Gas turbine, as a power generating machine, are widely used in the world today. The overall thermal efficiency of gas turbine is strongly dependent on the inlet temperature of the turbine. As the turbine inlet temperature increases, the problem of hot mainstream gas ingestion arises. Hot gas ingestion refers to the phenomenon of the mainstream hot gas ingests into the wheel space composed of the turbine disc and its adjacent casing due to the pressure difference between the main stream annulus and the wheel space. Once the hot mainstream gas flows into the wheel space, serious problems like creep, oxidation or thermal fatigue maybe caused, resulting in a shortened lifespan of the turbine components. To diminish the possibility of hot gas ingestion, a stream of cooling air is introduced from the compressor to the wheel space. Insufficient cooling air may result in an excessive thermal load on the turbine components, while a superfluous amount of cooling air would reduce the overall thermal efficiency. Therefore, a compromise must be made to satisfy the cooling demand while at the same time maintain a maximum thermal efficiency. To determine a minimum amount of cooling air, the flow characteristic of the rim seal should be investigated. Due to the rotation of the rotor and blades, the flow field is highly unsteady thus it’s important to carry out time-dependent simulations.Numerical simulation is carried out to investigate the flow field near the rim seal region with a time-dependent computational fluid dynamics (CFD) model of a complete turbine stage with a rim seal structure and a wheel space cavity to enhance the understanding of rim seal ingestion process and its unsteady flow characteristics. The full circle turbine stage has 33 nozzle guide vanes and 66 blades with a constant mainstream mass flow rate. Considering the axisymmetric characteristic of the flow field, a 1/33 sector is selected as computing region composing of one nozzle guide vane, two blades and a cavity sector, and periodic boundary conditions are applied to the partition surfaces. URANS numerical method is used with the commercial CFD code ANSYS CFX to conduct the simulation, with RANS numerical simulation providing the initial flow field results. In the simulation, the SST turbulent model is used. A stream of sealing gas is added into the cavity to prevent hot gas ingestion through the cavity bottom surface, which realized by adding a scaler transportation equation in the simulation. Three different rotating speeds (3000rpm, 4000rpm, 5000rpm) and two sealing structures (axial and radial) are considered in the simulation.The simulation results show that unsteady phenomena are observed in both axial and radial type of rim seals. And the unsteady phenomena are more distinct in axial rim seal structure than in the radial rim seal structure because the radial seal lip suppressed the interaction between the main stream and cavity flow. The Fast Fourier Transformation (FFT) is adopted to analyze the pressure fluctuation at various spot points including rim seal region, stator, cavity etc. These unsteady phenomena could lead to more severe hot gas ingestions. Increasing sealing flow rate and increasing rotating speed can stabilize the flow field in the rim seal region and cavity thus suppress the hot gas ingestion.

Investigation of Unsteady Flow Characteristics in Axial  Rim Seal

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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