Energy demand will be up by more than a quarter by 2040 [International Energy Agency data]. Given the dominance of combustion in meeting this demand, it is imperative to develop low-carbon, efficient gas turbine (GT) engines to reduce emissions impact and tackle the global warming as set by the Paris Agreement. In recent years lean premixed technology has attracted interest due to its potential of reduced emissions and high efficiency. However, lean combustion is prone to instabilities that may lead to unwanted oscillations, flame extinctions and flashbacks. Use of low or zero-carbon fuels like hydrogen is also limited because the high speeds needed to prevent flashbacks due the high low-heating values (LHV) can destabilise the vortex dynamics. Further development is thus required to achieve better efficiency and lower emissions, and effective flame holding techniques are crucial for this development. In ultra-compact combustor design, trapped vortex (TV) systems are implemented either in the primary zone or in the inter-turbine region to increase the resident time of combusting gases, resulting in better mixing, thus higher efficiency and lower emissions. Higher resident times also imply a shorter combustor, thus a lighter engine and less fuel consumption, also helping the process of hybridisation in multi-cycle devices. TV are locked stably within a cavity and thus are less sensitive to external disturbances even at high speeds, allowing use of low or zero-carbon fuels with high LHV like hydrogen. However, the process of flame stabilisation is rather complex because of the shear and boundary layer (BL) vortex dynamics, the strong heat transfer to the wall and the simultaneous occurrence of flame propagation and auto-ignition processes. The effective control of the flame dynamics requires a deep understanding of these processes.
This project aims to develop improved understanding of the fundamental processes governing flame stabilisation in TV systems for ultra-compact combustion design, and their potential to deliver improved flame stability and low emissions at high speed (subsonic) conditions in the context of lean premixed technology. In particular, the TV physics will be studied i) in presence of a radially accelerating flow representing the swirled flow dynamics at the entrance of the combustion chamber; and ii) in presence of an axially accelerating flow when the cavity is located within the converging duct near the combustor exit. Both swirled and axial acceleration can destabilise the vortex dynamics, so this dynamics has to be understood before TV systems can be effectively employed. The analyses will be conducted through high-fidelity large eddy simulations (LES), which represents a cost-effective tool as compared to expensive experimental investigations. In this way the effect of turbulence, equivalence ratio and cavity geometry can be explored in details via parametric study. Moreover, the performance of different alternative fuels and their implication in terms of flame holding and model performance can be evaluated for different TV designs. An improved model involving presumed PDF approaches based on mixed flamelets/perfectly stirred reactor will be developed to account for the aforementioned physics. The fundamental understanding for this development will be extracted from unprecedented detailed direct numerical simulation (DNS) and by using validation data from experiments provided by the project partners.
The outcomes of this project will significantly help the development of modern, low-carbon engines, and improve the understanding of the fundamental physics within these devices. Moreover, the project will lead to the development of CFD codes and models that can be used in industrial design cycles. Thus, this project is timely and strongly relevant for leading UK industries such as Rolls-Royce and other emerging industry, and will help them to maintain their leading role in the power-generation sector.

Potential Impact:
The development of trapped vortex systems for ultra-compact combustion (UCC) engines in the context of lean premixed technology, and predictive numerical tools to explore and understand the complex physics within these devices both have a strong long-term impact on the UK and the international industry. In particular, the collaboration with Rolls-Royce, which is a project partner, will aid to maintain the technological leadership in the power-generation sector within the UK. In addition to the UK strategic need for environmental impact due to the development of more efficient and low-carbon engine technology, UCC design implies a lighter structure, which implies less fuel consumption and can help the process of hybridization with electric sources and multi-cycles engines for high-speed transport flights and `green' power plants. Moreover, the better flame anchoring achieved through the use of trapped vortex cavities may allow effective use of fuels with high LHV like hydrogen (that produces no carbon) or syngases (i.e. gases with a percentage of hydrogen) to achieve a carbon-neutral strategy, meaning the amount of carbon produced is sustainable because balanceable by current technologies. The long term goal of the present research in understanding how to control the stabilisation process in these devices will thus strongly benefit the environment and consequently the health and quality of life of the society. Only considering that by 2040 the aviation sector will account to about 25% of the CO2 emissions of the entire transportation sector, shows the high impact that this research can have in the middle and long term. The lighter structure (less fuel per kilometre) allowed by ultra-compact devices also lead to decreasing the cost of travelling. The more efficient engine will reduce the cost of on-demand electricity produced in power-plants. This also plays in favour of local societies in developing economies.

This research will complement other EPSRC strategic researches on flame stabilisation based on different concepts such as plasma and electric fields stabilisation, flameless combustion and closed-loop engine generators, and thus add a significant contribution to the achievement of low- or zero-carbon technology. UK-based multinationals such as Siemens and Rolls-Royce (RR) are leading developers of lean premixed technology and thus will directly benefit from this research. Furthermore, the development of optimised, accurate and computationally-effective models for compressible flows, and in particular the statistical approaches explored in the present research, is of paramount importance to these industries as these models can be effectively employed within industrial design cycles of novel engines. This intent was declared at the First Workshop on Compressible Combustion (WCC) held in Aachen, Germany, in May 2019, where leaders of Siemens, Rolls-Royce and international academic groups met to discuss further advances within lean premixed technology and its modelling for relatively high speed (compressible) conditions. Thus, the proposed research is timely and will have a direct long-term impact to the above and other emerging UK industries.

The potential manufacturing of UCC devices and novel design concepts for low-carbon engines in the longer term will have further economical benefits. On the one hand, it will lead to additional researches to address further technological issues, thus creating employment for future academics; on the other hand, novel concepts may lead to creation of opportunities for emerging industries as well as further employment in leading industries in the aeronautical and power generation sectors (e.g. Rolls-Royce and Siemens within the UK, Alstom, GE, etc). The understanding and knowledge achieved within this and future related researches will thus help the formation of skilled people with specific expertise in a key area for the future of low-carbon technology.