Gas turbine engines are widely used to power aircraft flight, and in gas-fired power plants to generate electricity. While such engines are considered clean and efficient, they still produce some harmful emissions during combustion. One possibility to help reduce these is to change the amount of fuel used in aircraft engines, which can reduce the emission of pollutants such as nitrous oxides, which are bad for local air quality. Perhaps even more promising is the idea of switching to alternative fuels such as hydrogen, which can perhaps be used for power generation in order to completely eliminate the emission of greenhouse gasses such as carbon dioxide. However, when changing the amount or type of fuel, a common problem that engine designers meet is an issue called a «combustion instability». This project will help us better understand the physics which produce these unwanted instabilities, so that we can more easily design low- and even zero-emission engines in the future.
During an instability the flow of fuel and air pulsates. This causes flames in the combustion chamber to puff - often hundreds or even thousands of times a second. During each puff, a vortex ring-type structure is formed. These vortex structures look a bit like when somebody blows a smoke ring into the air. When such vortex structures meet the flame in a combustion chamber, they interact, causing the flame to change shape. Understanding exactly how and when the flames change shape is crucial to understanding the instability phenomena. Therefore, in order to understand this, we will measure the size and shape of the vortex structures, and the flame shape using high speed lasers and cameras. We will also use our measurements to help design a simple way to simulate the important changes to the flame shape, so that these can be predicted more easily in future. We will make these measurements for a wide range of different hydrogen fuel blends, to understand the issues relating to this type of fuel.
Three researchers have now joined the project, and together we have started to make measurements of the vortex structures and flame shape in a single isolated flame. These measurements involve introducing small particles into the flow, and illuminating these using a laser. We then measure the displacement of the particles in different regions. You can view a video of this technique in our lab here: https://twitter.com/NTNU_TCL/status/1403347486136283139?s=20
We take thousands of images to calculate the average size of the vortex structures, and we are currently processing these before we start our analysis. We have also designed some new equipment to help measure the vortex structures in a full annular combustor. An annular combustor has multiple flames which can interact with each other, and as is much closer to the design of a real gas turbine engine. We will begin testing in the annular combustor later this month, helping us understand the physics of the phenomenon in a relevant system.
At present we do not fully understand combustion instabilities in gas turbine combustors, and this lack of understanding hinders the development of low emission technology. The current proposal aims to generate a step change in our scientific understanding of flow field underlying this phenomena.
In order to achieve this aim a dedicated research group will undertake a wide ranging and ambitious programme of experimental and numerical work. A series of novel experiments will explore the phenomenology of vortex dynamics in thermoacoustically unstable hydrogen flames with the aim of forming new links between the flow field and flame response. Vortex scaling laws will then be developed and used to predict the behaviour and scaling of the heat release oscillations; creating a unique way of modelling this phenomena, using a more generalised and physically relevant description of the flame response. Low order vortex and flame modelling approaches will also be combined, to develop a new way to predict flame stability from first principles. Finally, flow asymmetry will be investigated as a new way to control the vortex dynamics and the stability of reacting flows.
This approach will dramatically improve our scientific understanding of this phenomena, and lead to the advancement of our predictive and design capability, allowing us to realise the operational and environmental benefits of modern low emission gas turbine technology. Furthermore, the new approach to modelling these instabilities and the study of fundamental flow regimes are very likely to stimulate future research, with a high potential to open up completely new research areas.