With the need for the development of novel hydrogen-compatible combustion devices, physical understanding of the flame behaviour and the identification of thermoacoustic instabilities at relevant combustor operating conditions for hydrogen-air swirl flames will help speed up the development of hydrogen combustors, in line with the UK government's net-zero vision. The proposed research will offer potential benefits to industry and contribute to the progress of science in the areas of fluid dynamics, turbulence and net-zero combustion. These include (i) An advanced Direct Numerical Simulation (DNS) database for hydrogen-air premixed swirl flames under representative combustor operating conditions. (ii) A comprehensive understanding and a detailed analysis of the behaviour of the Precessing Vortex Core (PVC) under non-reacting and reacting flow conditions. (iii) Identification of the combustor operating conditions for which hydrodynamic/thermoacoustic instabilities exist. (iv) An in-depth analysis on extinction strain rates and heat release rate for lean hydrogen premixed flames. The outcomes of this project will offer knowledge on the flame stability limits and will contribute to the development of hydrogen based power generation and propulsion devices (e.g. gas turbines used for power generation and aircraft engines).

The proposed UK Consortium on Turbulent Reacting Flows will perform high-fidelity computational simulations (i.e. Reynolds Averaged Navier-Stokes simulations (RANS), Large Eddy Simulation (LES) and Direct Numerical Simulations (DNS)) by utilising national High Performance Computing (HPC) resources to address the challenges related to energy through the fundamental physical understanding and modelling of turbulent reacting flows. Engineering applications range from the formulation of reliable fire-safety measures to the design of energy-efficient and environmentally-friendly internal combustion engines and gas turbines. The consortium will serve as a platform to collaborate and share HPC expertise within the research community and to help UK computational reacting flow research to remain internationally competitive. The proposed research of the consortium is divided into a number of broad work packages, which will be continued throughout the duration of the consortium and which will be reinforced by other Research Council and industrial grants secured by the consortium members. The consortium will also support both externally funded (e.g. EU and industrial) and internal (e.g. university PhD) projects, which do not have dedicated HPC support of their own. The consortium will not only have huge intellectual impact in terms of fundamental physical understanding and modelling of turbulent reacting flows, but will also have considerable long-term societal impact in terms of energy efficiency and environmental friendliness. Moreover, the cutting edge computational tools developed by the consortium will aid UK based manufacturers (e.g. Rolls Royce and Siemens) to design safe, reliable, energy-efficient and environmentally-friendly combustion devices to exploit the expanding world-wide energy market and boost the UK economy. Last but not least, the proposed collaborative research lays great importance on the development of highly-skilled man-power in the form of Research Associates (RAs) and PhD students of the consortium members, who in turn are expected to contribute positively to the UK economy and UK reacting flow research for many years to come.

This project focuses on the development, validation and documentation of a next-generation fully parallelised computa-tional fluid dynamics (CFD) code called HAMISH based on adaptive mesh refinement (AMR) which will enable high-fidelity Direct Numerical Simulations (DNS) of advanced turbulent reacting flows such as flame-wall interaction, localised ignition, and droplet combustion including atomisation processes. Such simulations cannot be achieved at present without limiting simplifications due to their prohibitive computational cost. AMR for large-scale highly-parallel simulations of compressible turbulent reacting flows is a significant new functionality which will offer major benefits in terms of computational economy for problems involving thin fluid-mechanical structures, e.g. resolution of both the flame and the boundary layer in flame-wall interaction, droplet surfaces in atomisation in spray combustion, shock waves in localised forced ignition, etc. Such structures have either been ignored or simplified severely in previous work due to the prohibitive computational cost of fixed global meshes, thus limiting the usefulness of the simulations. Hence AMR will offer a step-change in capability for the computational analysis of turbulent reacting flows, and will provide data with the degree of detailed physical information which is not currently available from simulations using existing CFD codes. The proposed software will be validated with respect to the results obtained from the well-proven uniform-mesh DNS code SENGA2, which has already been ported to ARCHER and is currently widely used by members of the UK Consortium on Turbulent Reacting Flows (UKCTRF). The newly developed code, HAMISH, will not only be ported to ARCHER, but also be prepared for architectures supporting accelerators thanks to OpenMP 4.5, which will support OpenACC, targeting a POWER8 cluster. As a part of this project, a detailed user guide will be produced at each new release of the code. This user guide will be made available on a website for public download along with the open-source version of the code and the associated documentation on code validation and user tutorials.

A small reduction in NOx emission per kilo-watt of generated power will have a significant reduction in environmental impact of combustion used for power generation. The MILD (Moderate or Intense Low-Oxygen Dilution) combustion technique offers an opportunity to drastically reduce emissions while improving thermal efficiency of furnaces and boil-ers. In gas turbines, though overall fuel-air mixture is fuel-lean and MILD combustion is not directly applicable, fuel-rich regions in the primary zone of the combustor exhibit localised MILD regimes, particularly for liquid fuel operation How-ever, the physical and chemical intricacies of this novel technique are not well understood and thus identifying key con-trol parameters for using this technique for power generation and industrial processes over wide range of conditions is challenging. This project aims to provide a step change in physical understanding and modelling of this combustion technique and to identify key control parameters. The aim is to investigate MILD combustion of high calorific value gaseous and liquid fuels for practical application using Direct Numerical Simulations (DNS) and Large Eddy Simula-tions (LES), with high-fidelity mathematical description for physical and chemical processes involved. The droplets of liquid fuel spray will be tracked using the Lagrangian approach while the gas phase is treated using the Eulerian ap-proach for the simulations. The effects of droplet diameter, equivalence ratio (both for gaseous and liquid fuels), extent of dilution by combustion products, volatility (by considering different fuels), turbulence intensity and its length scale on the burning rate, flame structure (in terms of chemical reaction pathways analysis and flame and flow topologies) and pollutants formation will be analysed based on a judicious parametric analysis based on three-dimensional detailed chemistry DNS data. In this project, the fundamental physical understanding extracted from DNS data will be utilised to develop high-fidelity models for engineering Computational Fluid Dynamics (CFD)-based simulations to identify key control parameters using LES after validating these models against the available experimental results. This project will provide (1) a ro-bust modelling framework for MILD combustion technique, which would be a cost-effective reliable tool for designing energy-efficient and clean gas turbines and industrial furnaces and (2) the key control parameters identified can help to design retro-fit "greener" combustion systems.

The new expanded UK Consortium on Turbulent Reacting Flows (UKCTRF) will further utilise the developments of High-Performance Computing (HPC) to offer improved fundamental understanding and modelling of turbulent reacting flows, which are pivotal in the effective usage of energy resources, development of reliable fire safety measures, and manipulation of the combustion processes to ensure environmental friendliness. These challenges are multi-faceted, and will require collaboration across a wide range of scientific areas. The UKCTRF brings together 40 experts (PI, 6 Co-Investigators, and 33 members) across 19 UK institutions, experienced in using HPC to enable concerted collaborative Computational Fluid Dynamics (CFD)-related fundamental and applied research on turbulent reacting flows to reduce duplication, and tackle challenges grander than individual attempts. Since its inception in 2014, the UKCTRF has achieved significant scientific and industrial impact with over 400 journal and conference papers which utilised ARCHER. The President of the Combustion Institute, Prof. J.F. Driscoll, has stated in his support letter that the publications of the UKCTRF members are among the best which help develop the minds of young researchers and the support letter from Rolls Royce states that as a result of the UKCTRF significant progress was made in the prediction of combustion phenomena with the help of HPC. Over the next 4 years, the consortium's goals are to: (i) further utilise HPC resources to conduct world-leading turbulent reacting flow research involving Reynolds Averaged Navier-Stokes (RANS), Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS); (ii) extract fundamental physical insights from simulations to develop high-fidelity modelling methodologies to study turbulent reacting flows relevant to power production, transportation and fire safety engineering; and (iii) ensure a forward-looking software development strategy to develop computationally efficient algorithms, and effectively exploit current and future developments of HPC hardware. The proposed research will build on the foundations of the current UKCTRF (2014-2019) and Flagship Software development (EP/P022286/1) projects and will address universal challenges of energy efficiency, sustainability and high-fidelity fire safety. The progress in HPC will enable this new incarnation of UKCTRF to reinforce existing strengths, but also address the following timely intellectual and industry-driven challenges: (i) simulation and modelling of multi-phase reacting flows (e.g. droplet and pulverised coal/biomass combustion); (ii) combustion analysis of biogas and low calorific fuels derived from coal gasification; (iii) flame-wall interaction; and (iv) combustion at elevated pressures, which have only recently become accessible due to the advancement of HPC.