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To cite this version:Michaël Bauerheim, Michel Cazalens, Thierry Poinsot. A theoretical study of mean azimuthal flow and asymmetry effects on thermo-acoustic modes in annular combustors. Proceedings of the Combustion Institute, Elsevier, 2015, vol. 35 (n°3) AbstractThe objective of this paper is to develop an analytical model to capture two symmetry breaking effects controlling the frequency and nature (spinning, standing or mixed) of azimuthal modes appearing in annular chambers: (1) Using two different burner types distributed along the chamber (2) Considering the mean azimuthal flow due to the swirlers or to effusion cooling. The ATACAMAC (Analytical Tool to Analyze and Control Azimuthal Modes in Annular Chambers) methodology is applied using the linearized acoustic equations with a steady and uniform azimuthal mean flow. It provides an analytical implicit dispersion relation which can be solved numerically. A fully analytical resolution is possible when the annular chamber is weakly coupled to the burners. Results show that symmetry breaking, either by mixing burners types or with a mean azimuthal flow, splits the azimuthal modes into two waves with different frequencies and structures. Breaking symmetry promotes standing modes but adding even a low azimuthal mean flow fosters spinning modes so that the azimuthal mean flow must be taken into account to study azimuthal modes.

An extension of the Large Eddy Simulation (LES) technique to two-phase reacting flows, required to capture and predict the behavior of industrial burners, is presented.While most efforts reported in the literature to construct LES solvers for two-phase flow focus on Euler-Lagrange formulation, the present work explores a different solution ('two-fluid' approach) where an Eulerian formulation is used for the liquid phase and coupled with the LES solver of the gas phase. The equations used for each phase and the coupling terms are presented before describing validation in two simple cases which gather some of the specificities of real combustion chamber: (1) a one-dimensional laminar JP10/air flame and (2) a non-reacting swirled flow where solid particles disperse [1]. After these validations, the LES tool is applied to a realistic aircraft combustion chamber to study both a steady flame regime and an ignition sequence by a spark. Results bring new insights into the physics of these complex flames and demonstrate the capabilities of two-fluid LES.

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The design of a clean combustion technology based on lean combustion principles will have to face combustion instability. This oscillation is often discovered late in engine development when unfortunately only a few degrees of freedom still exist to solve the problem. Individual component test rigs are usually not useful in detecting combustion instability at an early stage because they do not have the same acoustic boundary conditions as the full engine. An example of this unsteady activity phenomenon observed during the operation of a high-pressure core is presented and analyzed. To support the investigation work, two numerical tools have been extensively used: (1) experimental measurement of unsteady pressure and the results of a multidimensional acoustic code are used to confirm that the frequency variations of the observed modes within the operating domain of the high pressure core are due to the excitation of the first and second azimuthal combustor modes. The impact of acoustic boundary conditions for the combustor exhaust is shown to control the appearance and mode transition of this unsteady activity. (2) 3D reacting and nonreacting Large Eddy Simulations (LES) for the complete combustor and for the injection system cup alone suggest that the aerodynamic instability of the flow passing through the cup could be the noise source exciting the azimuthal acoustic modes of the chamber. Based on these results, the air system (cup) was re-designed in order to suppress this aerodynamic instability and experimental combustion tests confirm that the new system is free of combustion instability.