UNSTEADY ROTATING RADIAL LOAD OF A SINGLE STAGE RADIAL INFLOW TURBINE

摘要

Radial inflow turbines with pivoting stator vanes are known to keep a high level of efficiency over a large flow range. This turbine type maintains a good efficiency level at reduced speed because the turbine wheel is not very sensitive to negative incidence. For these reasons variable geometry radial inflow turbines are used where a large operating map is required. However, it is known from literature that impeller blades undergo significant unsteady aerodynamic forces due to upstream stator vanes, especially with transonic conditions at stator exit, which are quite common for these machines. From a mechanical point of view, the number of stator vanes is therefore crucial since it determines the excitation frequency. Over a large operating map, it is not possible to avoid all eigenmodes. The vane count shall avoid excitation of impeller critical eigenmodes, especially if their mode shape matches. Amongst unsteady phenomena occurring in turbomachinery, rotor/stator interactions are of primary importance. Independently from impeller eigenmodes, this study discusses the unsteady rotating radial load on the impeller that can be generated under specific blade count combinations, for a single stage radial inflow turbine. This radial load can indeed be large and impact the rotordynamics. The rotating frequency of the load is given by Tyler-Sofrin theory. In order to compute its amplitude, unsteady CFD is performed, with the use of the non-linear harmonic method implemented into FINE™/Turbo software from NUMECA. The effects of the fluid, of the operating conditions and of the rotational speed are also quantified. Under specific conditions, resonance may occur and is discussed.