Analyse and interpret results
  • Use common-sense
  • Use/develop knowledge of fluid dynamics
  • Check grid/mesh convergence
  • Check sensitivity of results to model parameters
conclusions
Conclusions
  • CFD is not a “magic bullet”
  • CFD is a powerful tool
  • Many pitfalls; many opportunities
  • CFD does not replace a smart designer
  • CFD provides great potential for improved pump-design process
  • CFD is (still) an art
questions and comments
Questions and comments
  • Thank you for your attention!
  • Questions and comments?
  • Presentation can be downloaded from: www.ts.wb.utwente.nl/kruyt/asme2005.pps
  • E-mail: n.p.kruyt@utwente.nl
literature
Literature
  • Baker, T.J. (2005). “Mesh generation: art or science”, Progress in Aerospace Sciences 41 29-63.
  • Craft, T.J. & Gerasimov, A.V. & Iacovides, H. & Launder, B.E. (2002). “Progress in the generalization of wall-function treatments”, International Journal of Heat and Fluid Flow 23 148-160.
  • Bradshaw, P. (1996). “Turbulence modelling with application to turbomachinery”, Progress in Aerospace Sciences 32 575-624.
  • Engström, T.F. & Gustavsson, L.H. & Karlsson, R.I. (2001). “Proceedings of Turbine 99 – Worskshop 2. The second ERCOFTAC Workshop on draft tube flow”, http://www.sirius.luth.se/strl/Turbine-99/.
  • Esch, B.P.M. van & Kruyt, N.P. (2001). “Hydraulic performance of a mixed-flow pump: unsteady inviscid computations and loss models”, Journal of Fluids Engineering 123256-264.
  • Jameson, A. (2001). “A perspective on computational algorithms for aerodynamic analysis and design”, Progress in Aerospace Sciences 37 197-243.
  • Milewski, W.M. (1997). “Three-dimensional viscous flow computations using the integral boundary-layer equations simultaneously coupled with a low-order panel method”, Ph.D. Thesis, MIT, Cambridge, USA.
  • Westra, R.W. & Kruyt, N.P. & Hoeijmakers, H.W.M. (2005). “An inverse-design method for centrifugal pump impellers”, 2005 ASME 5th International Symposium on Pumping Machinery, Paper FEDSM2005-77283.
  • Pope, S.B. (2000). “Turbulent flows”, Cambridge University Press, Cambridge, UK.