Final work : CFD Optimization for improved compressor efficiency and reliability

Abstract

High Cycle Fatigue (HCF) issues associated with forced response excitation in gas turbine compressors are of major concern given the modern design trends with complex airfoil stacking profiles and increased loading. Airfoil clocking has shown the potential to substantially reduce the unsteady loading in embedded or downstream rotor blades when the stator vanes have identical counts. This work aims to the development of a numerical optimization process using vane clocking to minimize Haigh utilization in rotor blades while increasing or maintaining performance.

Since the viscous vortical structures generated by the vanes not only influence the pressure field of the adjacent row but also the stages further downstream, different resonance crossings can be excited by the same engine. Thus, this approach is formulated by including the Haigh utilization for each excited crossing as objective functions and weighting factors are given according to the baseline configuration results. The forced response analysis is modeled using one-way fluid-structure interaction (FSI). The aeroelastic process is integrated into an Evolutionary algorithm (EA) based on the strategy of selection and inheritance to minimize the objective function within a region of interest. The generation of new members uses Kriging as a surrogate model. The high-fidelity process uses Non-linear Harmonic Balance (HB) simulations to evaluate the design parameters suggested for each generation.

The process implemented was tested using a conceptual design of a gas turbine compressor with multiple stages having the exact vane count. Results were obtained once the prediction and member results converged to the same design parameter values. The baseline configuration and the highest and lowest members in the Pareto rank were compared. The results evidenced that no Haigh utilization reduction for a specific crossing is achieved without increasing others. The weighting factors determine the trade-off between objective functions. The flow field exhibits a reduction of the aerodynamic forcing on the embedded rotor when the stator vanes are located such that the maximum contribution from upstream coincides with the minimum stimuli from the vane downstream. Furthermore, the impingement of the upstream stator wake in the downstream stator intensifies the unsteady pressure for stages further downstream. Finally, a decrease of 19.3%, 6.9%, and 14.3% for the three crossings with the highest Haigh utilization at the current configuration was achieved, preserving conflicting crossings under safe levels and without impacting the aerodynamic performance.