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Breaking the paradigm: A new approach to understanding and controlling combustion instabilities

Periodic Reporting for period 3 - TAIAC (Breaking the paradigm: A new approach to understanding and controlling combustion instabilities)

Reporting period: 2019-09-01 to 2021-02-28

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 is 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 is to understand the consequence of thermoacoustic instabilities in this realistic geometry. Therefore, the first objective of the project is to design a new unique pressurised annular combustion facility, which will be 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 is 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 will then characterise their behaviour and aim 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 is 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.
The project currently involves five scientific staff who are working to meet the project objectives, by addressing three main work packages.

The first work package involves the design, manufacture and testing of a new pressurised annular combustion facility. The facility has been commissioned and a wide range of experiments have been conducted, generating a particular type of thermoacoustic instabilities in a pressurized lab scale experiment for the first time. Analysing these instabilities has led to new insights into the physics of this phenomenon. The results demonstrate that operating the experiment at pressure can change the results, and have revealed features which do not exist in more simplified, less realistic setups. These results thereby significantly advance our understanding this phenomenon in practically relevant systems. These results have been published in 2 journal papers, and presented at 2 international conferences.

The second work package involves the development of a new type of acoustic forcing for annular combustors, in order to carefully study their controlled response. A series of experiments using this novel acoustic forcing were conducted, which demonstrated for the first time that the flame response in an annular combustor is dependent on the acoustic mode type, representing a breakthrough in our understanding of this phenomenon. Since this initial discovery, we have introduced a new framework allowing us to fully describe instabilities in annular systems. We have also used this understanding to refine numerical models, and found that when the mode type is correctly taken into account we can accurately capture certain unique features of the instability behaviour. These results have been published in 2 journal papers, with a third currently under review, which taken together offer a completely new way to describe instabilities in realistic annular geometry.

The third work package involves measuring and describing the full three-dimensional structure of unstable flames in an annular combustor. Conventional diagnostics involve integrated line-of-sight or planar methods, which are of limited use when applied to highly asymmetric flows. Therefore, a novel laser scanning method has been developed, allowing us to capture the flame structure slice by slice. A series of experimental measurements have been made, and these results are currently being processed and analysed, but it is expected that they will offer unique insight into the underlying physics of the flow inside a realistic annular geometry.
The development of the new pressurised facility has allowed us to investigate thermoacoustic instabilities under more realistic conditions than previously possible, therefore the measurements performed in this facility have significantly advanced our physical understanding of the phenomenon beyond the current state of the art.

The development of the new acoustic forcing system has also helped us to isolate different modes of oscillation in order to better understand how the flow behaves. This ability to accurately control the mode of oscillation in annular chambers goes beyond the current state-of-the-art, and is an important technical capability. Furthermore, during the analysis of results from this facility, we have also introduced a new framework to describe instabilities in annular geometry, which goes beyond the existing description suitable only for single isolated flames. This new framework has allowed numerical models to accurately predict certain characteristics of the response in annular combustors, significantly advancing the state-of-the-art in terms of our predictive capabilities.

Finally, the development of three-dimensional measurement methods applied to a complex asymmetric flow also goes beyond the current state-of-the-art planar and integrated line of sight methods, and will allow us to understand the full three-dimensional structure of these flows in detail.