Abstract
The flow in a 1.5-stage axial turbine is investigated by large-eddy simulations. The focus is on the ingress in the downstream wheel space. Two setups are considered. In the first, the full 360 deg annulus is included on a computational mesh with approx. 1 billion mesh cells. The second setup includes a single blade passage in a 22.5 deg segment. The computational mesh has approx. 75 million mesh cells accordingly. The flow fields in the downstream wheel space differ strongly. In the 22.5 deg setup, the disk pumping effect is much more pronounced than in the 360 deg setup and the fluid bulk rotates in the direction of the rotor rotation. In the 360 deg setup, the fluid rotates in the opposite rotor direction and the velocities feature a deflection at intermediate radii. The differences are caused by the instantaneous flow fields. In the upstream wheel space of the 360 deg setup, two large-scale rotating vortex structures are predicted, which create four pressure peaks that propagate downstream. They interact with the second stator and create a rotating flow structure in the downstream wheel space, which generates alternating ingress and egress. Due to their spatial extent, none of these structures can exist in the 22.5 deg setup. The results show that for the investigated combination of turbine geometry and operating condition, the analysis of the downstream wheel space requires the detailed prediction of the unsteady flow phenomena in the upstream wheel space, i.e., it cannot be performed using the 22.5 deg setup.
