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.
When predicting combustion instabilities in gas turbine combustion chambers, the complex geometry and three dimensional flow configurations are often neglected. However, these may have significant influence on the overall acoustic damping behavior of the system. An important element governing the flow inside a combustion chamber is the swirl atomizer nozzle. Therein, the flow is accelerated and a swirling fluid motion is imposed. At its exit considerable high flow velocities are reached and multiple shear layers are formed which discharge into the combustion chamber. To predict damping effects in these environments, acoustic-flow interaction processes need to be taken into account. These involve scattering and refraction of incident acoustic waves in shear layers, acoustic interaction with the unstable hydrodynamic shear layers as well as acoustic wall interaction processes. Their combined effect can be studied using acoustic scattering matrices. In this paper the acoustic scattering behavior of a lean injection system developed by Avio is predicted under non-reactive conditions and compared to experiments. The numerical method is very general and works as follows: First, the fluid dynamic field is computed using a Reynolds averaged Navier-Stokes turbulence model. Then, the linearized Navier-Stokes equations are solved in frequency space around the previously computed mean flow state. The complex three dimensionality of the nozzle configuration is taken into account as well as its corresponding flow field. Results are compared against experimental measurements of a swirl atomizer nozzle at atmospheric and elevated inlet temperatures. It is shown that the scattering behavior and therefore the acoustic-flow interactions are captured accurately.
The reduction of pollution and noise emissions of modern aero engines represents a key concept to meet the requirements of the future air traffic. This requires an improvement in the understanding of combustion noise and its sources, as well as the development of accurate predictive tools. This is the major goal of the current study where the low-order thermo-acoustic network (LOTAN) solver and a hybrid computational fluid dynamics/computational aeroacoustics approach are applied on a generic premixed and pressurized combustor to evaluate their capabilities for combustion noise predictions. LOTAN solves the linearized Euler equations (LEE) whereas the hybrid approach consists of Reynolds-averaged Navier–Stokes (RANS) mean flow and frequency-domain simulations based on linearized Navier–Stokes equations (LNSE). Both solvers are fed in turn by three different combustion noise source terms which are obtained from the application of a statistical noise model on the RANS simulations and a post-processing of incompressible and compressible large eddy simulations (LES). In this way, the influence of the source model and acoustic solver is identified. The numerical results are compared with experimental data. In general, good agreement with the experiment is found for both the LOTAN and LNSE solvers. The LES source models deliver better results than the statistical noise model with respect to the amplitude and shape of the heat release spectrum. Beyond this, it is demonstrated that the phase relation of the source term does not affect the noise spectrum. Finally, a second simulation based on the inhomogeneous Helmholtz equation indicates the minor importance of the aerodynamic mean flow on the broadband noise spectrum.
The design and optimization of turbines demands the use of fast low-fidelity tools. To obtain adequate results, loss correlations simplifying the complex turbine throughflow are implemented. Accounting for modern turbine designs and flow conditions, revisions of profile and secondary loss correlations were primarily focused upon, while improvements of the tip clearance loss correlations are difficult to achieve. Realistic engine-like conditions concerning variations of the tip clearance, blade loading and solidity are time- and cost-intensive to investigate.
This paper is focused on an extensive numerical study, intending to support experiments on tip clearance loss correlations. The losses of a high pressure axial turbine rotor are analyzed for different tip clearance gap heights and incidence angles at cruise condition. The results are contrasted with a cascade having comparable tip profile and gap heights. The cascade’s flow is comparable to the rotor, but with respect to experimental restrictions concerning inlet and outlet conditions.
Steady 3D calculations in the stationary and rotating frame were performed applying DLR’s turbomachinery CFD code TRACE using Menter’s SST k – ω turbulence model. The tip clearance loss coefficients were extracted from the flow field by post-processing data of an outlet plane and as massflow averaged global values. The findings are discussed referencing previous publications about the leakage flow system and tip clearance loss. Finally, a comparison to results from tip clearance loss correlations of Ainley-Mathieson and Dunham-Came is presented.
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