Massively parallel LES of azimuthal thermo- acoustic instabilities in annular gas turbines

Most of the energy produced worldwide comes from the combustion of fossil fuels. In the context of global climate changes and dramatically decreasing resources, there is a critical need for optimizing the process of burning, especially in the field of gas turbines. Unfortunately, new designs for efficient combustion are prone to destructive thermo-acoustic instabilities. Large Eddy Simulation (LES) is a promising tool to predict turbulent reacting flows in complex industrial configurations and explore the mechanisms triggering the coupling between acoustics and combustion. In the particular field of annular combustion chambers, these instabilities usually take the form of azimuthal modes. To predict these modes, one must compute the full combustion chamber comprising all sectors, which remained out of reach until very recently and the development of massively parallel computers. A fully compressible, multi-species reactive Navier-Stokes solver is used on up to 4096 BlueGene/P CPUs for two designs of a full annular helicopter chamber. Results show evidence of self-established azimuthal modes for the two cases but with different energy containing limit-cycles. Mesh dependency is checked with grids comprising 38 and 93 million tetrahedra. The fact that the two grid predictions yield similar flow topologies and limit-cycles enforces the ability of LES to discriminate design changes.