Theoretical Modelling of Ingress in Gas Turbine Rim Seals.

Engineers aim to improve the efficiency of gas turbines without sacrificing the life of materials. Ingress is the phenomenon where hot gas from the mainstream flow is ingested into the wheel-space between the stationary casing and the adjacent rotating disc. To prevent or reduce ingress, rim seals are fitted at the periphery of the discs and sealing air, which is directly bled from the compressor, pressurises the internal cavity. However, using too much sealing air reduces the efficiency, while too little can damage the materials due to high temperatures. Siemens currently use the University of Bath Orifice Model to design rim seals for their engines. Here, only the inertial effects are taken into account, while discharge coefficients are used to account for the viscous effects and other related losses. These coefficients cannot be determined theoretically and they are treated as empirical constants for each seal. The Orifice Model is used to predict ingress, and although it is in good agreement with concentration based experimental results, problems with pressure difference predictions arise for different rim seals geometries (i.e., axial and radial rim seals). Recently, Savov and Atkins proposed an alternative approach, where turbulent diffusion is used to model ingress as a viscous problem while ignoring the inertial effects.
This project aims to develop a new Control Volume Model for the prediction of ingress by uniting the Orifice Model and the viscous terms of turbulent diffusion model which are currently ignored by the Bath approach. The new model is expected to solve the problem of the pressure difference prediction while maintaining good agreement with concentration based effectiveness measurements. The student will combine the experimental and theoretical parts for a better understanding and interpretation of the results. Furthermore, it is desirable to involve CFD for validation purposes, however, at this stage, it is undecided to what extent.
At the start of the project the student will cover the available literature around ingress, while conducting experiments in the 1.5-stage turbine rig with the supervision of an experienced operator; later the student will be the main operator of the rig. The second phase includes the collection of pressure distribution and concentration effectiveness data which will inform the modelling. A parametric study of Siemens specific rim seal design will then be conducted. Finally, during the third phase, which overlaps with the second, the student and the supervisor will incorporate the experimental test results into the new theoretical model for validation.
Throughout the project, it is expected that colleagues from the Turbomachinery Research Centre (TRC) will be involved for the validation and verification of the model by comparing their results and improving their models and codes. This will constitute a 3.5 - year PhD programme and by 2022 a new Control Volume Model will be established.