Claus Heuwinkel

Experimental investigation of the acoustic damping of perforated liners with bias flow

In modern gas turbines thermo-acoustic instabilities can be very prominent. The amplification of these pressure oscillations can be suppressed by means of acoustic damping. Originally intended for cooling purposes, perforated liners with bias flow were found to have a significant damping eect. This paper presents the results of an experimental study on perforated liners. Three test objects have been examined at frequencies between 200-1400 Hz, with and without bias flow. The variables investigated in the parametric study were the jet velocity, the Mach number of the duct flow, the number of jets, the open-area-ratio of the liner, and the frequency. The acoustic properties, in terms of the reflection, transmission and dissipation coecients, have been determined for several configurations. In contrary to the conditions inside a combustion chamber, only cold air at ambient pressure was used throughout these measurements. In the frequency range of interest, the sound propagation was limited to plane waves. Successful eorts have been made that could reduce the errors contained in the final results to below 1%. This was achieved by optimizing the test facility, the measurement techniques, and the data analysis procedure. These optimizations were in particular: Consideration of viscosity and thermal conductivity losses at the duct wall, suppression of the influence of evanescent modes, a frequency dependent microphone calibration regarding amplitude and phase, and elimination of the influence of end reflections at the duct terminations. The experiments show that perforated liners can be used very eectively for acoustic damping. More than 50% of the sound energy could be absorbed over a broad range of frequencies, using a bypass mass flow of less then 2% of the main flow of the combustor. For some configurations the sound absorption exceeds 60% for a narrow band of frequencies. This was achieved by increasing the distance between the apertures. The result suggests that the jets for the narrow spaced liner interact with each other, which reduces their capacity to dissipate sound energy.

Impedance Deduction Based on Insertion Loss Measurements of Liners under Grazing Flow Conditions

This paper presents the results of insertion loss measurements and numerical impedance eduction of three dierent liner samples. An overview over the test rig and methodology is given and preprocessed results in terms of reection and transmission coecients as well as the energy dissipation are discussed. These coecients are calculated for discrete frequencies within the investigated frequency range. Subsequently, a numerical post processing is performed in the time domain and the educed impedance function for each sample and ow Mach number is presented. This post processing in the time domain uses an impedance model, which is based on the Extended Helmholtz Resonator with ve free parameters. The parameters of the model are tted via an optimization, which determines the whole frequency response by one optimization process. The comparison of measured and numerically evaluated energy coecients proves the usability of the tools for impedance evaluation under ow conditions. Finally the impedance results of the dierent samples are discussed, including a comparative study with Aermacchi data of the NLR ow tube and Aermacchi impedance tube experiments.

Characterization of a Perforated Liner by Acoustic and Optical Measurements

A comprehensive study of a perforated liner under bias and grazing ow is performed in three di erent test facilities. Microphone measurements are used to characterize the general performance of the liner. Laser Doppler Anemometry and Acoustic Particle Image Velocimetry measurements are applied to study the ow eld in the vicinity of the ori ces. The results allow a detailed interpretation of the relation between the damping performance and the ow structures, as well as a comprehensive comparison between the di erent test facilities and measurement techniques.

Comparison of Experimental and Numerical Results Concerning the Damping of Perforated Liners With Bias Flow

Perforated liners with bias flow are integrated in the wall of gas turbine combustors to suppress thermoacoustic instabilities. The suppression of these unstable pressure oscillations is a requirement for the safe and stable operation of a gas turbine while applying new combustion concepts concerning more efficiency and cleanliness. Previous experiments have shown the high potential of perforated liners absorbing sound energy and therefore minimize combustion instabilities. In this collaborative work, the absorption properties of a liner are determined from both experimental measurements and numerical simulations. In both cases the analysis is based on acoustic pressure data recorded at several axial positions upstream and downstream of the liner. In the experiments this data is acquired by microphone measurements and in the simulation it is numerically calculated applying a three dimensional compressible URANS approach. The dissipation coefficient of the liner is identified for plane wave propagation at ambient conditions while a grazing flow is present in the duct. Parameters are the bias flow velocity and the amplitude of the incident sound wave. Comparing the results of the highly accurate experiments and the simulation reveals the abilities and limits of the numerical approach to model the absorption effect. The results are in very good agreement for the case without bias flow. However, the discrepancy between the experimental and numerical results is increasing while a bias flow is present.Copyright

On the Excitation of a Zero Mass Flow Liner for Acoustic Damping

The Zero Mass Flow Liner is able to convert a perforated liner with minimal damping qualities into a very effective sound absorber. The damping improvement results from an additional acoustic excitation within the Zero Mass Flow Liner. This paper studies the damping performance of the Zero Mass Flow Liner while changing the parameters of this excitation. A strong dependency on the sound pressure level and frequency is revealed from the experiments. Though, a mode structure with a distinct pressure pattern seems to be of minor importance. However, it is of great benefit to operate the actuator at its natural resonances. This leads to a very good damping performance from a minimum of input power.

Establishment of a High Quality Database for the Modelling of Perforated Liners

Perforated liners, especially in combination with a bias flow, are very effective sound absorbers. Applied to gas turbine combustors they can suppress thermoacoustic instabilities and thus allow the application of new combustion concepts concerning higher efficiency and lower emissions. While the successful application of such a damping concept has been shown, it is still not possible to accurately predict the damping performance of a given configuration. This paper provides a comprehensive database of high quality experimental data. Variations of geometric, fluid mechanic and acoustic parameters have been studied, including realistic engine configurations. The results demonstrate each parameters influence on the damping performance. A low order thermo-acoustic model is used to simulate the test configurations numerically. The model shows a good agreement with the measurements for a wide range of geometries, Strouhal and bias flow Mach numbers.© 2010 ASME