The University of Edinburgh is purchasing a steady flow, high pressure (P < 120 bar) and temperature (T < 1000 K) optically accessible jet and spray research chamber. This chamber is unique within the UK. In addition, the university is also buying a single-cylinder optically accessible research engine. The chamber can be used to study sprays of all kinds; how they develop and react. The engine can be used to study transient fuel sprays as they interact with realistic in-cylinder flows. With this grant, the University of Edinburgh will acquire highly advanced laser diagnostics for multi-parameter measurements in the new chamber and engine, and in other related experimental devices, as a means to leverage the university's substantial equipment investment (£1.4 million) into a UK-wide Small Research Facility (SRF). The measurements to be acquired by this SRF include: a) A femtosecond laser system and ancillary devices (e.g. a second harmonic bandwidth compression system (SHBC), frequency resolved optical gating (FROG) to characterize the pulses etc.). The system will be used for hybrid picosecond/femtosecond rotational CARS (coherent anti-Stokes Raman spectroscopy), for line-image temperature and species (e.g. O2, N2, H2 etc.) in the jet/spray equipment, and ballistic imaging for investigation of primary breakup in highly atomizing sprays. b) High-speed (HS) 2-pulse, 532 nm wavelength laser and HS imaging systems for HS stereoscopic PIV, SLIPI imaging, and LII for particulate. A HS 1-pulse, 355/266 nm wavelength laser and HS intensifier system for HS PLIF, phosphors, and LITA. c) A phase Doppler instrument for droplet/particle size distribution and velocity in reactive jets and sprays The combined equipment and diagnostics will enable new studies on: a) Fuel sprays (including alternative fuels), and b) Supercritical materials synthesis (biofuels, pharmaceuticals, nano-catalysts, polymers etc.). Our research goals are multi-faceted. The research will enable more efficient combustion engines, reducing their impact on the climate. It will also make it possible to understand and then improve supercritical processing for materials synthesis, helping bring such products to market more effectively. In so doing we will address critical needs for both established industries and for key emerging industries across the UK.

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.

In recent times, immense efforts have been made in engine research to develop new concepts, from common-rail Diesel and GDI to HCCI (now primarily seen as an operating mode of the aforementioned), in order to meet increasingly-stringent demands for fuel consumption and emissions reductions. There is an urgent need for accurate simulation tools, ideally applicable to all engine types and operating modes, to assist engine designers to meet these targets. The modelling of turbulent reactive flows has always been a trade-off between capturing the complexity of the flow and the complexity of the chemical kinetics (with due regard for turbulent-chemistry interactions), due to the limitations in computer resources. The former is particularly demanding in ICE modelling and has tended in the past to receive most of the attention, but over the past decade increasing effort has gone into the combustion modelling, due to the availability of high-power and low-cost computers. IC engine development leads towards novel diesel combustion concepts that break the traditional NOx vs. PM trade-off of classic diffusion controlled combustion require. Pollutant formation in Diesel engines is mainly mixing controlled and a better understanding of the complex turbulence-chemistry interactions that strongly influence the formation and destruction of pollutants is required. The proposed research will extend the existing CFD methodology for engine simulation to accurately account for (i) mixing of fuel and oxidizer, especially due to large-scale motion, (ii) turbulence-chemistry interactions and (iii) cycle-to-cycle variations that cannot be predicted by current state-of-the-art three-dimensional Reynolds-averaged simulations (RAS). It is widely accepted that large-eddy simulation (LES) holds the largest potential of all present fluid dynamics models to accurately capture large-scale mixing and cyclic effects of in-cylinder motion, however, LES needs be combined with advanced combustion models for the correct treatment of the turbulence-chemistry interactions. Very recent studies have established the potential of the conditional moment closure (CMC) approach as a suitable combustion model for IC engines and as a suitable combustion sub-model in the LES context. This potential will now be exploited and the integration of CMC into LES for engine computations is at the core of this project. The research will primarily focus on the closure of the turbulent reaction rate term, associated turbulence-chemistry interactions and improvements to pollutant predictions. The effects of LES modelling on droplet motion, large-scale fuel-oxidizer mixing and the predictability of cyclic variations will be assessed. The LES-CMC approach will be validated by comparison with measurements from engine-like experiments of increasing complexity and trends for the dependence of NOx and soot emissions on engine operating conditions will be investigated.

Burning oil-based fuels accounts for approximately 31% of UK greenhouse gas emission as well as being a major culprit in toxic, irritant and carcinogenic pollutants on a national scale (www.naei.org.uk). Nearly all forms of transport currently rely on liquid fossil fuels, and this use of liquid fuel is likely to continue; the storage technology for electricity and hydrogen is not good enough completely to replace all use of energy-dense liquid fuels in heavy goods vehicles and aircrafts. It is necessary, therefore, to explore ways of reducing emissions and raising efficiency in the combustion of liquid fuel.Engine designers want computer programs to help them invent ways to use less fuel and produce less pollution. But the computational models currently available are not adequate to predict some important effects: such as how blends of future carbon-neutral bio-fuels will change engine performance.When a liquid fuel is injected in an engine, three interacting processes take place. First the liquid and gas display turbulence, i.e. they swirl and mix chaotically. Second, there is evaporation of the many compounds in the liquid fuel into gaseous fuel vapour. Third, there is combustion, i.e. the fuel combines with oxygen to form hundreds of different intermediate and final combustion products. Turbulence, evaporation and combustion are fundamentally difficult to compute. One reason is that they happen among a wide range of spatial and temporal scales and therefore need to be calculated with a very fine resolution. Another is that so many different chemical compounds are involved. As a result, simulating even a few milliseconds of a highly turbulent combustion process far exceeds the resources of the largest supercomputers in the world.This research proposal aims to solve these problems by providing an accurate, practical and rigorous model for the injection and combustion of liquid fuel blends. High-resolution simulation data will be analysed to provide fundamental information on the coupling among gasses and liquids. The resultant data will be used to devise a model that synthesises turbulence, evaporation and combustion processes into a unifying framework. This new model has great potential because it can describe evaporation and mixing processes, even those occurring at the smallest scales of the flow, in detail and at an acceptable computational cost.The proposed model will not only be useful for designing advanced combustion systems. The challenging combination of physics found in spray combustion is also found in a number of industrial processes, including spray-drying in food and chemical industries, spray-painting, and spray-forming of metal components. This research will also demonstrate how the new modelling can be used to improve design of more efficient industrial processes.Finally some processes that play a role in turbulent spray combustion also play a critical role in environmental processes affecting local air quality and global climate change. Environmental regulators at present often have to make high-impact environmental policy decisions without an accurate way to predict the consequences. The proposed model will be applied in this arena to help to provide this badly needed information to policy-makers, thus contributing to sound environmental policy.