Mesoscopic or microscopic gas turbines can be an interesting replacement for batteries as mobile energy supplies. A difficult consequence of small-scale turbomachinery is an increased rotor speed, in the order of 500 000 rpm and higher, turning bearing design into a challenging task. Air bearings are the only bearing type able to withstand the severe conditions of high speed and high temperature. However air bearings and more, in particular, aerodynamic bearings are prone to dynamic instabilities. Therefore unconventional bearing types such as foil bearings may present an interesting solution. This paper presents and discusses simulation techniques to predict the steady behaviour of a foil bearing. Furthermore, a method to calculate the dynamic properties is proposed. Using these dynamic stiffnesses and damping coefficients, a stability analysis is carried out. This analysis shows that, even without additional damping, a foil bearing is more stable than a rigid surface aerodynamic journal bearing with similar geometry but not as stable as is hitherto believed. However, due to its flexible nature, it is possible to improve the stability by simple means.

As the bottleneck for the successful application of ultra-miniature gasturbines is predominantly imposed by limitations in currently available high-speed bearing technology, new bearing concepts have to be developed. This paper reports on the achievement of a record speed of 1 203 000 rpm (= 7.2 million DN) on aerodynamic journal bearings. The concept and implementation of the flexible, damped bearing support are outlined and its effectiveness for the stabilisation of high-speed gas bearings is demonstrated experimentally. The following related topics are briefly studied: (i) characterisation of different elastomeric support materials; (ii) discussion of observed measurement artefacts at high speed; and (iii) an analysis of the frictional loss sources.

This paper describes the development of a microgasturbine with a rotor diameter of 20 mm. The target electrical power output lies around 1 kW. The total system fits in a cylinder with a diameter of 95 mm and a length of 120 mm. The system contains the same components as a large gasturbine generator: compressor, recuperator, combustion chamber, turbine, and electrical generator. Major challenges are the high rotational speed (500 000 r/min), high turbine inlet temperature (1200 K), and the efficiency of the components. Because of the small dimensions, the flow through compressor and turbine is characterized by relatively low Reynolds numbers. The higher flow losses and inherently lower efficiency require a higher blade tip speed (524 m/s) than for large turbines (300-400 m/s). To minimize wear and frictional losses, the rotor is mounted on aerodynamic bearings. To withstand the high centrifugal stresses, a high-strength steel is used for compressor and shaft. The turbine is made of a Si3N4-TiN ceramic composite to withstand the combination of elevated stress and temperature.

This paper concerns the dynamic characterization of rubber O-rings used to introduce damping in high speed gas bearing systems. O-shaped rubber rings composed of high temperature rubber compounds are characterized in terms of stiffness and damping coefficients in the frequency range 100-800 Hz. Simple formulas with frequency independent coefficients were identified to express the viscoelastic properties of the O-rings. The formulas proposed approximate the stiffness and damping coefficients of O-rings of general size.

For high-speed applications, gas lubricated bearings offer very specific advantages over other,more conventional bearing technologies: a clean and oil-free solution, virtually wear-free operation, lowfrictional losses, wide operating temperature range, etc. However, the principal drawback involved in theapplication of high-speed gas bearings concerns the dynamic stability problem. Successful applicationtherefore requires control of the rotor-bearing dynamics so as to avoid instabilities.After a detailed study of the dynamic stability problem and the formulation of a convenient stability criterium,a brief overview is given of the currently existing bearing types and configurations for improving the stability.In addition, three strategies are introduced: (i) optimal design of plain aerostatic bearings; (ii) modification ofthe bearing geometry to counteract the destabilising effects in the gas film; and (iii) introduction of dampingexternal to the gas film as to compensate for the destabilising effects.These strategies are worked out into detail leading to the formulation of a series of design rules. Theireffectiveness is validated experimentally at a miniature scale. In recent experiments a rotational speed of1.2 million rpm has been achieved with a 6 mm rotor on aerodynamic journal bearings, leading to a recordDN-number of 7.2 million.