NONLINEAR DYNAMIC ANALYSIS OF OIL-FREE TURBOMACHINERY

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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.