Direct Numerical Simulations of Localised Forced Ignition in Turbulent Mixing Layers: The Effects of Mixture Fraction and Its Gradient (original) (raw)
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2014
The effects of the characteristic width of the energy deposition profile and the duration of energy deposition by the ignitor on localised forced ignition of stoichiometric and fuel-lean homogeneous mixtures have been analysed using simplified chemistry three-dimensional compressible Direct Numerical Simulation (DNS) for different values of root-mean-square velocity fluctuation . The localised forced ignition is modelled using a source term in the energy transport equation, which deposits energy in a Gaussian manner from the centre of the ignitor over a stipulated period of time. It has been shown that the width of ignition energy deposition and the duration over which ignition energy is deposited have significant influences on the success of ignition and subsequent flame propagation. An increase in the width of ignition energy deposition (duration of energy deposition) for a given amount of ignition energy has been found to have a detrimental effect on the ignition event, which may...
Numerical investigation of flame propagation in a mixture reacting at the initial temperature
Combustion, Explosion, and Shock Waves, 1979
Transient flame propagation in fuel droplet arrays is investigated using three-dimensional numerical analyses with two different computational approaches. Flame propagation characteristics of fuel droplet arrays are first examined using a relatively new, but computationally expensive, method for two-phase reacting flow numerical simulations called the Level-Set approach, capable of tracking the gas-liquid interface movement. Next, the validity and effectiveness of the computationally cheaper Particle-Source-In-Cell (PSI-Cell) approach are verified for the prediction of flame propagation characteristics of fuel droplet arrays. The fuel constituting the droplets is n-decane (n-C 10 H 22). Reaction mechanism for n-decane combustion is described using a two-step overall reaction model, based on a widely used reduced two-step chemical scheme originally developed for kerosene flames. The reaction model is validated against a detailed reaction mechanism for n-decane combustion. Results of the investigations using Level-Set approach reproduce the three different modes of flame propagation in fuel droplet arrays as observed in experiment, and show that the flame propagation speed is accurately predicted for each mode. Radiative heat transfer has an appreciable influence on the flame propagation speed. Furthermore, the application of a Gaussian function filter to the calculation of source terms accounting for the liquid phase-gas phase interactions, in the computations using PSI-Cell approach is adopted. Using this technique, the three flame propagation modes are reproduced for the fuel droplet arrays, and their corresponding flame propagation speeds are also accurately predicted.
Numerical study of mild combustion in hot diluted diffusion ignition (HDDI) regime
Proceedings of the Combustion Institute, 2009
Mild Combustion is a process defined on the ground of well-identified external parameters, namely the temperature of reactants and the maximum allowable temperature increase. Although the definition is rigorous, it is necessary to identify unique, intrinsic structural properties of processes evolving in such conditions in the different, basic configurations that make Mild Combustion relevant from practical point of view. In this paper the configuration of opposed jets of hot air versus cold fuel/diluent mixture, here referred as Hot Diluted Diffusion Ignition (HDDI), has been considered. Distributions of temperature and heat release rate as a function of the mixture fraction are evaluated for different values of the external parameters, i.e. pre-heating temperature of the air, fuel dilution, pressure and strain rate. They have been used in order to identify combustion regimes on the ground of their location and broadness. In particular, it has been shown that a significant broadening of heat release distribution is associated to oxidant temperatures higher than the auto-ignition temperature of homogeneous charge for a characteristic time comparable to the convective characteristic time of the system, supporting the conceptual model of ''distributed oxidation". In the asymptotic condition with very diluted fuel where the pyrolysis region, typical of standard diffusion flames, is no longer present in the heat release profiles, the sub-domain Mild-HDDI Combustion is identified. In this regime the position of the maximum heat release is completely uncorrelated with the stoichiometric mixture fraction. In this case the oxidation takes place only where the autoignition can develop inside constrain of the residence time of the fuel in the system.
A numerical study of turbulence-flame interaction in mild combustion
2016
of the self-ignition process. e di erential di usion e ect plays an important role in the early stages of ignition, for cases presenting methane/hydrogen fuel mixture. If high is the level of hydrogen in the fuel blend, major stages of methane (CH 4) consumption pathway, from the CH 4 dehydrogenation to the carbon dioxide (CO 2) release, are signi cantly a ected by hydrogen (H 2) chemistry. In the latest stages of ignition, the methane pathway is also a ected by the drop in oxygen level. e in uence of turbulence on the di usion-chemistry interaction is studied by means of three-dimensional (3D) Direct Numerical Simulations modelling a methane/hydrogen circular jet mixing with a diluted oxidiser co-ow. e e ects of di erent fuel and oxidiser blends is also considered in the 3D study. In cases where large is the H 2 presence in the fuel jet, the presence of turbulent mixing has a minimal e ects on early stages of self-ignition, where instead di erential di usion still plays a major role. As turbulence develops, more marked di erences between 1D and 3D studies are observed. e role of turbulent mixing dominates over chemistry where the fuel blend includes a low amount hydrogen. For this con guration the temperature increment is strongly limited compared to corresponding 1D study. e outcome of this study is expected to be of use to other researches in MILD combustion, particularly those adopting existing RANS and LES models to MILD combustion cases.
DNS for Turbulent Premixed Combustion
Direct Numerical Simulations - An Introduction and Applications, 2020
Most of practical combustion occurs in turbulent flows which involve strong coupling between turbulence and chemical processes. The heat release from combustion alters the fluid properties such as density and viscosity and in turns affects the turbulence. Direct numerical simulations (DNS) provides a tool for obtaining both temporally and spatially resolved data in three dimension (3D). This chapter presents a brief overview of importance of DNS in turbulent combustion, the role of turbulence and identifies different combustion modes. The mathematical formulation and numerical implementation for DNS are introduced. The second half of this chapter presents DNS results for ignition in both homogeneous and stratified mixtures. It has been found that minimum ignition energy is required to obtain successful ignition in different turbulence regimes. An increase in turbulent velocity fluctuation may leads to a misfire. Additionally the difference between growing flames and those which are ...
Energies
Flame propagation statistics for turbulent, statistically planar premixed flames obtained from 3D Direct Numerical Simulations using both simple and detailed chemistry have been evaluated and compared to each other. To achieve this, a new database has been established encompassing five different conditions on the turbulent combustion regime diagram, using nearly identical numerical methods and the same initial and boundary conditions. The discussion includes interdependencies of displacement speed and its individual components as well as surface density function (i.e., magnitude of the reaction progress variable) with tangential strain rate and curvature. For the analysis of detailed chemistry Direct Numerical Simulation data, three different definitions of reaction progress variable, based on and mass fractions will be used. While the displacement speed statistics remain qualitatively and to a large extent quantitatively similar for simple chemistry and detailed chemistry, there ar...
Energies
In the present study, flame propagation statistics from turbulent statistically planar premixed flames obtained from simple and detailed chemistry, three-dimensional Direct Numerical Simulations, were evaluated and compared to each other. To this end, a new database was established encompassing five different conditions on the turbulent premixed combustion regime diagram, using nearly identical numerical methods and the same initial and boundary conditions. A detailed discussion of the advantages and limitations of both approaches is provided, including the difference in carbon footprint for establishing the database. It is shown that displacement speed statistics and their interrelation with curvature and tangential strain rate are in very good qualitative and reasonably good quantitative agreement between simple and detailed chemistry Direct Numerical Simulations. Hence, it is concluded that simple chemistry simulations should retain their importance for future combustion research...
Combustion Theory and Modelling
In order to obtain physical insights on ammonia combustion, which is characterised by exceptionally long ignition delays and increased NOx emissions, the autoignition dynamics of an ammonia/air mixture is analysed using the diagnostics tools derived from the Computational Singular Perturbation (CSP) methodology. The results are compared to the autoignition dynamics of a methane/air mixture of same initial conditions. Methane was chosen for comparison because, even though the two molecules have a formal similarity, the ignition delay of methane is more than 10 times shorter than the one of ammonia. By using the CSP diagnostics tools, we identified the dominant chemical pathways that relate to the explosive components that drive the system towards ignition for both cases. Furthermore, the reactions that hinder the ammonia ignition were identified. This led to the determination of an interesting difference in the electronic configuration of the molecules of the two fuels, which is the root of their drastically different oxidation dynamics. In particular, it was shown that the autoignition process starts with the formation of methyl (CH 3) and amine (NH 2) radicals, through dehydrogenation of methane and ammonia, respectively. In the methane case, the methyl-peroxy radical (CH 3-O-O-) then forms, which initiates a chemical runaway that lasts for approximately 2/3 of the ignition delay and leads to the gradual oxidation of carbon to CO 2. In the ammonia case, though, the structure of NH 2 is such that it is not possible to form NH 2-O-O-. As a result, the chemical runaway is suspended.