HCF Optimization of a High Speed Variable Geometry Turbine

Abstract

This paper describes the structural optimization of a high speed, 35mm tip diameter radial turbine wheel in a Variable Geometry Turbine (VGT) system, subjected to the wide range of aerodynamic loads experienced during the full operating cycle. VGTs exhibit a wide range of unsteady flow features, which vary as the nozzle vanes rotate through different positions during operation, as do the magnitudes and frequencies of the resulting pressure fluctuations experienced by the downstream turbine blades. The turbine wheel typically passes through a number of blade natural frequencies over their operating cycle, and there are a number of potential conditions where these unsteady aerodynamic loads can lead to resonant blade vibration. The focus of this work is on the development of a pragmatic design approach to improve the structural characteristics of a radial turbine blade with respect to High Cycle Fatigue (HCF), informed by detailed time-accurate Computational Fluid Dynamics (CFD) prediction of the unsteady pressure loads, coupled with FE vibration analysis to quantify the resulting blade vibration magnitudes. Unsteady CFD simulations are performed to determine the time-accurate pressure loads on the blades, and the results are used as input to forced response analysis to determine the peak alternating stress amplitudes. The detailed analysis results are then used to guide a subsequent parametric study in order to investigate the influence of key geometric parameters on the structural performance of the blade, with the optimum design identified through the use of a Goodman Diagram. The results quantify the influence of both blade thickness distribution and hub fillet details on the vibration characteristics of radial turbines.