Flow Control and Shape Optimization of Intermediate Turbine Ducts for Turbofan Engines

Doctoral thesis, 2008

Demands on improved efficiency, reduced emissions and lowered noise levels cause a strive towards high by-pass ratios of commercial turbofan engines. This results in an increased radial offset between the high- and low-pressure turbines. Thus the design of the intermediate turbine duct connecting the two turbines becomes more challenging. In this thesis the main focus is to explore techniques that enable design of turbine ducts with reduced length, larger radial offset or increased diffusion, so-called aggressive ducts. Response surface methodology and design of experiments techniques have been adopted to perform duct shape optimization based on CFD. A novel geometry parameterization suitable for both axisymmetric and non-axisymmetric endwalls, based on axial orthogonal polynomials and circumferential harmonics, has been introduced. Results show that shape optimization has the potential to reduce duct loss significantly. Flow control could be applied to avoid separation in highly aggressive ducts. Vortex generators are well-known, reliable and cost-effective passive flow control devices and have therefore been used in this work. Resolving the small scales of vortex generators requires fine grids and time consuming computations. Therefore a computational body-force model was developed and validated. The baseline separation in a very aggressive turbine duct was suppressed by an optimized vortex generator installation, defined using the body-force model. Steady RANS analyzes with different two-equation eddy viscosity models have been used to assess the aerothermal behavior of a state-of-the-art turbine duct. Detailed experimental data has been used to validate the CFD approach adopted throughout this work.