Unsteady Rotor-Stator Interaction of a Radial-Inflow Turbine With Variable Nozzle Vanes

Abstract

For radial turbines used in automotive turbochargers, the importance of variable flow capacity by means of a variable geometry system is getting higher under the growing demands for improved engine performance and reduced engine emissions. To realize a high-performance and aeromechanically-reliable turbine stage, the unsteady flow phenomena caused by the rotor-stator interaction and their impact on the mechanical integrity must be understood deeply. In the present paper, the periodic disturbance generated by the rotor-stator interaction of a research turbine stage is investigated. The research purposes are (i) to extract the flow phenomenon which is responsible for the blade excitation, (ii) to identify the operating condition at which the influence of the extracted phenomenon becomes stronger, and (iii) to clarify how and where the disturbance energy is fed into the blades. Three dimensional unsteady stage CFD simulations are conducted to investigate the unsteady stage interaction. Two parameters are mainly focused: the nozzle vane angle and the stage pressure ratio. By changing the former, the effect of different degrees of reaction can be examined, while by changing the latter, the effect of different Mach number levels can be evaluated. The unsteady blade loading is extracted from the CFD result and coupled with the blade displacement obtained from the eigen vibratory mode analysis to examine the aeromechanical influence of the unsteady loading on the impeller blade excitation at various operating conditions. The nozzle shock wave and nozzle clearance flow are identified as the principal phenomena for the impeller blade excitation. At the mean section of the impeller blade the nozzle shock wave impinges on the S/S and diffracts on the P/S periodically, these two processes constitute high unsteady blade loading at the impeller L/E. At the shroud section the nozzle clearance flow generates high fluctuation in the relative flow direction to the impeller which results in high unsteadiness in the blade loading. These two phenomena are more important at vane closed conditions due to the higher nozzle loading. The higher the pressure ratio, the higher the normalized loading, though once the nozzle shock wave is established the normalized loading does not increase appreciably. Most of the excitation energy enters the blade at the impeller L/E at the closed condition, while it enters the blade both at the L/E and T/E at the open condition.