Development of ultra-compact combustors for low-carbon technology using trapped vortex concepts

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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.