Analysis of Single-Blade Passage and Full Circumference Large-Eddy Simulations of Turbine Rim Seal Flows

Abstract

The flow in a 1.5-stage axial flow turbine with 16 vanes and 32 blades is investigated by large-eddy simulations with special focus on the analysis of the hot gas ingress through the rim seal into the upstream wheel space. Two setups with different solution domains are used. In the first large-eddy simulation, the full circumference of the turbine is resolved on a mesh with approximately 1 billion mesh cells. The second setup includes only a single-blade passage and is solved on a mesh with approximately 75 million cells. The analysis shows that the flow fields inside the upstream wheel space, i.e., between the disk of the upstream stator and the rotor disk strongly differ. In the 360 deg domain, two large-scale rotating vortex structures are observed, which have a strong impact on the ingress of main annulus gas into the wheel space. These structures cannot be captured in the single-blade passage domain. The instantaneous flow fields are further analyzed by dynamic mode decomposition to determine similarities and differences in the two flow fields. The comparison of the modes from the two setups reveals that the hot gas ingress from modes at the blade passing frequency generates a reduced sealing efficiency only at the outer radius of the wheel space. The additional presence of large-scale modes, however, reduce the sealing efficiency also in the inner wheel space. These results suggest that a single-blade passage simulation cannot be used for a reliable prediction of the hot gas ingress for the investigated turbine setup and operating condition.