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Research Papers: Gas Turbines: Structures and Dynamics

Rotordynamic Performance of an Oil-Free Turbo Blower Focusing on Load Capacity of Gas Foil Thrust Bearings

[+] Author and Article Information
Tae Ho Kim, Yong-Bok Lee

Energy Mechanics Center,  Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Songbuk-gu, Seoul, Korea, 136-791

Tae Young Kim

 Maritime Research Institute, Hyundai Heavy Industries Co. LTD, 1 Jeonha-dong, Dong-gu, Ulsan, Korea, 687-792

Kyong Ho Jeong

Rotating Machinery Group, Hwang Hae Electric, 679-1, Gojan-Dong, Namdong-Gu, Inchun Metropolitan City, Korea, 405-819

J. Eng. Gas Turbines Power 134(2), 022501 (Dec 02, 2011) (7 pages) doi:10.1115/1.4004143 History: Received April 12, 2011; Revised April 19, 2011; Published December 02, 2011; Online December 02, 2011

Engineered design of modern efficient turbomachinery based on accurate model predictions is of importance as operating speed and rate power increase. Industrial applications use hydrodynamic fluid film bearings as rotor support elements due to their advantages over rolling element bearings in operating speed, system stability (rotordynamic and thermal), and maintenance life. Recently, microturbomachinery (< 250 kW) implement gas foil bearings (GFBs) as its rotor supports due to its compact design without lubricant supply systems and enhanced stability characteristics. To meet the needs from manufacturers, the turbomachinery development procedure includes a rotordynamic design and a gas foil journal bearing (GFJB) analysis in general. The present research focuses on the role of gas foil thrust bearings (GFJBs) supporting axial load (static and dynamic) in an oil-free turbo blower with a 75 kW (100 HP) rate power at 30,000 rpm. The turbo blower provides a compressed air with a pressure ratio of 1.6 at a mass flow rate of 0.92 kg/s, using a centrifugal impeller installed at the rotor end. Two GFJBs with a diameter of 66mm and a length of 50 mm and one pair of GFTB with an outer diameter of 144 mm and an inner diameter of 74 mm support the rotor with an axial length of 493 mm and a weight of 12.7 kg. A finite element rotordynamic model prediction using predicted linearized GFJB force coefficients designs the rotor-GFB system with stability at the rotor speed of 30,000 rpm. Model predictions of the GFTB show axial load carrying performance. Experimental tests on the designed turbo blower; however, demonstrate unexpected large amplitudes of subsynchronous rotor lateral motions. Post-inspection reveals minor rubbing signs on the GFJB top foils and significant wear on the GFTB top foil. Therefore, GFTB is redesigned to have the larger outer diameter of 166 mm for the enhanced load capacity, i.e., 145%, increase in its loading area. The modification improves the rotor-GFB system performance with dominant synchronous motions up to the rate speed of 30,000 rpm. In addition, the paper studies the effect of GFTB tilting angles on the system performance. Insertion of shims between the GFTB brackets changes the bearing tilting angles. Model predictions show the decrease in the thrust load capacity by as large as 86% by increase in the tilting angle to 0.0006 rad (0.03438 deg). Experimental test data verify the computational model predictions.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Oil-free 75 kW (100 HP) turbo blower. Rated speed of 30 krpm (500 Hz).

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Figure 2

Three dimensional (3D) view of centrifugal impeller design developed using CFB analysis. one eighth (1/8) sectional area of whole analysis domain. Provided by manufacturer.

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Figure 3

Gas foil bearings. (a) Journal bearing (GFJB) with a single top foil and a single bump layer. (b) Thrust bearing (GFTB) with eight top foils and eight bump layers.

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Figure 4

Finite element (FE) model of oil-free turbo blower rotor-GFBs system

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Figure 5

Predicted natural frequency versus rotor speed. Cylindrical and conical mode natural frequencies denoted.

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Figure 6

Predicted logarithmic decrement versus rotor speed. Cylindrical and conical mode natural frequencies denoted. Mode shapes illustrated at 30 krpm.

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Figure 7

Schematic view of developed 75 kW oil-free turbo blower (upper) with four eddy current sensors installed (lower in photos)

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Figure 8

Waterfall plot of rotor response amplitude measured at rotor thrust runner end in vertical direction. Speed up from 3 krpm to 18 krpm and coastdown to 0 rpm.

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Figure 9

Photos of GFTB top foil surfaces after test to 18 krpm. Motor side GFTB (left) and cooling fan side GFTB (right).

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Figure 10

Predicted GFTB load capacity versus rotor speed for two GFTB outer radii of 72 mm (original) and 83 mm (modified). Load capacities predicted at minimum film thickness of 5 μm.

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Figure 11

Modified GFTB with increasing outer radius of 83 mm

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Figure 12

Waterfall plot of rotor response amplitude measured at rotor thrust runner end in vertical direction for modified GFTB configuration with outer radius of 83 mm. Speed up from 0 krpm to 30 krpm.

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Figure 13

GFTB tilting conditions. Null tilting angle (upper) and tilting angle of θ° (lower). Tilting angles applied by installation of shims between GFTB back plates.

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Figure 14

Load capacity versus GFTB top foil number for increasing GFTB back plate tilting angles (θ) of 0.0 rad (0.0 deg), 0.0002 rad (0.01146 deg), and 0.0006 rad (0.03438 deg). Rotor speed of 30 krpm.

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Figure 15

Waterfall plot of rotor response amplitude measured at rotor thrust runner end in vertical direction for modified GFTB configuration with outer radius of 83 mm. GFTB back plate tilting angles (θ) of 0.0002 rad (0.01146 deg). Speed up from 0 krpm to 16 krpm.

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Figure 16

Waterfall plot of rotor response amplitude measured at rotor thrust runner end in vertical direction for modified GFTB configuration with outer radius of 83 mm. GFTB back plate tilting angles (θ) of 0.0006 rad (0.03438 deg). Speed up from 0 krpm to 12 krpm.

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Figure 17

GFJB synchronous stiffness coefficient versus speed

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Figure 18

GFJB synchronous damping coefficient versus speed

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