Prediction of Axial Compressor Blade Vibration by Fluid-Structure Interaction by J. D Brandsen Supervisors: Dr S. J. van der Spuy Prof G. Venter Faculty of Engineering at Stellenbosch University Department of Mechanical and Mechatronic Engineering 1/19
Overview FSI 2/19
Flutter in Turbomachinery FSI 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 Högskolan in Sweden. Also has 25 other partners, including Stellenbosch University and the Council for Scientific and Industrial Research (CSIR). 3/19
(continued) Vibration Excitation System FSI 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
(continued) Vibration Excitation System Vibration excitation system fitted to Rofanco test bench (images from Van der Spuy et al (2012)): FSI 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
(continued) Blade Row Vibration Modes FSI Nodal diameter (ND) modes: 2 ND mode 0 ND mode Rotation Rotation ND Wave propagation + ND 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. 6/19
FSI Goal of thesis project: Construct a FSI model of the vibration excitation system. Purpose of FSI model was two-fold: 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 FSI Monolithic approach: Structural equations + Fluid equations Staggered approach: Structural equations Single dedicated solver CFD solver Data transfer Fluid equations FE solver 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
FSI 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
FE Model of First Rotor Blade Row FSI FE model created of a single blade and verified. Multiple copies of single blade FE model then combined: 3 copies (0 ND FE model) 21 copies (+2 ND FE model) Single blade FE model created using SOLSH 190 elements. Each blade constrained in cantilever fashion at root. Material properties were those of aluminium. 10/19
(continued) CFD Model of Vibration Excitation System FSI 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 0 ND CFD model Sinusoidal velocity selected so that velocity profile at interface matched experimental profile. Approach already demonstrated by Raubenheimer (2011). 11/19
FSI (continued) CFD Model of Vibration Excitation System 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. in model of 7 exciters, 21 rotor blades Nozzle jets set to fire out of phase. +2 ND CFD model 12/19
FFTs of blade deformation FSI 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. 0 ND FSI model +2 ND FSI model 13/19
(continued) FFTs of blade deformation FSI 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
(continued) Blade formation for 0 ND mode FSI Phase angles of 660 Hz components for the 0 ND mode: 32 hole rotors 16 hole rotors Blade 2 Blade 3 Blade 2 Blade 3 Ideal 0 0 0 0 FSI model -2 4-3 3 Phase angles from 0 ND FSI model all within 5 of ideal values. Blades deemed to be vibrating in 0 ND mode. Phase angles of 660 Hz components for the +2 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 Phase angles from +2 ND FSI model all within 3 of ideal values. Blades deemed to be vibrating in +2 ND mode. 15/19
(continued) Visualisation of Blade Deformation Simulation of scenario where vibration excitation system is set to 660 Hz, 2.5 bar and the 0 ND mode: FSI Phase angles showed that the 660 Hz components of motions of the blades are all in phase. However, visualisation shows that overall motions of the blades are not in phase. 16/19
FSI Correlation between results of FSI models and experimental data was satisfactory: 660 Hz components of tip displacement perpendicular to root all within 6% of experimental data. 660 Hz components of bending strain all within between 10% and 20% of experimental data. Both 0 ND FSI model and +2 ND FSI model therefore an acceptable recreation of vibration excitation system. Phase angles of 660 Hz components of blade motions show: Vibration excitation system should be able to excite the 0 ND mode and the +2 ND mode. Provided excitation frequency is close to 660 Hz. 17/19
Acknowledgements The financial assistance of the National Research Foundation (NRF) towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at, are those of the author and are not necessarily to be attributed to the NRF. Thank you to Project BALLAST for the financial assistance provided for this thesis project. 18/19
Thank you 19/19