Improvement of High Pressure Turbine Air Systems by De-Swirl Nozzles

Conventional two-stage high pressure turbine (HPT) air system concepts usually are based on exclusive use of compressor delivery air to cool HPT blades and vanes and to seal the gaps between rotors and stators. This air is expensive in terms of engine efficiency, since work has been done on it from all stages of the compressor and the air is lost for the main thermodynamic engine cycle. It is also very hot, leading to strong thermal loading of the turbine material. Improvements in this area thus lead to an immediate reduction of fuel consumption and increased cycle life of the turbine discs. On static components, it is common practice to use pre-swirl nozzles in order to reduce the relative total temperature of downstream disc and blades. To feed the interstage cavity between both HPT discs, the air is transferred through the first rotor disc or drive arm. In a conventional system, the air is passed on through straight holes, thus no benefit is taken from the internal energy potential of the cooling air, where work output from the flow would lead to an immediate air temperature reduction. The advanced air system presented in this paper uses de-swirl nozzles in the rotating part to extract energy from the fluid. By this, the air temperature for the downstream static part is reduced on the one hand and in addition, the overall turbine efficiency is increased due to the contribution from the fluid. This paper will cover the effects of an air system design change from a conventional to an advanced HPT air system on the Rolls-Royce BR715 aeroengine based on numerical analysis and test data. An overview of the change to the flow field and prospect of current research programmes in this field will be given.