Fluid Mechanics of Ingress in a 1.5-Stage Gas Turbine Experimental Facility

Abstract

The gas turbine engine is undoubtedly one of the most versatile engines, with applications ranging from aircraft, maritime and locomotive propulsion to electricity generation. Increasing fuel costs and strict environmental legislation demand increasingly efficient engines. Engine efficiency can be improved by operating at a turbine entry temperature (TET) beyond the melting point of the turbine components. To enable this, compressor flow is diverted to the turbine for cooling and sealing the cavities (or wheel-spaces) formed between adjacent stator and rotor discs. Insufficient sealing flow reduces component operating life and superfluous use reduces the benefits of increased TET.Rim-seals, fitted at the periphery of the rotor discs, are used to minimise the ingress of hot gas and therefore reduce the amount of sealing flow required. These rim-seals are designed using computational fluid dynamics (CFD) software which require experimental validation in facilities operating at engine-simulated conditions as well as experimental rigs operating at low Reynolds number.This thesis presents: (i) the design, assembly and commissioning of a new 1.5-stage turbine experimental facility, (ii) measurements of ingress through generic seals in an upstream and downstream wheel-space and (iii) parametric studies of the performance of eight Siemens proprietary seals. The new 1.5-stage test facility is designed to investigate ingress into the wheel-spaces upstream and downstream of a rotor disc. The flow structure inside the wheel-spaces is representative of the one found in engines with the rig operating at incompressible flow conditions, far removed from the harsh environment of the engine which is not conducive to experimental measurements. The test facility features interchangeable rim-seal components, offering significant flexibility and expediency in terms of data collection over a wide range of sealing-flow rates. The rig was specifically designed to enable an efficient method of ranking and quantifying the performance of generic and engine-specific seal geometries.The radial variation of concentration effectiveness based on carbon dioxide gas concentration, pressure and swirl is measured to explore, for the first time, the flow structure in both the upstream and downstream wheel-spaces. For both single and double radial-clearance seals, the measurements show that the concentration effectiveness in the core is equal to that on the stator and that both distributions are virtually invariant with radius. These measurements confirm that mixing between ingress and egress is essentially complete immediately after the ingested fluid enters the wheel-space, and that the fluid from the boundary-layer on the stator is the source of that in the core. The swirl in the core is shown to determine the radial distribution of pressure in the wheel-space.The variation of concentration effectiveness with sealing flow rate in the upstream and the downstream wheel-spaces is obtained and found to be independent of rotational Reynolds number for a common flow coefficient in the mainstream annulus. A simple theoretical orifice model was fitted to the experimental data showing good agreement between theory and experiment for all cases. This observation is of great significance as it demonstrates that the orifice model can accurately capture the variation of effectiveness with sealing flow rate in both wheel-spaces. The driving mechanism for ingress in the downstream wheel-space is identified using concentration effectiveness measurements taken in a rotationally-induced (RI) ingress experiment. The measurements of the RI test were found to be equal to those of the externally-induced (EI) ingress test, showing that RI ingress dominates in the downstream wheel-space.Three parametric studies including eight Siemens proprietary seals were performed in the downstream wheel-space using measurements of carbon dioxide concentration, swirl ratio and pressure. In the first study it was proven that the inter-blade gaps in engines have a negative effect on the performance of the seals. In the second study it was shown that a significant decrease in ingress can be achieved by using a compound angel-wing/radial-clearance seal as opposed to a simple angel-wing seal. In the third study the negative effect of the introduction of a buffer cavity in the design of seals for the downstream wheel-space was revealed. In all cases, the majority of the wheel-space was dominated by the expected flow structure and good agreement between the theoretical orifice model and experiments was observed despite the complex geometry of the seals.

Date of Award	1 Mar 2017
Original language	English
Awarding Institution	University of Bath
Supervisor	Carl Sangan (Supervisor) & Gary Lock (Supervisor)