Radial Inflow Turbine: Synchronous Vibration Failure

Abstract

Radial inflow turbine is frequently used in small gas turbine application where ruggedness and simplicity are prime requisites. For compactness, the radial turbine is mounted back to back with a radial compressor resulting in an overhung rotor in both compressor and turbine wheel. In such configuration the structural integrity of the radial turbine is crucial since due to its large inertia its failure could result in an uncontained egress of the engine. This paper presents one class of radial inflow turbine wheel failure due to high cycle fatigue of synchronous nature caused by two different excitation sources, namely mechanics and aerodynamics. While the first source of excitation is associated with the dynamics of the rotor, the second one is related to the dynamic of fluid flow and the thermal characteristics of the combustor, expressed in terms of hot streak. These sources of excitation can act individually or in combination. Two distinct types of failures were illustrated — blade mode — and disc-blade coupling mode. The failure phenomenon is characterized by fatigue crack originated at location corresponding to maximum dynamic stress for each type of failures, followed by the released of one portion of the blade. Analytical methods including finite element method, rotor dynamics analysis, and computational fluid dynamics are used to illustrate the root cause of such failure and also to its underlying solutions. Laser vibrometry and optical method were used to obtain the blade dynamic characteristics and to validate the solutions.