Ingo Röhle

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.

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.

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.

A fiber-optical microphone based on a Fabry-Perot interferometer applied for thermo-acoustic measurements

A high-temperature resistant fiber-optical microphone (FOM) was developed and successfully applied in a combustion chamber (~1.2 × 105 Pa, ~1400 K gas temperature) with thermo-acoustic oscillations resulting in a sound pressure level of 154 dB at the dominant frequency. The core of the optical set-up used for the FOM is a Fabry–Perot interferometer. To create an acoustical sensor based on this type of interferometer, a new method of generation and postprocessing of the interference signal was developed. The simple replaceability of the used membrane material allows the adaptation of the sensor sensitivity to the projected field of application.

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.

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

Doppler global velocimetry for the analysis of combustor flows

Abstract The principle of Doppler global velocimetry (DGV) and a DGV system optimized for time averaged three-component velocity measurements is described in this paper. Furthermore, the design of the different components of the DGV-system as well as the manner of its operation is presented. The volumetric, time averaged, three-component velocity distribution was acquired in the isothermal flow of a low NOx, staged combustion chamber sector from Rolls-Royce Deutschland. The combustor was developed within the German public-funded Engine 3E program. On the basis of the collected data, the complex flow phenomena in the combustor could be analyzed in detail and supported by CFD calculations. A recently developed, pulsed Nd:YAG laser now enables planar, time-averaged, three-component DGV application in combusting flow fields. Measurements were carried out in a single-nozzle, kerosene combustion chamber model, operated under atmospheric pressure. The successful measurements demonstrated the capability of DGV as a new tool for combustion research. It was possible to separately measure the gas velocity and the velocity of the fuel droplets, a promising capability of DGV for two phase flow analysis.

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.