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Research Papers: Gas Turbines: Structures and Dynamics

Prediction of Gas Thrust Foil Bearing Performance for Oil-Free Automotive Turbochargers

[+] Author and Article Information
Luis San Andrés

Mast-Childs Professor
Fellow ASME
Department of Mechanical Engineering,
Texas A&M University,
College Station, TX 77843
e-mail: LSanAndres@tamu.edu

Keun Ryu

Assistant Professor
Department of Mechanical Engineering,
Hanyang University,
Ansan, Gyeonggi-do 426-791, South Korea
e-mail: kryu@hanyang.ac.kr

Paul Diemer

Director of Engineering
BorgWarner Turbo Systems,
Arden, NC 28704
e-mail: pdiemer@borgwarner.com

The description is brief and incomplete. A perturbation analysis is conducted to find Reynolds equations for the equilibrium pressure field (P0) and a complex perturbed pressure field (Pz). These PDES are coupled to the structure FE Eq. (9).

Λ ≫ 1 denotes a regime of operation dominated by fluid compressibility effects, while αc > 1 signifies a bearing with a stiff under-spring layer (hard bumps).

The bearing compliance factor (αc) increases from 0.12 (at the highest temperature and lowest load) to 0.50 (at the lowest temperature and highest load).

1Corresponding author.

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 10, 2014; final manuscript received July 26, 2014; published online September 30, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(3), 032502 (Sep 30, 2014) (10 pages) Paper No: GTP-14-1364; doi: 10.1115/1.4028389 History: Received July 10, 2014; Revised July 26, 2014

Green technologies are a mandate in a world concerned with saving resources and protecting the environment. Oil-free turbocharger (TC) systems for passenger and commercial vehicles dispense with the lubricant in the internal combustion engine (ICE), hence eliminating not just oil coking, but also suppressing nonlinear behavior, instability and excessive noise; all factors to poor reliability and premature mechanical failure. The work hereby presented is a stepping stone in a concerted effort toward developing a computational design tool integrating both radial and thrust foil gas bearings for oil-free automotive TCs. The paper presents the physical analysis and numerical model for prediction of the static and dynamic forced performance of gas thrust foil bearings (GTFBs). A laminar flow, thin film flow model governs the generation of hydrodynamic pressure and a finite element plate model determines the elastic deformation of a top foil and its support bump strip layers. For a specified load, the analysis predicts the minimum gas film thickness, deformation and pressure fields, the drag torque and power loss, and the axial stiffness and damping force coefficients, respectively. Open source archival test data on load capacity and drag torque serves to benchmark some of the model predictions. Next, predictions are obtained for a GTFB configuration designed for an oil-free TC operating at increasing gas temperatures, axial loads, and shaft rotational speeds. The largest drag torque occurs at the highest temperature since the gas viscosity is also highest, whereas the largest load determines operation with a minute film thickness that sets a limit for the manufacturing tolerance. 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 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 frequency, whereas the operating speed and the ensuing applied thrust load determine the largest changes. The model predicts, as an excitation frequency (ω) increases, a GTFB axial stiffness (Kz) that hardens and a damping coefficient (Cz) that quickly vanishes. The most important finding is that CzΩ/Kz ≈ γ = the material loss factor for the bearing. Hence, the success of foil bearing technology relies on the selection of a metal underspring structure that offers the largest mechanical energy dissipation characteristics.

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References

Figures

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Fig. 1

Schematic view of a three pad thrust foil bearing

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Fig. 2

Schematic view of a bump strip layer and top foil in a TFB. Rotor surface speed = Ωr.

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Fig. 3

Schematic view of top foil supported on linear undersprings and depiction of a finite element for structural elasticity analysis

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Fig. 4

Measurements and current predictions. Test data taken from Ref. [16]. Drag torque versus shaft speed for applied load = 40 N. TFB geometry in Table 1.

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Fig. 5

Measurements and current model predictions. Test data taken from Ref. [16]. Drag torque versus applied load at 21 krpm. TFB geometry in Table 1.

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Fig. 6

Predicted minimum film thickness and maximum top foil deformation versus applied load for operation at 21 krpm. TFB geometry in Table 1.

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Fig. 7

Contours of predicted film thickness, gas film pressure, and top foil deformation for operation at 21 krpm and two applied loads: (a) 50 N and (b) 180 N. TFB geometry in Table 1.

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Fig. 8

Predictions versus applied load for speed 21 krpm: (a) synchronous speed stiffness and damping coefficients and (b) ratio of force coefficients. TFB geometry in Table 1.

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Fig. 9

Axial load (normalized with respect to maximum load) acting on GTFB versus shaft speed (normalized with respect to maximum shaft speed)

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Fig. 10

GTFB for oil-free TC: minimum film thickness and maximum foil deformation versus axial load (and speed) for operation at 21 °C, 140 °C, and 250 °C. (a) Minimum film thickness normalized with respect to minimum film thickness at maximum shaft speed. (b) Maximum foil deformation normalized with respect to minimum film thickens at maximum shaft speed.

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Fig. 11

GTFB for oil-free TC: drag power loss and friction factor versus shaft speed (and load) for operation at gas temperatures of 21 °C, 140 °C, and 250 °C. (a) Drag power loss normalized with respect to maximum power loss. (b) Friction factor.

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Fig. 12

GTFB for oil-free TC: synchronous speed stiffness and damping coefficients and (CzΩ/Kz) versus axial load (and speed) for operation at gas temperatures of 21 °C, 140 °C, and 250 °C. (a) Stiffness at synchronous speed. (b) Damping at synchronous speed. (c) CzΩ/Kz.

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Fig. 13

GTFB for oil-free TC: axial stiffness versus frequency ratio for operation at three rotor speeds and gas temperature of 250 °C

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Fig. 14

GTFB for oil-free TC: axial damping versus frequency ratio for operation at three rotor speeds and gas temperature of 250 °C

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Fig. 15

GTFB for oil-free TC: ratio of force coefficients (Czω/Kz) versus frequency ratio (ω/Ω) for operation at three rotor speeds and gas temperature of 250 °C

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