NONLINEAR DYNAMIC ANALYSIS OF OIL-FREE TURBOMACHINERY

Oil-free turbomachinery is an emerging technology defined as high speed rotating machinery that operates without oil-lubricated rotor supports. The term is generally understood to refer to gas-bearing technology, in particular, foil-air bearings. Such bearings support the shaft by means of an air cushion bounded by a flexible foil structure. The introduction of the foil structure resolves the problems associated with the very tight radial clearance required by a plain air bearing. With a foil-air bearing, while the shaft is stationary, there is either a slight clearance or a preload between shaft and bearing. As the shaft turns, a pressure is generated, which pushes the foil boundary away, allowing the shaft to become completely airborne. A solid lubricant coating on the shaft and/or top foil allows for the brief rubbing interval during start-up and shutdown. Recent technological breakthroughs in the USA in solid lubricant technology will enable the widespread use of such bearings in turbomachinery, particularly gas turbine machinery. This has resulted in intensive research in oil-free turbomachinery motivated by its technological and environmental benefits for both military and civil applications (e.g. turbochargers that run up to 180,000rpm and engines for small aircraft). As stated by NASA, the foremost challenge for this technology is the design of an oil-free turbine engine to power 21st century aircraft .Foil-air bearings, like conventional oil bearings, are nonlinear elements that are capable of introducing undesirable nonlinear effects into the dynamic response of the system. These effects may involve sudden jumps in the vibration amplitude, non-synchronous vibration and self-excited vibration. These effects exacerbate vibration and introduce fatigue. Hence, to guarantee structural integrity, the deployment of these bearings in practical machinery necessitates rotordynamic analysis that takes account of the bearing nonlinearity. The ability to make reliable quantitative predictions of such effects enables the engineer to account for/mitigate them in the design. Moreover, such analysis provides the basis of a much-needed knowledge database for in-service monitoring. However, such calculations are hampered by the prohibitive computational cost introduced by the complexity of the bearing model. Consequently, dynamic analysis has so far been restricted to a highly simplified rotordynamic system. The proposed project researches novel methods that enable the efficient nonlinear dynamic analysis of practical oil-free turbomachinery. These methods will be experimentally validated in a study that provides a much-needed insight into the nonlinear dynamics of such systems. The deliverables of this project will be:i. A suite of computer software algorithms for efficient nonlinear dynamic analysis based on three novel approaches (Galerkin reduction, Harmonic Balance, System Identification). ii. An original-design test-rig for experimental validation of the computational methods.iii. A report on the validation of the methods, focussing on both computational and experimental issues.The proposed research is novel since: (a) It will give the UK a foothold in oil-free turbomachinery technology, raising the UK's scientific profile - such research has to date been confined mainly to the US; (b) It will research the prediction of the nonlinear dynamics of practical oil-free turbomachinery (e.g. an oil-free turbocharger); (c) It will do so through the three novel approaches mentioned above; (d) It will produce an original-design test-rig for the validation of the methods developed and investigation of nonlinear phenomena.The work will be carried out by a post-doctoral researcher over a period of three years.

The main deliverable of this project is a suite of efficient computer software algorithms which would allow, for the first time, the nonlinear dynamic analysis of realistic oil-free turbomachinery. An original-design test rig will enable experimental validation and provide an insight into the dynamics of foil-air bearings. The ability to predict and understand nonlinear dynamic phenomena is essential for guaranteeing the structural integrity of such machinery. Hence, this project will significantly advance oil-free technology on an international level by facilitating the safe application of foil-air bearings to a wide range of turbomachinery. This, in turn, will significantly contribute towards the ongoing process of making such technology more affordable for civilian applications. The technological and environmental benefits of oil-free turbomachinery ultimately translate into an improved quality of life to the general public as a result of cleaner, more reliable machinery that is much less bulky and more economical to run. The elimination of an oil lubrication system means higher reliability, no scheduled maintenance and weight reduction, which is a bonus for aerospace applications. It also means less power consumption, and hence less pollution. Also, with oil lubrication, some of the oil mist generated at high engine speeds will find its way into high temperature areas, resulting in its combustion and extra pollution. Due to the tight clearances, if a foil-air bearing failure occurs, the presence of the foil restrains the shaft from excessive movement and so, damage is confined to the bearings and shaft surfaces. Foil bearings can handle severe environmental conditions such as sand and dust ingestion without any need for air filters. They are self-acting, so do not require a power source (unlike magnetic bearings). Moreover, the load capacity of air bearings increases as speed increases in direct contrast to rolling-element bearings, making them ideal for small high speed turbomachinery. The outcomes of this research will give the UK a foothold in this beneficial emerging technology, raising the UK's world research standing. This project will form the basis of a specialist research group which would be key to facilitating post-project technology transfer to UK industry. The establishment of oil-free turbomachinery research in the UK is envisaged as a two-stage process. The proposed project constitutes the first stage: the development of a significant body of publicly-funded academic research and associated technical skills. These outcomes would then provide the basis for the second stage which would see closer industrial involvement and technology transfer. Technology transfer would involve the embedding of the algorithms into industrial rotordynamic codes and the exploitation of the test rig for experimental testing of foil-air bearings for industrial applications. Such technology transfer will benefit UK turbomachinery manufacturers in their quest to design greener and more economical machinery products. In the longer term, their UK-based client companies, who supply components, would also benefit if they branch into the supply (and possibly design and manufacture) of foil-air bearings (design and manufacture of these bearings is currently almost exclusive to the US companies). This project is endorsed by a leading US foil-air bearing manufacturer (Mechanical Solutions Inc). This is appropriate since this project constitutes the first stage of the establishment of UK research into oil-free turbomachinery. In order to facilitate close post-project industrial collaboration, the awareness of industry to the ongoing project activities and achievements will be raised through two means: (a) Conference participation (in addition to journal publication); (b) A small measure of collaboration with a UK turbomachinery manufacturer (Cummins Turbo Technologies Ltd) during the project itself.