Prediction of Low-Engine-Order Excitation due to a Non-Symmetrical Nozzle Ring in a Radial Turbine by Means of the Nonlinear Harmonic Approach

Abstract

Abstract Low engine order (LEO) excitation in a turbomachine stage can be induced by non-uniform inflow conditions, manufacturing tolerances or in-service wear. LEOs are known to excite significant forced response vibration amplitudes that can easily cause High Cycle Fatigue (HCF) failure of blades. The accurate prediction of LEO excitation usually requires high-fidelity CFD models of the full annulus of the machine due to the loss of symmetry leading to excessive computational cost. Previous investigation showed that the aerodynamic excitation stemming from the blade-passing-frequency in a vaned radial inflow turbine can be accurately predicted by using the NonLinear Harmonic (NLH) method at highly reduced computational costs. In the current paper, the feasibility of the NLH method for the prediction of LEO excitation due to geometrical asymmetries is investigated for the same test object. An exact digital replica of the nozzle guide ring is created using measured throat width data. NLH simulations resolving different combinations of frequencies and a time-marching calculation are conducted with the new model involving this digital replica. The results show that a NLH model including small number of certain frequencies is able to predict the occurring LEO excitation sufficiently accurate. By comparing results from subsequent forced response analysis with measured vibration amplitudes, a satisfactory agreement was found confirming this conclusion.