Microsoft PowerPoint - GT2014-25940 Thrust FB ppp11
Luis San Andrés Mast-Childs Professor, Fellow ASME Texas A&M University
Research sponsored by BorgWarner Turbo Systems
Prediction of Gas Thrust Foil Bearing Performance for Oil-Free Automotive Turbochargers
Keun Ryu Assistant Professor
BorgWarner Turbo Systems
Proceedings of ASME Turbo Expo 2014: Turbine Technical Conference and Exposition, June 16-20, 2014, Düsseldorf, Germany
ASME GT2014-25940
Gas bearing for turbochargers
In support of OIL-FREE systems eliminate lubrication systems  and seals No oil coking and seal failure! reduce overall system weight, complexity. extend maintenance intervals. increase system efficiency due to low drag power losses. Higher ICE efficiency and lesser emissions. Green technology
http://www.aeronautics.nasa.gov/oil_free.htm
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• In early 1999, NASA & Miti & BorgWarner (Schwitzer) demonstrated oil-free turbocharger with gas foil bearings.
• The revamped S410 turbocharger was installed on the gas stand and operated at temperatures over 650°C and shaft speeds to 120 krpm.
• NASA Glenn Research Center spearheaded the oil-free turbocharger project with foil bearings from Mohawk Innovative Technologies (MiTi®) and turbocharger technology from BorgWarner (Schwitzer).
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• Series of corrugated foil structures (bumps) assembled within a bearing sleeve.
• Integrate a hydrodynamic gas film in series with one or more structural layers.
Current Applications: ACMs, micro gas turbines, turbo expanders.
Reliable Tolerant to misalignment and
debris, also high temperature. Damping from dry-friction and
operation with limit cycles. Excessive drag and wear during
rotor startup and shutdown . Need coatings to reduce friction.
Gas foil bearings – Bump type http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110011144.pdf
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Iordanoff (1998): Established limit of GFTB operation at high speeds.
Somaya et al. (2009): Analysis and testing of thrust bearings with viscoelastic supports.
Dykas (2006), Dickman (2010), Stahl (2012): Experimental results on static load and drag torque.
Lee and Kim (2011): Prediction and measurements for TFBs enhanced by hydrostatic pressurization.
Lee et al. (2008 - 2013): GFTBs integrated into turbo compressors and turbochargers. Report on-going analyses and test data.
Heshmat et al (1983): Coupled gas film pressure field to elastic surface deformation field via a simplified uniform stiffness model.
Zhou et al. (2012): Introduced novel TFB punching dimples on the top foil to act as the underspring element.
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Year Topic 2011-12 Thrust Foil Bearings: Computational modeling – Prediction of
dynamic force coefficients, performance characteristics (load capacity, drag power loss etc.)
2008-13 Metal Mesh Foil Bearings: construction, verification of lift off performance and load capacity, identification of structural stiffness and damping coefficients, identification of rotordynamic force coefficients
2008-10 Performance at high temperatures, temperature and rotordynamic measurements. Extend nonlinear rotordynamic analysis
2007-09 Thermoelastohydrodynamic model for prediction of GFB static and dynamic forced performance at high temperatures
2005-07 Effect of feed pressure and preload (shims) on stability of FBS. Measurements of rotordynamic response. Rotordynamic measurements: instability vs. forced nonlinearity?
2005-06 Model for ultimate load capacity, Isothermal model for prediction of GFB static and dynamic forced performance
2004-09 Measurement of static load capacity, Identification of structural stiffness and damping coefficients. Ambient and high temperatures
TAMU research on foil bearings
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Objective & Tasks
To develop predictive tool to perform the engineering design of thrust foil bearings for automotive turbochargers.
Implement FE model of top foil and integrate to gas film analysis.
Validate predictions from model with limited published test data.
Predict static and dynamic forced performance of gas thrust foil bearings for PV turbochargers.
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3 31 0
r r r r r t

Reynolds eq: Laminar flow and isothermal conditions
Film thickness (h)
T P e r
h h h w
( , ) ( , )( ), ,r r a B TFw f P P K D
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gas film
Support disk
Line weld
Support disk
Line weld
Top foil: 2D Finite Elements
- Neglect curvature: Small ratio of top foil deflection to its radius (~0.001) - Negligible interactions between bumps
- 2D flat SHELL (thin plate) finite element, Anisotropic material (No membrane stresses)
x
r : rotor surface peed
Mechanical impedance: z z zZ K i C
Exact advection model to solve the partial differential equations (PDEs) for the pressure fields in the gas film.
Control volume method for numerically stable and accurate solution at arbitrary operating conditions.
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Validation: TFB Geometry
NO available reference with enough description of TFB to benchmark any predictive tool.
Number of pads, NPAD 6 Outer diameter, Do 0.1016 m Inner diameter, Di 0.0510 m Pad arc extent, ΘP 45o
Pad taper extent, ΘT 15o
Pad taper, h 0.050 m* Top foil material Inconel X-750
Thickness, tTF 0.150 mm Bump foil material Inconel X-750
Thickness, tBF 0.102 mm Pitch, s0 5.00 mm*
Half length, l0 1.60 mm* Height, hBF 0.500 mm*
Friction coefficient, μf 0.10* Bump stiffness/area, KB 6.44 N/mm3
Structural loss factor, γ 0.20*
* Assumed value based on the authors’ practical experience
Dickman, J. R., 2010, “An Investigation of Gas Foil Thrust Bearing Performance and its Influencing Factors,” MS Thesis, Case Western Reserve University, Cleveland, OH.
s0
tt
tb
l0
Good correlation => Validates model for prediction of bearing static load performance.
Load = 40N (W/AreaTB=0.06 bar [0.95 psi])
D ra
g to
rq ue
[N m
Validation: Drag torque vs. load
Model predictions agree well with test data for loads < ~120 N (2.85 psi)
Largest difference & Sudden increase
D ra
g to
rq ue
[N m
Prediction: Min. film thickness vs. Load
As load increases, the min. film thickness decreases exponentially as the top foil deformation increases linearly.
Large speed number (696) and compliance factor (αc up to 5.25) denote large compressibility effects and a very soft underspring.
26 ))((
e
O
50 N
180 N
effect)
Sags
Predicted film thickness is too small at highest load. Surface roughness effects must play an important role in the generation of drag torque and dissipation power.
26 ))((
e
O
Prediction: Stiffness and damping vs. load
Both stiffness and damping coefficients increase with load since the film thickness decreases.
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(Cz/Kz) γ=0.2 Apparent at large load or high speed.
γ=0.2
Prediction of Performance for Thrust Foil Bearing in OIL-FREE TURBOCHARGER
Outer diameter, Do 0.054 m Inner diameter, Di 0.026 m
Structural loss factor, γ (design) 0.32
*TFB configuration is proprietary*
Axial load vs. shaft speed
Axial load on GTFB (normalized with respect to maximum load) versus shaft speed (normalized with respect to maximum shaft speed).
Load from balance of
thrust loads generated in
compressor wheels
Surface roughness is
important
As the shaft speed and thrust load increase, the minimum film thickness decreases.
Operation at a higher temperature leads to a larger film thickness, gas viscosity increases with temperature!
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Max. foil deformation vs. axial load
As shaft speed and thrust load increase, the maximum elastic deformation of the top foil increases linearly.
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Drag power loss vs. speed
Drag power increases with shaft speed (and load) and with gas temperature (higher viscosity).
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Friction factor vs. speed
Small friction factor makes the gas TFB an extremely attractive support for an oil-free TC. Nearly frictionless!
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Stiffness vs. axial load
Stiffness increases linearly with applied load, a typical condition for a soft TFB. Bearing compliance factor (αc) 0.12 ~ 0.50
Be
at 250°C
Damping vs. axial load
Bearing damping is most affected by temperature at the lowest load (and shaft speed) condition, while at high speed the damping coefficient is the largest
Damping normalized
at 250°C
Nearly constant Cz/Kz independent of gas temperature or load condition, structural damping is vital for mechanical energy dissipation.
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250oC
As frequency increases, the TFB stiffness hardens (increases) by ~50% at the low speed (low load) condition, and ~16% for the high speed (high load).
Stiffness normalized
at 250°C
250oC
Damping is largest for low speed and low load conditions Very small at γ=0 & vanishing quickly as frequency
increases. Dry-friction as a loss factor determines the magnitude of damping!
Damping normalized
at 250°C
z
z

The success of foil bearing technology relies on the selection of a metal underspring structure that offers the largest mechanical energy dissipation.
γ=0.32
speed number (gas compressibility
effect)
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While airborne, the drag friction factor for the bearing is small, ranging from 0.009 to 0.015, thus demonstrating the advantage of an air bearing technology over engine oil lubricated bearings. The largest drag occurs at the highest temperature since the gas viscosity is also highest. The synchronous speed axial stiffness increases with operating speed and load, whereas the axial damping coefficient remains nearly invariant. The operating gas temperature plays an insignificant role on the variation of the force coefficients with excitation frequency.
Conclusions GTFB designed for use in an oil-free turbocharger
ASME GT2014-25940
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The operating speed and the ensuing applied thrust load determine the largest change in the TFB force coefficients. As an excitation frequency increases, a TFB axial stiffness that hardens and an axial damping coefficient that decreases rapidly. The most important finding is that Cz/Kz ≈ γ = the material loss factor for the bearing.
Conclusions
ASME GT2014-25940
Learn more http://rotorlab.tamu.edu
Resultant shear forces (Q) and
bending moments (M) for distributed
load q=P-Pa in a shell element.
0
x Q
0yxx x
Shell FE & Underspring connections
[KG] : global FE stiffness matrix [ek] : top foil FE stiffness matrix [ks] : spring FE stiffness matrix
Assembly of structural stiffness matrix
4-node shell FE and linear spring
x
' 1B BK K i Structural loss factor (γ)
1
Nem
e
e
Global stiffness matrix decomposition
KG : global FE stiffness matrix, FG : external force vector UG : displacement vector, L : lower triangular matrix
DECOMPOSITION Performed off- line, prior to computations coupling structure to thin gas flow (Reynolds equation)
Computational efficiency greatly enhanced
Cholesky Decomposition Forward/backward substitutions
T G G GK L L
LG x =FG