An investigation into combined film and internal cooling of turbine blades

The primary users of gas turbines are being impacted by rising fossil fuel prices and stringent government targets for reducing carbon-dioxide emissions. This is putting increasing pressure on gas turbine manufacturers to improve engine efficiencies so that their products remain competitive. One way of improving the efficiency of a gas turbine is to raise the turbine entry temperature (TET). Present-day engines operate with TETs as high as 2000K, which is well above the melting point of the alloys from which first-stage turbine blades are made. Two cooling techniques are employed to prevent damage to the blades from high TETs: film cooling, where a thin film of coolant introduced to the external surface of the blade reduces the driving-temperature for heat transfer; and internal cooling, where coolant is passed through a series of passages within the blade to convect heat from the internal surfaces. The air for this cooling is taken from the compressor at a penalty to engine efficiency: for every 1% of air drawn from the compressor a 1% drop in isentropic efficiency follows.

Relatively few experimental studies have investigated coupled film and internal cooling; consequently there are insufficient published data for validation of the models used to predict blade metal temperatures. There is little margin for error in these predictions: the life of a blade can be reduced by half if the temperature at which it operates is 10K higher than predicted. As a result, blades are often superfluously cooled at the expense of engine efficiency. Validated models would enable blade cooling schemes to be designed with more confidence. This would reduce design conservatism, enabling more efficiently cooled designs with an associated improvement in engine efficiency. It would also reduce the costly risk of re design or in-service replacement of inadequately cooled blades.

The proposed project will design and build a highly-modular rig for obtaining fluid dynamic and heat transfer information on test pieces subjected to coupled film and internal cooling. The rig will make use of the University of Bath's state-of-the-art EPSRC funded Versatile Fluid Measurement System (VFMS), enabling high-precision measurements of heat transfer coefficients and temperatures on the surface of the test pieces, and the concentration field and three component velocities in the fluid volume above the film cooling holes. The flexibility of the facility combined with the unparalleled fidelity of measurement techniques offered through the VFMS will make it a highly novel and extremely useful platform for studying combined film-internal cooling.

Findings from the project will provide unique insight into the fundamental science of the research problem and will supply Siemens - the industrial partner in this proposal - with data to validate their models and inform design methodology. The data will also be made available to workers in the wider gas turbine technical community and academia.

This project will directly benefit the project collaborators. Siemens employee 1500 people at their UK site in Lincoln, making it the biggest engineering employer in the city. The company has recently invested £35 million in expanding its Lincoln operations, a commitment that will see it remain a key economic contributor to the local area for years to come. The site designs and manufactures small to medium size gas turbine engines (5 to 15MW) for the global power generation and oil and gas markets. 1700 engines are currently in operation in over 90 countries.

Small improvements in the efficiency and reliability of an engine can significantly reduce operating costs for the customer, which, in highly challenging markets, offers a competitive advantage that may directly impact sales. Siemens anticipate that the findings from this project will lead to improvements in the thermal analysis methods used to predict blade metal temperatures. The following benefits have been identified [1]:

- Siemens are targeting an additional 2 engine sales per year by 2024 (estimated net profit increase of ~£400k). This project will help improve their blade cooling design methods, identified as a key area for increasing their competitiveness to meet these targets.
- Better blade temperature predictions early in the design cycle will reduce cooling design modifications (saving time and money); it will also reduce the likelihood of a blade entering service with a shorter than predicted life (reducing service costs and engine downtime).
- Cooling schemes can be designed with more confidence, reducing the need for superfluous cooling to account for uncertainties in predictions (the life of a blade can be reduced by half if the temperature at which it operates is 10K higher than predicted). Reductions in the supply of cooling flow to the blades will benefit engine efficiency. This brings direct financial benefit to the engine operator - a 0.1% saving in efficiency would result in a fuel saving of >£15k per annum for a 13MW engine. Reducing fuel burn will also reduce emissions of carbon-dioxide (a clear benefit to society).
- More accurate temperature predictions will reduce the need for expensive component temperature validation testing. A typical thermal paint test for a HP blade design costs up to £60,000; Siemens anticipate that the project will reduce the frequency of these tests, bringing significant cost savings to the company.

These benefits will decrease Siemens' development costs and improve the efficiency and reliability of their engines. This will play a part in ensuring that they remain competitive, helping to secure the future of their UK based business operation in Lincoln (important to the local and national economy).

There is also impact from this project to the wider industrial gas turbine community and academics involved in blade cooling research. An online repository will be setup to enable individuals to access experimental data from the project for validation of their conjugate CFD models (there is currently a lack of published experimental data on this topic). Experiments will be conducted on a test facility that uses state-of-the-art measurement techniques available through the University of Bath's EPSRC funded Versatile Fluid Measurement System. These novel techniques will be of real interest to academics working in the fields of fluid dynamics and heat transfer; it is anticipated that the successful demonstration of these techniques by this project will lead to their uptake by others, facilitating exciting new scientific research.

There will also be impact from this project in terms of people - the project will provide an opportunity for a student to improve their research skills by obtaining a PhD. Subsequent employment is likely to be in the UK (either in turbomachinery or a university research setting), bringing an additional highly skilled worker into the labour market.

[1] Siemens Lincoln