Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - TAIAC (Breaking the paradigm: A new approach to understanding and controlling combustion instabilities)

Teaser

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...

Summary

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 a huge range of different modes can exist, which cause different flow behaviours. 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.

Work performed

The project currently involves three scientific staff who are working to meet the project objectives.

The first work package involves the design and manufacture of a new pressurised annular combustion facility. We have completed the design of this, and it is currently being manufactured. We have also been working to develop the laboratory, and to purchase and make ready the necessary measurement equipment and control systems to operate this equipment. The rig is planned to be operational by Autumn this year. The major achievement is the completion of the test rig design.

The second work package involves the development of a new type of acoustic forcing for annular combustors. A researcher has been working to develop control hardware and software in order to control the oscillation pressure field inside the combustion chamber, and we have a working system in place. We have made experimental measurements using this control system, and have submitted several papers which is currently under review. We have also presented these results at the 2017 American Physical Society conference (APS DFD). The main achievements are the technical ability to control the acoustics inside our combustor, and new insight into the dynamics during strongly spinning modes.

We have also made several smaller studies to examine the effect of system symmetry on thermoacoustic instabilties, using different flow configurations and acoustic dampers to vary the symmetry around the annular chamber. We have presented the investigation into varying the injector swirl level at the 2017 American Physical Society conference (APS DFD), and will present work on acoustic dampers at the 2018 ASME Turbo Expo. We have also submitted some of this work as a journal paper which is currently under review. The major achievement here is new understanding of the flow physics, and a better understanding of how to control these instabilities with damping devices.

Finally, in order to understand the dynamics of these systems, we have also been studying the effect of acoustic forcing on simpler systems, such as a canonical round jet flow. We have presented this work at the 16th European Turbulence Conference, and also at the 2017 American Physical Society conference (APS DFD), and are currently preparing a journal publication for this work. Again, the major achievement of this work is the ability to better understand the physics of flows subject to large pressure oscillations.

Final results

The development of the new pressurised facility will allow us to investigate thermoacoustic instabilities under more realistic conditions than previously possible, therefore the measurements performed in this facility are likely to significantly advance our physical understanding of the phenomenon beyond the current state of the art.

The development of the acoustic forcing system is already helping us to isolate different modes of oscillation in order to understand how the flow behaves during these, in order to make more accurate assessments of the heat release oscillations generated during such instabilities. 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. We will be using this new method to conduct a wide ranging study of different flow conditions in order to build up our understanding of how instability modes affect the flow behaviour in our combustors, and the important implications of this for engine manufacturers.

We will also be hiring an additional scientific researcher who will be responsible for developing three dimensional measurement methods which will also go beyond the current state-of-the-art planar and integrated line of sight methods, and allow us to understand the full three-dimensional structure of these flows.