Global Flame Characteristics Research Articles

Energy balance and global characteristics of metal dust flames

Recently demonstrated high values of radiative loss during metal combustion require a new approach to describing dust flames. In particular, given strong light emission, the adiabatic flame temperature (AFT) concept is not applicable due to high radiative losses. Accordingly, global flame characteristics such as an expansion factor become unrelated to the AFT. That expansion factor can be directly inferred from the flame geometry, and therefore, its analysis offers a simple path to justify the need for a detailed energy balance in metal dust combustion. The analysis can also provide insight into peculiarities of the temperature distribution within the flame. In the current paper, previously published data on flow velocities in a metal dust flame are used for the expansion factor analysis. A relatively low value of the obtained expansion factor is reconciled with the advanced comprehension of metal particle combustion.

Combustion characteristics of syngas laminar microjet diffusion flames

Two-dimensional numerical investigations of laminar microjet syngas (H2 - CO) diffusion flames are carried out in the present study. A detailed parametric study on the effects of fuel jet velocity and fuel composition on global flame characteristics is analyzed. Results show that the flame height increases with an increase in fuel jet velocity, and it decreases with an increase in hydrogen content in the fuel mixture. The dependence of fuel composition and jet velocity on the rate of tube wall heat recirculation is explored. The radial extent of flame increases with increase in hydrogen content in the fuel mixture. The analysis of key reactions in the standoff region revealed the significant contribution of HO2 radical in the formation of high-temperature reaction zone. Flame extinction is observed at lower fuel jet velocities, as the composition of hydrogen in fuel mixture increases from 10 to 90%. Near extinction, the combined effect of axial and radial diffusion results in the formation of spherically shaped flames.

Fuel Effects in Turbulent Premixed Pre-vaporised Alcohol/Air Jet Flames

To study combustion fundamentals of complex fuels under well-defined boundary conditions, a novel Temperature Controlled Jet Burner (TCJB) system is designed that can stabilise both gaseous or pre-vaporised liquid fuels. In a first experimental exploratory study, piloted turbulent jet flames of pre-vaporised methanol, ethanol, 2-propanol and 2-butanol mixtures are compared to methane/air as a reference fuel. Complementary one-dimensional laminar flame calculations are used to provide flame parameters for comparison. Blow-off and flame length as global flame characteristics are measured over a wide range of equivalence ratios. For fuel rich conditions, blow-off limits correlate well with extinction strain rate calculations. Differing flame lengths from lean to rich conditions are explained partly by different flame wrinkling that is assessed using planar laser-induced fluorescence imaging of the hydroxyl radical (OH-PLIF). A study of Lewis-number effects indicates that they have substantial influence on flame wrinkling. Lean alcohol/air flames, opposed to methane/air, have a Lewis-number greater than unity. This impedes curvature development, which promotes relatively large flame lengths. In contrast, across stoichiometric conditions, all alcohol/air mixture Lewis-numbers decrease significantly. At such conditions, alcohol/air flames show alike or even larger wrinkling compared to methane/air flames. However, quantitatively, the differences in flame length and wrinkling observed among the flames can neither be explained alone by Lewis-number differences, nor other global mixture parameters available from 1D laminar flame calculations. This study shall therefore emphasise the need for more detailed experimental analyses of the full thermochemical state of laminar and turbulent flames fuelled with complex fuels.

An extended artificial thickening approach for strained premixed flames

A method is presented to consistently treat artificial flame thickening in strained flows. Originally, the artificial thickened flame (ATF) approach was derived from a coordinate transformation for unstrained, freely propagating flames. Using the same transformation for a strained counterflow configuration, it is shown that global flame characteristics such as the consumption speed cannot be recovered. These observations, obtained using detailed and tabulated chemistry approaches, are then studied with an analysis of the deficiencies. Characteristic flame properties e.g. consumption speed and flame thickness, are compared, discussing the interplay of strain and flame thickening. The effect of strain on the individual budgets of the progress variable transport equation, including the spatial gradients, is then analyzed in composition space. Based on the findings, in the context of tabulated chemistry, a method is proposed to account for strain, reproducing the correct strain-dependent consumption speed while at the same time keeping the flame resolvable on coarse grids. The validity of this approach is finally demonstrated in 1D counterflow simulations with different strain rates and 2D Tsuji burner simulations with locally varying strain rates.

Application of deep artificial neural networks to multi-dimensional flamelet libraries and spray flames

The “curse of dimensionality” has limited the applicability and expansion of tabulated combustion models. While the tabulated flamelet model and other multi-dimensional manifold approaches have shown predictive capability, the associated tabulation involves the storage of large lookup tables, requiring large memory as well as multi-dimensional interpolation subroutines, all implemented during runtime. This work investigates the use of deep artificial neural networks to replace lookup tables in order to reduce the memory footprint and increase the computational speed of tabulated flamelets and related approaches. Specifically, different strategic approaches to training the artificial neural network models are explored and a grouped multi-target artificial neural network is introduced, which takes advantage of the ability of artificial neural networks to map an input space to multiple targets by classifying the species based on their correlation to one another. The grouped multi-target artificial neural network approach is validated by applying it to an n-dodecane spray flame using conditions of the Spray A flame from the Engine Combustion Network and comparing global flame characteristics for different ambient conditions using a well-established large-eddy simulation framework. The same framework is then extended to the simulations of methyl decanoate combustion in a compression ignition engine. The validation studies show that the grouped multi-target artificial neural networks are able to accurately capture flame liftoff, autoignition, two-stage heat release and other quantitative trends over a range of conditions. The use of neural networks in conjunction with the grouping mechanism as performed in the grouped multi-target artificial neural network produces a significant reduction in the memory footprint and computational costs for the code and, thus, widens the operating envelope for higher fidelity engine simulations with detailed mechanisms.

Local and global flame characteristics in a liquid kerosene fueled supersonic combustor equipped with a thin strut

The investigations of local and global flame characteristics in a liquid kerosene fueled supersonic combustor based on the strut flame holder were conducted. A thin strut with the thickness of 6 mm is equipped in the center of the main flow, and there are two rows of oxygen injectors at the tailing edge of it to enhance the combustion. The room-temperature liquid kerosene is chosen as the fuel in the flush wall combustor without any cavity and other flame holders. The experimental condition is at flight Mach number of 6, and the Mach number is 2.8 at the entrance of the combustor, with stagnation state Tt=1680 K, Pt=1.87 MPa. The flame images in different equivalence ratios and oxygen mass flow rates are well captured and reproduced by the high-speed camera. Experimental results show that the flame mode could be divided into two modes, namely, the local flame mode and the global flame mode. With the injection of the oxygen, a local oxygen enriched zone forms in the recirculation zone, and the local flame generates in this location. The transition process from local flame to global flame is influenced by the equivalence ratio and the oxygen mass flow rate. There are three flame stabilization patterns being found accompanied with the changing of the equivalence ratios, and different stabilization patterns correspond to different flame modes. The main function of the injected oxygen is to establish the local flame and to strength it, then the enhanced local flame will act as the pilot flame to ignite the extra fuel to form the global flame. In addition, the oxygen will improve the efficiency of the global combustion to a certain degree. All in all, these results are valuable for the optimization of the fuel and oxygen injection strategy.

Properties of lean turbulent methane-air flames with significant hydrogen addition

We examine the combustion of mixed H2-CH4-air fuels using two-dimensional simulations that incorporate detailed kinetics and a mixture-averaged model for differential species transport. The mixtures range from lean H2-air at ϕ=0.37 to lean CH4-air at ϕ=0.7. For each mixture, we compute the quasi-steady propagation of a flame into flow with superimposed low-level turbulent fluctuations, so that the resulting flames are in the laminar flamelet regime. We examine the resulting global flame characteristics, and quantify how the chemistry depends on local flame curvature. We then examine in more detail how the methane chemistry is modulated by the presence of hydrogen. In particular, we find that the local methane burning speed shows a strong positive correlation with local flame curvature when sufficient hydrogen is added to the mixture. Moreover, for higher hydrogen concentrations, the mixtures exhibit cellular burning patterns that are traditionally associated with thermodiffusively unstable fuel mixtures. Various pathways for the oxidation of methane are identified and are shown to be amplified considerably in the presence of H2 combustion, which varies considerably along the flame surface.

Trajectory and Characteristics of Buoyancy and Momentum Dominated Horizontal Jet Flames From Circular and Elliptic Burners

Relative effects of buoyancy and momentum on the characteristics of horizontally oriented circular (Circ) and elliptic (E) burner flames in a quiescent environment over a wide range of jet exit velocities are presented. The major axis of the elliptic burner was oriented horizontally and vertically (referred to as Emaj and Emin flames, respectively). Propane was used as fuel and a small amount of hydrogen was piloted to attach flames to the burner. Global flame characteristics such as flame dimensions, centerline trajectory, emission indices (EI) and radiative fraction, and in-flame transverse concentration and temperature profiles were measured. At a jet exit Reynolds number (Rej) of 2000, based on the area-equivalent diameter of the burner, the flame characteristics were affected by the burner geometry and its orientation. Also, the vertical dimension of the burner exit dictated buoyancy effects. At Rej=12,500, the influence of burner geometry or its orientation was negligible. Elliptic burner flames exhibited lower liftoff and blowout velocities than circular burner flames. Furthermore, the flame stability and nitric oxide emissions were not much affected by the orientation of elliptic burner. Although the elliptic burners produced higher EINO at lower jet exit velocities, the variation in EINO among three burners (Circ, Emaj, and Emin) was insignificant at higher velocities. Some effects of buoyancy on EICO were observed at lower jet exit velocities and the EICO was the lowest for the burners with largest buoyancy flux. Elliptic burner flames produced greater peak flame temperature than the corresponding circular burner flames under most conditions.

An experimental and numerical study on the global and detailed kinetics of premixed and nonpremixed flames of chloromethane, methane, oxygen and nitrogen

A comprehensive experimental and numerical study has been conducted on the global flame characteristics and detailed oxidation kinetics of both premixed and nonpremixed flames of chloromethane, methane, oxygen and nitrogen. The experimental phase involves the determination of the laminar burning velocities of premixed flames and of the extinction stretch rates of nonpremixed flames in the counterflow configuration. The experimental results demonstrate and quantify the significant reduction in the burning intensities of these flames with increasing chlorine content. Together with those in the literature, they are then compared with the numerically calculated values obtained with detailed kinetic schemes and transport descriptions. These comparisons, while being fair in most situations, also indicate inadequacies in the existing chlorine-related kinetic sub-mechanisms and/or transport descriptions. As such, the calculated results show a weaker burning intensity than observed and the peaking of the laminar burning velocity on the rich instead of the lean side of stoichiometry. The calculated species profiles within the flame structure allow further identification of the role of the chlorinated species in the flame response, including an increase in the formation of higher-order hydrocarbons indicating a strong tendency to form soot. Finally, based on a one-step overall reaction interpretation, the influences of chlorine content on the responses of premixed and nonpremixed flames are found to be similar.