Tobias Waumans

High-speed bearings for micro gas turbines: stability analysis of foil bearings

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.

Micropower generation with microgasturbines: A challenge

Abstract 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.

A Semi-Analytical Method for the Solution of Entrance Flow Effects in Inherently Restricted Aerostatic Bearings

The behaviour of aerostatic bearings is strongly influenced by the entrance flow effects nearby feeding sources. A basic understanding of these flow phenomena and accurate prediction of their relevant parameters are therefore essential in the design and optimisation process of any aerostatic bearing application. The subject matter of this paper has for long been the topic of extensive research. An overview of the different approaches found in literature shows mostly methods based on empirical formulas with a validity limited to the experimental conditions that produced them. The proposed solution method uses the technique of separation of variables to convert the boundary-layer equations describing the laminar channel flow into an initial value problem. This allows the exact calculation of the pressure profile from gap entrance up to the attainment of viscous flow. Knowledge of the pressure distribution near the gap entrance together with the mass flow rate leads to the determination of an expedient coefficient of discharge, allowing a more practical lumped parameter formulation. The results are compared with experimental data from various sources and the agreement is found to be remarkably good, indicating that a laminar flow model is adequate in characterising the entrance flow over a wide range of working parameters.Copyright

Integrated solution for micro power generation

Battery systems have a much lower energy density than fuels. The use of fuel based micro power generation units instead of batteries can therefore become interesting in certain applications. This paper describes the problems that occur when designing such a power generation unit. It is shown that operational conditions and geometrical restrictions impose tough requirements on the design of different components. The solutions as found in traditional fuel based generation systems cannot simply be downscaled, but several concepts have to be fundamentally rethought

Dynamic Characterization of Rubber O-Rings: Squeeze and Size Effects

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.