1 Department of Mechanical Engineering, Technical University of Denmark2 Solid Mechanics, Department of Mechanical Engineering, Technical University of Denmark
From a rotordynamic point of view there are two phenomena that limit the widespread of traditional gas lubrication: 1) low damping makes operation across critical speed dangerous, as even low level of unbalance can generate large vibration responses; 2) an upper bound to supercritical operation is determined by the appearance of subsynchronous whirl instability. In fact, postponing the onset speed of instability poses one of the greatest challenges in high-speed gas bearing design. A great deal of research is devoted to attack such issues, where most propose passive designs such as compliant foil bearings, tilting pad and flexure pivot gas bearings. These solutions proved to be effective in improving static and dynamic properties of the bearings, however issues related to the manufacturing and accuracy of predictions has so far limited their applications. Another drawback is that passive bearings offer a low degree of robustness, meaning that an accurate optimization is necessary for each application. Another way of improving gas bearings operation performance is by using active control systems, transforming conventional gas bearings in an electro-mechanical machine component. In this framework the main focus of this thesis is the theoretical modeling, numerical simulation and experimental rotordynamic testing of a flexible rotor supported by hybrid aerostatic-aerodynamic gas journal bearing equipped with an electronic radial air injection system. Experimental results on a specially designed test-rig are backed by a comprehensive mathematical model that couples a finite element model of a flexible rotor, a thermohydrodynamic model based on a modified form of the Reynold’s equation for hybrid aerostatic-aerodynamic lubrication of compressible fluid, a piezoelectric injection system and a proportional-derivative controller. It is shown that synchronous vibrations can be effectively addressed ensuring safe operation across the critical speeds; whirling instability is suppressed; intervening on the software, rather than the hardware can modify the response of the system. Optimum tuning of the control loop is addressed experimentally, showing dependency on the supply pressure and, less prominently, the rotational velocity. Moreover, additional research is carried out in order to perform a feasibility study on a new kind of hybrid permanent magnetic – aerodynamic gas bearing. This new kind of machine is intended to exploit the benefits of the two technologies while minimizing their drawbacks. The idea is to improve the poor start-up and low speed operation performance of the gas bearing by using magnetic forces to lift the rotor. At high speeds the dynamic characteristics of the gas bearing can also be modified by using the same magnetic forces.