The aerodynamic design of the low pressure air delivery ducts for a cooled cooling air system

As aero gas turbines strive for higher efficiencies and reduced fuel burn, the trend is for engine overall pressure ratio to increase. This means that engine cycle temperatures will increase and that cooling of various engine components, for example the high pressure turbine, is becoming more difficult. One solution is to employ a cooled cooling air system where some of the compressor efflux is diverted for additional cooling in a heat exchanger fed by air sourced from the by-pass duct. Design of the ducting to feed the heat exchangers with coolant air is challenging as it must route the air through the scenery present in the existing engine architecture which leads to a convoluted and highly curved system. Numerical predictions using ANSYS Fluent demonstrated that a baseline design was unsuitable due to large amounts of flow separation in the proximity of the heat exchangers. This paper is mainly concerned with the aerodynamic design of this component of the duct. In order to produce a viable aerodynamic solution a numerical design methodology was developed which significantly enhances and accelerates the design cycle. This used a Design of Experiments approach linked to an interactive design tool which parametrically controlled the duct geometry. Following an iterative process, individually optimized 2D designs were numerically assessed using ANSYS Fluent. These designs were then fed into an interactive 3D model in order to generate a final aerodynamic definition of the ducting. Further CFD predictions were then carried out to confirm the suitability of the design. RANS CFD solutions, generated, using a Reynolds stress turbulence model, suggested that the new design presented significant improvement in terms of diffusion and flow uniformity.