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.
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.
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.
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.
4. Future work
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