Search

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.

This thesis considers the control of combustion oscillations, motivated by the susceptibility of lean premixed combustion to such oscillations, and the long and expensive development and commissioning times that this is giving rise to. The controller used is both closed-loop, employing an actuator to modify some system parameter in response to a measured signal, and adaptive, meaning that it is able to maintain control over a wide range of operating conditions. The controller is applied to combustion systems with annular geometries, where instabilities can occur both longitudinally and azimuthally, and which require multiple sensors and multiple actuators for control. One of the requirements of Lyapunov-based adaptive control which is particularly troublesome for combustion systems is then addressed: that the sign of the high-frequency gain of the open-loop system is known. We address it by using an adaptive controller which employs a Nussbaum gain, and successfully apply it experimentally to combustion oscillations in a Rijke tube. Another type of fluid-acoustic resonance is then considered: the compressible flow past a shallow cavity. We start by finding a linear model of the cavity flow's dynamics, or its 'transfer function', which we identify from direct numerical simulations. We compare this measured transfer function to that given by a conceptual model which is based on the Rossiter mechanism, and which models each component of the flow physics separately. We then look at using closed-loop control to eliminate these cavity oscillations. We start by designing a robust H₂ controller based on a balanced reduced order model of the system, the model being provided by the Eigensystem Realization Algorithm (ERA). The robust controller provides closed-loop stability over a much wider Mach number range than seen in previous studies. Finally, we look at the suitability of the adaptive controller, earlier developed for combustion oscillations, for the cavity. Based on some general properties of the cavity flow, and by using collocated control, the oscillations are eliminated at all Mach numbers tested in the range 0.4 ≤ M ≤ 0.8.