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Autoignited lifted flames of dimethyl ether (DME) in laminar nonpremixed jets with high-temperature coflow air have been studied experimentally. When the initial temperature was elevated to over 860 K, an autoignition occurred without requiring an external ignition source. A planar laser-induced fluorescence (PLIF) technique for formaldehyde (CH2O) visualized qualitatively the zone of low temperature kinetics in a premixed flame. Two flame configurations were investigated; (1) autoignited lifted flames with tribrachial edge having three distinct branches of a lean and a rich premixed flame wings with a trailing diffusion flame and (2) autoignited lifted flames with mild combustion when the fuel was highly diluted. For the autoignited tribrachial edge flames at critical autoignition conditions, exhibiting repetitive extinction and re-ignition phenomena near a blowout condition, the characteristic flow time (liftoff height scaled with jet velocity) was correlated with the square of the ignition delay time of the stoichiometric mixture. The liftoff heights were also correlated as a function of jet velocity times the square of ignition delay time. Formaldehydes were observed between the fuel nozzle and the lifted flame edge, emphasizing a low-temperature kinetics for autoignited lifted flames, while for a non-autoignited lifted flame, formaldehydes were observed near a thin luminous flame zone. For the autoignited lifted flames with mild combustion, especially at a high temperature, a unique non-monotonic liftoff height behavior was observed; decreasing and then increasing liftoff height with jet velocity. This behavior was similar to the binary mixture fuels of CH4/H2 and CO/H2 observed previously. A transient homogeneous autoignition analysis suggested that such decreasing behavior with jet velocity can be attributed to partial oxidation characteristics of DME in producing appreciable amounts of CH4/CO/H2 ahead of the edge flame region.

Due to their natural coupling of low-temperature chemistry and transport, cool flames are a valuable platform for drawing fundamental understandings of complicated phenomena relevant to real engines. In this study, self-sustaining partially premixed cool flames of dimethyl ether are investigated in detail through the use of an ozone-assisted counterflow burner. A double cool flame with distinct diffusion flame and premixed flames sides is visibly observed at increased fuel loading and equivalence ratio. The examination of double cool flames and double hot flames through planar laser-induced fluorescence measurements reveals the stark differences in the role of CH 2 O in each. The results show that while CH 2 O is one of the main product species of the cool flame via low-temperature chemistry, for the hot flame it is only a short-lived intermediate produced in the preheat zone. Comparisons of experimental results with numerical calculations based upon detailed chemical kinetic models show fair agreement on the double cool flame structure but a noticeable discrepancy in the prediction of the second-stage ignition limit, which triggers the transition from cool flames to hot flames. The critical strain rate for second-stage ignition is shown to be much more sensitive to fuel addition on the premixed side of the double flame than on the diffusion side. A mechanism for second-stage ignition in partially premixed cool flames is proposed based upon numerical modeling and experimental observations: H 2 O 2 is primarily formed in the premixed cool flame, diffuses toward the stagnation plane, and then finally decomposes into OH radicals upon approaching the cool diffusion flame.

The ignition process of a methane diffusion impinging flame was investigated experimentally, using high speed schlieren/stereo imaging and image processing techniques. Two types of flame can be identified during the ignition process: premixed like flame with weak blue colour and diffusion flame with yellow-reddish colour. The 3D structure of blue flame after enhancement and yellow flame has been reconstructed and analysed seperately. The ratio between green and blue colour intensity is used to provide indications of the global fuel/air mixture state of the blue flame, which is found to correlate well with the flow conditions and flame propagation characteristics.

The influences of fuel dilution, inlet velocity, and gravity on the shape and structure of laminar coflow CH4-air diffusion flames were investigated computationally and experimentally. A series of nitrogen-diluted flames measured in the Structure and Liftoff in Combustion Experiment (SLICE) on board the International Space Station was assessed numerically under microgravity (mu g) and normal gravity (1g) conditions with CH4 mole fraction ranging from 0.4 to 1.0 and average inlet velocity ranging from 23 to 90 cm/s. Computationally, the MC-Smooth vorticity-velocity formulation was employed to describe the reactive gaseous mixture, and soot evolution was modeled by sectional aerosol equations. The governing equations and boundary conditions were discretized on a two-dimensional computational domain by finite differences, and the resulting set of fully coupled, strongly nonlinear equations was solved simultaneously at all points using a damped, modified Newton's method. Experimentally, flame shape and soot temperature were determined by flame emission images recorded by a digital color camera. Very good agreement between computation and measurement was obtained, and the conclusions were as follows. (1) Buoyant and nonbuoyant luminous flame lengths are proportional to the mass flow rate of the fuel mixture; computed and measured nonbuoyant flames are noticeably longer than their 1g counterparts; the effect of fuel dilution on flame shape (i.e., flame length and flame radius) is negligible when the flame shape is normalized by the methane flow rate. (2) Buoyancy-induced reduction of the flame radius through radially inward convection near the flame front is demonstrated. (3) Buoyant and nonbuoyant flame structure is mainly controlled by the fuel mass flow rate, and the effects from fuel dilution and inlet velocity are secondary.

The combustion characteristics of lean non-premixed Thai synthetic natural gas on improved inverse diffusion flame (IDF) burner were investigated. Air and fuel nozzles were improved from normal diffusion flame burner (NDF) and set as inside and outside exits on burner, respectively. The combustion flames were studied with variations of fuel and air velocity. The luminous flame length was lower and premixed flame length was higher by designed IDF burner due to enhance of mixing between inside air and ambient air. When the equivalence ratio decreased, lower flame length and flame temperature were obtained. Moreover, the highest premixed combustion flames and the maximum temperature were observed equivalence ratio between Φ = 0.76-0.80 on lean non-premixed synthetic Thai natural gas flames with designed IDF burner.

The structure and dynamics of swirled, strongly pulsed, turbulent jet diffusion flames were examined experimentally in a co-flow swirl combustor. The dynamics of the large-scale flame structures, including variations in flame dimensions, the degree of turbulent flame puff interaction, and the turbulent flame puff celerity were determined from high-speed imaging of the luminous flame. All of the tests presented here were conducted with a fixed fuel injection velocity at a Reynolds number of 5000. The flame dimensions were generally found to be more impacted by swirl for the cases of longer injection time and faster co-flow flow rate. Flames with swirl exhibited a flame length up to 34% shorter compared to nonswirled flames. Both the turbulent flame puff separation and the flame puff celerity generally decreased when swirl was imposed. The decreased flame length, flame puff separation, and flame puff celerity are consistent with a greater momentum exchange between the flame and the surrounding co-flow, resu...

Ethylene/air laminar diffusion flames were studied at sub-atmospheric pressures to simulate a non-buoyant environment and at super-atmospheric pressures for comparison, at fuel flow rates of 0.48 mg/s and 1.16 mg/s. Flame properties including flame geometry, soot formation and temperature field of the flames were studied. Overall, luminous flame height decreased with decreasing pressure to the point of visible luminosity disappearance, resulting in blue flames at a near vacuum. Flame width increased with decreasing pressure until the flame was almost spherical. Soot formation was also found to decrease with decreasing pressure and existed at very negligible concentrations in a near vacuum. Subatmospheric peak soot volume fractions ranged from about 0.1 ppm to 0.93 ppm at 0.48 mg/s, whereas at 1.16 mg/s, peak soot volume fractions were substantially higher. At subatmospheric pressures, higher fuel flow rates produced flames with higher soot concentrations. Soot production was restricted to an annular region with this annular region shifting closer to the flame centerline with increasing height from the burner. At locations about halfway between the burner rim and the flame tip, soot volume fraction decreased with increasing radial distance from the flame centerline. These results are consistent for both high and low pressure flames. The annular soot formation region was also located further from the flame centerline for the higher flow rate flames because of the difference in luminous flame shape. At 0.48 mg/s, the fraction of carbon in the fuel converted into soot was between 0.1 % and 1.2 % in the sub-atmospheric pressure range, from 0.2 atm to 1 atm.

An experimental technique for measuring a temperature field in an axisymmetric non-luminous flame is developed using the flame reaction technique combined with the inverse Abel transformation. Flame visualization is carried out using alkali metal solution of Potassium (K), which is supplied to a premixed methane/air flame in a form of spray mist. The basic principle of this technique is based on the measurement of local emission intensity distribution visualized by the flame reaction, which is a function of temperature according to the Maxwell–Boltzmann statistics of thermodynamics. The relationship between the local intensity and the temperature is obtained from the calibration study, in which the local intensity is evaluated from the line of sight intensity by the inverse Abel transformation, and the temperature is measured by thermocouple. This technique is successfully applied to the measurement of local temperature distribution in steady and flickering premixed methane/air flame. The temperature field in the flickering flame indicates that the local temperature oscillates periodically with the flickering frequency, and the highest temperature is found along the flame front and in the merging region.

Coaxial jet spray flames of kerosene and oxygen are experimentally studied over a pressure range of 0.1–1.0 MPa to determine the relationship between flame structure, droplet behavior, and soot formation region, which varies with changes in pressure. The direct images and chemiluminescence spectra show that the spray flames have three regions: the blue flame region, which has a peak of CH* and C 2 * radical chemiluminescence, luminous flame region caused by soot emission, and blue emission region caused by CO 2 emission. With increase in ambient pressure, the flame length shortens drastically, the luminous flame region envelopes the blue flame region, and the blue emission becomes more intense. The result of phase-Doppler anemometry shows that a large number of small droplets evaporate and disappear near the burner, and the evaporation of large droplets also occurs rapidly under high pressure. The result of temperature measurements shows that high-temperature regions appear near the burner. The flame temperature drastically decreases along the central axis, and a minimum temperature point appears. This point moves upstream with increase in ambient pressure because evaporation of the droplets occurs further upstream. A laser-induced incandescence measurement shows that the soot volume fraction does not monotonously increase or decrease with increase in ambient pressure. The soot volume fraction at the central axis becomes low upstream and high downstream. As pressure increases, the vertical position at which the peak of soot volume fraction appears at the central axis moves upstream.

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.

Saturation currents and chemiluminescence, especially at the CH ∗ and C 2 ∗ wavelengths, are measured for a range of small, laminar methane flames during progressive addition of air, with the principal objective of distinguishing between pure diffusion flames, premixed flames of compositions falling between the upper and lower flammability limits, and the broad range of aerated flames lying in between these regimes. Flame areas defined by the loci of maximum luminosity and by schlieren contours were recorded, so that saturation current densities, CH ∗ and C 2 ∗ emission per unit flame area, as well as burning velocities could be deduced. For admixtures of less than 70 vol.%, air appears to act, surprisingly, as an inert diluent as regards saturation currents, so that saturation currents are essentially proportional to fuel flow alone. Much the same applies to chemiluminescence. However, schlieren contours, which were recorded both to provide a basis for burning velocity measurements and to explore density changes in the reactants, indicated the presence of a burner – stabilised propagating reaction zone ahead of the luminous flame surface starting at around 50 vol.% and possibly even at lower air admixtures. This evidence of a steep change in refractive index is indicative of a premixed reaction zone involving the added oxygen, which however generates no chemi-ionization and emits no light. Even photographing the flame by radiation emitted at the CH ∗ and C 2 ∗ wavelengths shows no sign of its existence. Its burning velocity is about 10 cm/s, when stabilized by the surrounding diffusion flame. The most plausible rationale for these observations is the formation of syngas by the partial oxidation of methane. The subsequent burning of CO and H 2 is known to occur without chemi-ionization or appreciable light emission.

The common misconception that hydrogen flames are not visible is examined. Examples are presented of clearly visible emissions from typical hydrogen flames. It is shown that while visible emissions from these flames are considerably weaker than those from comparable hydrocarbon flames, they are indeed visible, albeit at reduced light levels in most cases. Detailed flame spectra are presented to characterize flame emission bands in the ultraviolet, visible and infrared regions of the spectrum that result in a visible hydrogen flame. The visible blue emission is emphasized, and recorded spectra indicate that fine spectral structure is superimposed on a broadband continuum extending from the ultraviolet into the visible region. Tests were performed to show that this emission does not arise from carbon or nitrogen chemistry resulting from carbon-containing impurities (hydrocarbons) in the hydrogen fuel or from CO2 or N2 entrainment from the surrounding air. The spectral structure, however, is also observed in methane flames. The magnitude of the broadband emission increases with flame temperature in a highly nonlinear manner while the finer spectral structure is insensitive to temperature. A comparison of diffusion and premixed H2 flames shows that the fine scale structure is comparable in both flames.

Experimental studies of the length of vertical isolated burning flames with hydrogen and methane outflow from cylindrical nozzles of diameter 2, 3, 5 and 10 mm into the atmosphere at Re 0 = 100-6800 have been made. The data obtained have been approximated by simple calculated dependences. The application of various methods for recording the length of a luminous flame (visual, black-and-white photography, video and digital cameras) has made it possible to not only explain the discrepancies between sizes but also record (at short exposures) elementary bending bright cylindrical jets creating, when superimposed on the another, one known spindle-shaped form of the flame.

Spectral optical techniques, including light extinction and laser induce fluorescence and incandescence measurements, are combined to characterize large-molecule soot precursors and soot in a slightly sooting flame of benzene at atmospheric pressure. Light absorption coupled to in-situ light scattering measurements and ex-situ Atomic Force Microscopy also allowed the evaluation of particle sizes. In the benzene flame high molecular mass structures with typical sizes of 3–4 nm are formed in the main oxidation region of the flame. The radical-rich flame environment in which these compounds are formed promotes their dehydrogenation increasing the level of their aromaticity. As a result, nanoparticles with typical sizes of about 5 nm, absorbing and fluorescing in the visible are formed. These compounds reach a maximum concentration just before the appearance of incandescent soot particles.

Two-dimensional numerical simulations are applied to spray flames formed in a laminar counterflow and the effects of radiation on spray flame characteristics and soot formation are studied. N-Decane (C10H22) is used as the liquid fuel, and the droplet motion is calculated by the Lagrangian method. A single-step global reaction is employed for the combustion reaction model. A kinetically based soot model with a flamelet model is used to predict soot formation. Radiation is taken into account using the discrete ordinate method. The results show that radiation strongly affects the spray flame behavior and soot formation. Without the radiation model, flame temperature and soot volume fraction are greatly overestimated. The soot is formed in the diffusion flame regime, and its radiation emission increases with the increase in the equivalence ratio of the droplet fuel. This trend is in good agreement with that of the luminous flame behavior observed in the experiments.

In this study, we have examined co-flowing diffusion flames formed in a triple port burner. The coannual burner consists of three concentric tubes. Air flows in both inner (central) and outer tubes, and fuel flows in the annulus between these air tubes. These velocities are defined as U1A, U2F, and U3A (or U3N), respectively. The fuel is propane. Two diffusion flames are formed in the boundary of fuel and air. For comparison, nitrogen is ejected in the outer tube to examine the mutual effect of two flames. The luminous flame height was obtained by direct photographs. A laser induced incandescence (LII) technique was applied to examine instantaneous soot concentration. To discuss the mixing of fuel and air, a NO-PLIF technique was used, where NO is added in the fuel flow. Results show that, as the fuel velocity, U2F, is increased, the flame height becomes larger. When nitrogen is ejected in the outer tube, the flame height is larger. However, the flame temperature for both cases is almost the same. The LII measurement shows that there are two soot regions in two luminous flame zones, which are emerged in the downstream. The soot concentration in the inner flame is higher than that in outer flame when U1A U3A. The minimum soot concentration is achieved when U1A and U3A are the same.

This paper shows the results of n-octane two droplets combustion in DC electric field following our previous study. Fiber-supported two droplets are burned under microgravity in air at atmosphere pressure and room temperature. Droplets are arrayed in the directions of electric field with 10 mm distance between droplets and in this condition both flames are individual and have very little interaction between them without an electric field. The equivalent applied elctric field is around 0 to 120 kV/m. The results show that flames are principally deformed to cathode with electric field, however, luminous flame at anode-side end is observed for anode-side flame while blue flame is observed at anode-side end of cathode-side flame. In addition, the increase in burning rate constants for both flames are observed and the increase in burning rate constant for cathode-side flame is larger than that for anode-side flame. The increase in burning rate constant is consistent to information from flame intensities. For cathode-side flame, an analogy to droplet combustion in force convection is discussed in terms of flame deformation and burning rate constants and a relation between them shows good agreement to that under forced convection quantitatively. The increase for anode-side flame is also mentioned from flame deformations.

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.

In nonpremixed combustion, edge flames can form as a region of flame propagation or flame recession. Forwardly propagating edge flames, as occur in lifted flames, have a local gas velocity at the flame edge that is from unburned partially premixed fuel and air into the flame. These flames represent an ignition process, and permit the flame itself to either stabilize against an incoming gas stream or propagate into unburned fuel and air. Negative edge flames represent the opposite case of a local gas velocity from burned products through the flame edge. The negative edge flame represents a local extinction process, and occurs, for example, during vortex-induced extinction of a nonpremixed flame sheet. A technique for generating steady negative edge flames in a standard counterflow burner is presented, which permits detailed examination of their properties. A coannular counterflow burner is used to create a strain gradient that quenches a central diffusion flame. Unlike previous research on strain-induced flame edges, the axisymmetric flow field ensures gas flow from products through the edge. Measurements of the edge flame's sensitivity to global strain rates and fuel mixtures are presented, along with measurements of the edge flame structure using OH fluorescence and CH emission imaging.

The effects of radiation heat transfer on the soot formation and oxidation process in laminar diffusion flames have been studied experimentally using a “radiation shield” for an ethylene flame and a laser heating technique for propylene flames. The soot volume fraction of ethylene diffusion flames was measured for two different radiation boundary conditions. One is the “radiation shield” boundary condition (AL), established by placing the flame inside a highly polished aluminum cylinder, and the other is the fully absorbing radiation boundary condition (BB), obtained with a “black body cylinder enclosure”. The soot formation and oxidation processes are enhanced under the “radiation shield” boundary condition. A second set of experiments was conducted for propylene diffusion flames around the sooting conditions. A non-sooting flame can be converted to a sooting flame when a laser light heats up a flame at a height of 7mm above the burner (HAB), where soot particles are formed. On the contrary, a sooting flame can be changed to a non-sooting flame when the flame is heated with a laser light at 13mm HAB, where soot particles are oxidized. In this study, the absorbed amounts of radiation energy, the soot volume fraction, and the increased soot temperatures were measured.

An attempt has been made to examine the characteristics of ion current obtained from Low-Oxygen, High-Temperature Air Combustion. Non-luminous zone is seen in the upstream region and luminous zone is seen in the downstream region in low-oxygen, high-temperature air flame, while only the luminous region is seen in the high-temperature air flame. The sharp spike like signals are seen on the ion current obtained from the luminous zone of both high-temperature air and low-oxygen, high-temperature air flames. The spike like signal is not seen on the ion current obtained from the non-luminous zone of low-oxygen, high-temperature air flame. The ion current looks quite similar with that obtained from turbulent premixed flames in the reaction sheet regime. It implies that the local structure of low-oxygen, high-temperature air flame is not the flameless combustion but shmilar to that of turbulent premixed flames in the reaction sheet regime, in which the local reaction zone is separately defined from burnt and unburnt gases.

A premixed flame stabilized on a circular porous plug burner produces a uniform, steady luminous flame front. Throughout much of the parameter range hydrocarbon-oxygen mixtures form spiral-shaped fronts. In methane-oxygen flames at low pressure, the flame exhibits a sequence of states as a control parameter is decreased. These states include periodic rotation of a spiral front; precession of the spiral front in a direction opposite to its rotation, corresponding to doubly periodic petals-out meandering; and nonperiodic states with intermittent jumps associated with linear excursions of the tip, which occur after the spiral front has reached the boundary of the circular burner. We use Karhunen-Loeve (KL) analysis to find the coefficients of the dominant KL spatial eigenfunctions. Their phase space portraits and power spectra provide a description of the dynamics as flow rates are reduced and the system destabilizes. We discuss how these experimental results relate to previous theoretical studies that assume Euclidean symmetry for the experimental configuration.

The morphologies of deposits on 15-μm diameter silicon carbide (SiC) fibers have been investigated with a scanning electron microscope (SEM) in co-flowing, propane/air laminar diffusion flames. The average size of the soot particles deposited in the luminous flame edge is strongly dependent on their position in a typical unconfined heavily sooting flame, but this position dependence is not discernible in a confined flame under a reduced oxidizer condition. The transition to soot from the condensed-phase deposits could be observed in the radial deposition of sooting flames. Two kinds of processes that mature soot particles are produced exist in a heavily sooting flame: one is the transition from the condensed-phase deposits; the other is the aggregation of the smaller soot particles (or chains of them) carried along the particle path line. The fiber-like structures, which are about 10-nm thick, were found outside the luminous flame surface, and are thought to be made under high temperature oxidation environment.

This paper reports a PIV measurement technique for application to the combusting flow with a luminous flame. The experiments are conducted on the velocity field and the flame structure of spray combustion in a combustion chamber model. The results with and without combustion indicate that the oncoming flow velocity from the burner is enhanced by the chemical reaction to promote the re-circulating flow inside the chamber model. The simultaneous visualization of flame structure showed the distribution of luminous flame, the unburned droplets, and the soot structure generated in a flame.

In order to clarify flame base structure of turbulent lifted diffusion flame, two flames (Re=4 200 and 6 700) have been experimentally investigated in the configuration of a coflow nozzle. Fuel is methane/hydrogen mixture. These flames were visualized by the schlieren method and OH radical luminescence was also captured by an ICCD camera to characterize the flame bases. Quantitative argumentation of the flame bases has been done on the temperature fields measured by the Rayleigh scattering method; especially, on the temperature dissipation rate and the scalar dissipation rate. Flame bases of turbulent lifted diffusion flames are exposed to small-scale turbulence and emit the light from OH radicals strongly. Both of the vortex-flame interaction and the partial premixing are possibly related to the phenomenon. On the other hand, the scalar structure of the flame base seems to be close to the conventional flamelet structure to show lognormal probability density distributions of the scalar dissipation rate.

The structure of flame propagating through lycopodium dust clouds has been investigated experimentally. Upward propagating laminar flames in a vertical duct of 1800 mm height and 150×150 mm square cross-section are observed, and the leading flame front is also visualized using by a high-speed video camera. Although the dust concentration decreases slightly along the height of duct, the leading flame edge propagates upwards at a constant velocity. The maximum upward propagating velocity is 0.50 m/s at a dust concentration of 170 g/m 3 . Behind the upward propagating flame, some downward propagating flames are also observed. Despite the employment of nearly equal sized particles and its good dispersability and flowability, the reaction zone in lycopodium particles cloud shows the double flame structure in which isolated individual burning particles (0.5–1.0 mm in diameter) and the ball-shaped flames (2–4 mm in diameter; the combustion time of 4–6 ms) surrounding several particles are included. The ball-shaped flame appears as a faint flame in which several luminous spots are distributed, and then it turns into a luminous flame before disappearance. In order to distinguish these ball-shaped flames from others with some exceptions for merged flames, they are defined as independent flames in this study. The flame thickness in a lycopodium dust flame is observed to be 20 mm, about several orders of magnitude higher than that of a premixed gaseous flame. From the microscopic visualization, it was found that the flame front propagating through lycopodium particles is discontinuous and not smooth.

This article reports observations of structures consistent with triple flame reaction zones in the stabilization region of turbulent nonpremixed jet flames. Previous studies of laminar mixing layers and nonpremixed jets show Hi brachial structures, however reports of triple flame structures in turbulent flowfields are sparse, The asymmetric coflow, which facilitates visualizing the luminous flame structure, is described and the observed double and triple flame structures are discussed. Flame luminosity data is presented and the relevance to CH planar laser-induced fluorescence (PLIF) studies of leading edge flame structures is discussed. Furthermore, connection is drawn to select theoretical studies of triple flames and partially premixed combustion.

This experimental investigation analyzes the soot formation effects of oxygen concentration in the oxidizer stream (O 2 + N 2 ) ventilating laminar jet nonpremixed methane flames. The base flame incorporates air as the oxidizer; two additional flames, with respective oxygen concentrations of 50% and 100% in the ventilating coflow, are also examined. The microstructure of soot collected from selected flame locations is determined combining thermophoretic sampling and transmission electron microscopy. A laser-light extinction technique is employed along with tomographic inversion to measure the soot volume fraction distributions within the three flames. The results indicate that soot surface growth and oxidation rates in the methane/50% oxygen flame are higher compared to the respective rates in the methane/air base flame. The rate of soot inception becomes stronger with increasing oxygen content in the oxidizer stream. Soot yields diminish with increasing oxygen concentration, as do luminous flame spatial dimensions. Soot aggregate data on the soot annulus suggest a higher degree of agglomeration under oxygen-enriched conditions. Finally, the fractal dimensions of selected soot aggregate samples are measured to be 1.64 (methane/air flame) and 1.65 (methane/50% oxygen flame), being similar to previously published values for carbonaceous soot.

Visible spectral characteristics of cross-sectional emissions from a partially premixed methane/air flame and a propane/air flame have been investigated. An optical train with a two-axis scanning mirror system was used to record line-of-sight emission spectra from 354nm to 618nm, and inversion technique was applied to obtain cross-sectional emission spectra. By analyzing the reconstructed emission spectra, cross-sectional intensities of CH and C2 radicals were separated from the background emission. The blue flame edge and yellow flame edge were also obtained by image processing technique for edge detection with color photographs of the flames. These edges were compared with radial distributions of CH, C2 radicals and background emission. The CH radicals were observed at the blue flame edge. The background emission was generated by soot precursors at upstream of the flame and by soot at downstream of the flame. The C2 radicals in the propane/air flame were more noticeable than those in the methane/air flame.

The shapes (luminous flame boundaries) of round luminous nonbuoyant soot-containing hydrocarbon/air laminar jet diffusion flames at microgravity were found from color video images obtained on orbit in the Space Shuttle Columbia. Test conditions included ethylene- and propane-fueled flames burning in still air at an ambient temperature of 300 K, ambient pressures of 35-130 kPa, initial jet diameters of 1.6 and 2.7 mm, and jet exit Reynolds numbers of 45-170. Present test times were 100-200 s and yielded steady axisymmetric flames that were close to the laminar smoke point (including flames both emitting and not emitting soot) with luminous flame lengths of 15-63 mm. The present soot-containing flames had larger luminous flame lengths than earlier ground-based observations having similar burner configurations: 40% larger than the luminous flame lengths of soot-containing low gravity flames observed using an aircraft (KC-135) facility due to reduced effects of accelerative disturbances and unsteadiness; roughly twice as large as the luminous flame lengths of soot-containing normal gravity flames due to the absence of effects of buoyant mixing and roughly twice as large as the luminous flame lengths of soot-free low gravity flames observed using drop tower facilities due to the presence of soot luminosity and possible reduced effects of unsteadiness. Simplified expressions to estimate the luminous flame boundaries of round nonbuoyant laminar jet diffusion flames were obtained from the classical analysis of Spalding (1979); this approach provided Successful Correlations of flame shapes for both soot-free and soot-containing flames, except when the soot-containing flames were in the opened-tip configuration that is reached at fuel flow rates near and greater than the laminar smoke point fuel flow rate.

Soot zone structures of counterflow and co-flow diffusion flames have been studied experimentally using the soot extinction-scattering, polycyclic aromatic hydrocarbon fluorescence, and laser Doppler velocimetry measurements. The counterflow flame has been numerically modelled with detailed chemistry. Results show that two different categories of sooting flame structures can be classified depending on the relative transport of soot particles to flames. These are the soot formation-oxidation flame and the soot formation flame. The soot formation-oxidation flame characteristics are observed in counterflow flames when located on the fuel side and in normal co-flow flames. In this case, soot particles are transported toward the high temperature region or the flame and experience soot inception, coagulation-growth, and oxidation. The soot formation flame characteristics are observed in counterflow flames when located on the oxidizer side and in inverse co-flow flames. In this case, soot particles are transported away from the flame without experiencing oxidation and finally leak through the stagnation plane in counterflow flames or leave the flame in inverse co-flow flames. Sooting limit measurements in both flames also substantiate the two different sooting flame structures and their characteristics.

The structure of a luminous, laminar, methane-air premixed flame has been investigated using gas chromatograph-mass spectrometer (GC-MS) ; a luminous flame is realized owing to energy recirculation by radiation transferred from the luminous flame to a porous medium at the upstream side of the combustion space. The results show that in the early stage of reaction the methane breaks down to C2H2 through a pathway similar to that of the conventional methane-oxygen premixed flame, i.e., CH4→CH3→C2H6→C2H4→C2H2 Many species, such as C2H2, C4H2 and C6H6, appear in the upstream region of the luminous zone ; these species play an important role in soot nucleation. Furthermore, when the concentration of these species exceeds a critical point, that is, for an equivalence ratio larger than 1.8, a luminous flame is observed.

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.

A new way of using thermography for radiative objects like pool flames has been developed by the author. The thermography method, which has already been applied to large pool fires, is applied to small pool flames in this paper. The radiative characteristics and flame structure of small pool flames are considered by examining flame temperature distribution from a radiation point of view. In addition, it is shown that pool flame structure for various fuels becomes clearer using the standard deviation and the coefficient of variation obtained from thermographic data.

A droplet cloud of liquid fuel produced by an ultrasonic atomizer was ignited by a spark, and the ball of flame propagating outwards was observed in detail in order to elucidate the mechanism of flame propagation and the complicated group combustion behavior of spray flames. For this purpose, the instantaneous images of droplet clusters, OH-radical chemiluminescence and C2-band flame luminosity were taken simultaneously, and the light emission signals in OH- and CH-bands, Mie-scattering signal from droplets, and the size and velocity of droplets were monitored simultaneously in time series. It was found that a nonluminous flame propagated ahead of the luminous flame, that droplets disappeared in the luminous flame zone due to their rapid evaporation, and that a number of small droplet clusters burned in the diffusion combustion mode associated with solid-body emissions inside a ball of flame.

We investigated the effects of a high-temperature environment on diesel spray combustion and formation and oxidation of soot in the flame by eliminating the effects of wall temperature and boundary layer as much as possible using a rapid compression machine which has large combustion chamber. When gas temperature increased, short ignition delay and elongation of combustion was observed by analyzing the cylinder pressure diagram. However disappearance time of luminous flame did not change. Recombination of thermal dissociation must be considered as apparent heat release. Higher surrounding gas temperature results in increased soot formation time and extended low-density soot region in the flame, in spite of almost the same time histories of total amount of soot.

Planar images of laser-induced flurescence from OH · radicals and elastic scattering from soot particles are presented in time-varying, laminar CH4/air diffusion flames burning in a co-flowing, axisymmetric configuration at atmospheric pressure. Acoustic forcing is used to phase lock the periodic flame flicker to the pulsed laser system operating at 10.13 Hz. For conditions where the tip of the flame is clipped, the intensity of the light scattered by the soot particles increases dramatically (by more than a factor of 7 for the maximum signals at a point) compared to a steady-state, laminar flame with the same mean fuel flow velocity. Comparison of the scattering signals integrated along the flame radius is carried out in the steady-state and time-varying flames as a function of height above the burner. Time-varying flames exhibit a larger range of combustion conditions than observed in corresponding steady-state flames, including different residence times, temperature histories, local stoichiometries, and strain and scalar dissipation rates. Thus, their investigation promises to yield new insights into a wide variety of chemistry-flowfield interactions which are prominent in turbulent combustion.

Premixed hydrocarbon-air flames invariably show two separate reaction zones. In the primary zone, the combustible gas mixture burns principally to carbon monoxide, hydrogen, and water, and in the outer mantle, or secondary diffusion flame, the hot gases burn with atmospheric oxygen to carbon dioxide and water. Teclu [J. Prakt. Chem. 44, 246 (1891)] and Smithells and Ingle [Trans. Chem. Soc. 61, 204 (1892)] independently demonstrated the existence of these two zones in various premixed hydrocarbon-air flames, using the flame separator. This device consists of a wide glass or silica tube fitted over the bunsen type burner to form an extension above the inner burner port. The primary combustion then occurs at the inner burner port, while the pale blue secondary diffusion flame is maintained at the top of the outer glass tube. An alternative method of separation of premixed hydrocarbon-air flames consists of sheathing the flame with an inert gas to lift off or separate the secondary diffusonzone. The interconal zone of flames separated by these methods are extended in length and exhibit very low radiative background. The interconal zone also contains the hottest part of the flame, and can be viewed without interference from radiation produced in a secondary diffusion zone that would normally surround it in separated flames. It is the hot interconal zone of premixed flames that is most frequently employed in analytical flame photometry, because it is in this region that the greatest population of atoms occurs when elements are introduced into the flame by nebulization of solutions of their salts. Thus, separated flames may be employed with advantage in thermal emission, atomic absorption, and atomic fluorescence spectroscopy. This paper describes the separation of the air-acetylene and nitrous oxide-acetylene flames, and some applications of these flames in analytical flame spectroscopy.

Characteristics of laminar methane-oxygen diffusion flames were studied at sub-critical and supercritical pressures up to 100 atmospheres. The influence of pressure on soot formation and on the structure of the temperature field was investigated over the pressure range of 10 to 90 atmospheres in a high pressure combustion chamber using a non-intrusive, line-of-sight spectral soot emission diagnostic technique. Two distinct zones characterized the appearance of a methane and oxygen diffusion flame: an inner luminous zone similar to the methane-air diffusion flames and an outer blue flame zone. The flame height, marked by the visible soot radiation emission, was reduced by over 50% over the pressure range of 10 to 100 atmospheres at the higher methane flow rate. At the lower methane flow rate, above 10 atmospheres, the luminous flame zone started collapsing and solid carbonaceous structures appeared on the fuel nozzle rim. Further increase in pressure resulted in shrinking of the luminous zone into mostly solid material and only the blue flame zone was visible. For the higher methane flow rate, between 10 and 40 atmospheres, the soot levels increased with increasing pressure; however, above 40 atmospheres the soot concentrations reached a plateau and then began decreasing with increasing pressure.

A droplet suspension of liquid fuel produced by an ultrasonic atomizer was spark-ignited. The flame ball propagating outwards was visualized in order to elucidate the mechanism of flame propagation and the complicated group structure of spray flames. It was found that a nonluminous flame was propagating continuously through a gas-phase mixture followed by luminous flamelets, so that a number of small-scaled droplet clusters with luminous emissions were observed behind the flame front propagating in premixed combustion mode. This fact implies that the group structure of a spray flame is not simple but complex.

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.

A laminar methane-air diffusion flame has been extensively probed for its temperature, velocity, and stable species' concentration profiles. The data obtained were used to validate a theoretical model capable of characterizing flame macrostructure and useful in predicting the effects of flow rate, equivalence ratio, fuel and air preheat, and nozzle size on the thermal and aerodynamic fields established in confined, axisymmetric, laminar diffusion flames. The model utilizes the Burke and Schumann flame sheet concept to locate the stoichiometric fuel-oxygen interface and, hence, the points of heat release. The model eliminates the restrictions of the classical Burke-Schumann model in that allowances are made for natural convection effects and variable thermodynamic and transport properties. The stoichiometric fuel-oxygen interface was found to coincide with the outer edge of the primary reaction zone and represented a surface through which an amount of oxygen in stoichiometric proportion to the amount of fuel fired diffuses. The inner edge of the reaction zone was defined by the profile of the luminous flame core and was found to be well fitted by the theoretical predictions when the ratio of oxygen and fuel diffusing to the flame front was assigned a value 0.88 times the stoichiometric coefficient. The concentration and temperature profiles could be generalized by plotting the results as a function of the local-equivalence ratio.

The effects of radiant heat fluxes on the stability range and burning velocity of a laminar premixed flame were examined by inserting a water-cooled Bunsen burner into a radiant heat recirculation-type furnace. The intensity of the radiant heat flux in the furnace was altered by regulating the surface temperatures of the furnace wall and downstream permeable plate. It was found that the flame blowoff and flashback limits were hardly affected by a radiation field as intense as the one surrounded by a refractory whose temperature was between 1250K and 1450K, and that no perceptible changes appeared either in the laminar burning velocity or flame thickness.

An experimental study on flame propagation through an interface between layered methane and air, propane and air, and methane and oxygen in inert baths has been made with a combustor having a length of 10 cm. Detailed flame structure was investigated by means of schlieren photography and a hot wire anemometer. The premixed flame, the diffusion flame, and the convection flame just like those propagating in line appeared in the limited combustor space, but one after another with some time lags in between. The diffusion flame appeared about 80 msec after the premixed flame passed, which equivalently corresponds to about 10 cm of space existing between the premixed flame and the forefront of the diffusion flame. An anomalous structure was observed in the premixed flame. That is, a post burning occurs immediately behind the conventional flame front which forms double flame fronts. The temperature in between these is flat, and the separated gap width increases towards lean and rich mixtures. Steady fla...

Swirl type combustor for study of spray combustion has been designed. Using this combustor luminous flame and non-luminos flame can be produced by shifting the spray nozzle position without changing the operating conditions. From the experimental studies the following results were obtained. (l) Supplying combustion air from tangential port, swirl number of S=0.99 was obtained. The circulation length was nearly equal to the combustion tube diameter. (2) Flame appearances were controlled by diffusion process of fuel spray and combustion air. If fuel spray was injected into a reverse flow in torroidal vortex, combustion process was so fast that non-luminous flame was obtained. (3) Paying attention to the primary combustion, combustion efficiency of swirl-type combustion was found better in the case of luminous flame, and it was shown combustion performances were controlled by the flow pattern and the spray directions.

Planar visualization was employed to study flame structure and low frequency flame oscillation. Two distinct vortices were visualized in the flames studies: large toroidal vortices outside the luminous flame and small roll-up vortices inside the luminous flame. The flame oscillation frequency and the convective velocity of the toroidal vortices were measured for ethylene, methane, and propane diffusion flames over a wide range of test conditions. The frequency was typically in the range 10 to 20 Hz and the convective velocity was approximately, 0.8 m/s. The frequency of the toroidal vortices was found to correlate with the flame oscillation frequency. The potential effects of the toroidal vortices on the flame dynamics at low fuel flow rates are discussed, for example, low frequency flame oscillation, non-linear flame bulge motion, and quenching of the luminous flame surface.