14th European Conference on Turbomachinery Fluid dynamics & Thermodynamics
We quantify the sensitivity of turbine acoustic impedance to aerodynamic design parameters, using an extension of the semi-actuator disk model for cambered blades. Acoustic impedance boundary conditions are important for predicting thermoacoustic stability of gas turbine combustors. Non-linear time-domain computations of turbine vane and stage cascades with acoustic forcing provide validation data. Discretising cambered aerofoils into a series of connected disks improves reflection coefficient predictions, reducing error by an order of magnitude compared to a flat-plate assumption. A parametric study of turbine stage designs using the analytical model shows acoustic impedance is a weak function of degree of reaction and polytropic efficiency. The design parameter with the strongest influence is flow coefficient, followed by Mach number and axial velocity ratio. We provide the combustion engineer with improved tools to predict impedance boundary conditions, and suggest thermoacoustic stability is most likely to be compromised by a change to turbine flow coefficient.