Experimental and theoretical modelling of hot-gas ingestion through gas-turbine rim seals

This proposal is in the EPSRC portfolio research area of fluid dynamics and aerodynamics (maintained) and will contribute to the experimental capability and measurement instrumentation knowledge base of the science community. The primary industrial impact of the research will be improvement in energy efficiency, which is one element of the EPSRC energy theme.

The gas turbine engine is an adaptable source of power and has been used for a wide variety of applications, ranging from the generation of electric power and jet propulsion to the supply of compressed air and heat. Competition within the industry and, more recently, environmental legislation from government have exerted pressure on engine manufacturers to produce ever more cleaner and efficient products.

The most important parameter in governing engine performance and life cycle operating costs is the overall efficiency. High cycle efficiency depends on a high turbine entry temperature and an appropriately high pressure ratio across the compressor. The life of turbine components (vanes, blades and discs) at these hot temperatures is limited primarily by creep, oxidation or by thermal fatigue. It is only possible for the turbine to operate using these elevated mainstream gas temperatures (as hot as 1800 K) because its components are protected by relatively cool air (typically 800 K) taken from the compressor. However, this cooling comes at a cost: as much as 15-25% of the compressor air bypasses combustion to provide the required coolant to the combustor and turbine stages.

Ingress is one of the most important of the cooling-air problems facing engine designers, and considerable international research effort has been devoted to finding acceptable design criteria. Ingress occurs when hot gas from the mainstream gas path is ingested into the wheel-space between the turbine disc and its adjacent casing. Rim seals are fitted at the periphery of the system, and a sealing flow of coolant is used to reduce or prevent ingress. However, too much sealing air reduces the engine efficiency, and too little can cause serious overheating, resulting in damage to the turbine rim and blade roots.

It is proposed to build a new fully-instrumented rotating-disc rig to measure the flow structure and heat transfer characteristics of hot gas ingress in an engine-representative model of gas-turbine wheel-spaces. An annular single-stage turbine will create an unsteady circumferential distribution of pressure, which in turn will create the ingestion of hot air in the wheel-spaces. The rig will be designed specifically for optical access, with transparent rotating and stationary discs coated with thermochromic liquid crystal and illuminated by a strobe light synchronised to the disc frequency. This will be a new, bold application of the advanced thermal-imaging technology developed at Bath and will provide both qualitative 'thermal visualisation' and quantitative measurements of heat transfer coefficient in the regions on the rotating and stationary surfaces affected by ingress. Miniature unsteady pressure transducers, pressure taps, pitot tubes, fast-response thermocouples and concentration probes will also be used inside the seal annulus and in the upstream and downstream wheel-spaces.

In parallel with the experimental programme, new theoretical models developed at Bath will be used extensively in the analysis and interpretation of the experimental data obtained from the new rig. These generic models will be of use to any gas turbine manufacturer, and here this will be demonstrated by specifically translating them into the engine-design methodology used at Siemens. The research will generate unique and practically-useful data which can be rapidly exploited. The successful completion and implementation of this research through improved secondary air system design should result in a competitive advantage for the UK-based company.

The impact of this research will benefit the economy and society. The primary beneficiary is the UK power generation industry, represented directly by Siemens Industrial Turbomachinery and its workforce in Lincoln. At Lincoln, 1,600 employees design, manufacture and maintain small gas turbines (5-15 MW) in the world market. In this very competitive market, small service and performance changes can have significant impact directly on sales as these strongly influence the life-cycle cost of the products which is a major key performance indicator to customers.

Siemens anticipate that the new rim-seal component expected from this research will improve the thermal efficiency of the gas turbine by 0.4%. The tangible impact is then demonstrated by the following anticipated direct benefits [1].

1. With this new technology introduced into the engine, it is estimated that it will lead to an additional engine sale per year. With 5% profit margin, this yields a net profit increase of £210k per year for the company.

2. The design tool will allow accurate predictions of the level of ingress, which will lead to a reduced number of turbine-disc validation tests using thermal paint. It is estimated there will be one fewer test per three-year period, saving £60k, i.e. a cost saving of £20k per year.

3. The reduction in the level of ingress into the disc cavities will extend the creep life of the stage 2 rotor blade by approximately 20%. The stage 2 rotor blades are limited by the number of start cycles and running hours. This is expected to influence ten engines per year with the stage 2 rotor blade rows costing £25k. This is an expected cost saving of 10 x £25k x 0.2 = £50k per year.

4. The improved thermal efficiency performance of 0.4% will deliver large fuel savings and greener energy. The estimated savings for a single 12.9 MW generator set is £73,000 per engine per year. Reduced fuel burn will reduce environmentally-unfriendly emissions (mainly carbon-dioxide).

There is impact from this research in terms of knowledge. The research will generate practically-useful data which can be rapidly exploited. The programme will demonstrate how, using theoretical models, ingress data obtained from experimental rigs can be extrapolated to engine-operating conditions. The experimental data and the theoretical models will be generic and will be of benefit to gas turbine manufacturers in both the power and aerospace sectors. In addition, the work should benefit other experimental research workers - in particular those making heat transfer and fluid dynamic measurements - and the data generated should be useful for code validation by the computational fluid dynamics community.

There is impact from this research in terms of people. The Bath group has a strong record of research students moving on to employment in the UK gas turbine industry and academia and there will be additional impact in terms of training, at PhD and post-doctoral level, five research workers.

[1] Figures supplied by John Maltson, Senior Engineer, Siemens Industrial Turbomachinery, Lincoln.