A novel FE BulkFlow Model for Improved Predictions of Force Coefficients in OffCentered Grooved Oil

1
A novel FE Bulk-Flow Model for Improved
Predictions of Force Coefficients in Off-Centered
Grooved Oil Seals
Luis San Andrés Mast-Childs Professor
Adolfo Delgado Research Assistant
TRC-SEAL-1-08
28th TRC Annual Meeting 2008
TRC Project 32513/1519 T7
2
Oil seal in a compressors1
Oil Seals
Oil seals are commonly used to prevent leakage of
process fluid in centrifugal compressors.
- Locked oil seal rings can induce instability in
compressors. - Seals are grooved to reduce
cross-coupled stiffness and lower lock-up forces
3
Damping, Cross-coupled Stiffness Inertia
2L
c
Journal
Constant pressure
2-land seal (deep groove divides lands)
Short length seal
4
Test Grooved Oil Seals
Childs et al., (2006)
Single groove and multiple groove oil seal
(single clearance)
Childs et al., (2007)
One groove with groove depths (5c,10c,15c)
Results
Added mass versus eccentricity ratios Childs et.
al
Force coefficients are underpredicted (grooved
seal)
Groove does not effectively separate seal lands
Large added mass coefficients (30 kg)
Experimental Results
5
Grooved Oil Seal
?
Experiments
Predictions
Groove does not effectively separate seal lands
Groove should reduce crossed-coupled stiffness
and damping coefficients by a factor of four
At most Kxy (1 land) 2 Kxy(2 lands) Cxx (1 land)
2 Cxx(2 lands)
?
Kxy (1 land) 4 Kxy(2 lands) Cxx (1 land) 4
Cxx(2 lands)
Large added mass coefficients , increasing with
increasing groove depth
?
Null (neglected) added mass coefficients
Need for better predictive models
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Improved predictive model
TRC-SFD-2-07

• Bulk flow, centered operation, incompressible
  fluid
• Qualitative observations of laminar flow field
• Boundary Conditions
• Characteristic groove depth

oil supply, Ps
feed plenum groove
mid-land groove
Ps- Pd gt0 Pd discharge pressure
Pd
Pd
z
y
Streamlines in axially symmetric grooved annular
cavity.
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Bulk flow model
Centered operation
Separate flow regions
No fluid advection
Oil supply
3
4
n
1
2
n1
IV
N
III
II
I
zN
zIV
zI
zIII
zII
Reynolds eqn with temporal fluid inertia
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Finite Element Solution
Off-centered operation
x qR
Film thickness
Reynolds eqn. with temporal fluid inertia
h
W
w
e
R
Y
De
X
9
Assemble system of equations, impose boundary
conditions and solve
FEM for solution of Pressure field
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Excel GUI XFEGLOSeal
User inputs -Fluid properties Density and
viscosity -Operating conditions Inlet and outlet
pressures, static journal eccentricity.
-Geometry Rotor diameter, clearance, groove
depth, number of grooves, inlet and outlet land
length, inter-groove length, groove length.
(XFiniteElementGroovedLaminarOilSEAL)
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Test Grooved Oil Seal FE mesh
xRq
Clearance 86 mm
Journal diameter 117 mm
Childs, D. W., Graviss, M., and Rodriguez, L. E.,
2007, The Influence of Groove Size on the Static
and Rotordynamic Characteristics of Short,
Laminar-Flow Annular Seals, ASME J. Tribol,
129(2), 398-406.
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CFD simulations show streamline separating flow
regions IS a physical boundary delimiting the
domain for squeeze film flow due to journal
radial motions.
Effective groove depth
Ps

Pa
Test seal
10c
Laminar flow
15c
Ps supply pressure Pa ambient pressure
Inner groove close up (CFD -Pressure driven flow)
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Shaft speed 10,000 rpm
Test data
Static eccentricity ratio 0, 0.3, 0.5, 0.7
Load
Supply pressure 70 bar
Journal center locus indicates seal operates with
oil cavitation at the largest test eccentricities
Journal locus
Seal operating conditions
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10,000 rpm, 70 bar
Smooth Seal
Grooved Seal
Predicted leakage correlates well with
experiments for both smooth land and grooved seal
(ch 7c)
(cg 15c)
Leakage
15
10,000 rpm, 70 bar
Cxx
Smooth Seal
Model predicts accurately reduction in direct
damping due to inner land groove.
Grooved Seal
Cyy
Smooth Seal
(ch 7c)
(cg 15c)
Grooved Seal
Direct Damping
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70 bar
Eccentricity ratio0
Smooth Seal
e 0, 0.3
Grooved Seal
Kxy
Model effectively predicts reduction in
cross-coupled stiffness due to mid-land groove.
Eccentricity ratio0.3
Smooth Seal
Grooved Seal
(ch 7c)
(cg 15c)
Kxy
Cross-coupled Stiffness
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10,000 rpm, 70 bar
Experimental data shows relatively large added
mass coefficients. Predictions correlate well
with experimental results.
Mxx
Grooved Seal
Added mass coefficients are larger for grooved
seal
Smooth Seal
Classical theory 1 predicts 1/10 of test value
(ch 7c)
(cg 15c)
1 Reinhardt, F., and Lund, J. W., 1975, The
Influence of Fluid Inertia on the Dynamic
Properties of Journal Bearings, ASME J. Lubr.
Technol., 97(1), pp. 154-167.
Added Mass
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Conclusions

• Predictions accurately capture the reduction of
  force coefficients on oil seals due to the
  addition of circumferential grooves.
• Predicted force coefficients (K,C,M) correlate
  well with experimental data.
• Boundary conditions reproduce well physical
  system.
• A groove does not fully uncouple adjacent film
  lands!!

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Boundary Conditions
Flow continuity is automatically satisfied at
boundaries
Constant static pressure at exit plane
PPexit
Null dynamic pressure at exit plane
Zeroth Order Pressure Field
Laminar flow
First Order Pressure Field
Constant static pressure at inlet plane
Null axial flow rate (geometrical symmetry)
In the occurrence of oil cavitation (Pcav0), the
first order dynamic pressure field vanishes
For both the zeroth and first order fields the
pressure field be periodic in the circumferential
direction
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Grooved Oil Seals
Predictive Models
Semanate and San Andrés, (1993)
- Bulk flow equation model
Kxy (1 land) 4 Kxy(2 lands) Cxx (1 land) 4
Cxx(2 lands)
- Grooves should reduce force coefficients by a
factor of four, i.e.
-Fluid inertia effects not predicted (considered
negligible)
Baheti and Kirk, (1995)

• - Reynolds and energy equation (Finite element
  solution)
• Grooves should effectively isolate seal lands
• Cross-coupled stiffness and damping coefficients
  are reduced by 60 for grooved configurations