Prediction of thermoacoustic instability in gas turbine combustors

This thesis numerically predicts thermoacoustic instability in two typical gas turbine combustors. Incompressible large eddy simulation (LES) is used as the main numerical tool to simulate the heat release response of a flame to upstream acoustic perturbations, resulting in linear and nonlinear flame response models. These flame models are then combined with analytical acoustic wave models to predict thermoacoustic instability, and for those unstable operations, the resulting nonlinear limit cycle oscillations. Other features relevant to thermoacoustic instability are also studied, including entropy wave transport within the combustor flow fields, and the axial variation of the heat release response along a long flame length, etc. This thesis has found that (i) the incompressible LES performed by BOFFIN and OpenFOAM can both accurately simulate the reacting flow in realistic gas turbine combustors, and capture the forced flame heat release responses to upstream acoustic perturbations; (ii) although different combustion and chemical reaction models lead to different unforced flame behaviour, their influences on the forced flame responses are relatively smaller, suggesting the use of simpler chemistry to reduce computational costs and providing more options for combustion modelling; (iii) both the low order network-based and the Helmholtz equation-based thermoacoustic solvers are validated by accurate thermoacoustic predictions; (iv) the entropy wave transport within a realistic combustor flow is dominated by the dispersive advection, affected by both mean and unsteady large-scale flow features (with the latter having higher effect). The generated entropy noise is likely to affect the thermoacoustic modes at high flow speed conditions; (v) the axial variation of the heat release response along a very long flame needs to be accounted for in the network-based thermoacoustic prediction, which is done by splitting the continuous flame into multiple segments, each treated as an individual flame and represented by a compact flame model. All of the above findings significantly help to gain insight into the prediction of thermoacoustic instability for realistic gas turbine combustors.Open Acces