Peter Vleugels

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.

Collector Pressure Losses in Micro Heat Exchangers

As collector losses are expected to play a crucial role in micro heat exchangers, an experimental method is developed to determine these losses. Experiments are performed on a micro heat exchanger consisting of 42 parallel microchannels positioned in a 6 by 7 matrix, with hydraulic diameters in the range of 260-280 mum. The proposed method is successfully applied. From the experimental results it can be concluded that the pressure drop of a small collector performs according to classical correlations. However, in a wider collector design the pressure drop clearly deviates from macro scale behaviour