Study of Gas Turbine Ingress using Computational Fluid Dynamics

Abstract

The ingestion of hot mainstream gas into the wheel-space between the rotor and stator discs is one of the most important internal cooling problems for gas turbine designers. To solve this problem, engineers design a rim seal at the periphery of wheel-space and direct a sealing flow from the internal cooling system to prevent ingress. The main aim of this thesis is to build a simple computational model to predict the sealing effectiveness of externally-induced ingress for engine designers. The axisymmetric model represents a gas turbine wheel-space and provides useful information related to the fluid dynamics and heat transfer in the wheel-space. At the same time, this model saves much computation time and cost for engine designers who currently use complex and time-consuming 3D models.

The computational model in this thesis is called the prescribed ingestion model. Steady simulations are carried out using the commercial CFD code, ANSYS CFX with meshes built using ICEM CFD. Boundary conditions are applied at the ingress inlet of the model using experimental measurements and a mass-based averaging procedure. Computational parameters such as rotational Reynolds number, non-dimensional sealing flow rate and thermal conditions on the rotor are selected to investigate the fluid dynamics and heat transfer at typical experimental rig operating conditions. Different rim seal geometries are investigated and results are compared with experimental data.

In addition to the prescribed ingestion model, two typical axisymmetric rotor-stator system models without ingress are established. The aim of these rotor-stator models is to investigate the fluid dynamics and heat transfer of the wheel-space in the situation without ingress. The effects of geometry and turbulence model also are studied in these simulations. Most results from these simulations are in good agreement with experimental data from the literature, which enhances confidence in the prescribed ingestion model.

For the prescribed ingestion model, the axial-clearance seal (the simplest rim-seal geometry) is first simulated for externally-induced ingress. The sealing effectiveness (on both the stator and rotor), fluid dynamics (in terms of swirl ratio and velocity profiles) and heat transfer (in terms of Nusselt number) are investigated and are shown to be in reasonable agreement with the experimental results from a rig at the University of Bath. In these computations, the mass-weighted average boundary condition is tested with the momentum-weighted average boundary conditions. Different layouts of ingress inlet and egress outlet are also tested and the optimal one is applied to other seal geometries. For heat transfer simulations, different thermal wall boundary conditions are investigated to understand the effect of these conditions on the distribution of Nusselt number on the rotor.

In addition to the axial-clearance seal, three other seals are presented in this thesis: a radial-clearance seal, a double axial-clearance seal, and a radial-axial-clearance combination seal. In accordance with the axial-clearance seal, the sealing effectiveness and fluid dynamics in the wheel-space are investigated for these seals and compared with experimental data. The performance of these four seal geometries is ranked.

It is suggested that the prescribed ingestion model is a compact, relatively straight-forward tool for engine designers. It allows the designer insight into the fluid dynamics and heat transfer of rim seals, and supports the experimental data collected at the University of Bath.

Date of Award	31 Dec 2013
Original language	English
Awarding Institution	University of Bath
Supervisor	Gary Lock (Supervisor) & Michael Wilson (Supervisor)