Study on Damping of Nozzleless Radial Turbine and Its Role in Forced Response

Abstract

Abstract. This paper systematically investigates the factors influencing radial turbine damping, including aerodynamic and material damping. Aerodynamic damping is identified as the dominant contributor, accounting for over 98% of the total damping in the first vibration mode. The key factors affecting aerodynamic damping, including pressure ratio, nodal diameter, vibration modes, and their coupling effects, are discussed. It is revealed that aerodynamic damping correlates linearly with pressure ratio in the typical operating range, except at low pressure ratios, where strong vortices induce nonlinearity and hence unsteady damping. A vortex-filtering method is proposed for reliable prediction at low pressure ratios. The nodal diameter significantly affects the aerodynamic damping ratio. A simplified theoretical derivation further proved their harmonic relationship for rapid assessment. The influence of vibration mode lacks a clear pattern, as changes affect both fluid and solid properties. An inverse correlation between turbine damping and vibration frequency near resonance is introduced, offering a rapid assessment method for damping. Lastly, the damping prediction method is applied to assess the forced response of radial turbines via two experimental cases. The fluid and structure method (FSI) with full consideration of damping accurately predicts the radial turbine's vibration amplitude with an averaged error of 7.1%. Additionally, the method diagnoses high cycle fatigue failure in several marine turbocharger turbines, where the traditional empirical constant damping model fails. It is confirmed that the turbine blade failure resulted from the rare second excitation, mainly due to the lack of damping. Inaccurate damping prediction or neglecting its variation leads to incorrect assessment of radial turbine operating risk.