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Because ammonia is one of the most promising candidates for energy carrier in the future, various applications of ammonia as a fuel are currently considered. One medium for utilizing ammonia is by introducing it to coal-fired boilers. To the best of our knowledge, this paper is the first to report the fundamental mechanism of the flame propagation phenomenon for pulverized coal/ammonia co-combustion. The effects of the equivalence ratio of the ammonia-oxidizer mixture on the flame propagation velocity of pulverized coal/ammonia co-combustion in turbulent fields were clarified by the experiments employing a unique fan-stirred constant volume chamber. The flame propagation velocities of pulverized coal/ammonia co-combustion, pure ammonia combustion, and pure pulverized coal combustion were compared. As expected, the flame propagation velocity of pulverized coal/ammonia was higher than that of the pure pulverized coal combustion for all conditions. However, the comparison of the flame propagation velocities of pulverized coal/ammonia co-combustion and that of the pure ammonia combustion, revealed that whether the flame propagation of the pulverized coal/ammonia was higher than that of the pure ammonia combustion was dependent on the equivalence ratio of the ammonia-oxidizer. This unique feature was explained by a mechanism including three competing effects proposed by the authors. In the ammonia lean condition, the positive effects, which are the strong radiation from the luminous flame and the increment of local equivalence ratio by the addition of volatile matter, are larger than the negative effect, which is the heat absorption by coal particles in preheat zone. In the ammonia rich condition, the effect of an increment of the local equivalence ratio by the addition of volatile matter turns into a negative effect. Consequently, the negative effects overcome the positive effect in the ammonia rich condition resulting in a lower flame propagation velocity of pulverized coal/ammonia co-combustion.

Combustion of a seven-component surrogate for a research grade 87 octane gasoline mixed with 10% ethanol is investigated experimentally and numerically from the perspective of an isolated droplet burning under conditions that promote one-dimensional gas transport. The numerical analysis included a kinetic mechanism comprised of 398 species and 24,814 reactions and a soot model that accounted for nucleation, surface growth, coalescence/aggregation of soot particles, and luminous flame radiation. Measurements of droplet and flame diameters were made for an initial droplet diameter (Do) of approximately 0.63 mm. The simulations agreed well with the measurements including the location of the soot shell. Preferential vaporization was revealed by simulations of the liquid concentrations in the droplet. Predicted peak soot volume fractions coincided with temperatures between 1300 K and 1400 K as a soot inception temperature. Simulations were also carried out for Do between 0.25 mm and 5 mm to explore the effect of radiation and Do on burning. Below 0.25 mm radiation was negligible and burning rates and flame temperatures converged to a single value. Increasing Do up to 1.8 mm lowered the burning rate with luminous radiation having a strong effect. When radiation was entirely removed from the model the burning rate was nearly constant. Above Do = 2 mm droplets extinguished almost immediately after ignition. The flame temperature decreased with increasing Do while it increased when radiation was omitted. The simulations show that soot precursors including polyaromatic hydrocarbons were concentrated around the soot shell.

Accepted: 2015-03-13, 資料番号: SA1150197000

From the image processing of high speed colour images of flame ignition, areas of dominant infrared emission have been noticed, which has stimulated the more in depth investigation in this paper. Two test cases for propane and methane with co-flow air were carried out. By applying selective digital image enhancement technique, the weak chemiluminescence-induced visible flame and infrared signals are simultaneously resolved together with the much stronger visible soot radiation. It is found that the pockets of soot only emitting infrared signal exists in both propane and methane ignition process cases. In the ignition process, the chemiluminescence-induced blue flame is observed first, which is followed by soot only having infrared emission. The visible orange coloured sooty flame only appears afterwards. The soot only emitting infrared is found in between the blue flame and the visible sooty flame. The co-flow air is found to accelerate the ignition process and it also brings forward soot formation. The investigation demonstrates that a proper image enhancement technique is essential in the further understanding of combustion process taking place when using a high speed camera.

9th Mediterranean Combustion Symposium, Rhodes, GREECE, JUN 07-11, 2015; International audience; An experimental study was conducted to investigate the effects of the Oxygen Index (OI) in an ethylene laminar inverse diffusion flame (IDF). The 01 was varied from 21% to 37% and its influence was measured in terms of the flame height, soot volume fraction, soot temperature and radiant fraction. The stoichiometric flame height was measured by the spontaneous emission of CH* radicals and was found to decrease when the OI increases. In contrast, the luminous flame height increases with OI because soot can still form and grow beyond the reaction zone. Radial profiles of soot volume fraction and soot temperature were obtained by means of a Modulated Absorption/Emission (MAE) technique. The line-of-sight intensities, integrated along the optical path and captured by ECCD camera at two wavelengths were inverted using deconvolution and regularization techniques in order to obtain radial profiles of soot volume fraction and temperature. The increase in OI enhances soot volume fraction due to higher temperatures and soot formation rates. Both the local and integrated soot quantities increase with OI. The radiant fraction of IDF increases with the OI in a similar way to the integrated soot volume fraction. (C) 2015 Elsevier Inc. All rights reserved.

This paper reports on the application of independent component analysis (ICA) to 2D cycle-resolved images of the luminous combustion, collected in a port fuel injection spark ignition optically accessible engine. The method of ICA is employed for the identification of the independent spatial components, which along with the corresponding time dependent coefficients, represent the spatiotemporal evolution of the luminous combustion during a single cycle and over a number of cycles. ICA is applied on the data preprocessed by proper orthogonal decomposition (POD). POD-filtered ICA permits to determine only few dominant and relatively easy interpretable independent components. Successively, and for comparison, ICA is applied to the non-truncated data. It is demonstrated that ICA applied to single cycle permits to extract independent structures, clearly separated in time. The time dependent coefficients correlate well with the integral flame luminosity, and characterize time evolution of the combustion pattern in the chamber. The analysis over several cycles shows that independent components carry information about the dominant morphology of the cyclic variations. The low correlation of the corresponding (in terms of time succession) components for successive cycles is in agreement with the high spatial variability of the combustion process in spark ignition engines, mainly due to the combustion of fuel pockets created in the combustion chamber by the injection process. A different approach is then proposed, enabling separation of sources corresponding to objects moving in the field of view. Each whole sequence of 2D images (video) is considered as a single observed mixture of independent signals. Both linear and nonlinear mixing models are considered, and artificial examples are used to illustrate features and limitations of linear ICA when applied to nonlinear mixing. The procedure is successfully applied to a video reporting the motion of a luminous flame over a portion of the cylinder head. ICA separates the moving source from the background, with a mechanism explained by a nonlinear mixing model.

We have investigated the downward flame spread over a thin solid fuel. Hydrogen, methane, or propane, included in the gaseous product of pyrolysis reaction, is added in the ambient air. The fuel concentration is kept below the lean flammability limit to observe the partially premixing effect. Both experimental and numerical studies have been conducted. Results show that, in partially premixed atmospheres, both blue flame and luminous flame regions are enlarged, and the flame spread rate is increased. Based on the flame index, a so-called triple flame is observed. The heat release rate ahead of the original diffusion flame is increased by adding the fuel, and its profile is moved upstream. Here, we focus on the heat input by adding the fuel in the opposed air, which could be a direct factor to intensify the combustion reaction. The dependence of the flame spread rate on the heat input is almost the same for methane and propane/air mixtures, but larger effect is observed for hydrogen/air mixture. Since the deficient reactant in lean mixture is fuel, the larger effect of hydrogen could be explained based on the Lewis number consideration. That is, the combustion is surely intensified for all cases, but this effect is larger for lean hydrogen/air mixture (Le < 1), because more fuel diffuses toward the lean premixed flame ahead of the original diffusion flame. Resultantly, the pyrolysis reaction is promoted to support the higher flame spread rate.

© 2015 The Combustion Institute. The accuracy and computational expense of various radiation models in the simulation of turbulent jet flames are compared. Both nonluminous and luminous methane-air nonpremixed turbulent jet flames are simulated using a comprehensive combustion solver. The combustion solver consists of a finite-volume/probability density function-based flow-chemistry solver interfaced with a high-accuracy spectral radiation solver. Flame simulations were performed using various k-distribution-based spectral models and radiative transfer equation (RTE) solvers, such as P-1, P-3, finite volume/discrete ordinates method (FVM/DOM), and line-by-line (LBL) accurate Photon Monte Carlo (PMC) methods, with and without consideration of turbulence-radiation interaction (TRI). Various spectral models and RTE solvers are observed to have strong effects on peak flame temperature, total radiant heat source and NO emission. The P-1 method is found to be the computationally least expensive RTE solver and the FVM the most expensive for any spectral model. For optically thinner flames all radiation models yield excellent accuracy. For optically thicker flames the P-3 and the FVM with advanced k-distribution methods predict radiation more accurately than the P-1 method with any spectral model when compared to the benchmark LBL PMC. The LBL PMC yields exact results with sufficient number of samples and is found to be less expensive than the FVM (for all spectral models) and the P-3 (for some spectral models) in statistically stationary combustion simulations. TRI is found to drop the peak temperature by close to 150. K for a luminous flame (optically thicker) and 25-100. K for a nonluminous flame (optically thinner).

51st AIAA/SAE/ASEE Joint Propulsion Conference (July 27-29, 2015.Hilton Orlando) Orlando, Florida, USA., The pintle injector is a promising candidate of the propellant injection systems for a rocket engine with deep throttling capability which is essential for future space transportation missions. However, studies focusing on combustion phenomena in a rocket engine with a pintle injector is rather limited. In this study, optical measurements inside an ethanol/liquid oxygen rocket engine combustor with a planar pintle injector are conducted to clarify spray and flame structures in the pintle injector. Combustion tests where the chamber pressure is 0.36 to 0.40MPa and O/F is 1.15 to 1.40 are conducted. Effects of the injection configuration on spray structures are also evaluated. High speed imaging techniques are used to observe the flame and spray structures under hot fire conditions. Strong chemiluminescence of CH is observed in the vicinity of the impinging point of two propellants. Luminous flame is observed intermittently in the vicinity of the faceplate and the upper wall of the combustor with the direct observation of the flame. Characteristic exhaust velocity efficiency with oxidizercentered configuration is lower than fuel-centered configuration due to the large amount of propellant impinging on the upper wall of the combustor. Periodical phenomena with the frequency of approximately 300Hz which can be related to atomization processes are also observed., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: AC1600102000, レポート番号: AIAA 2015-3845

The flame jet width and flame jet velocity of the burnt gases of a premixed Bunsen-type flame are important parameters for quantifying the heat-transfer rates of these flames. In this paper a simple expression is derived to estimate the flame jet width and flame jet velocity of burnt gases of a free flame after expansion over the flame front for the special case of plug flow in and above the burner. The results of particle image velocimetry experiments on three different flame types having different oxygen concentrations are presented. The model is validated with these measurements and shows good agreement. Deviations occur, however, when the curved flame front area of the flame tip is not negligible compared to the total flame area.

To examine the details of the Mg−CO 2 combustion consisting of the gas-phase reactions and the surface reactions, we tried to separate the Mg−CO 2 flame from the surface reactions. For this purpose, we formed a stable Mg−CO 2 counterflow diffusion flame between the Mg vapor and a CO 2 stream by using a Mg vaporizer with many small ejection holes. The Mg−CO 2 counterflow diffusion flames contained two types of flames: the luminous flame and the dark flame. In the luminous flame, the homogeneous reaction of Mg with CO 2 forming gaseous MgO and CO occurred, as well as the condensation of MgO. The dark flame was observed when the Mg ejection velocity was small. The heterogeneous reaction of Mg with CO 2 forming condensed MgO and C occurred. The change from the luminous flame to the dark flame was caused by the heat loss to the Mg vaporizer. The flame stability limits diagram of Mg−CO 2 counterflow diffusion flame was obtained, in which the Mg ejection velocity is plotted against the stagnation stream velocity gradients. There exists a critical value of the Mg ejection velocities, below which the flame can never be stabilized. This limit is due to the thermal quenching of the flame near the Mg vaporizer. Also, the critical value of the stagnation stream velocity gradients exists and is due to the chemical limitations on the combustion rate in the flame zone. This flame stability limits diagram was similar to those of the ordinary gaseous fuel-oxidizer counterflow diffusion flames.

International audience; A series of outdoor experiments were conducted in a fire tunnel to measure the emission of infrared radiation from wildland flames, using a FTIR spectrometer combined with a multispectral camera. Flames of different sizes were produced by the combustion of vegetation sets close to wildland fuel beds, using wood shavings and kermes oak shrubs as fuels. The nongray radiation of the gas-soot mixture was clearly observed from the infrared emitted intensities. It was found that the flame resulting from the combustion of the 0.50 m long fuel bed, with a near-zero soot emission, may be considered as optically thin and that the increase in bed length, from 1 to 4 m, led to an increase in flame thickness, and therefore, in flame emission with contributions from both soot and gases. A further analysis of the emission was conducted in order to evaluate effective flame properties (i.e., emissivity, extinction coefficient, and temperature). The observation of emission spectra suggests thermal nonequilibrium between soot particles and gas species that can be attributed to the presence of relatively cold soot and hot gases within the flame.

A detailed analysis of the unburned material heat-up during upward flame spread over small slabs of PMMA is provided using a numerical model. The two-dimensional time-dependent Favre-averaged Navier–Stokes equations coupled with sub-models for turbulence, combustion, soot formation, and radiation are solved for the gas phase. The modelling of condensed phase processes is based on the one-dimensional heat conduction equation and pyrolysis is treated as a phase change using the latent heat approach. Pyrolysis heights, xp, up to 0.6 m are considered. Model results agree with data on the pyrolysis front trajectory and heat release rate per unit width, Q ˙ ′ , as a function of xp from experiments in which the lateral air entrainment is expected to have no significant effect on flame spread. It is shown that the average luminous flame height, xfl, defined by the height at which the total wall heat flux is equal to 10 kW/m2, correlates with Q ˙ ′ 1.25 for Q ˙ ′ 20 kW / m , whereas it correlates with Q ˙ ′ 2 / 3 for Q ˙ ′ 20 kW / m , as deduced from experiments. Appropriate scaling of the total wall heat flux distribution is found in terms of ξ = ( x - x p ) / ( x fl - x p ) . This scaling allows us to locate the continuous flame region between ξ=0 and 0.4 and the plume region above ξ=1.6, the intermittent flame region being located between the two. Both continuous and intermittent flames are found to play an important role in the process of heating the unburned solid fuel whereas the plume contribution is negligible.

Flame height is defined by the experimentalists as the average position of the luminous flame and, consequently is not directly linked with a quantitative value of a physical parameter. To determine flame heights from both numerical and theoretical results, a more quantifiable criterion is needed to define flame heights and must be in agreement with the experiments to allow comparisons. For wall flames, steady wall flame experiments revealed that flame height may be defined by a threshold value on the wall heat flux. From steady wall flame measurements, three definitions of flame height from wall heat flux are retained: the first is based on the continuous flame while the two others are based on threshold values of 4 kW/m2 and 10 kW/m2. These definitions are applied to determine flame heights from a two-dimensional time-dependent CFD model used to describe flame spread on a vertical slab of PMMA. Results show that the predicted flame heights are consistent with the available data of the literature. Defining flame height by threshold values on the wall heat flux of 4 and 10 kW/m2 allows the correlation of the wall heat flux in terms of (x-xp)/(xfl-xp), which is the dimensionless characteristic length scale for upward flame spread. It is also found that the continuous flame is not a characteristic length for the heat transfer to the unburnt fuel and is not really appropriate to define flame height in upward flame spread. © 2007 Elsevier Ltd. All rights reserved.

Flame heights of co-flowing cylindrical ethylene-air and methane-air laminar inverse diffusion flames were measured. The luminous flame height was found to be longer than the height of the reaction zone determined by planar laser-induced fluorescence (PLIF) of hydroxyl radicals (OH) because of luminous soot above the reaction zone. However, the location of the peak luminous signals along the centerline agreed very well with the OH flame height. Flame height predictions using Roper’s analysis for circular port burners agreed with measured reaction zone heights when using values for the characteristic diffusion coefficient and/or diffusion temperature somewhat different from those recommended by Roper. The fact that Roper’s analysis applies to inverse diffusion flames is evidence that inverse diffusion flames are similar in structure to normal diffusion flames.

The purpose of the present paper is to develop a mathematical model for predicting thermal field and heat transfer inside short length/height ratio, rectangular and nonhomogeneous material pipe connecting a combustion chamber with outdoors, while the gas combustion temperatures are in the range of 2 000 to 3 000 K and the pipe is made of heat-resistant material. The used approach is based on a finite difference simulation and makes certain idealizations and approximations in order to get to an engineering workable and at the same time adequate, solution. Main assumptions are an equivalent temperature cavity to describe radiant heat transfer due to both the combustion chamber luminous flame and outdoors. The proposed model shows a very good agreement with experimental results coming from an experimental set-up used for materials testing, that is embodied in a refractory rectangular pipe, at high temperature.

A novel diagnostic technique, which makes use of laser light scattered by soot particles, was used in an effort to identify the flame sheets within a natural gas diffusion flame. Soot particles, inherently created and consumed in the flame, were used as the scattering medium, which obviated the need for externally supplied seed material. Since no foreign material was added to the flame, the current technique can be considered truly nonintrusive. The soot distribution within a large buoyant natural gas diffusion flame is argued to be a reasonable marker of the flame sheets. Measurements made in 47.4-190 kW natural gas flames stabilized on a 0.5 m diameter burner show that the flame sheets are highly wrinkled and convoluted surfaces. The flame sheets are distributed fairly uniformly within the instantaneous volume of the flame, based on images of the associated soot, and the instantaneous flame volume is devoid of soot for 40-60% of the time. When soot is present, it is observed as thin sheets which become narrower in regions where the average strain rate is estimated to be greater.