Gas turbine engines are widely used for aviation and for power generation, and there is a currently a focus on reducing harmful emissions from these. When new gas turbine engines are designed, they can experience a phenomenon known as a thermoacoustic instability, which can cause large and potentially damaging pressure oscillations to occur. Thermoacoustic instabilities are more likely to occur in new low emission systems, therefore we need the ability to predict when these will occur, in order to avoid costly mistakes being made during the design phase of a new engine. The current project aims to improve our understanding of how these instabilities occur in relatively realistic conditions, so we will be able to design better low emission engines in future. This project was therefore important for society, as it will ultimately make it easier to design new low emission gas turbine engines, and other combustion systems such as rockets or boilers. This will allow us to generate clean energy and allow future power plants to operate using a wider variety of fuels.
Gas turbine engines often feature annular combustion chamber geometry, and the aim of the current project was to understand the consequence of thermoacoustic instabilities in this realistic geometry. Therefore, the first objective of the project was to design a new unique pressurised annular combustion facility, which was used to understand this phenomenon under engine realistic conditions. While thermoacoustic instabilities occur spontaneously, when they do so in annular geometry there are a huge range of parameters that influence these. Therefore, the second objective was to develop a method of forcing oscillations onto the flow inside a combustor in order to isolate certain behaviours, and to simplify their study. We then characterised their behaviour and aimed to describe it so that the behaviour can be modelled in future, which will ultimately give us a way to predict when such instabilities occur. The flow inside realistic combustors is complex and highly three-dimensional. Therefore, to help us understand these complex flow fields, the final objective was to develop new 3D measurement methods and apply these to be understand these instabilities, in order to describe them in much more detail than has been previously possible.
At the end of the project, all major objectives have been met. It can be concluded that the response of combustion instabilties at elevated pressure do differ somewhat from those at atmospheric pressure, meaning the boundary conditions are important. The work published on this topic forms a valuable reference for future studies. A method of forcing was successfully applied, and used to examine in detail the response of different modes of oscillation. It was found that the response does depend on the mode of oscillation, but that a relatively simple empirical model can be used to describe all responses. Finally, new methods were developed to describe the three-dimensional structure of asymmetric flames. In conclusion, the project has resulted in a better understanding of thermoacoustic instabilities in annular geometry. However, it has also left open questions, particularly with regard to the flame response to different modes of oscillation. This will be pursued in future work.
