Combustors of modern gas turbines for power generation and mechanical drive are predominantly operated in premixed mode, which is sensitive to coupling between flame dynamics and combustor acoustics. In practice, combustor flames tend to drive instabilities at certain eigenfrequencies of the systems, according to the classical Rayleigh criterion. In order to guarantee combustor stability in the entire operation range of the engine it has to be avoided under all circumstances that the flame excites the system beyond its damping potential. One option to accomplish this is to provide sufficient damping capabilities of the combustor system so that the decay of acoustic energy inside the system always exceeds the excitation provided by the flame. Experimental methods for the determination of combustor damping rates therefore may become a valuable tool for combustor design in the future. In the past, methods with different accuracy, complexity and capabilities have been developed to gain experimental access to decay rates of the acoustic energy inside combustor systems. In this study we compare accuracy and capabilities of three different time-domain methods that allow the determination of pressure decay rates from experimental dynamic pressure traces: A simple exponential fit to the measured dynamic pressure, a method based on the decay of acoustic energy and a newly developed statistical method are examined. In the first step, the methods are tested using artificially generated test signals. The simple signals decaying exponentially with the known rate α are of pure sinusoidal shape and have discrete frequencies. As practical dynamic pressure traces are in general corrupted with noise, in the second step of the analysis a certain amount of random noise is added to the test signal. The last step of the analysis involves realistic pressure signals obtained from a simple duct with throughflow. The results obtained from the different methods are compared with each other and differences regarding performance, accuracy, robustness as well as computational costs are presented.

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The thermoacoustic stability of gas turbines depends on a balance of acoustic energy inside the engine. While the flames produce acoustic energy, other areas like the impingement cooling system contribute to damping. In this paper, we investigate the damping potential of an annular impingement sleeve geometry embedded into a realistic environment. A cold flow test rig was designed to represent real engine conditions in terms of geometry, and flow situation. High quality data was delivered by six piezoelectric dynamic pressure sensors. Experiments were carried out for different mean flow velocities through the cooling holes. The acoustic reflection coefficient of the impingement sleeve was evaluated at a downstream reference location. Further parameters investigated were the number of cooling holes, and the geometry of the chamber surrounding the impingement sleeve. Experimental results show that the determining parameter for the reflection coefficient is the mean flow velocity through the impingement holes. An increase of the mean flow velocity leads to significantly increased damping, and to low values of the reflection coefficient.

When modeling gas turbine sound emissions an important boundary condition is the acoustic reflection coefficient at the engine exhaust. Here, the flow discharges into atmosphere at elevated temperature and high flow velocity. The exit reflection coefficient governs the proportion of engine core noise that is radiated into the far-field as well as the acoustic energy contained inside the engine. In addition to pure jet noise, the core noise transmitted through the exit boundary contributes to the overall acoustic emission of an engine, particularly in the audible low frequency regime. In this paper we investigate the plane-wave reflection coefficient at the jet exhaust for a series of different jet Mach numbers and temperatures by solving the linearized Navier-Stokes equations (LNSEs) in frequency space using a finite element method. This approach accounts for effects of mean flow, i.e. scattering and refraction in shear layers, as well as acoustic interaction with unstable shear layers and entropy fluctuations. Their combined effect may cause amplification or attenuation of incident acoustic waves. Applicability and accuracy of the LNSE approach are validated for a set of ambient flow cases at different Mach numbers where a broad base of experimental data and theoretical models is available from literature. Consecutively, the numerical experiment is extended to significantly higher flow temperatures for which few publications exist. The results for elevated flow temperatures show a considerable decrease in the reflection coefficient magnitude with increasing flow temperature. These computations are compared to results obtained from the theory derived by Munt [1,2] and differences are assessed.