André Fischer

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.

Determination of the impedance for lined ducts with grazing flow

Passive means of acoustically damping devices like perforates or - more generally - liners play an important role in industrial applications to protect the community e.g against power plant noise´. In aero-engines, Helmholtz resonator type liners serve as substantial noise abatement means in regions of cold flow. An important physical property to characterize lining devices is the impedance. Several impedance eduction methods exist in literature, most of which are mathematically complicated or need substantial computation time. This paper presents a new impedance eduction method, which is relatively simple to apply, robust, and cheap in terms of computational effort. The method is derived and validated with NASA benchmark data. Its accuracy and applicability is demonstrated and approved.

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.

Acoustic PIV: Measurement of the acoustic particle velocity using synchronized PIV‐technique

This paper outlines a technique for measuring the acoustic particle velocity and the flow field simultaneously by applying synchronized particle image velocimetry (PIV). As test set‐up a squared acrylic glass chamber was chosen. One side of the test section is connected to a loudspeaker, which allows a sinusoidal excitation of the chamber. To point out constrains of this method the investigation includes an analysis of excitation amplitude and frequency as well as the effect of the mean flow magnitude. Therefore a small PC fan can be mounted inside the test section to produce an adjustable mean flow. It can be shown that for a low number of averaged images (80) reasonable results can be achieved up to a certain level of fan rotation speed. Beyond this level the turbulence sensitivity increases and more images are necessary for the calculations. However, the acoustic particle velocity can be computed in the presence of turbulent flow. The presented method called acoustic PIV is a non intrusive technique, a...

Indirect Combustion Noise: Investigations of Noise Generated by the Acceleration of Flow Inhomogeneities

The entropy noise mechanism was experimentally investigated under clearly defined flow and boundary conditions on a dedicated test setup. Previous experimental research on the topic of entropy noise could draw only indirect conclusions on the existence of entropy noise due to the complexity of the physical mechanism. In order to reduce this complexity, a reference test rig has been set up within this work. In this test rig well controlled entropy waves were generated by electrical heating. The noise emission of the entropy waves accelerated in an adjacent nozzle flow was measured accurately and therewith a qualitative and quantitative determination of the entropy noise source mechanism could be experimentally accomplished. In addition to this, a parametric study on the quantities relevant for entropy noise was conducted. The results were compared to a one-dimensional theory by Marble & Candel. In a next step investigations on a combustor test rig showed a broadband noise generation mechanism in the frequency range between 1 and 3.2 kHz. The combustor rig was set up with a similar outlet-nozzle geometry like the reference test rig (EWG) and provided therefore outlet-boundary conditions like in real-scale aero-engines (outlet Mach number = 1.0). It was found that this broadband noise has a strong dependency on the nozzle Mach number in the combustor outlet. The summed-up broadband sound pressure level increases exponentially with the nozzle Mach number. However, investigations of comparable cold flow conditions did not show this behavior. Since the results of the reference experiment with artificially generated entropy waves did not show this exponential increase with the nozzle Mach number, this leaves the conclusion, that this additional noise is generated by the interaction of small-scale fluctuations, e.g. in entropy or vorticity, with the turbulent nozzle flow in the combustion chamber outlet nozzle.

Measurements of Density Pulsations in the Outlet Nozzle of a Combustion Chamber by Rayleigh-Scattering Searching Entropy Waves

The development of measurement techniques, which enable temporal and spatial highly resolved density investigations even in harsh environments, is essential. Rayleigh scattering is a noninvasive optical measurement technique permitting such investigations. A Rayleigh-scattering measurement system is set up, providing a new insight into fluid mechanical processes in turbomachines. In this paper, Rayleigh scattering is used for the detection of density oscillations in the optical accessible convergent-divergent outlet nozzle of a small scale combustion test rig at various power consumptions and equivalence ratios. Until now, this part of the combustion chamber is sparsely investigated due to the challenging measurement conditions. The temporal density oscillation inside the nozzle can be shown up to 4 kHz as well as its spatial distribution. Systematic errors of the setup are investigated. Spectra of pressure and density oscillations are compared. Measurements with nonreacting air flow are conducted to study flow induced density fluctuations. Entropy noise related correlations between density and pressure fluctuations are found. Therewith, the builtup Rayleigh-scattering system enables investigations of the presumed region of indirect noise generation.

Off-line phase-averaged particle image velocimetry and OH chemiluminescence measurements using acoustic time series

In order to analyze unsteady flow phenomena in combustion facilities two phase-sorting methods have been developed and investigated for the retrieval of phase-resolved data from (randomly) sampled single-shot data such as PIV recordings or chemiluminescence imagery in a post-processing step. This is made possible by simultaneously recorded continuous time traces of reference data (e.g., pressure signal). Using this off-line method synchronous phase-locked PIV and OH chemiluminescence visualizations could be recovered from data obtained in two different combustion facilities. This paper also presents some of the theoretical background necessary for the application of two different phase-sorting algorithms.

Broadband Entropy Noise Phenomena in a Gas Turbine Combustor

Broadband sound emission in the region of 1 kHz to 3 kHz probably related to entropy noise was found in a model gas turbine combustor. Entropy noise is caused by the acceleration of fluctuating hot spots in the outlet nozzle of aero-engine combustors. It was predicted in literature in the late 70’s. However, the entire relevance of entropy noise concerning the total noise emission of a combustor is not confirmed until now. Disagreements still exist in the dominating propagating frequencies. This paper presents a broadband noise phenomenon appearing from 1 kHz to 3 kHz in a model aero-engine combustor. In this frequency range the sound pressure level depends on the Mach number as the entropy noise theory expected. Experiments are performed for different types of combustion chambers varying in length and material at various power consumptions and equivalent ratios. A scale factor had been chosen to double the length of the combustion chamber, to allow the investigation of different states of temperature mixing in the exhaust flow following the primary flame zone. Due to the decomposition of the acoustic field into forward and backward propagating waves only the emitted sound power can be considered in the analysis. It can be shown that the broadband noise seems to be independent of changes in geometry or material. A similar cold test flow condition was investigated with respect to the broadband noise phenomena and compared to the reactive case.© 2008 ASME

The acoustic-particle velocity in the vicinity of a liner: a PIV-CAA comparison

A comparison between measurements and simulations of the acoustic-particle-velocity field in a rectangular duct in the near field of a single-degree-of-freedom, Helmholtz-resonator liner is presented. For the measurements, at low-velocity (Ma=0.01) grazing flow condition, the non-intrusive measurement technique particle-image velocimetry (PIV) is applied. The computational aeroacoustic (CAA) simulation is performed by solving the linearised Euler equations in the time domain, without background flow (Ma=0). The CAA solution uses an extended-Helmholtz-resonator acoustic-impedance model to represent the liner. A strong similarity between the simulated and the measured acoustic-particle-velocity distributions is observed. This suggests that the PIV-based technique enables the visualisation of the acoustic-velocity field, on the one hand. On the other hand, it confirms that the given acoustic-impedance model effectively simulates the response of a liner, in terms of the near-field acoustic-particle velocity.

Investigation of the Correlation of Entropy Waves and Acoustic Emission in Combustion Chambers

The entropy noise mechanism was experimentally investigated under clearly defined flow and boundary conditions on a dedicated test setup. Previous experimental research on the topic of entropy noise could draw only indirect conclusions on the existence of entropy noise due to the complexity of the physical mechanism. In order to reduce this complexity, a reference test rig has been set up within this work. In this test rig well controlled entropy waves were generated by electrical heating. The noise emission of the entropy waves accelerated in an adjacent nozzle flow was measured accurately and therewith an experimental proof of entropy noise could be accomplished. In addition to this, a parametric study on the quantities relevant for entropy noise was conducted. The results were compared to a one-dimensional theory byMarble& Candel. In a next step investigations on a combustor test rig showed a broadband noise generation mechanism in the frequency range between 1 and 3.2 kHz. The combustor rig was set up with a similar outletnozzle geometry like the reference test rig (EWG) and provided therefore outletboundary conditions like in real-scale aero-engines (outlet Mach number = 1.0). It was found that this broadband noise has a strong dependency on the nozzle Mach number in the combustor outlet. The summed-up broadband sound pressure level increases exponential with the nozzle Mach number. However, investigations of comparable cold flow conditions did not show this behavior. Since the results of the reference experiment with artificially generated entropy waves did not show this exponential increase with the nozzle Mach number, this leaves the conclusion that this additional noise is generated by the interaction of small-scale fluctuations, e.g. in entropy or vorticity, with the turbulent nozzle flow in the combustion chamber outlet nozzle.