Non-Synchronous Vibrations of Turbomachinery Airfoils

22 Pages·2012·2.5 MB·English

Non-Synchronous Vibrations of Turbomachinery Airfoils
600
500
NSV
hz400
!, ncy, 300 Flutter
ue F.R.
q
Fre200
100 SFV
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Rotor Speed, !, RPM
Kenneth C. Hall, Je!rey P. Thomas, Meredith Spiker & Robert E. Kielb
Department of Mechanical Engineering and Materials Science
Edmund T. Pratt, Jr. School of Engineering
Duke University
9th National Turbine Engine High Cycle Fatigue Conference
Pinehurst, North Carolina
Outline
¥ Objectives of the present work.
¥ Description of non-synchronous vibration (NSV), review.
¥ Some preliminary results of a conventional time-marching simula-
tion of NSV.
1. 3D front stage compressor
¥ The harmonic balance method — a nonlinear eigenvalue formula-
tion.
¥ Computational results.
1. 2D vortex shedding.
2. 2D compressor instability.
¥ Conclusions and future work.
Objectives of Present Study
Objectives:
¥ To develop an understanding of the most signi(cid:222)cant types of NSV,
with emphasis on fan & compressor blades & vanes.
¥ Todevelopane"cientcomputationaltooltopredictNSVfrequen-
cies (campbell diagram) and modal force.
¥ To develop a design approach.
Existing capability
¥ Time domain simulations can capture NSV phenomena, but at a
high computational cost.
Our approach:
¥ Frequencydomain(harmonicbalance)methodstomodelnonlinear
(cid:223)uid mechanics instabilities.
¥ Novelsearchtechniquesto(cid:222)ndnonlineareigenvalues(frequencies)
of NSV drivers.
What is NSV?
Classical Aeroelastic Phenomena:
¥ Forced Response — Synchronous with engine order excitations.
¥ Flutter — Non-Synchronous vibrations at low to moderate reduced
frequencies.
600
500
NSV
hz400
!, y, Flutter
nc300
ue F.R.
q
Fre200
100 SFV
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Rotor Speed, !, RPM
¥ Non-synchronous vibration (NSV) — Coherent (cid:223)ow instability.
¥ Separated (cid:223)ow vibration (SFV) — Broadband (cid:223)ow instability.
Non-Synchronous Vibration
Characteristics of NSV:
¥ Blades excited by a coherent (cid:223)uid dynamic instability (e.g.
Strouhal shedding).
¥ High amplitude response possible, especially when the excitation
frequency is near the blade natural frequencies.
¥ Blade motion is frequency and phase locked.
¥ Flutter design parameters are well within the stable region — not
(cid:223)utter.
¥ Occurs in blades & vanes of fans, compressors and turbines and
can cause high cycle fatigue failures.
NSV is (cid:210)missing line(cid:211) on Campbell diagram. Although NSV frequencies
are in(cid:223)uenced by blade motion, our initial research will emphasize the
role of (cid:223)uid dynamic instabilities only.
Experimental Evidence of NSV
Airfoil strain gauge Casing pressure measurement
Fluid Dynamic Instabilities
¥ A number of potential phenomena may potentially con-
tribute to NSV, including; dynamic boundary-layer separation,
shock/boundary-layerdynamics,vortexshedding,tip(cid:223)ow/vortices,
hub vortices, rotating stall, combustion instabilities.
¥ Fluid dynamic instabilities are main driver.
¥ Bladedynamicsplayasecondaryrole,with(cid:223)uidinstability(cid:210)locking
on(cid:211) to blade natural frequency.
Time-Marching Simulation of NSV
¥ Numericallymodeled(cid:222)vepassagesofC1compressorusingTURBO
time marching simulation.
¥ TURBO simulation included tip clearance and turbulence model.
¥ (Model also included wakes from upstream inlet guide vane)
¥ Blades modeled as rigid (no aeroelastic coupling).
Near Midspan Near Tip
! "# $ ! "# $
C1 Compressor
¥ TURBO simulation shows (cid:223)uid dynamic instability involves tip
leakage vortex from one blade interacting with neighboring suc-
tion side blade.
¥ Unsteady (cid:223)uid dynamic (cid:210)eigenmode(cid:211) dominated by unsteadiness
near the tip.
¥ Numerical simulation provided useful insight into physical mech-
anisms of NSV, but required signi(cid:222)cant computer resources
(turnaround time for one case was months).
Previous Studies for Cascades
¥ Mailach et al. (1999, 2000 & 2001)
— 4 Stage LSRC & Linear Cascade
— Tip Flow Instability
— Multi-Cell Circumferentially Traveling Wave
— Near Stall Line with Large Tip Clearance (> 2%)
— Strouhal-type Number Proposed
¥ Marz et al. (1999)
— Low Speed Fan Rig
— Tip Flow Instability
— Near Stall Line with Large Tip Clearance
— CFD Frequency Prediction 8% Higher Than That Measured
¥ Camp (1999)