Combustion chambers are the heart of gas turbines, which are the cornerstone of aviation and the most efficient way to generate significant power. Extensive research on portions of real combustors has revealed novel processes of combustion dynamics physics. Predicting how these physics play out in real, more complex systems is crucial. With the support of the Marie Skłodowska Curie Actions programme, the ANNULIGhT project characterised combustion dynamics in full annular gas turbine combustors. The new methodologies, tools and fundamental understanding presented in more than 40 journal publications are now state of the art in the field and will support the development of next-generation low-emission, zero-carbon gas turbines for propulsion and power.
Thermo-acoustic instabilities, lean blow-off and reignition
Most jet engines rely on annular (doughnut-shaped) combustors with multiple injectors around the circumference to stabilise the flames. “Thermo-acoustic instabilities occur when acoustic (pressure) waves interact with flames and are amplified, causing the combustion system to resonate. This occurred with the Saturn V rocket. In annular combustors, these pressure waves travel around the annulus and can be strong enough to severely damage the engine and cause failure,” explains James Dawson of the Norwegian University of Science and Technology and ANNULIGhT coordinator. Lean blow-off and reignition are interrelated challenges. Combustion with more air than fuel minimises nitrogen oxide emissions, but too much air can extinguish the flames. Engineers ensure that engines can be reignited; however, their understanding of the processes is limited because they cannot see into the engine when running tests – they only know whether the engine is on or not.
Full annular combustor investigations yield critical insight
“ANNULIGhT has several unique laboratory-scale annular combustors that allow us to see what happens in a real engine, like when you ignite one of the flames and it travels around the combustor igniting all the injectors (light around). Experiments using high-speed cameras and lasers enable us to capture the essential complexity of real engines. These are combined with high-fidelity numerical simulations which provide even more detail,” explains Dawson. The team discovered that symmetry breaking is crucial to control of thermo-acoustic instabilities in annular geometries. This led to new computational methods and tools to identify key parameters to prevent these instabilities and improve designs. Visualising lean blow-off as well as ignition and light around has provided vital insight into how to make engines safer. ANNULIGhT has demonstrated that the rich variety of system responses occurring in annular geometries can only be revealed with full annular combustor geometries. The enhanced understanding of the physics of engine relight and the new physics-based tools to better predict and prevent combustion dynamics will support the design of next generation combustors.
Academia and industry united to fuel combustor development
The strong collaboration in ANNULIGhT is already yielding practical progress. Experiments and numerical simulations showed that very high rotational flows suppress thermo-acoustic instabilities, and the phenomenon was explained theoretically. This has led to developments in spinning combustion technology from partner Safran Helicopter Engines. Dawson concludes: “Rocket science gets all the hype, but the gas turbine is an incredibly complex and beautiful machine. I hope the next time you sit on an aeroplane and look out your window to see this technical wonder, you ask yourself ‘how does it work?’.” ANNULIGhT has the answer.
ANNULIGhT, combustor, annular, gas turbine, thermo-acoustic, lean blow-off, reignition