Identification and Correction of Rotor Instability in an Oil-Free Gas Turbine

The direction of advanced gas turbines and other turbomachinery has been towards oil-free designs, enabled by the significant improvements of high temperature foil bearings. The advantages of oil-free gas turbines have been studied and shown to be realistic. However, the oil-free technology is still at an early stage in its development relative to conventional oil lubricated turbomachinery systems which have been studied and manufactured for about 100 years, and the bearings even longer. Oil-free gas turbines are most successful as a system design initiated with oil-free bearings. Making these successful designs requires knowledge of the strengths and weaknesses of integrating oil-free bearings. A common example is foil bearings, the type typically considered for oil-free gas turbines. These bearings are lower in damping than their oil lubricated counterparts. Therefore special considerations are made by the experienced oil-free gas turbine designer early in the design process. Knowledge of the opportunities for instability that are not as common in conventional turbomachinery provides value to the final design. This paper presents the identification and correction of rotor instability in an oil-free microturbine of a 65 kW system. The manufacturer put significant effort into identifying the root cause of the seemingly random occurrences of rotor instability, in order to improve yield for acceptance tests. Through the application of conventional rotordynamics theory and techniques, combined with 3-D imaging of complex cast parts, the root cause was identified as an Alford’s-type force at the turbine driven by critical machined and cast features of the turbine wheel that would not have been important in a conventional oil lubricated turbomachine. A successful corrective process has been put in place, providing final confirmation of the root cause.