Turbine blade cooling systems are among the technologies being developed at the Osney Thermofluids Laboratory, part of Oxford University’s Department of Engineering Science, which officially opened last September.
Jet engine efficiency largely depends on the design of the hottest part at the core of the engine. The faster the engine spins, and the hotter the gas, the better the engine performs. However, the gas is so hot, over 1600°C, that it approaches the melting point of the turbine blades. New ways of cooling these blades are needed if engines are to operate at higher temperatures and burn less fuel.
Osney says its new lab combines wind tunnels, test facilities, measurement techniques, and computational modelling to understand and improve current jet engines, with an emphasis on the core turbine. One technique developed in the lab is used to perform careful measurements of the heat transfer characteristics of new blade designs.
The Turbine Test Facility was relocated to the new laboratory from QinetiQ Farnborough with funding provided by the South East England Development Agency. This flagship facility enables tests to be carried out on blades rotating at realistic speeds, reproducing the conditions in a real turbine.
In work funded by the EC and EPSRC, the Oxford team is measuring heat transfer on the turbine tip in the presence of "hot-streaks" pockets of excessively hot fluid that form due to partial mixing of the combustion fluid.
"The most difficult part of the program was developing a method for mounting a sensor that could withstand the 30,000 g acceleration that is experienced at the tip of the rotor blade. The clearance gap between the rotor and shroud was less than a millimeter, so very small sensors were required," said Kam Chana, Commercial and Technical Director, Oxford Turbine Research Facility.
"By analyzing the mixing process in a low-speed wind tunnel we managed to calculate how to inject flow that mixed to give the same flow pattern as in a real combustor. The trick was then to build a simulator which operated at very high pressure so we could get the equipment into the test facility," said Tom Povey of Oxford’s Department of Engineering Science.
Using the system the team managed to simulate very strong hot streaks for the first time, generating data that is now being used by manufacturers during the engine design process. The team is currently working on methods of cooling this complex part of the engine.
"Even today the design of turbine cooling systems remains one of the most challenging processes in engine development," said Paul Stein, Chief Scientific Officer, Rolls-Royce. "The Oxford UTC team have developed new strategies for laboratory testing which simulate the complex flows in hot turbine parts of jet engines very effectively."
Oxford University research has also already resulted in an accurate low-cost technique for measuring jet engine capacity, the flow-path area of the vanes in the core of the engine that is vital in ensuring that an engine’s components work together efficiently.
Instead of running engines continuously to measure the capacity, which is very costly and time-consuming, Povey developed an approach in which full scale engine hardware can be tested in just 10 s, reducing the test time by a factor of approximately 1000.
"This reduction in time means that the total energy required to run the experiment is also reduced by a factor of one thousand," said Povey. "The exciting thing about this research is that we have demonstrated accuracies that allow us to measure and understand even very small changes in capacity."