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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.

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This paper describes the research carried out in the European Commission co-funded project LEMCOTEC (Low Emission Core Engine Technology), which is aiming at a significant increase of the engine overall pressure ratio. The technical work is split in four technical sub-projects on ultra-high pressure ratio compressors, lean combustion and fuel injection, structures and thermal management and engine performance assessment. The technology will be developed at subsystem and component level and validated in test rigs up to TRL5. The developed technologies will be assessed using three generic study engines (i.e. regional turbofan, mid-size open rotor, and large turbofan) representing about 90% of the expected future commercial aero-engine market. Two additional study engines from the previous NEWAC project will be used for comparison. These are based on intercooled and intercooled-recuperated future engine concepts. The compressor work is targeting efficiency, stability margin and flow capacity by improved aerodynamic design. High-pressure and intermediate-pressure compressors are addressed. The mechanical and thermo-mechanical functions, including the variable-stator-systems, will be improved. Axial-centrifugal compressors with impeller and centrifugal diffuser are under investigation too. Three lean burn fuel injection systems are developed to match the technology to the corresponding engine pressure levels. These are the PERM (Partially Evaporating Rapid Mixing), the MSFI (Multiple Staged Fuel Injection) and the advanced LDI (Lean Direct Injection) combustion systems. The air flow and combustion systems are investigated. The fuel control systems are adapted to the requirements of the ultra-high pressure engines with lean fuel injection. Combustor-turbine interaction will be investigated. A fuel system analysis will be performed using CFD methods. Improved structural design and thermal management is required to reduce the losses and to reduce component weight. The application of new materials and manufacturing processes, including welding and casting aspects, will be investigated. The aim is to reduce the cooling air requirements and improve turbine aerodynamics to support the high-pressure engine cycles. The final objective is to have innovative ultra-high pressure-ratio core-engine technologies successfully validated at subsystem and component level. Increasing the thermal efficiency of the engine cycles relative to year 2000 in-service engines with OPR of up to 70 (at max. condition) is an enabler and key lever of the core-engine technologies to achieve and even exceed the ACARE 2020 targets on CO2, NOx and other pollutant emissions: • 20 to 30 % CO2 reduction at the engine level, exceeding both, the ACARE 15 to 20% CO2 reduction target for the engine and subsequently the overall 50% committed CO2 and the fuel burn reduction target on system level (including the contributions from operations and airframe improvements), • 65 to 70 % NOx reduction at the engine level (CAEP/2) to attain and exceed the ACARE objective of 80% overall NOx reduction (including the contributions from both, operational efficiency and airframe improvement), reduction of other emissions (CO, UHC and smoke/particulates) and • Reduction of the propulsion system weight (engine including nacelle without pylon).

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