Ultra-high-speed set-up

Record speed: 1 200 000 rpm on aerodynamic bearings

1. Set-up

A special set-up (see figure 1) is used to test new bearing geometries. This set-up uses a simple 6 mm diameter caliper pen as rotor. This rotor is supported on aerodynamic radial bearings and aerostatic thrust bearings. Two symmetrically placed nozzles drive the pen by means of a small Pelton turbine in the center of the rotor. Shaft whirl is measured via optical probes mounted radially at both ends of the rotor.

Fig. 1. Ultra-high-speed set-up.

2. Measurements

Driven by compressed air

The rotor was first tested with the turbine driven by compressed air, reaching a speed of 685 000 rpm at full throttle. The maximal speed was clearly determined by the nozzle performance, NOT by bearing instability. The air speed at the exit of the nozzle is limited to sonic speed (about 343 m/s). When comparing this to the rotor's circumpherential speed (215 m/s), we see that the Pelton turbine is operating well above its point of maximum efficiency (this point lies at half the sonic speed). Figure 2 shows a waterfall diagram for a controlled run-up experiment from 0 to 683 280 rpm.

Figure 3 shows the spectra of the rotor whirl measured at both ends of this rotor, at the maximal speed.

Fig. 2. Waterfall diagram for controlled run-up experiment. Turbine driven by compressed air. Fig. 3. Spectra of the rotor whirl at the maximal speed obtained with the air-driven turbine.

Driven by helium

To obtain higher speeds, the turbine nozzles are fed with compressed helium, because helium has a much higher sonic speed than air (1007 m/s versus 343 m/s at room temperature). Regarding turbine performance, the increased velocity more than compensates for the lower density of helium. A record speed of 1.2 million rpm was obtained. Helium may flow into the aerodynamic bearing gap, but this has no influence on the bearing performance as both air and helium have very similar viscosities.

Fig. 4. Waterfall diagram for controlled run-up experiment. Turbine driven by helium.

3. Comparison

The table below compares gas bearing speed records obtained by institutes and companies around the world. To make a fair comparison between small and large rotors, the DN-number should be considered: diameter x speed. The highest DN-number was obtained by J. W. Beams in 1937, a time when considerable effort was put in the development of ultra-centrifuges for uranium enrichment. An important 'trick' was the use of hydrogen as bearing gas, because its low viscosity postpones rotor instability with respect to air, and its high speed of sound increases the speed at the nozzle outlets.

To our knowledge, we realised the fastest air bearings in the world. Furthermore, our bearings are of the aerodynamic (self acting) type, requiring no external supply of compressed gas or energy. This is quite important for application in microgasturbines.

Author Institute Year Diameter [mm] Speed [rpm] DN-number [mm.rpm] Bearing type Subtype
G. Belforte Politecnico di Torino 2008 37 75 000 2 775 000 Aerostatic O-ring support
H. Signer NASA Lewis Research Center 1973 120 25 000 3 000 000 Ball bearings
C. Zwyssig ETH Zürich 2008 3.175 1 000 000 3 175 000 Ball bearings
S. Tanaka Tohoku University 2003 4 1 250 000 5 000 000 Aerostatic Hydroinertia
S. Tanaka Tohoku University 2009 8 642 000 5 136 000 Aerodynamic Foil bearings
- Bruker Biospin 2005 1.3 4 200 000 5 460 000 Aerostatic O-ring support
F. D. Doty Doty Scientific 2006 4 1 380 000 5 520 000 Aerostatic O-ring support
A. Epstein MIT 2006 4.2 1 700 000 7 140 000 Aerostatic Small length/diameter ratio
T. Waumans K.U.Leuven - PMA 2010 6 1 200 000 7 200 000 Aerodynamic Rigid surface, externally damped
J. W. Beams University of Virginia 1937 9 1 300 000 11 700 000 Aerostatic Hydrogen fed
J. W. Beams University of Virginia 1946 0.521 37 980 000 20 130 000 Magnetic In vacuum

4. Future work

These new aerodynamic bearings will now be implemented on the turbo-shaft set-up and later on in the complete gasturbine.

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