Analysis of Wheel Space Ingress in a One-Stage Axial Turbine

Abstract

The ingress of main annulus gas into the wheel space of an one-stage axial flow turbine with 30 stator and 62 rotor blades is analyzed based on the results of a large-eddy simulation (LES) of the full 360 circumference of the turbine stage. The Navier-Stokes equations are solved using a finite-volume solver based on a Cartesian mesh with cut cells for the wall surfaces. The rotating blades are tracked by a level set, which is transported with a semi-Lagrangian method. The cut cell formulation at the boundaries with a flux redistribution method ensures conservation of mass, momentum, and energy. The LES is performed for a low cooling gas mass flow rate, at which several acoustic modes are excited in the wheel space. The focus of the analysis is on the determination of the main annulus gas ingress contributed by the various modes. For that purpose, the radial velocity component is filtered at the frequencies of the various modes and the time and azimuthally averaged main annulus gas ingress is computed as a function of the individual modes of the radial velocity fluctuations. The results show that the acoustic modes generating the largest amplitude of radial velocity fluctuations do not cause substantial ingress. This can be attributed to the relatively long time scale of the turbulent mixing of main annulus gas with the cooling gas compared to the time scale of the acoustic modes. Only modes at low frequencies generate substantial ingress, which is confirmed by the energy spectra of main annulus gas concentration variations.