Impact of a cooled cooling air system on the external aerodynamics of a gas turbine combustion system

To manage the increasing turbine temperatures of future gas turbines a cooled-cooling air system has been proposed. In such a system some of the compressor efflux is diverted for additional cooling in a heat exchanger located in the by-pass duct. The cooled air must then be returned, across the main gas path, to the engine core for use in component cooling. One option is do this within the combustor module and two methods are examined in the current paper; via simple transfer pipes within the dump region or via radial struts in the pre-diffuser. This paper presents an experimental investigation to examine the aerodynamic impact these have on the combustion system external aerodynamics. This included the use of a fully annular, isothermal test facility incorporating a bespoke 1.5 stage axial compressor, engine representative outlet guide vanes, pre-diffuser and combustor geometry. Area traverses of a miniature five-hole probe were conducted at various locations within the combustion system providing information on both flow uniformity and total pressure loss. The results show that, compared to a datum configuration, the addition of transfer pipes had minimal aerodynamic impact in terms of flow structure, distribution and total pressure loss. However, the inclusion of pre-diffuser struts had a notable impact increasing the pre-diffuser loss by a third and consequently the overall system loss by an unacceptable 40%