The efficiency of a gas turbine engine is maximised by operating the gas turbine engine at the highest temperature and pressure possible. However, this objective is limited by the temperature capability of the materials from which the components of the gas turbine engine are manufactured. The components, combustion chamber components and turbine components, of the gas turbine engine operating at the highest temperatures are generally cooled using air bled from a compressor of the gas turbine engine to increase the operating temperature limit. However, the air bled from the compressor is already at a high pressure and a high temperature and thus the potential for cooling these components is limited.
The air bled from the compressor may be cooled prior to being supplied to the components to be cooled, so that the maximum operating temperature of the gas turbine engine may be increased and consequently the efficiency increased. The air bled from the compressor and cooled in this manner is generally known as cooled cooling air. A typical arrangement supplies air bled from the compressor of the gas turbine engine through a duct to a heat exchanger and then supplies the cooled air through another duct or ducts to the components of the gas turbine engine to be cooled. The heat exchanger may transfer heat from the air bled from the compressor to air in a bypass duct of a turbofan gas turbine engine or may transfer heat to fuel supplied to the gas turbine engine.
A turbine disc, for example a high pressure turbine disc, of the gas turbine engine is one of the components of the gas turbine engine which requires cooling. The cooled cooling air must be supplied across the flow path of the core of the gas turbine engine as part of the ducting process. One means of accomplishing this is to flow the air through hollow vanes, or hollow struts, across the compressor exit diffuser. The hollow vanes, or hollow struts, may be the compressor high pressure outlet guide vanes or structural struts.
Large rearward loads are typically exerted by the structure used to mount the high pressure turbine nozzle guide vanes which is transferred by the compressor high pressure outlet guide vanes or structural struts into the engine casing. Thus the high pressure outlet guide vanes or structural struts perform a major structural duty. The cooled cooling air is significantly cooler than the air supplied from the compressor exit diffuser to the combustion chamber of the gas turbine engine. Thus, there would be a large temperature differential across the wall of any hollow vanes, or hollow struts, extending across the compressor exit diffuser. This temperature differential will induce further stresses into structural vanes, or structural struts, which are already carrying significant structural loads from the high pressure nozzle guide vanes.
These thermally induced stresses may significantly reduce the operating life of the compressor high pressure outlet guide vanes or structural struts in the compressor exit diffuser. The provision of a manifold radially outwardly of the high pressure outlet guide vanes may make the system complicated and difficult to manufacture and may result in the manufacture of a separate ring of high pressure outlet guide vanes and a separate ring of hollow struts which are bolted together and this may increase the weight of the gas turbine engine and may introduce a step in the inner and/or outer surfaces of the compressor exit diffuser which may produce a loss of aerodynamic performance.
Therefore the present invention seeks to provide a novel gas turbine engine cooling arrangement which reduces or overcomes the above mentioned problem.