Assessment of Low Engine Order Excitation of a Contaminated Turbocharger Radial Turbine Stage Using a Nozzle Ring Only Computational Model

Abstract

The use of critical fuels such as heavy fuel oil (HFO) usually leads to deposits of combustion residues in the turbocharger turbine stage, foremost in the nozzle ring where flow channels can get partially or completely clogged. The resulting change in geometry is heavily asymmetrical and therefore induces low engine order (LEO) excitation, which may lead to resonant excitation of rotor blades and potentially to high cycle fatigue (HCF) failure. The accurate prediction of LEO excitation is computationally intensive especially, in the context of a probabilistic analysis. The current study investigates the suitability of using a simplified model that only contains the nozzle ring in the assessment of LEO excitation to reduce computational costs. The aerodynamic excitation is assessed from the circumferential variation of the nozzle ring outflow. The results obtained with this simplified model are compared to results of validated forced response analyses. Two deteriorated nozzle rings are used for validation purposes, a generic one having one passage blocked by 90%, and a digital replica of a real contaminated nozzle ring. It is shown that the simplified model is suitable for the qualitative prediction of the aerodynamic excitation if the contamination patterns feature sufficiently large differences and that the computational costs can be greatly reduced.