Friedrich Bake

Investigation of Entropy Noise in Aero-Engine Combustors

Strong evidence is presented that entropy noise is the major source of external noise in aero-engine combustion. Entropy noise is generated in the outlet nozzles of combustors. Low-frequency entropy noise, which was predicted earlier in theory and numerical simulations, was successfully detected in a generic aero-engine combustion chamber. It is shown that entropy noise dominates even in the case of thermo-acoustic resonances. In addition to this, a different noise generating mechanism was discovered that is presumably of even higher relevance to jet engines: There is strong evidence of broad band entropy noise at higher frequencies (I to 3 kHz in the reported tests). This unexpected effect can be explained by the interaction of small scale entropy perturbations (hot spots) with the strong pressure gradient in the outlet nozzle. The direct combustion noise of the flame zone seems to be of minor importance for the noise emission to the ambiance. The combustion experiments were supplemented by experiments with electrical heating. Two different methods for generating entropy waves were used, a pulse excitation and a sinusoidal excitation. In addition, high-frequency entropy noise was generated by steady electrical heating.

Fundamental Mechanism of Entropy Noise in Aero-Engines: Experimental Investigation

Entropy noise caused by combustors increases rapidly with rising Mach number in the nozzle downstream of the combustion chamber. This is experimentally shown with a dedicated test facility, in which entropy waves are generated in a controlled way by unsteady electrical heating of fine platinum wires immersed in the flow. Downstream of the heating module called entropy wave generator (EWG), the pipe flow is accelerated through a convergent-divergent nozzle with a maximum Mach number of 1.2 downstream of the nozzle throat. Parameters like mass flux of the flow, nozzle Mach number, amount of heating energy, excitation mode (periodic, pulsed, or continuously), and propagation length between EWG and nozzle have been varied for the analysis of the generated entropy noise. The results are compared with the results of a one-dimensional theory found in early literature.

Experimental investigation of the entropy noise mechanism in aero-engines

It is assumed by theory, that entropy noise emitted by combustion systems increases rapidly with rising Mach number in the nozzle downstream of the combustion chamber. Model experiments have been carried out to verify the existence of this sound generating mechanism. A dedicated test facility was built, in which entropy waves are generated in a controlled way by unsteady electrical heating of fine platinum wires immersed in the flow. Further experiments have been carried out in a model combustor test rig where a broadband noise phenomenon, presumably related to indirect noise generation mechanisms, was found.

Establishment of a High Quality Database for the Acoustic Modeling of Perforated Liners

Perforated liners, especially in combination with a bias flow, are very effective sound absorbers. When appplied to gas turbine combustors, they can suppress thermo-acoustic 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 parameter 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 and Strouhal and bias flow Mach numbers.

Comparative Study of Impedance Eduction Methods, Part 1: DLR Tests and Methodology

A number of methods have been developed at NASA Langley Research Center for eduction of the acoustic impedance of sound-absorbing liners mounted in the wall of a flow duct. This investigation uses methods based on the Pridmore-Brown and convected Helmholtz equations to study the acoustic behavior of a single-layer, conventional liner fabricated by the German Aerospace Center and tested in the NASA Langley Grazing Flow Impedance Tube. Two key assumptions are explored in this portion of the investigation. First, a comparison of results achieved with uniform-flow and shear-flow impedance eduction methods is considered. Also, an approach based on the Prony method is used to extend these methods from single-mode to multi-mode implementations. Finally, a detailed investigation into the effects of harmonic distortion on the educed impedance is performed, and the results are used to develop guidelines regarding acceptable levels of harmonic distortion

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.

Indirect Combustion Noise Generation in Gas Turbines

The paper investigates the indirect combustion noise, which is generated during the acceleration of the convected entropy nonuniformities of the combustion products in the outlet nozzle of the combustion chamber. The generation mechanism of the indirect noise is proven experimentally and through numerical simulation. Probe microphones and fast thermocouple probes were used to measure pressure and temperature uctuations. The generation of indirect noise is veried via the phase relationship between thermocouple and microphone signals. The o w eld in the combustion chamber is simulated by means of an unsteady RANS computation. Self excited oscillations are used for the computation of the direct and indirect noise generation of the combustion chamber. Since the related frequencies are low and the corresponding scales much larger than the turbulent scales, a CAA-method is employed for both the propagation of sound waves as well as entropy perturbations. It is shown that the CAA method is capable to describe the acoustical properties of the combustion system found in the experiments when the URANS simulation is used as input. The experimental results also show that indirect combustion noise may contain high frequency noise contributions, which are generally attributed to turbine noise.

Indirect Combustion Noise: Noise Generation by Accelerated Vorticity in a Nozzle Flow

The noise generation by accelerated vorticity waves in a nozzle flow was investigated in a model experiment. This noise generation mechanism belongs, besides entropy noise, to the indirect combustion noise phenomena. Vorticity as well as entropy fluctuations, originating from the highly turbulent combustion zone, are convected with the flow and produce noise during their acceleration in the outlet nozzle of the combustion chamber. In the model experiment, noise generation of accelerated vorticity fluctuations was achieved. The vorticity fluctuations in the tube flow were produced by injecting temporally additional air into the mean flow. As the next step, a parametric study was conducted to determine the major dependencies of the so called vortex noise. A quadratic dependency of the vortex noise on the injected air amount was found. In order to visualise and classify the artificially generated vorticity structures, planar velocity measurements have been conducted applying Particle Image Velocimetry (PIV).

Sound Generation in the Outlet Section of Gas Turbine Combustion Chambers

Indirect combustion noise is investigated experimentally and numerically. This noise is generated in the outlet nozzle of combustion chambers if the entropy of the medium is nonuniform, which is the case in the exhaust of combustors. The contribution to the total noise emission of aeroengine combustors is not known. A test rig for the experimental investigation of this noise emission in the presence of swirl is first described. The indirect noise is generated in an exchangeable convergent‐divergent nozzle at the exit of the combustor. The noise radiation is studied in a circular exhaust pipe with probe microphones using a radial mode analysis of the microphone signals. First results of the measured sound fields are reported. The experimental situation will be studied numerically with a 4 th order accurate CAA‐method, which is first validated with theoretical results of the literature for the cases of a compact nozzle or diuser and incoming entropy and sound waves in a one‐dimensional mean flow. The agreement with the results of sound generation due to incoming entropy waves and the sound reflection and transmission for incoming sound waves is very good. The method is then applied to the more realistic cases of non‐compact nozzles and it is found that the amplitudes of the generated waves are substantially smaller in comparison to the one‐dimensional theory. The sound generation of real cases like a swirling hot‐spot and entropy waves in a one‐dimensional flow through a convergent‐divergent nozzle as well as plain entropy waves in the swirl flow of the experimental setup are finally studied and the noise emission is computed.

Comparison of impedance eduction results using different methods and test rigs

This thesis is motivated by the need for noise control in aircraft engine with orifices and perforated liner. The presence of high-level acoustic excitation, different flow situations either bias flow, grazing flow or any combination in the aircraft engine, makes the acoustic behavior complex due to the interaction between sound and flow over the lined wall. Both systematic acoustic prediction of aircraft engines and liner optimization necessitate progress in impedance measurement methods by including the effect of the complex flow situations. The aim of the present thesis is to experimentally study the change in acoustic properties of orifices and perforated liners under bias or grazing flow.In order to study the effect of different combinations of bias flow and high-level acoustic excitation, an in-duct orifice has been investigated with finely controlled acoustic excitation levels and bias flow speeds. This provides a detailed study of the transition from cases when high-level acoustic excitation causes flow reversal in the orifice to cases when the bias flow maintains the flow direction. Nonlinear impedance is measured and compared, and a scattering matrix and its eigenvalues are investigated to study the potentiality of acoustic energy dissipation or production. A harmonic method is proposed for modelling the impedance, especially the resistance, which captures the change in impedance results at low frequencies compared with experimental results.The presence of grazing flow can increase the resistance of acoustic liners and shift their resonator frequency. So-called impedance eduction technology has been widely studied during the past decades, but with a limited confidence due to the interaction of grazing flow and acoustic waves. A comparison has been performed with different test rigs and methods from the German Aerospace Center (DLR). Numerical work has been performed to investigate the effect of shear flow and viscosity. Our study indicates that the impedance eduction process should be consistent with that of the code of wave propagation computation, for example with the same assumption regarding shear flow and viscosity. A systematic analysis for measurement uncertainties is proposed in order to understand the essentials for data quality assessment and model validation. The idea of using different Mach numbers for wave dispersion and in the Ingard-Myers boundary condition has been tested regarding their effect on impedance eduction. In conclusion, a local Mach number based on friction velocity is introduced and validated using both our own experimental results and those of previous studies.