Prediction of Axial Compressor Blade Vibration by of Axial Compressor Blade Vibration by Modelling Fluid-Structure Interaction Faculty of Engineering at Stellenbosch University Department of ... Background Flutter in Turbomachinery Background Goal of Project FSI

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  • Prediction of Axial Compressor Blade Vibration

    by Modelling Fluid-Structure Interaction

    Faculty of Engineering at Stellenbosch University

    Department of Mechanical and

    Mechatronic Engineering

    by J. D Brandsen

    Supervisors:

    Dr S. J. van der Spuy

    Prof G. Venter

    1/19

  • Overview

    Background

    Goal of Project

    FSI Modelling Approaches

    Experimental Work

    Numerical Modelling

    Results

    Conclusions

    2/19

  • Background

    Flutter in Turbomachinery

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    Flutter is the vibration of a mechanical system:

    At or near natural frequencies of system.

    Caused by instability. Does not require disturbance.

    Aerodynamic forces feed energy into system. Amplitude

    increases with time.

    Cause of high cycle fatigue failure in turbomachinery.

    Project FUTURE initiated to improve methods used to model

    and design for flutter.

    Project FUTURE is coordinated by Kungliga Teknista

    Hgskolan in Sweden.

    Also has 25 other partners, including Stellenbosch University

    and the Council for Scientific and Industrial Research (CSIR).

    3/19

  • Background (continued)

    Vibration Excitation System

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    As part of Project FUTURE, the CSIR have developed a

    vibration excitation system:

    Designed to excite the first rotor blade row of an axial

    flow compressor.

    Designed to make the blade row vibrate at the desired

    frequency and in the desired mode shape.

    Injects air into compressor flow path thereby causing

    velocity perturbations.

    Stellenbosch University responsible for demonstrating

    capabilities of vibration excitation system.

    Vibration excitation system was therefore fitted to the

    Rofanco compressor test bench.

    4/19

  • Background (continued)

    Vibration Excitation System

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    Vibration excitation system fitted to Rofanco test bench

    (images from Van der Spuy et al (2012)):

    Rofanco compressor (manufactured by Royston Fan Co. Ltd.):

    Three identical stages (43 rotor blades, 41 stator blades).

    36 inlet guide vanes (removed for excitation system).

    Each exciter consists of a DC servo motor fitted with a special

    rotor disk.

    Two types of rotor disk: 32 hole rotor disk, 16 hole rotor disk. 5/19

  • Background (continued)

    Blade Row Vibration Modes

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    Phase difference = 360

    x (no. of NDs)/(no. of blades)

    Vibration excitation system designed to excite 0 ND, +1 ND,

    +2 ND, +3 ND, -1 ND, -2 ND and -3 ND modes.

    Nodal diameter (ND) modes:

    +

    Rotation

    Wave propagation

    ND

    ND

    Rotation

    2 ND mode 0 ND mode

    6/19

  • Goal of Project

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    Goal of thesis project:

    Construct a FSI model of the vibration excitation system.

    Purpose of FSI model was two-fold:

    Numerical tool for carrying out experiments digitally.

    Will complement the existing experimental data.

    Restrictions placed on FSI model due to time constraints:

    Single setting simulated: excitation frequency of 660 Hz

    and a supply pressure of 2.5 bar.

    Needs to only be able to simulate the 0 ND mode and

    the +2 ND mode of the system.

    Must be able to accurately recreate component of blade

    motion occuring at excitation frequency (660 Hz).

    7/19

  • FSI Modelling Approaches

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    Monolithic approach:

    Staggered approach:

    Structural equations

    +

    Fluid equations

    Structural equations

    Fluid equations

    Single dedicated solver

    CFD solver

    FE solver

    Data transfer

    Staggered approach preserves software modularity.

    Ansys CFX and Ansys Mechanical available at start of project.

    Staggered approach already demonstrated for turbomachinery

    by Im and Zha (2012), Gnesin et al (2000). 8/19

  • Experimental Work

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    Measurements of velocity perturbations required for

    boundary conditions of FSI model.

    Velocity profile measured for a frequency of 650 Hz and

    supply pressure of 2.5 bar.

    Velocity profiles measured for 0 ND mode of the system:

    32 hole rotors 16 hole rotors

    9/19

  • Numerical Modelling

    FE Model of First Rotor Blade Row

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    FE model created of a single blade and verified.

    Multiple copies of single blade FE model then combined:

    Single blade FE model created using SOLSH 190 elements.

    Each blade constrained in cantilever fashion at root.

    Material properties were those of aluminium.

    3 copies (0 ND FE model) 21 copies (+2 ND FE model)

    10/19

  • Numerical Modelling (continued)

    CFD Model of Vibration Excitation System

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    To save computation time, number of cells kept to a minimum.

    Set up for model of 14 exciters and 42 rotor blades.

    Periodic boundaries used to reduce model to three rotor

    blades and a single exciter (0 ND CFD model).

    Each exciter nozzle jet modelled by applying a sinusoidal

    velocity to patch boundary

    Sinusoidal velocity

    selected so that velocity

    profile at interface

    matched experimental

    profile.

    0 ND CFD model

    Approach already

    demonstrated by

    Raubenheimer (2011).

    11/19

  • Numerical Modelling (continued)

    CFD Model of Vibration Excitation System

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    When vibrating in the

    +2 ND mode, period of

    travelling wave is half of

    rotor blade row.

    Model must therefore

    contain half of rotor.

    Seven copies of 0 ND

    CFD model used to make

    +2 ND CFD model.

    Results in model of

    7 exciters, 21 rotor blades

    Nozzle jets set to fire out

    of phase.

    +2 ND CFD model

    12/19

  • Results

    FFTs of blade deformation

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    Two modes simulated:

    Frequency of 650 Hz, Pressure of 2.5 bar, 0 ND mode.

    Frequency of 650 Hz, Pressure of 2.5 bar, +2 ND mode.

    Run for 4500 time steps at a time step size of 5.4112 x 10-5 s.

    +2 ND FSI model 0 ND FSI model

    13/19

  • Results (continued)

    FFTs of blade deformation

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    Data of Van der Spuy et al (2012) shows amplitude of 660 Hz

    component of tip displacement perpendicular to root should

    be:

    0.089 mm for 0 ND mode for the 32 hole rotors.

    0.105 mm for +2 ND mode for the 32 hole rotors.

    In both cases, amplitudes predicted by FSI models all within

    6% of experimental data.

    Data of Van der Spuy et al (2012) shows amplitude of 660 Hz

    component of bending strain, 6.1 mm from root, should be:

    0.093 mm/m for 0 ND mode for the 32 hole rotors.

    0.109 mm/m for +2 ND mode for the 32 hole rotors.

    As with tip displacement, amplitudes predicted by FSI

    models all within between 10% and 20 % of experimental

    data for both cases.

    14/19

  • Results (continued)

    Blade formation for 0 ND mode

    Background

    Goal of Project

    FSI Modelling

    Approaches

    Experimental

    Work

    Numerical

    Modelling

    Results

    Conclusions

    Phase angles of 660 Hz components for the 0 ND mode:

    Blade 2 Blade 3 Blade 8 Blade 14

    Ideal 17.1

    34.3

    120

    223

    FSI model 18.8

    31.3

    118

    222

    32 hole rotors 16 hole rotors

    Bl