Abstract
Lean, premixed flames are is particularly susceptible to flame instabilities caused, for example, by thermoacoustic instabilities. Those self-exited oscillations are driven by the complex interaction between acoustic modes and the heat release fluctuations of the flame. As the resulting high amplitude pressure oscillations can lead to increased pollutant emissions and in increased mechanical strain the development of methods for prediction and control of combustion instabilities is essential for further development of gas turbines. The present study focuses on one of the most important effects responsible for thermoacoustic feedback in modern gas turbines: The formation and convection of swirl fluctuations caused by acoustic excitation.
The generation of swirl fluctuations itself is an interesting physical phenomenon due to the appearing decoupling mechanisms which leads to a time separation of the influences of one single acoustic excitation pulse: Superimposed to the direct effect of acoustic excitation on the flame, swirl instabilities are induced at the swirl generator, which are moved as inertial waves at bulk velocity and also generate heat release fluctuations when interacting with the flame, which influence the thermoacoustic stability of the system.
In our work we address the problem by linearizing the governing flow equations around the temporal mean flow obtained from high fidelity Large Eddy Simulations (LES). The resulting equations are discretized via a continuous Galerkin approach. With our approach, the linearized state can be forced in frequency domain, i.e. in our case a harmonic flow perturbation is applied at the inlet of the domain. The linearized equations then describe the response of the mean flow to this perturbation. The linearization of the flow in combination together with the treatment in frequency domain allows to describe the flow dynamics at low numerical cost and at the same time provides new possibilities for their analysis.
The first focus of the study is the generation of swirl fluctuations in a model swirler. Since this effect cannot be reduced to two dimensions, a three-dimensional analysis is performed. The temporal mean flows obtained by LES serve as input to the analysis. Periodic forcing is applied at the inlet, which leads to the generation of swirl fluctuatins at the swirler. Results of our linearized tool are compared against forced cases of the high fidelity LES.
Subsequently, the convection of the swirl waves in the mixing duct downstream of the swirler is described. Here, the homogeneity of the wave packets in azimuthal direction can be taken advantage of and the analysis is performed in a two-dimensional domain. Again, velocity forcing, is applied at the inlet of the domain. This time the spatial, complex valued forcing is obtained from the LES as well as from the linearized description of the generated swirl waves, which we compare against each other. We are able to model the subsequent convection in downstream direction of the perturbations. Again, the correct description of these results is compared against the high fidelity LES. Preliminary results show very good agreement with the validation data.
This work makes its valid contribution in analyzing the formation and transport of swirl fluctuations caused by acoustic excitation in modern gas turbine fuel injectors.
Analysis of Generation and Convection of Inertial Waves in Swirled Fuel Combustion Systems Using Linearized Navier-Stokes Equations
Category
Student Poster Presentation
Description
Submission ID: 17501
ASME Paper Number: GT2020-16016
Authors
Patrizia Pschera TU Berlin
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