Modern turbofan engines are designed with pressure ratios as high as 40 and with bypass ratios in excess of 8. Multiple spools with substantial radial offset are essential to achieve the desired design and off-design performance and s-shaped ducts are necessary to deliver the flow from the exit of a turbomachine to the inlet of the following one in the minimum possible space to reduce the weight and the size of the gas turbine, without the risk of incurring in flow separation. In the absence of practical design rules or performance correlations for annular s-shaped ducts, a practical design approach could be the use of Computational Fluid Dynamics (CFD) for performance evaluation together with optimisation techniques for seeking the best design. Numerical optimisation methods (either deterministic or stochastic) can be linked to CFD solvers to provide a more thorough exploration of the design space, trying to produce a better design than the datum one, subject to a number of constraints. This paper reports the development and testing of an automatic framework for design optimisation of s-shaped ducts and its initial application to the optimisation of an axial-symmetric inter-compressor duct.

This paper first reviews the application of design optimization techniques to axial compressor blade design, commenting on the feasibility of this approach for highly constrained, highly dimensional problems. It then analyzes an innovative method to characterize the design space, based on proper orthogonal decomposition (POD). The method was used to generate a reduced order model (ROM) from the true complex system, and revealed its potential to find optimum solutions in selected regions of the design space.