Browsing by Author "Agarwal, Parash"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Open Access Comparison of hydrogen micromix flame transfer functions determined using RANS and LES(ASME, 2019-11-05) McClure, Jonathan; Abbott, David; Agarwal, Parash; Sun, Xiaoxiao; Babazzi, Giulia; Sethi, Vishal; Gauthier, Pierre Q.Hydrogen has been proposed as an alternative fuel to meet long term emissions and sustainability targets, however due to the characteristics of hydrogen significant modifications to the combustion system are required. The micromix concept utilises a large number of miniaturised diffusion flames to improve mixing, removing the potential for local stoichiometric pockets, flash-back and autoignition. No publicly available studies have yet investigated the thermoacoustic stability of these combustion systems, however due to similarities with lean-premixed combustors which have suffered significant thermoacoustic issues, this risk should not be neglected. Two approaches have been investigated for estimating flame response to acoustic excitations of a single hydrogen micromix injector element. The first uses analytical expressions for the flame transfer function with constants obtained from RANS CFD while the second determines the flame transfer function directly using unsteady LES CFD. Results show the typical form of the flame transfer function but suggest micromix combustors may be more susceptible to higher frequency instabilities than conventional combustion systems. Additionally, the flame transfer function estimated using RANS CFD is broadly similar to that of the LES approach, therefore this may be suitable for use as a preliminary design tool due to its relatively low computational expense.Item Open Access Injector design space exploration for an ultra-low NOx hydrogen micromix combustion system(ASME, 2019-11-05) Agarwal, Parash; Sun, Xiaoxiao; Gauthier, Pierre Q.; Sethi, VishalThe depletion of fossil fuel resources, as well as the increasing environmental concerns have become the driving forces towards the research and development necessary for the introduction of alternative fuel such as hydrogen into civil aviation. Hydrogen is a suitable energy source primarily because it is free of carbon and other forms of impurities and is also the most abundant element in the universe. The advantages of using Liquid Hydrogen (LH2) for civil aviation extends beyond carbon-free mission level emissions; LH2 combustion can potentially reduce NOx emission by up to 90%, providing long-term sustainability and unrivalled environmental benefits. The paper presents a simplified parametric analysis to investigate the influence of various injector design parameters on a hydrogen micromix combustor reactive flow field. The main characteristics investigated are the flame structure (shape and position), the aerodynamic stabilization of the flame and the resulting NOx emissions. The design parameters include variations in the air-feed dimensions and the hydrogen injection diameter. A suitable numerical model was established by comparing various turbulence modelling approaches, reaction mechanisms and turbulence-chemistry interaction modelling schemes. The predictive capabilities, and limitations, of each of these modelling approaches, are assessed. The numerical challenges and limitations associated with modelling H2/air combustion at high pressure and temperature conditions are detailed. The influence of varying the injector design parameters on the mixing and hence the NOx characteristics is assessed.Item Open Access Numerical investigation into the impact of injector geometrical design parameters on hydrogen micromix combustion characteristics(American Society of Mechanical Engineers, 2021-01-11) Sun, Xiaoxiao; Agarwal, Parash; Carbonara, Francesco; Abbott, David; Gauthier, Pierre Q.; Sethi, BobbyHydrogen micromix combustion is a promising concept to reduce the environmental impact of both aero and land-based gas turbines by delivering carbon-free and ultra-low-NOx combustion without the risk of autoignition or flashback. As a part of the ENABLEH2 project, the current study focuses on the influence of design parameters on the micromix hydrogen combustion injectors. This study provides deeper insights into the design space of a hydrogen micromix injection system via numerical simulations. The key geometrical design parameters of the micromix combustion system are the sizing of the air gates and the hydrogen injector orifices together with the offset distance between air gate and hydrogen injection, the mixing distance and the injector to injector spacing. This paper first presents results of the numerical simulation of four designs, down selected from a series of combinations of the key design parameters, including cases with low and high momentum flux ratio, weak and strong flame-flame interaction. It was discovered that the hydrogen/air mixing characteristics, and flame to flame interactions, are the main factors influencing the combustor gas temperature distributions, flame lengths and the corresponding NOx production. The current study then focused on the effect of air gate geometry on the mixing characteristics, flame shape and temperature distribution. The momentum flux ratio was kept constant throughout this investigation by keeping the air gate area constant. Variations of the original baseline air gate design were studied, followed by a study of various novel air gate geometries, including circular, semi-circular and elliptical shapes. It is concluded that NOx production is influenced by a number of factors including jet penetration flame interactions and air gate shape and that there is a “Sweet Spot” that results in the lowest practicable NOx production. Flatter and wider air gate shapes tend to yield the lowest temperature and consequently the lowest NOx. Reduced interaction between flames also tends to reduce NOx and by manipulating hydrogen penetration, there is the potential to further reduce the NOx production.