Experimental measurements of the effect of hot gas ingestion on rotor discs

The highly adaptable gas turbine engine is one of the most frequently utilised sources of power in the modern age. Derivatives exist in applications ranging from the generation of electric power and jet propulsion to the supply of compressed air and heat. The world market today is driven by increasing fuel costs and reducing CO2 emissions brought about from new environmental legislation. Competition within the industry and the external pressure from government has compelled 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 cycle efficiency. High efficiency depends on a high turbine inlet temperature and an appropriately high pressure ratio across the compressor. In order for elevated mainstream gas temperatures (can be as high as 1800 K) to be viable, the turbine components must be protected from these temperatures, far above their metallurgical limit. Relatively cool air (typically around 800 K) bled from the compressor is used to extend the life of these turbine components, which would otherwise be limited by creep, oxidation or by thermal fatigue. However, cooling air is expensive: approximately 20% of the compressed air is used for cooling and not combustion. Effective use of the cooling air is therefore key to designing an efficient gas turbine.

One of the most important cooling-air problems facing gas turbine designers today is the ingestion of hot mainstream gases into wheel-spaces between the turbine discs and their adjacent casings. 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 (with an associated increase in fuel consumption and CO2 emissions), and too little can cause serious overheating, resulting in damage to the turbine rim, blade roots and disc: the correct sealing balance is therefore of critical importance.

A recently completed experimental programme (funded in part by EPSRC grant EP/G096107/1) has successfully modelled ingestion into a single-stage gas turbine (referred to here as the 'single-stage facility'). The programme had great success both in terms of industrial impact and academic publication, improving the design of gas turbine rim seals through extensive experimental measurements made on the stationary turbine disc (stator) and in the wheel-space between the discs. This new research will build on the previous achievements by transforming the single-stage facility to allow for experimental heat transfer measurements to be made on the rotating disc (rotor). Surface temperatures measured in a transient experiment using new infra-red technology will be used in conjunction with new specifically developed analysis techniques to increase the accuracy of adiabatic wall temperature measurements. The data will directly lead to adiabatic sealing effectiveness distributions on the rotor. The experiments, which would be conducted under fluid dynamic conditions representative of those found in engines, will lead to crucially important rotor metal temperatures at engine operation conditions for use by the designer. This would be a world first: many research workers have measured the minimum coolant flow rate necessary to prevent ingress using measurements on the stator, but no one has simultaneously measured the heat transfer from the gas to the rotating turbine disc after the gas has entered the wheel-space.

As a result of the successful completion of this experimental research, an extensive database of rotor disc heat transfer data will be used to improve the design of gas turbine secondary air systems at Siemens. The UK-based company will receive a competitive advantage, both in exploiting the practically-useful data generated from the research and also in significantly influencing the 1D design methodology within the company.

The impact of this research will benefit the economy and society. The UK power generation industry, represented directly by Siemens Industrial Turbomachinery and its workforce in Lincoln, will be the principal beneficiary. At Lincoln, 1,600 personnel are employed to design, manufacture and maintain small gas turbines (5-15 MW) for worldwide distribution. Owing to the competitiveness of the gas turbine market, small service and performance changes can have significant and direct impact on unit sales. These changes strongly influence the life-cycle cost of the products which is a major performance indicator to customers.

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

1. The development of this new technology and its introduction into the engine will lead to an anticipated additional engine sale per year. With 5% profit margin, this yields a net profit increase of £210k per year for the company.

2. The extensive database of experimental data for a wide range of rim-seal configurations will allow accurate predictions of the level of ingress in the engine. These predictions will directly 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 thermal effects of ingestion on the rotor disc 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 an extensive database of practically-useful data which can be rapidly exploited by gas turbine manufacturers in both the power and aerospace sectors. The experimental data will be generic and should also benefit other experimental research workers - in particular those making heat transfer and fluid dynamic measurements - and will also be critically important for code validation by the computational fluid dynamics community.

There is impact from this research in terms of people. There will be impact in terms of training, at PhD level, a research worker who is likely to progress to employment in the UK gas turbine industry and/or academia.

[1] Figures supplied by Siemens Industrial Turbomachinery, Lincoln.