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.