60184 - Application of Large Eddy Simulation for Ha-Class Combustion System Design to Mitigate Combustion Instabilities (Frequency, and Amplitude) 

Enabled by national commercialization of massive shale resources, Gas Turbines continue to be the backbone of power generation in the US. With the ever-increasing demand on efficiency, GT combustion sections have evolved to include shorter combustion lengths and multiple axial staging of the fuel, while at the same time operating at ever increasing temperatures.

To reduce the development cost and time of the new units, HA combustion development program has leveraged extensive use of CFD analysis through out the design cycle. In addition to  Reynolds Averaged Navier Stokes (RANS) based tools, the program has also utilized the computationally expensive Large Eddy Simulation (LES) tools. These simulations have been conducted to influence the combustor aerodynamics, and the fuel and air mixing in order to impact thermo acoustic and emission characteristics of combustors and to increase the operability window accordingly. A large amount of computational resources have been devoted towards the use of LES tools for predicting combustion instabilities (amplitude & frequency), for both the  engine cofiguration (multiple combustor cans) as well as the high pressure in house test rig (single combustor can situated within the high pressure lab cavity).  Simulations have been conducted for different design candidates over a wide range of operating parameters. The results of these simulations have provided invaluable insight into the combustion dynamics tones, and their amplitudes that could be excited in the engine, very early in the design cycle.  This has consequently enabled the mitigation of the undesired tones through simple hardware design changes and /or through conducting operations in a different fuel split space.

This paper presents the results of very detailed Large Eddy Simulations of one (or two) combustor can(s) for a 7HA GE Gas Turbine Engine over a range of operating parameters.  The model of the simulated combustor can(s) includes all the details of the combustor from the compressor diffuser to the end of the stationary part of the first stage of the Turbine. It includes the geometries of multiple pre-mixers within the can and the complete design features for axial fuel staging.

The CharLES flow solver developed by Cascade Technologies was used for these simulations. CharLES is a suite of massively-parallel CFD tools designed specifically for multiphysics LES. The models and numerical methods in CharLES are carefully formulated for accuracy, robustness, and efficiency in high-fidelity engineering simulations. The numerical discretization uses a novel entropy-preserving scheme to achieve numerical stability on unstructured grids with low numerical dissipation. Consequently, the scheme can effectively simulate turbulent combustion without dissipating the high-frequency eddies that are important for fuel/air mixing or acoustic waves that sustain thermoacoustic instabilities.

Thermo acoustic results from LES were validated first in the physical GE lab and then in engine testing.  Both the trend as well as the predicted amplitudes for the excited axial dominant combustion mode matched the data produced in the lab and in the engine. The simulations also revealed insight into the ingestion of hot gases by different hardware pieces that may occur when  machine operates under medium to high combustion dynamics amplitudes. This insight then informed the follow-on design changes which were made to the existing hardware to mitigate the problems encountered.