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.