Improving Turbine Efficiency by Combining the Effects of Rim Seals and End-wall Contours in the Presence of Purge Flow

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 new environmental legislation stimulating reduced CO2 emissions. Competition within the industry and the external pressure from government has compelled engine manufacturers to produce ever cleaner and more 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 to optimise efficiency, modern gas turbines operate at temperatures far beyond the metallurgical limit of the components (vanes, blades, seals and discs). The efficiency is influenced by the internal-air systems which provide cooling and sealing air. Purge (or sealing) flow is used to prevent the ingress of hot, mainstream gas through rim-seal gaps into the wheel-space between the turbine disc (rotor) and its adjacent casing (stator). The mixing between the egress of this purge flow and the mainstream gases alters the behaviour of the secondary flow-field near the hub endwall and results in a deterioration of aerodynamic performance. Designers use non-axisymmetric end-wall contouring (EWC) and leading-edge fillets to influence the static pressure field and guide the secondary end-wall flow to reduce losses. The interaction between the purge and mainstream flows, and its influence on the secondary flow-field, is complex and unsteady; further, designers need to ensure that any improvements resulting from EWC do not detrimentally affect the performance of the rim seal or compromise the mechanical integrity of turbine components.

Industry is moving towards a combined design of the rim-seal geometry, seal-clearance profile and mainstream end-wall contours, principally through the use of computational fluid dynamics (CFD). The proposed project will build a new experimental facility specifically designed to investigate the fundamental nature of the egress-mainstream interaction. The facility will feature independently interchangeable components: EWC profiles, blade-fillet geometries, rim-seals and rim-seal exit profiles. The research programme will employ the recently funded Versatile Fluid Measurement System (VFMS), awarded to the University of Bath by the EPSRC Strategic Equipment Panel in 2014. The unique VFMS will provide quantitative flow visualisation, for the first time, tracking the plumes of egress in three dimensions as it passes through the downstream rotor and the unsteady velocity field. Simultaneous measurements of the rim-seal effectiveness will ensure that performance benefits are not achieved at the expense of increased ingress.

The data will be used to validate a complementary CFD programme conducted at Siemens, with the aim to gain fundamental insight into the governing fluid-dynamics and to translate new design concepts to the engine. The experimental and computational programmes are symbiotic in nature with academia and industry working together to translate new ideas and understanding into future engines.

The impact of this research will benefit the economy and society. Since 2008 the research at the University of Bath has had significant impact at Siemens Industrial Turbomachinery, on its workforce in Lincoln and the local and wider UK economy. At Lincoln, 1,600 employees design, manufacture and maintain gas turbines in the power ranges from 5 MW to 15 MW. The majority of the production is exported, with over 3500 engines in operation in 90+ countries around the world. In addition to the direct investment and employment within Lincoln, where salaries from the high-value employment bring over £48M into the local economy, the business generated by Lincoln supports a UK supply chain that provides goods and services worth over £160M. 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 performance indicator to customers.
The direct impact of this research programme will be to change and improve the design practices used at the company. More broadly, the impact of this research will contribute significantly to the company's current level of technology and competitiveness in the power generation industry. The financial benefit of this impact (the improvement in thermal efficiency as a result of the expected 1.4% increase in stage efficiency) is expected to be as follows [1].

1) With this new technology introduced into the engine, the thermal efficiency gain will reduce engine temperature at a fixed power level thereby offering increased component life and reduced service costs, or increased power for a fixed operating temperature. Based on a 15 MW power generation engine typically costing £470/KW the increased power available represents a product cost reduction of over £70K. The reduced product cost/KW, a key market measure, will increase the company's competitiveness against it rivals. This increase in competitiveness will lead to additional engine sales, where each additional sale will lead to an increase in gross profit margin of £500K to £1m.
2) The reduction in the level of ingress into the disc cavities extends the creep life and hence serviceability of the discs and hot blades. These components represent a major factor in the service costs paid by an operator and hence represent a key element in the long term service agreements the company sells as part of its service and support package. Increasing the life of these parts therefore makes the company's service plans more attractive to the customer and secures the jobs in its service organisation. Each additional long term service agreement secured has a sale value between £250K and £500K per year for 5 to 10 years.
3) The improved thermal efficiency performance of 1% 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 CO2) by over 430 tonnes per year. This is equivalent to off-setting the average car running over 2 million miles. Over a typical engine life total reductions will amount to over 8600 tonnes.

A great wealth of quantitative flow visualisation and sealing effectiveness data from various generic parametric studies will provide a considerable database of fluid mechanics measurements which can be rapidly exploited by gas turbine manufacturers in both the power and aerospace sectors. The data will be particularly useful for the engine designer and also be critically important for code validation by the CFD community. New experimental techniques will be developed, which will be of great interest to experimenters in the areas of fluid dynamics and heat transfer. The CO2-PLIF technique, applied for the first time to a rotating system, and the demonstration of V3V in air, will be world-leading.

[1] Figures supplied by Siemens, Lincoln