Impact of fuel supply impedance on combustion stability of gas turbines

In the development of gas turbines the prevention of thermoacoustic instabilities plays an important role. The present study analyzes the influence of the acoustic impedance of the fuel supply system on combustion stability in a generic configuration representative of practical lean-premix combustors. Transient Computational Fluid Dynamics (CFD) of turbulent reacting flow and system identification (SI) are combined to obtain a description of the combustion dynamics in terms of two flame transfer functions, which describe the response of heat release rate to fluctuations of velocity and equivalence ratio, respectively. In this way, the mixing and transport of the fuel from the injector to the flame, the kinematic response of the flame to upstream flow fluctuations, and combined effects like the perturbation of flame speed and position due to equivalence ratio perturbations are all captured. The flame transfer functions obtained are combined with a network model for the system acoustics in such a way that results from a single CFD simulation can be used to investigate a wide variety of combustor and fuel supply configurations in a quantitative and very efficient manner. It is demonstrated that a change of the fuel supply impedance can significantly influence the amplitude of equivalence ratio fluctuations as well as the relative phase of the two transfer functions, and thereby provide a means for control of combustion instabilities.