Nancy Kings

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.

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).

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.

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.

Indirect combustion noise: Experimental investigation of the vortex sound generation in accelerated swirling flows

Combustion noise consists of direct noise related to the unsteady combustion process itself and indirect noise. As known, indirect noise is produced when entropy fluctuations originating from the combustor are accelerated through the turbine. According to the characterisation of the flow by pressure, entropy and vorticity perturbations accelerated vorticity fluctuations are likewise expected to generate indirect noise. In a model experiment the sound generation through the acceleration of vorticity fluctuations was studied. Within a swirl free and a swirling tube flow, vorticity fluctuations were generated artificially by injecting temporarily additional air into the mean flow. The spatial and temporal changes of the velocity field were determined with Hot-Wire Anemometry measurements. During the acceleration of the vorticity fluctuations in a choked convergent-divergent nozzle pressure disturbances are generated. The produced acoustic waves were detected downstream of the nozzle. Direct and vortex sound was identified and separated by varying the distance between the air-injection inlet and the nozzle in case of a swirling flow. The number of inlet ports for the air-injection was modified for a constant injected mass flow rate yielding to amplitude differences in the generated vortex sound. In addition, the intensity of the swirling flow and of the vorticity fluctuation was varied. Increasing the air-injection into the mean flow augments the generated indirect noise.

Indirect combustion noise: Experimental investigation of the vortex sound generation mechanism

uctuations, originating from the unsteady combustion process, are convected with the ow and their acceleration in the outlet nozzle of the combustion chamber or in the turbine generates further noise - the indirect combustion noise. In the model experiment the controlled injection of additional air into a tube ow was used for the generation of the uctuating vorticity yielding temporally to a swirling ow. The sound generation during the acceleration of this articial vortex structure in a convergent-divergent nozzle was measured. The acoustic pressure waves were recorded downstream of the nozzle, simultaneously the velocity components upstream. The spatial and temporal resolution of the velocity eld and therewith of the vortex structure was determined with Hot-Wire Anemometry measurements. With the distance variation between the air-injection inlet and the nozzle the identication and separation of direct and vortex sound was achieved. In a parametric study, the intensity of the swirling ow was inuenced

Optical measurement of acoustic pressure amplitudes—at the sensitivity limits of Rayleigh scattering

Rayleigh scattering is a measurement technique applicable for the determination of density distributions in various technical or natural flows. The current sensitivity limits of the Rayleigh scattering technique were investigated experimentally. It is shown that it is possible to measure density oscillations caused by acoustic pressure oscillations noninvasively and directly. Acoustical standing waves in a rectangular duct were investigated using Rayleigh scattering and compared to microphone measurements. The comparison showed a sensitivity of the Rayleigh scattering technique of 75 Pa (7·10(-4) kg/m(3)) and a precision of 14 Pa (1·10(-4) kg/m(3)). Therefore, it was also shown that Rayleigh scattering is applicable for acoustic measurements.

Numerical Investigation of Indirect Noise Generation by Accelerated Vorticity

Combustion noise of aero engines originates from unsteady combustion processes which in turn lead to vortical and temperature fluctuations. These so-called vorticity and entropy waves are convected from the combustor into the turbine where their acceleration results in an additional sound release, namely the indirect noise. In the present study the noise generation by accelerated vorticity waves is investigated in a convergent-divergent nozzle representing the simplest model of the flow through the turbine. A hybrid CFD/CAAapproach is applied which consists of RANS mean flow simulations followed by frequency domain simulations of linear acoustic and vortical fluctuations based on the linearized Navier-Stokes equations (LNSEs). Vorticity waves are excited by a body force term which is deduced from an analytical solution of the linearized vorticity equation. By this means, the indirect noise released by the accelerated vorticity waves is computed and compared with experimental measurements. The numerical simulation captures in general the coupling mechanisms relevant to indirect noise.

Broadband indirect noise generation by accelerated vorticity

In the framework of research on core noise in aero-engines the generation of broadband indirect noise is investigated in detail. Hereby, the noise generation due to turbulent velocity fluctuations accelerated in a transonic nozzle flow is in the focus. The approach follows the idea to link the turbulent velocity components, longitudinal and lateral, measured upstream of the nozzle to the downstream detected acoustic pressure fluctuations. An important aspect is the transition of the turbulent velocity fluctuations and their change during the acceleration process through the nozzle. A theoretical model is used to estimate this change in the turbulence quantities. The comparison between the measured acoustic field and the velocity fluctuations indicates a relation of the longitudinal velocity fluctuations to the acoustic pressure fluctuations in the range from 4 − 200Hz and of the lateral velocity fluctuations to the acoustic pressure fluctuations in the range from 201 − 4000Hz. This suggests the thesis that the change of the different turbulent velocity fluctuations generate sound in different frequency bands in the downstream propagating acoustic field.