Aerodynamic Design and Optimization of Intermediate Ducts for High By-pass Ratio Jet Engines

Licentiatavhandling, 2006

Demands on improved efficiency and reduced noise levels cause a strive toward high by-pass ratio commercial aero engines. This results in multi-spool aero engines with small high-pressure ratio cores and large fans. The low-pressure system has a lower rotational speed and a larger radius than the high-pressure core system. Hence intermediate S-shaped transition ducts are needed to connect the two systems. In this thesis the main focus is on exploring ways of designing more aggressive intermediate ducts. The use of duct endwall shape optimization based on computational fluid dynamics analysis has been investigated. The response surface methodology combined with design of experiments has been the optimization technique of choice. An efficient geometry parameterization based on orthogonal polynomials as basis functions has been introduced. The results show that significant duct loss reduction is achievable if optimization is included in the design process. Some possible mechanisms for reducing losses and suppressing boundary layer separations are also indicated. As duct designs become more aggressive flow control could be included in the optimization process to avoid separation. Vortex generators are well-known reliable and cost-effective passive flow control devices and have therefore been investigated in this work. Resolving the small scale devices require fine grids and thus time consuming computations. For this reason a tuning-free body-force vortex generator model was developed and tested. In the case studied an 80% reduction in the number of grid cells was possible using the model.