A Novel Computational Model for Tilting Pad Journal...
Transcript of A Novel Computational Model for Tilting Pad Journal...
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A Novel Computational Model for Tilting Pad Journal Bearings
with Soft Pivot Stiffness
Yujiao TaoResearch Assistant
Dr. Luis San AndrésMast-Childs Professor
32nd Turbomachinery Research Consortium Meeting
TRC 32514/15196B
Year II
May 2012
22
JustificationPivot flexibility reduces the force coefficients in heavily loaded tilting pad journal bearings (TPJBs).
XLTRC2 TFPBRG code shows poor predictions for TPJB force coefficients.
W, static load
Y
X
Housing
Pad
Pivot
Fluid film
Journal
Ω , Journal speed
δ, Pad tilt angleξ
η
Research objective:To develop a code, benchmarked by test data, to predict
the K-C-M coefficients of TPJBs. Code accounts for thermal energy transport and the (nonlinear) effects of pivot flexibility.
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Tasks completed• Completed derivation of reduced
frequency force coefficients for TPJBs
• Developed iterative search scheme to update pad radial and transverse displacements
• Constructed GUI for new TPJB code as per XLTRC2 standards
• Compared predictions from the TPJB codeto test data
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• Completed derivation of reduced frequency force coefficients for TPJBs
Includes pivot NL deformations
• Developed sound iterative search scheme to update the pad radial and transverse displacements
• Constructed GUI for new TPJB code as per XLTRC2 standards
• Compared predictions from the enhanced TPJB code to test data
Tasks completed
55( ) ( ) ( ) ( )
cos sin
cos sinp X Y
piv p p piv d p p
h C e e
r R
θ θ
ξ θ θ η δ θ θ
= + + +
− − + − −
Film thickness in a padCp : Pad radial clearance
CB = Cp-rp Bearing assembled clearance
Rd= Rp+t : Pad radius and thicknessrp : Pad dimensional preload
δp : Pad tilt angleξpiv, ηpiv : Pivot radial and transverse
deflections
Unloaded Pad
Journal
P
Ω
X
Y
δp
η
ξ
θp
θh
eRJ
WX
RB
ξpiv
OB
RP
WY
OP
ηpiv
P’
Θp
t
θL
Bearing CenterPad CenterFluid film
Loaded Pad Film thickness:
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• Laminar flow• Includes temporal fluid inertia
effects• Average viscosity across film
3 3 2 2
2 2
112 12 2 12J
h P h P h h h hR z z t t
ρθ μ θ μ θ μ
⎧ ⎫ ⎧ ⎫∂ ∂ ∂ ∂ ∂ Ω ∂ ∂+ = + +⎨ ⎬ ⎨ ⎬∂ ∂ ∂ ∂ ∂ ∂ ∂⎩ ⎭ ⎩ ⎭
On kth pad
h : fluid film thickness P : hydrodynamic pressure
μ : lubricant viscosity Ω : journal speed
RJ : journal radius
Reynolds equation for thin film bearingY
X
Housing
Pad
Pivot
Fluid film
Journal
Ω , Journal speed
W, static load
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Thermal energy transport in thin film flowsT: film temperature
h : film thickness
U,W: circ. & axial flow velocities
μ, ρ, Cv : viscosity & density, specific heat
hB, hJ : heat convection coefficients
TB, TJ : bearing and journal temperatures
Ω : journal speed
Neglects temperature
variations across-film. Use bulk-flow
velocities and temperature
( ) ( ) ( ) ( )
22 2212
12 2
v B B J JC U h T W h T h T T h T TR z
R RW Uh
ρ
μ
⎡ ⎤∂ ∂+ + − + −⎢ ⎥∂ Θ ∂⎣ ⎦
⎛ ⎞Ω Ω⎡ ⎤= + + + −⎜ ⎟⎢ ⎥⎜ ⎟⎣ ⎦⎝ ⎠CONVECTION + DIFFUSION= DISSIPATION(Energy Disposed) = (Energy Generated)
8
ΩR
Upstream pad
Journal
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Pad inlet thermal mixing coefficientInlet thermal mixing coefficient λ (0< λ <1) is empirical parameter.
in s hF F Fλ= +
in in s s h hF T FT F Tλ= +
Downstream pad
FhTh
FsTs
FinTin
Fs , Fh , Fin Volumetric flow ratesTs , Th , Tin Fluid flow temperatures
λ ~0.6-0.9 for conventional lubricant feed arrangements with deep grooves and wide holes.
λ small (<< 1) for TPJBs with LEG feed arrangements and scrappers.
Hot oil Mixing oil
Cold oil
Is λ constant for all conditions?
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Nonlinear pivot deflection & stiffnessPivot deformation is typically nonlinear depending on the load (Fpiv), area of contact, hardness of the materials, surface conditions.
• Sphere on a sphere
Pivot and housing:Ep , Eh Elastic modulus νp ,νh Poisson’s ratiosDp , Dh Diameter of the curvature L Contact length
( )22 22
31 11.04 P H H P
piv pivP H P H
D DFE E D D
υ υξ⎛ ⎞− − −
= +⎜ ⎟⎝ ⎠
2
2
2 (1 ) 4 ( )2 ln3 2.15
piv P H H Ppiv
piv
F LED D D DLE F
υξ
π⎛ ⎞− −
= +⎜ ⎟⎜ ⎟⎝ ⎠
( )2 22 233
1 10.52 1 pivH P P Hpiv
H P P H
FD DD D E E
υ υξ⎛ ⎞ ⎛ ⎞− − −
= + +⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠
1 piv
piv pivK Fξ∂
=∂
( ) ( ) ( )2 3 4
0 1 2 3 4piv piv piv piv pivF a a a a aξ ξ ξ ξ= + + + +piv
pivpiv
FK
ξ∂
=∂
• Sphere on a cylinder
•Cylinder on a cylinder
• Load-deflection function(empirical)
*Kirk, R.G., and Reedy, S.W., 1988, J. Vib. Acoust. Stress. Reliab. Des., 110(2), pp. 165-171.
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Tasks completed• Completed derivation of reduced
frequency force coefficients for TPJBs• Developed sound iterative search scheme to
update the pad radial and transverse displacements
• Constructed GUI for new TPJB code as per XLTRC2 standards
• Compared predictions from the enhanced TPJB code to test data
Convergence to the pad and journal equilibrium positions
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Iterative scheme to find pad equilibrium positionJournal displacement, pivot radial and transverse displacements converge to equilibrium solution for TPJB with flexible pivots
Flow chart of iterative scheme
Set initial journal center displacements
Check convergence on loads (W0)
Find tilt angle for kth pad
Estimate the initial pivot radial displacement for kth pad
Find pivot radial and transverse displacements for kth pad
Find tilt angle for kth pad
Update e
Check convergence on kth pad tilt angle and pivot displacements
End of procedure
Check convergence on loads (W0)
Update e Take TPJB with flexible pivots, find pads tilt angles, radial and transverse displacements and journal eccentricity
Estimate the pivot radial displacement and journal eccentricity
Take TPJB with rigid pivots, find pads tilt angles and journal eccentricity
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Tasks completed• Completed derivation of reduced
frequency force coefficients for TPJBs• Developed sound iterative search scheme to
update the pad radial and transverse displacements
• Constructed GUI for new TPJB code as per XLTRC2 standards
• Compared predictions from the enhanced TPJB code to test data
Fortran program and Excel GUI
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Excel GUI and Fortran code• Modifications/enhancements to XLPRESSDAM® code • FEM to solve Reynolds equation (hydrodynamic pressure)• Uses control volume method to solve energy transport eqn
Excel GUI
Parameters of pivot: type, radii of contact & material properties (E,ν)
Different pads: geometry parameters
•Sphere on a sphere•Cylinder on a cylinder•Sphere on a cylinder•Rigid pivot•Load-deflection function
1414
Tasks completed• Completed derivation of reduced
frequency force coefficients for TPJBs• Developed sound iterative search scheme to
update the pad radial and transverse displacements
• Constructed GUI for new TPJB code as per XLTRC2 standards
• Compared predictions from TPJB code to test data
Bearings tested by Childs and students(TurboLab)
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Predictions for a four-pad TPJB(Childs and Harris*) Four pad, tilting pad bearing (LBP)
*Childs, D.W., and Harris, H., 2009, ASME, J. Eng. Gas Turbines Power, 131, 062502
Number of pads, Npad 4Configuration LBPRotor diameter, D 101.6 mm (4 inch)Pad axial length, L 101.6 mm (4 inch)Pad arc angle, ΘP 73o
Pivot offset 65%Pad preload, 0.37, 0.58Nominal bearing clearance, CB 95.3 μm (3.75 mil)
Measured bearing clearance, CB54.6 μm (2.15 mil)99.6 μm (3.92 mil)
Pad inertia, IP 7.91kg.cm2 (2.70lb.in2)Oil inlet temperature ~40 oC (104 oF)Lubricant type ISO VG32, DTE 797Oil supply viscosity, μ0 0.032 Pa.s
Pr
Specific load, W/LD 0 kPa-1,896 kPa (275 psi)Journal speed, Ω 4 krpm-12 krpm
Cold conditions
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CB=55 μm
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Predictions for a four-pad TPJB
*Harris, H., 2008, Master Thesis, Texas A&M University, College Station, TX.
Specific load, W/LD 1.9 MPa (275 psi)Journal speed, Ω 4 krpm-12 krpm
Lubricant arrangements: Spray bar blocker, By pass cooling
Pivot type: Ball-in-socket pivot
Measured pivot stiffness: 350 MN/m
Nominal cold bearing clearanceCB=100 μm
CB=55 μmCB=100 μm
Measured cold bearing clearances on Pads #1 and #3 are ~40% smaller than the nominal cold clearance.
X
Y
2.45o
Θp=73o
W
Measured cold bearing clearance
CB=95 μm
Pad 1
Pad 4
Pad 2
Pad 3
X
Y
W
Pad 1
Pad 4
Pad 2
Pad 3
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0
20
40
60
80
100
0 400 800 1200 1600 2000Specific Load (kPa)
Jour
nal D
ispl
acem
ent
( μm
)Journal eccentricity vs. static load
-eY
eX
Rotor speed Ω =10 krpm
Rotor speed Ω =6 krpm
Predicted journal eccentricity correlates well with measurements.
XY
W XY
W
Symbols: test dataLines: prediction
*Childs, D.W., and Harris, H., 2009, ASME, J. Eng. Gas Turbines Power, 131, 062502
0
20
40
60
80
100
0 400 800 1200 1600 2000
Specific Load (kPa)
Jour
nal D
ispl
acem
ent
( μm
)
-eY
eX
Max. 275 psi
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0
10
20
30
40
0 400 800 1200 1600 2000Specific Load (kPa)
Tem
pera
ture
Ris
e at
Pad
Tr
ailin
g Ed
ge (o C
)Film temperature rise vs. static loadmeasured pad sub-surface temperature rise at pad trailing edge
Rotor speed Ω=6 krpm
X
Y
WPad 3
Pad 2
Pad 4
Pad 1
Trailing edges
Input: inlet thermal mixing coefficient λ=0.5
At 6 krpm, film temperature rises at pad trailing edges are
considerable even with no load applied
Pad 1Pad 2Pad 3
Pad 4
*Harris, H., 2008, Master Thesis, Texas A&M University, College Station, TX.
Symbols: test dataLines: prediction
Max. 275 psiOil Inlet temperature ~40oC
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0
10
20
30
40
0 400 800 1200 1600 2000Specific Load (kPa)
Tem
pera
ture
Ris
e at
Pad
Tr
ailin
g Ed
ge (o C
)
Film temperature rise vs static loadmeasured pad sub-surface temperature rise at pad trailing edge
X
Y
WPad 3
Pad 2
Pad 4
Pad 1
Trailing edges
Input: inlet thermal mixing coefficient λ=0.95
Film temperatures underpredicted at 10 krpm.
Film heats little with load
Rotor speed Ω=10 krpm
Pad 1Pad 2Pad 3
Pad 4
*Harris, H., 2008, Master Thesis, Texas A&M University, College Station, TX.
Symbols: test dataLines: prediction
Max. 275 psi
Effectiveness of spray bar blocker diminishes
Oil Inlet temperature ~40oC
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200
400
600
800
1000
0 400 800 1200 1600 2000
Specific Load (kPa)
TPJB
Stif
fnes
s C
oeffi
cien
ts
(MN
/m)
200
400
600
800
1000
0 400 800 1200 1600 2000
Specific Load (kPa)
TPJB
Stif
fnes
s C
oeffi
cien
ts
(MN
/m)
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Stiffness coefficients
Rotor speed Ω=10 krpm
Rotor speed Ω=6 krpm
λ=0.5
λ=0.95
KYY
KXX =KYY, TFBBRG
Kpiv=350 MN/m
Kpiv=350 MN/m
Very soft pivot produces
~ constant K’s, invariant with
load and speed.
XY
W XY
W
Symbols: test dataLines: prediction
*Childs, D.W., and Harris, H., 2009, ASME, J. Eng. Gas Turbines Power, 131, 062502
TFBBRG code in XLTRC2-TPJB model with rigid pivotKXX
Prediction KXX=KYY
KYY
KXX =KYY, TFBBRG
KXX
Max. 275 psi
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Rotor speed Ω=10 krpm
Rotor speed Ω=6 krpm
λ=0.5
λ=0.95Soft pivot renders
nearly constant damping coefficients. Good correlation with
test data
Damping coefficients Symbols: test dataLines: prediction
*Childs, D.W., and Harris, H., 2009, ASME, J. Eng. Gas Turbines Power, 131, 062502
CYY
CXX =CYY, TFBBRG
CXX
CYY
CXX =CYY, TFBBRG
CXX
XY
W XY
W
Synchronous speed coefficients
Prediction CXX=CYY
0
200
400
600
800
1000
0 400 800 1200 1600 2000Specific Load (kPa)
TPJB
Dam
ping
Coe
ffici
ents
(k
N.s
/m)
0
200
400
600
800
0 400 800 1200 1600 2000Specific Load (kPa)
TPJB
Dam
ping
Coe
ffici
ents
(k
N.s
/m)
Max. 275 psi
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Predictions for a five-pad TPJB(Wilkes and Childs*) Five pad, tilting pad bearing (LOP)
*Wilkes, J.C., 2011, PhD. Thesis, Texas A&M University, College Station, TX.
Pr
Specific load W/LD: 3,132 kPa (454 psi)Journal speed Ω: 4.4 krpm-13.1 krpm
X
Y
Pad
W
Journal
Fluid film
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0
200
400
600
800
1000
0 5 10 15 20
Pivot Radial Deflection (μm)
Pivo
t Rad
ial S
tiffn
ess
(MN
/m)
0246
8101214
0 5 10 15 20
Pivot Radial Deflection (μm)
Piv
ot R
adia
l For
ce (k
N)
Pivot stiffness & hot bearing clearance
Pivot load-deflection function
Pivot stiffness
Hot bearing clearance
Rocker back pivotPivot stiffness-deflection function
CB,cold-CB,hot=α(Thot-Tcold)Bearing clearance decreases due to thermal expansion of the rotor and pad surfaces.
α=0.396 μm/oC
Pivot radial force
Pivot radial stiffness
*Wilkes, J.C., 2011, PhD. Thesis, Texas A&M University, College Station, TX.
EXPERIMENTAL
Hot bearing clearance CB: 48 μm~58 μmNominal cold CB=68 μm
Empirical
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Pad bending stiffnessPad bending stiffness*
kpad =5.4644×104Mp+1.1559×105 (N.m/rad)
MP ,Pad bending moment from fluid film
*Wilkes, J.C., 2011, PhD. Thesis, Texas A&M University, College Station, TX.
/2 /22 2
/2 /2
sin sinP T
L P
L L
p p pL L
M P R d dZ P R d dZβ β β βΘ Θ
− Θ − Θ
= − =∫ ∫ ∫ ∫
Equivalent pad-pivot stiffness: series pivot + pad bending21 1
eq piv pad
lk k k
= +
12 P Pl R= Θ
Derived from experiments
Pad ½ length
β
RP
MPMP
OP
Used in code TPJB
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0
20
40
60
80
0 500 1000 1500 2000 2500 3000 3500Specific Load (kPa)
Jour
nal D
ispl
acem
ent (
μm
)
0
20
40
60
80
0 500 1000 1500 2000 2500 3000 3500
Jour
nal D
ispl
acem
ent (
μm
)
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Journal eccentricity slightly under/over predicted at the
low/high rotor speeds.
*Wilkes, J.C., 2011, PhD. Thesis, Texas A&M University, College Station, TX.
Journal eccentricity vs. static load
Symbols: test dataLines: prediction
-eY
-eY
Max. 454 psi
X
Y
W
Rotor speed Ω =13.1 krpm
Rotor speed Ω =4.4 krpm
cB= 57μm<-eY
cB= 49μm~-eY
λ=0.8
λ=0.9
cold CB=68 μm
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Rotor speed Ω =13.1 krpm
0
200
400
600
800
0 500 1000 1500 2000 2500 3000 3500Specific Load (kPa)
TPJB
Stif
fnes
s C
oeffi
cien
ts
(MN
/m)
0
200
400
600
800
0 500 1000 1500 2000 2500 3000 3500
TPJB
Stif
fnes
s C
oeffi
cien
ts
(MN
/m)
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Static stiffness coefficients over
predicted at large specific loads
KYY
KXX
KYY
KXX
K-C-M model
*Wilkes, J.C., 2011, PhD. Thesis, Texas A&M University, College Station, TX.
Static stiffness coefficients Symbols: test dataLines: prediction TPJBDashed: Wilkes preds.
Max. 454 psi
X
Y
W
Rotor speed Ω =4.4 krpmλ=0.8
λ=0.9
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0
100
200
300
400
0 500 1000 1500 2000 2500 3000 3500Specific Load (kPa)
TPJB
Dam
ping
Coe
ffici
ents
(k
N.s
/m)
0
100
200
300
400
0 500 1000 1500 2000 2500 3000 3500Specific Load (kPa)
TPJB
Dam
ping
Coe
ffici
ents
(k
N.s
/m)
27
Damping coefficients vary little with static load.
CYY
CXX
CYY
CXX
*Wilkes, J.C., 2011, PhD. Thesis, Texas A&M University, College Station, TX.
Damping coefficients
X
Y
W
Max. 454 psi
λ=0.8
λ=0.9Rotor speed Ω =13.1 krpm
Rotor speed Ω =4.4 krpm
Synchronous speed coefficients
Symbols: test dataLines: prediction TPJBDashed: Wilkes preds.
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-150
-100
-50
0
50
100
0 500 1000 1500 2000 2500 3000 3500
Virtu
al M
ass
Coe
ffici
ents
(kg)
-150
-100
-50
0
50
100
0 500 1000 1500 2000 2500 3000 3500Specific Load (kPa)
Virtu
al M
ass
Coe
ffici
ents
(kg)
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Large negative virtual masses at 4.4 krpm.
Dynamic stiffness increases with frequency.
MYY
MXX
MYY
MXX
*Wilkes, J.C., 2011, PhD. Thesis, Texas A&M University, College Station, TX.
Virtual mass coefficients
X
Y
W
Rotor speed Ω =13.1 krpm
Rotor speed Ω =4.4 krpmλ=0.8
λ=0.9
Symbols: test dataLines: prediction TPJBDashed: Wilkes pred.
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0
100
200
300
400
500
600
700
800
0 50 100 150 200 250 300 350Excitation Frequency (Hz)
Rea
l par
t of i
mpe
danc
e co
effic
ient
s R
e(Z
) (M
N/m
)
0
100
200
300
400
500
600
700
800
0 50 100 150 200 250 300 350Excitation Frequency (Hz)
Rea
l par
t of i
mpe
danc
e co
effic
ient
s R
e(Z
) (M
N/m
)
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Real part
At 4.4 krpm, dynamic stiffness HARDENs at frequencies ω > 2Ω.
Re( ZXX)
Re( ZYY)
Synchronous frequency
Re( ZXX)
Re( ZYY)Synchronous frequency
Re(Z)=K-Mω2
*Wilkes, J.C., 2011, PhD. Thesis, Texas A&M University, College Station, TX.
Impedances Specific load = 227psi
X
Y
W
Rotor speed Ω =13.1 krpm
Rotor speed Ω =4.4 krpm
Symbols: test dataLines: prediction TPJBDashed: Wilkes pred.
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0
100
200
300
400
500
0 50 100 150 200 250 300 350Excitation Frequency (Hz)
Imag
inar
y pa
rt of
impe
danc
e co
effic
ient
s Im
(ZXX
) (M
N/m
)
0
100
200
300
400
500
0 50 100 150 200 250 300 350Excitation Frequency (Hz)
Imag
inar
y pa
rt of
impe
danc
e co
effic
ient
s Im
(Z)
(MN
/m)
30
At 4.4 krpm, predicted damping is frequency dependent for ω>2Ω
Im( ZXX)
Im(ZYY)
Synchronous frequency
Im( ZXX)
Im( ZYY)Synchronous frequency
Ima(Z)=Cω
*Wilkes, J.C., 2011, PhD. Thesis, Texas A&M University, College Station, TX.
Impedances
Rotor speed Ω =13.1 krpm
Rotor speed Ω =4.4 krpm
Imaginary part
Specific load = 227psi
X
Y
W
Symbols: test dataLines: prediction TPJBDashed: Wilkes pred.
3131
Predictions for other test TPJBsKulhanek and Childs, 2012, ASME, J. Eng. Gas Turbines Power, 134, 052505 1-11.• TPJB code delivers good predictions for stiffness and damping by estimating the (actual) hot bearing clearances and using a constant pivot radial stiffness.
In the works. Will be part of forthcoming Y. Tao M.S. Thesis (TRC report 2012)
Delgado et al., 2010, ASME Paper GT2010-23802• TPJB code takes TPJB pivots as rigid and uses hot bearing clearances.• Predicted stiffness and damping correlate well with test data (45 psi).
Tschoepe and Childs, 2012, not yet published• TPJB code uses measured pivot load-deflection function and hot bearing clearances.• Predicted stiffness and damping are in agreement with test TPJB data.
3232
Conclusions• For TPJBs with very soft pivots (Kpiv<<Kfilm), pivot stiffness determines bearing stiffness.
• Film temperatures at no load condition are high. At high rotor speeds (> 10 krpm), LEG and spray bar blockers have less effectiveness in cooling a bearing.
• Bearing & pad clearances change due to thermal expansion & mechanical deformation of the rotor & pad surfaces. Using nominal cold bearing & pad clearances is a BAD idea.
•A-priori knowledge of pivot stiffness and bearing & pad clearances is required to obtain accurate predictions of TPJB performance.
• Bearing & pad clearances change a lot due to thermal expansion & mechanical deformation of the rotor & pad surfaces. Using nominal cold bearing & pad clearances is a BAD idea.
3333
2012 Proposal to TRC (2 years)
Hydrodynamic pressure P Pad surface elastic & thermal deformations change bearing & pad clearances
Kδ = P + C∆TK, Pad stiffness matrix
P, Fluid film pressure vectorC, Mechanical-thermal stiffness
matrixδ, Pad displacement vector
Objective: Enhance TPJB code to accurately predict pad surface deformations
Hot oil flow
Pivot constraint
FE pad structural analysis by Yingkun Li
3434
Proposed work 2012-2013• Build a 3-D FE model of commercial pads (ANSYS® or
SolidWorks®) to obtain pad stiffness matrix. Reduce model with active DOFs, perform structural modal analysis for easy off-line evaluation of pad surface deformations and pivot deflections.
• Implement oil feed arrangements (LEG, spray bar blockers etc.) in the FE model
• Construct new Excel GUI and Fortran code for XLTRC2
• Digest more test data and continue to update predictions using enhanced code.
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TRC BudgetYear III
Support for graduate student (20 h/week) x $ 2,200 x 12 months $ 26,400Fringe benefits (0.6%) and medical insurance ($197/month) $ 2,522Travel to (US) technical conference $ 1,200Tuition & fees three semesters ($227/credit hour) $ 9,262Other (PC+software+storage supplies) $ 1,600
2012-2013 Year III $ 40,984
Enhanced TPJB code will model current (commercial) TPJBsand improve predictions of force coefficients with minimum User expertise for specification of empirical parameters.
2012-2013 Year III
3636
Questions (?)