Bernd Mühlbauer

Numerical Investigation of the Fundamental Mechanism for Entropy Noise Generation in Aero-Engines

Abstract n nIn the present work, the generation and propagation of entropy noise is computed using a compressible URANS approach in combination with appropriate acoustic boundary conditions. The Entropy Wave Generator (EWG) experiment is best suited for validating the proposed approach and for investigating the acoustic sources of entropy noise. The EWG involves a non-reactive tube flow, where entropy modes are imposed to an incoming air flow by using a heating module. The generated temperature non-uniformities are accelerated with the mean flow downstream in a convergent divergent nozzle, thus producing entropy noise. Simulation results of pressure fluctuations and their spectra for a defined standard configuration as well as for diff erent operating points of the EWG agree very well with the respective experimental data. Additionally, an analysis of the acoustic sources was performed. For this purpose the acoustic sources caused by the acceleration of density inhomogeneities were calculated. For the first time, a numerical method is introduced for the localization of the acoustic sources of entropy noise in acceleration/deceleration regions.

Evaluation of the RPM Approach for the Simulation of Broadband Combustion Noise

The derivation and validation of a broadband combustion noise model is presented. The random particle-mesh approach for combustion noise is a hybrid computational fluid dynamics/computational aeroacoustics method and reliesonthestochastic reconstructionofcombustionnoisesources inthe timedomain.Thestochastic reconstruction of unsteady sound sources based on statistical turbulence quantities from a reacting Reynolds-averaged Navier– Stokes simulation is realized with the random particle-mesh method. In the present paper, the modeled combustion noise sources are derived for the use in conjunction with the linearized Euler equations for the computation of the acoustic propagation. Two open, nonpremixed, turbulent jet flames (DLR-A and DLR-B), which differ in their fuel outletvelocityandtheirrespectiveReynoldsnumber,areusedforthevalidationoftheparticle-meshforcombustion noise approach. Results of the reacting flow computations and the subsequent acoustic simulations are compared with measurements. Excellent agreement is found between the computed narrow band sound spectra and the experimental data.

Broadband combustion noise simulation of open non-premixed turbulent jet flames

Numerical broadband combustion noise simulations of open non-premixed turbulent jet flames applying the Random Particle-Mesh for Combustion Noise (RPM-CN) approach are presented. The RPM-CN approach is a hybrid Computational Fluid Dynamics/Computational Aeroacoustics (CFD/CAA) method for the numerical simulation of turbulent combustion noise, based on a stochastic source reconstruction in the time domain. The combustion noise sources are modeled on the basis of statistical turbulence quantities, for example achieved by a Reynolds averaged Navier-Stokes (RANS) simulation, using the Random Particle-Mesh (RPM) method. RPM generates a statistically stationary fluctuating sound source that satisfies prescribed one- and two-point statistics which implicitly specify the acoustic spectrum. Subsequently, the propagation of the combustion noise is computed by the numerical solution of the Linearized Euler Equations (LEE). The numerical approach is applied to the DLR-A, the DLR-B and the H3 flames. The open non-premixed turbulent jet flames differ in the mean jet exit velocity, therefore in their respective Reynolds number, and in the fuel composition. Computed radial profiles of the reacting flow field are compared to experimental data and discussed. Computed sound pressure level spectra of the DLR-A and DLR-B flames and acoustic intensity level spectra of the H3 flame at different microphone locations are presented and compared to measurements.

Numerical Investigation of Entropy Noise and its Acoustic Sources in Aero-Engines

In the present work, the generation and propagation of entropy noise was simulated applying a compressible three dimensional URANS CFD approach. To this end, a test rig, the so-called Entropy Wave Generator (EWG) was modeled. The EWG implies a non-reactive tube flow where entropy modes are induced to the air flow by a heating module. The generated temperature nonuniformities are accelerated in a convergent-divergent nozzle and excite entropy noise. Simulation results of pressure fluctuations and power spectra in the standard configuration as well as simulations of different operating points of the EWG are in very good agreement with measurements. Additionally, entropy noise was deduced for realistic gas turbine conditions with calculated sound pressure level above 160 dB, which evidences the relevance of entropy noise in gas turbines. The numerical investigation is completed by the analysis of the acoustic sources. For this purpose the acoustic sources caused by the acceleration of density inhomogeneities suggested by Dowling [1] were calculated. Thus, for the first time a method is introduced to locate the acoustic sources of entropy noise in the acceleration region.Copyright

Fundamental Mechanism of Entropy Noise in Aero-Engines: Numerical Simulation

Entropy noise is generated by entropy non-uniformities being accelerated for example in the turbine section downstream of gas-turbine combustion chambers. The entropy noise was experimentally investigated in the test facility Entropy Wave Generator (EWG) [1, 2]. The EWG induces entropy waves by supplying energy pulses to a tube flow. The air stream is accelerated in a convergent-divergent nozzle where entropy noise is excited. In order to investigate entropy noise generation mechanism the numerical simulation of thermo-acoustics was done. This paper shows results concerning the numerical simulation of a reference test case of the EWG applying a compressible URANS approach. The simulations indicate that both fully reflective and non-reflective boundary conditions are inappropriate for modeling of the propagation of entropy noise in this case. The change of cross sectional area in the downstream tube section at a certain position requires the application of a partially reflective boundary condition in the numerical simulation. The variation of the length of the tube section upstream the nozzle proves the existence of entropy noise. Changing the tube length downstream the nozzle results in a phase shift of the superposed downstream and partially reflected upstream propagating pressure fluctuations. Furthermore, the influence of the amount of energy supplied to the system on the generated entropy noise is shown. All numerical results show a good agreement with experimental data.Copyright

Evaluation of the RPM-CN approach for broadband combustion noise prediction

The derivation and validation of a broadband combustion noise model is presented. The applied RPM-CN approach, a hybrid CFD/CAA approach relies on the stochastic reconstruction of combustion noise sources in the time domain. The stochastic reconstruction is conducted by the RPM method out of statistical turbulence quantities which can be delivered by a reacting RANS simulation. In the present work, the modeled combustion noise sources are derived for the use in conjunction with the LEE for the computation of the acoustic propagation. The DLR-A and the DLR-B ames, both non-premixed open jet ames which dierentiate in the fuel outlet velocity and the respective Reynolds number, are used for the validation of the RPM-CN approach. Results of the reacting ow computations and the subsequent acoustic predictions are compared to measurements and discussed. The reliability and accuracy of the RPM-CN approach will be demonstrated by a good agreement of the computed sound pressure level spectra with the experimental data.

Numerical Simulation of Broadband Combustion Noise With the RPM-CN Approach

A new numerical approach called RPM-CN approach is applied to predict broadband combustion noise. This highly efficient hybrid CFD/CAA approach can rely on a reactive RANS simulation. The RPM method is used to reconstruct stochastic broadband combustion noise sources in the time domain based on statistical turbulence quantities. Subsequently, the propagation of the combustion noise is computed by solving the acoustic perturbation equations (APE-4). The accuracy of the RPM-CN approach will be demonstrated by a good agreement of the simulation results with acoustic measurements of the DLR-A flame. The high efficiency and therefore low computational costs enable the usage of this numerical approach in the design process.Copyright

Numerical RANS/URANS simulation of combustion noise

In the present work, numerical simulation tools for two different combustion noise source mechanisms are presented. The generation and propagation of entropy noise is computed directly using a compressible CFD approach in combination with appropriate acoustic boundary conditions. The EntropyWave Generator (EWG) experiment is taken for validation of the proposed approach and for evaluating the acoustic sources of entropy noise. Simulation results of pressure fluctuations and their spectra for a defined standard test configuration as well as for different operating points of the EWG agree very well with the respective experimental data. Furthermore, a new numerical approach called RPM-CN approach was developed to predict broadband combustion noise. This highly efficient hybrid CFD/CAA approach can rely on a reactive RANS simulation. The RPM method is used to reconstruct stochastic broadband combustion noise sources in the time domain based on statistical turbulence quantities. Subsequently, the propagation of the combustion noise is computed by solving the acoustic perturbation equations (APE-4). The accuracy of the RPM-CN approach will be demonstrated by a good agreement of the simulation results with acoustic measurements of the DLR-A flame. The high efficiency and therefore low computational costs enable the usage of this numerical approach in the design process.