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The broadband component of fan noise has grown in relevance with the utilization of increased bypass ratio and advanced fan designs. Thus, while the attenuation of fan tones remains paramount, the ability to simultaneously reduce broadband fan noise levels has become more desirable. This paper describes improvements to a previously established broadband acoustic liner optimization process using the Advanced Noise Control Fan rig as a demonstrator. Specifically, in-duct attenuation predictions with a statistical source model are used to obtain optimum impedance spectra over the conditions of interest. The predicted optimum impedance information is then used with acoustic liner modeling tools to design liners aimed at producing impedance spectra that most closely match the predicted optimum values. Design selection is based on an acceptance criterion that provides the ability to apply increased weighting to specific frequencies and/or operating conditions. Constant-depth, double-degree of freedom and variable-depth, multi-degree of freedom designs are carried through design, fabrication, and testing to validate the efficacy of the design process. Results illustrate the value of the design process in concurrently evaluating the relative costs/benefits of these liner designs. This study also provides an application for demonstrating the integrated use of duct acoustic propagation/radiation and liner modeling tools in the design and evaluation of novel broadband liner concepts for complex engine configurations.

The occurrence of combustion instability dynamics known, as “screech, howl and growl,” in the combustors of gas turbine engines is a very difficult challenge for engineers. The very high amplitude pressure oscillations caused by combustion dynamics, are not only detrimental to the operation of the engine and combustor, but the difficulty in predicting and remedying these problems can lead to significant costs and delays in engine development. The coupling of the unsteady heat release in the flame with the natural acoustic resonance modes of the combustor duct causes the phenomena of combustion dynamics. To improve our understanding of stability characteristics in such complex systems, encountered in many industrial applications, the flame structure of an atmospheric swirl-stabilized burner, containing dilution and cooling air holes and fed with natural gas fuel, was systematically investigated for various inlet temperatures, pressure drops and air-fuel ratios. Experiments were also designed and conducted with the goal to understand better the phenomena of combustion dynamics that were experienced. More specifically, six acoustic pressure transducers were incorporated in the combustor and in the upstream duct to measure the acoustic field and the acoustic impedance characteristics at specified locations of interest. A one-dimensional wave propagation model is presented to predict the acoustic frequencies and damping of resonance modes, based on the geometry of the test rig, the flow conditions, and the acoustic impedance characteristics of the terminations of the combustor. This paper will present the acoustic analysis of the test data in the light of the above-mentioned theoretical modeling. The limitations of the current test rig are pointed out and changes in the rig design are discussed for future research.Copyright © 2010 by ASME

The phenomena of combustion dynamics strongly depends on the energy release (flame) dynamics of combustion systems. The temporal Fast Fourier Transform (FFT) and spatial Proper Orthogonal Decomposition (POD) methods were used to analyze high speed energy release (chemiluminescene) videos on a natural gas powered gas turbine burner. The considered case has a single dominant acoustical frequency. Both methods provide an insight into the temporal and spatial distribution of energy release rate dynamics associated with the acoustic pressure oscillation. In the FFT analysis, the spatially distributed amplitude and phase angle at the selected acoustic frequency can be secured, and the energy release dynamics (motion) associated with the considered acoustics can be visualized. The trigger mechanism for combustion dynamics was unveiled in the spatial phase angle distribution at the acoustic frequency. In the POD analysis, the first few modes have been found to contain a significant portion of energy associated with the considered dynamics system. The first POD mode contains 19% of energy and its associated temporal and spatial feather is very similar to those calculated by the FFT analysis. The POD Mode #2 and #3 contains less energy, 7% and 5% respectively and their temporal behavior is also associated with the acoustical frequency. However, their spatial structures contain higher spatial frequency components compared to that of Mode #1. The unveiled spatial structure by Mode #2 and #3 cannot be easily discovered from the FFT amplitude and phase information since it was washed out by the higher energy spatial structure from Mode #1. A very significant finding in this research is that the POD can be used for reduced order set of modeling which can capture the energy release dynamics by truncating out the “noise.” The POD also represents an effective means of “spatial frequency filtering,” based on the POD related energy distribution, which enables the sharpening of the energy release rate spatial structure information.

This paper investigates the combustion dynamics of a single cup gas turbine combustor sector, a swirl-stabilized burner with dilution and cooling air holes. The burner was fueled by natural gas and was operated at one atmospheric pressure with various air inlet temperatures, air pressure drops, and air–fuel ratios. Acoustic emissions as functions of operating conditions were measured. Chemiluminescence imaging with a high-speed CCD camera and simultaneous dynamic pressure measurements has been used to characterize the combustor’s behavior. Imaging of the combustion energy release rate has provided an insight on the flame structure and its interaction with the dilution air jets. The combustion dynamics were correlated well with the heat release rate in the primary zone or in the dilution air jet region. Two low-frequency combustion dynamics modes (fuel-rich combustor domes with frequency around 420Hz, and fuel-lean combustor domes with frequency around 280 Hz) were identified by a dynamic microphone, dynamic pressure transducers, and a high-speed CCD camera. A two-dimensional distribution of the heat release rate dynamics (amplitude and phase angle) reveals different combustion dynamics driving mechanisms for fuel-lean and fuel rich modes. The acoustic emissions for the fuel-lean combustion mode correlated well with the energy release rate dynamics of the swirling jet inside the combustor dome. The acoustic emissions for the fuel-rich mode were associated with the unsteady chemical energy release rate from interactions between the dilution air jets and the fuel-rich mixture convected from the combustor dome.

This paper presents the results of an investigation into the acoustic impedance characteristics of porous materials such as porous foams and fibrous mats. Most of previous experimental investigations focused on the characteristics of fibrous materials only. However, Yu, Kwan and Yasukawa did include some foam materials in th eir investigation. Their experiments were conducted in an impedance tube, which allowed measurement up to 6000 Hz. The empirical correlations of Delany and Bazley were derived from measurements conducted between 250 and 4000 Hz. The data presented by Lee a nd Selamet 4 had an upper limit of just over 3000 Hz. The apparatus developed by Syed and Ichihashi at the University of Cincinnati is capable of accurate measurements up to 9000 Hz. Therefore it allows an investigation of the acoustic characteristics of p orous materials over a much wider frequency range.