Session: 04-20: Combustion Noise
Paper Number: 82971
82971 - Modeling the Convection of Entropy Waves in Strongly Non-Parallel Turbulent Flows Using a Linearized Framework
In recent years, entropy waves and the question how to control them has shifted into focus of gas turbine combustion engineers. Entropy waves are pockets of varying temperature in the burnt gases, which are mainly caused by an inhomogeneous distribution of fuel. If accelerated in the transition duct between combustion chamber and the first turbine stage, they emit sound waves. These sound waves can either feedback into a thermoacoustic cycle or they exit the engine through the turbine stages and add to the noise emission of the engine. Both of these consequences are highly unwanted effects, not only but especially in flight engines.
Numerous experimental and numerical studies were conducted to enhance our understanding of entropy waves and their consequences. Studies using simple analytical models on the other hand are valuable tools to analyze the effect of entropy waves on the thermoacoustic stability of the combustion chamber. Furthermore, they have shown that transport phenomena, such as mean flow shear dispersion, and diffusive processes can lead to a mitigation of the entropy waves while they are convected from the flame towards the transition duct. These analytical models, however, are based on strong assumptions, such as a restriction to one dimensional transport, no turbulent diffusion, or velocity profiles, which do not vary in streamwise direction. These conditions, however, are hardly met in real combustion chambers, which makes their direct application in industry troublesome.
In our study, we apply a recent method to investigate the transport of entropy waves in a three-dimensional, strongly non-parallel flow field. The method is based on the governing equations of the flow linearized around its temporal mean state. The temporal mean flow, which needs to be provided as input to our methodology, takes into account the effect of turbulent shear dispersion. The impact of turbulent transport on the convection of the entropy waves is considered by an eddy diffusivity. The resulting set of equations is discretized in the three-dimensional domain using tetrahedral, continuous-Galerkin finite elements.
We apply the method to the experiment of a turbulent duct flow of rectangular cross section. A pulsating hot jet in crossflow penetrating the boundary layer in the duct creates pockets of varying temperature (or entropy waves) which are convected along the duct by the highly turbulent, non-parallel flow. The temporal means of the flow, as well as the validation data is obtained by Large Eddy Simulations of the configuration.
Our study shows that the presented methodology can reproduce the Strouhal number dependent mitigation of entropy waves in the mixing duct which is observed in the experiment and the LES. Unlike the LES, the method allows to quantify the contributions of the different effects that lead to this mitigation. It is shown that the strongly non-parallel flow region at the jet in crossflow significantly contributes to both mean flow shear dispersion and turbulent diffusion. In the more parallel flow field downstream of the jet in crossflow, the effect of turbulent diffusion in cross-streamwise direction is dominant, while turbulent diffusion in streamwise direction plays a subordinate role, especially for low frequencies.
Presenting Author: Thomas Ludwig Kaiser TU Berlin
Presenting Author Biography: After concluding his studies at TU Munich focussing on CFD and thermofluiddynamics, Thomas went to IMFT Toulouse/ France to obtain his PhD. There he focussed on thermoacoustics and linear stability analysis of swirling flows. Since 2018 he is having a PostDoc Position at TU Berlin.
Authors:
Thomas Ludwig Kaiser TU BerlinNicolas Noiray ETH Zürich
Quentin Male ETH Zürich
Kilian Oberleithner TU Berlin
Modeling the Convection of Entropy Waves in Strongly Non-Parallel Turbulent Flows Using a Linearized Framework
Paper Type
Technical Paper Publication