Double-flame Structure Research Articles

A NUMERICAL INVESTIGATION OF METHANE AIR PARTIALLY PREMIXED FLAMES AT ELEVATED PRESSURES

A numerical investigation of high pressure, counterflow methane-air partially premixed flames (PPFs) is presented to characterize the effect of pressure on flame structure. Four different mechanisms, namely GRI-2.11, GRI-3.0, the San Diego, and the C2 mechanisms have been examined. The mechanisms have been validated by comparing the predicted laminar flame speeds with experimental data. While there is excellent agreement between the measured and predicted laminar flame speeds based on these mechanisms, there are noticeable discrepancies amongst the mechanisms for the prediction of high-pressure PPFs. The GRI 3.0 mechanism is used to examine the detailed PPF structure at elevated pressures. At low pressures, the PPF exhibits a typical double flame structure that consists of rich premixed and nonpremixed reaction zones, and the separation distance between the reaction zones decreases with increasing pressure. However, for pressure above 10 atm, the separation distance becomes nearly independent of pressure. In addition, a critical pressure is observed, above which the PPF structure exhibits anomalous behavior, characterized by the presence of endothermic reactions and the production of CO in the region between the two reaction zones. A rate of production analysis revealed that the anomalous behavior is attributable to the following reactions: H + H2O → OH + H2, CH2CO + M → CH2 + CO + M, and HCO + M → H + CO + M. The critical pressure at which this behavior is first observed increases with the increase in equivalence ratio and/or strain rate. Thermal radiation is found to have a negligible effect on the flame structure for the range of conditions investigated.

Investigations on double-state behavior of the counterflow premixed flame system

The counterflow flame system established between lean-methane–air and lean-hydrogen–air streams is investigated experimentally and numerically. A two-dimensional model known as UNICORN was used for the simulation. Detailed measurements for temperature and species concentrations were obtained along the centerline using Raman spectroscopy. A double-state behavior for this flame system was identified in the numerical simulations, which was later confirmed by the experiments. For the given flow conditions, the flame system can have either a single-flame or a double-flame structure depending on the way those conditions were achieved. Detailed comparisons were made between measurements and calculations for the two flame structures. Calculations for various lean methane–air mixtures and stretch rates were performed to understand the double-state behavior of counterflow premixed flames. It was found that the flame system exhibits double-state behavior only for leaner ( ϕ CH 4 < 0.74 ) methane–air mixtures. Aerodynamic and chemical structures of the flames in different stretch-rate regimes were analyzed. When stretch rate on the flame system is increased, the flame transitions from a double-flame to a single-flame structure due to aerodynamic-cooling process. When stretch rate is decreased, the flame does not transition back to the double-flame structure due to stretch effects on molecular diffusion. However, for ( ϕ CH 4 > 0.81 ) , decrease in stretch rate increases flame temperature due to lack of stretch-induced cooling and returns the flame structure to a double-flame one. For a narrow range of equivalence ratios (0.74–0.81) counterflow premixed flames exhibit a hysteresis property.

FUNDAMENTAL STUDIES OF A PARTIAL PREMIXED COUNTER-FLOW COMBUSTION SYSTEM AND ITS EFFECT ON NOx EMISSIONS

The use of air staging as a technique for reducing NOX emissions from process industry burners has been employed for many years but lower levels of NOX and high thermal efficiencies can be achieved using a partial premixed combustion mode. The development of partial premixed burners has only gradually increased because of the increased risk of flash back and the difficulties of control. The current study presents both an experimental and a numerical investigation on a laboratory-scale counter-flow combustion system of the partial premixed mode. The system aims to duplicate the mixing process observed within an industrial burner, which produces a double flame structure. Results have shown that the nozzle configuration has a direct influence on NOX emissions.

A numerical investigation of flame liftoff, stabilization, and blowout

The effects of fuel stream dilution on the liftoff, stabilization, and blowout characteristics of laminar nonpremixed flames (NPFs) and partially premixed flames (PPFs) are investigated. Lifted methane-air flames were established in axisymmetric coflowing jets. Because of their flame suppression characteristics, two predominantly inert agents, CO2 and N2, were used as diluents. A time-accurate, implicit algorithm that uses a detailed description of the chemistry and includes radiation effects is used for the simulations. The predictions are validated using measurements of the reaction zone topologies and liftoff heights of both NPF and PPF. While an undiluted PPF is stabilized at the burner rim, characterized by significant radical destruction and heat loss to the burner, the corresponding undiluted NPF is lifted and stabilized in a low-velocity region extending from the wake of the burner. Detailed comparison of diluted NPF with PPF reveals that the base structures of both the flames are similar and exhibit a double flame structure in the near-field region, where the flame stabilization depends on a balance between the reaction rate and the scalar dissipation rate, which could also be interpreted as a balance between the edge-flame speed undergoing its local scalar dissipation rate and the local flow velocity. As diluent concentration is increased, the flames become weaker, move downstream along the stoichiometric mixture fraction line, and stabilize at a location where they can find a local flow field that has a lower scalar dissipation rate. Further increase of the diluent concentration moves the flames further downstream into the far-field region, where both the NPF and PPF exhibit a triple flame structure, and the flame stabilization mechanism also involves a balance between the triple flame speed and local flow velocity. The PPFs, however, shift to a higher liftoff height and blow out at a lower diluent concentration compared to the NPF, which can withstand larger amounts of dilution. In addition, both NPF and PPF are stabilized at lower liftoff heights and blowout at a lower diluent concentration, when they are diluted with N2 compared to that with CO2. The observed effects of fuel stream dilution and partial premixing on flame liftoff and blowout can be explained using the existing flame stabilization theories.

Evaluation of the equivalence ratio of the reacting mixture using intensity ratio of chemiluminescence in laminar partially premixed CH 4-air flames

An experimental study was performed to investigate the effects of partially premixing, varying the equivalence ratios from 0.79 to 9.52, on OH ∗, CH ∗ and C 2 ∗ in laminar partially premixed flames. The signals from the electronically excited states of OH ∗, CH ∗ and C 2 ∗ were detected through interference filters using a photo multiplier tube, which were processed to the intensity ratios ( C 2 ∗ / CH ∗ , C 2 ∗ / OH ∗ and CH ∗/OH ∗) to determine a correlation with the local equivalence ratios. Furthermore, the consistency between the results of the tomographic reconstruction; Abel inversion technique, image with CCD (Couple Charged Detector) camera and the local radical intensity with PMT was investigated. The results demonstrated that (1) the flames at Φ ⩽ 1.36 exhibited classical double flame structure, at Φ ⩾ 4.76, the flames exhibited non-premixed-like flame structure and the intermediate flames at 1.36 < Φ < 4.76 merged, (2) the peak intensities of OH ∗, CH ∗ and C 2 ∗ were maximal at Φ = 0.86, 0.91 and 1.06, (3) the intensity ratio of CH ∗/OH ∗ was nearly proportional to 1/ Φ for the 1 dimensional results with PMT, with a correlation coefficient of 0.99 at the equivalence ratios of 0.79–9.52. These results are in good agreement with those obtained from the two dimensional results with ICCD. However, it is difficult to predict the relationships between the equivalence ratios and intensity ratios of C 2 ∗ / CH ∗ and C 2 ∗ / OH ∗ .

Comparison of the structures of methane–air and propane–air partially premixed flames

A comparison of flame structures between methane–air and propane–air laminar partially premixed flames has been made through the centerline concentration distributions of selected species measured using gas chromatography. The concentrations of fuel, major species like O 2, CO and CO 2 and those of the intermediate hydrocarbons like C 2H 6, C 2H 4, C 2H 2 and CH 4 (for the propane flame only) have been compared. Distinct double flame structures are observed for the experimental conditions under study. With approximately the same equivalence ratio and jet velocity for the primary mixture, the height of the inner flame for propane is less than that of methane. The peak concentration of C 2H 6 in the propane flame is found to be only a little higher than that in the methane flame, while the peak concentrations of C 2H 4 and C 2H 2 are much greater in the propane flame than in the methane flame. In a methane partially premixed flame, the hydrocarbon concentrations drop from their peak values very rapidly at the inner flame tip, but in the propane flames it is more gradual. In a methane partially premixed flame, CO is formed at the inner flame and burns at the outer flame to CO 2. Similar distributions of CO and CO 2 are found in the propane flame also. However, the peak CO concentration in the propane flame is found to be higher than in methane flame. A radial measurement of species distribution for a particular case in the propane partially premixed flame is also done to ascertain the species distributions across the flame.

Liftoff characteristics of partially premixed flames under normal and microgravity conditions

An experimental and computational investigation on the liftoff characteristics of laminar partially premixed flames (PPFs) under normal (1-g) and microgravity (μ-g) conditions is presented. Lifted methane–air PPFs were established in axisymmetric coflowing jets using nitrogen dilution and various levels of partial premixing. The μ-g experiments were conducted in the 2.2-s drop tower at the NASA Glenn Research Center. A time-accurate, implicit algorithm that uses a detailed description of the chemistry and includes radiation effects is used for the simulations. The predictions are validated through a comparison of the flame reaction zone topologies, liftoff heights, lengths, and oscillation frequencies. The effects of equivalence ratio, gravity, jet velocity, and radiation on flame topology, liftoff height, flame length, base structure, and oscillation frequency are characterized. Both the simulations and measurements indicate that under identical conditions, a lifted μ-g PPF is stabilized closer to the burner compared with the 1-g flame, and that the liftoff heights of both 1-g and μ-g flames decrease with increasing equivalence ratio and approach their respective nonpremixed flame limits. The liftoff height also increases as the jet velocity is increased. In addition, the flame base structure transitions from a triple- to a double-flame structure as the flame liftoff height decreases. A modified flame index is developed to distinguish between the rich premixed, lean premixed, and nonpremixed reaction zones near the flame base. The 1-g lifted flames exhibit well-organized oscillations due to buoyancy-induced instability, while the corresponding μ-g flames exhibit steady-state behavior. The effect of thermal radiation is to slightly decrease the liftoff heights of both 1-g and μ-g flames under coflow conditions.

Concentration measurements of selected hydrocarbons in methane/air partially premixed flames using gas chromatography

Concentrations of selected hydrocarbons (viz. CH 4, C 2H 6, C 2H 4 and C 2H 2) have been measured in CH 4/air partially premixed flames for different primary mixture equivalence ratio ( ϕ) and primary jet velocity using gas chromatography. Visual observations of the flames are also made using digital photography. An axi-symmetric, co-flowing burner is placed on a traversing stage and a quartz probe is used for the collection of the gas sample by placing it at appropriate locations with respect to the burner. The double flame structure is observed with the increase in the premixing air with the fuel. At a high premixing air, the inner flame turns from blue to a greenish one. Hydrocarbons are detected only inside the inner flame structure of the partially premixed flames at all equivalence ratios studied and the concentrations of the non-fuel hydrocarbons are much less than in the non-premixed flames at most of the locations. The peak C 2H 6 and C 2H 4 concentrations are found to be the maximum in a partially premixed flame with an intermediate ϕ, while peak C 2H 2 concentration continually decreases with the decrease in ϕ. The partial premixing decreases the radial transport of species at lower elevations mainly because of the high axial velocity of the primary jet. At higher elevations within the inner flame the radial distribution of C 2H 4 and C 2H 2 concentrations show the peak value to exist at the centerline.

Structure of partially premixed n-heptane–air counterflow flames

To avoid the complexities associated with the droplet/vapor transport and nonuniform evaporation processes, a fundamental investigation of liquid fuel combustion in idealized configurations is very useful. An experimental–computational investigation of prevaporized n-heptane nonpremixed and partially premixed flames established in a counterflow burner is described. There is a general agreement between various facets of our nonpremixed flame measurements and the literature data. The partially premixed flames are characterized by a double flame structure. This becomes more distinct as the strain rate decreases and partial premixing increases, which also increases the separation distance between the two reaction zones. The peak partially premixed flame temperature increases with increasing premixing of the fuel stream. The peak CO 2 and H 2O concentrations are relatively insensitive to partial premixing. The CO and H 2 peak concentrations on the premixed side increase as the fuel-side equivalence ratio decreases. These species are transported to the nonpremixed reaction zone where they oxidize. The C 2 species have peaks in the premixed reaction zone. The concentrations of olefins are ten times larger than those of the corresponding paraffins. The oxidizer is present in partially premixed flames throughout the combustion system and there are no regions characterized by simultaneous high temperature and high fuel concentration. As a result, pyrolysis reactions leading to soot formation are greatly diminished.

Characteristics and structure of inverse flames of natural gas

Characteristics and structure of nominally non-premixed flames of natural gas are investigated using a burner that employs simultaneously two distinct features: fuel and oxidiser direct injection, and inverse fuel and oxidiser delivery. At low exit velocities, the result is an inverse diffusion flame that has been noted in the past for its low NO x emissions, soot luminosity, and narrow stability limits. The present study aimed at extending the burner operating range, and it demonstrated that the inverse flame exhibits a varying degree of partial premixing dependent on the discharge nozzle conditions and the ratio of inner air jet and outer fuel jet velocities. These two variables affect the flame length, temperature distributions, and stability limits. Temperature measurements and Schlieren visualisation show areas of enhanced turbulent mixing in the shear region and the presence of a well-mixed reaction zone on the flame centreline. This reaction zone is enveloped by an outer diffusion flame, yielding a unique double-flame structure. As the fuel–air equivalence ratio is decreasing with an increase in the inner jet velocity, the well-mixed reaction zone extends considerably. These findings suggest a method for establishing a flame of uniform high temperature by optimising the coaxial nozzle geometry and flow conditions. The normalised flame length is decreasing exponentially with the air/fuel velocity ratio. Measurements demonstrate that the inverse flame stability limits change qualitatively with varying degree of partial premixing. At the low premixing level, the flame blow-out is a function of the inner and outer jet velocities and the nozzle conditions. The flame blow-out at high degree of partial premixing occurs abruptly at a single value of the inner air jet velocity, regardless of the fuel jet velocity and almost independent of the discharge nozzle conditions.

Approximating the chemical structure of partially premixed and diffusion counterflow flames using FPI flamelet tabulation

Various strategies have been proposed to tabulate complex chemistry for subsequent introduction into fluid mechanics computations. Some of them are grounded on laminar flame calculations, which are useful to seek out key relations linking a few control parameters with relevant species responses. The objective of this paper is to estimate whether approaches based on premixed flamelets (FPI or FGM) can be extended to partially premixed and diffusion flames. Prototypes of nonpremixed laminar and strained counterflow flames are simulated using fully detailed chemistry. The configuration studied is a jet of methane/air mixture opposed to an air stream. A set of reference flames is then obtained, to which FPI results are compared. By varying the equivalence ratio of the free stream of methane/air mixture, from stoichiometry up to pure methane, premixed, partially premixed, and diffusion flames are analyzed. When the fresh fuel/oxidizer mixture equivalence ratio takes values within the flammability limits, excellent results are obtained with FPI. When this equivalence ratio is outside the flammability limits, diffusive fluxes across isomixture fraction surfaces lead to a departure between the FPI tabulation and the reference detailed chemistry flames. This is associated mainly with the appearance of a double-flame structure, progressively evolving into a single diffusion flame when the fuel side equivalence ratio is further increased. Using an improved flame index to distinguish between premixed and diffusion flame burning, hybrid partially premixed combustion is reproduced from a combination of FPI and diffusion flamelets.

TEMPERATURE AND CO CONCENTRATION MEASUREMENTS IN A PARTIALLY PREMIXED CH4/AIR COFLOWING JET FLAME USING COHERENT ANTI-STOKES RAMAN SCATTERING

In a partially premixed methane/air flame, temperature and CO concentration measurements were conducted using coherent anti-Stokes Raman spectroscopy. Additionally, qualitative two-dimensional OH laser-induced fluorescence imaging was applied. Measurements were performed at different positions downstream of the burner's exit for equivalence ratios (φ) between 1.7 and 4.0. A distinct double-flame structure was observed. Accurate temperature and CO concentration results were achieved, even under sooting conditions. Temperature measurements close to the burner (5 mm downstream) indicated a temperature peak of 2000 K while further downstream at 10 and 20 mm a high-temperature plateau could be found. The CO concentration increases with decreasing φ starting below a particular threshold value. It was a maximum at the inner flame due to the formation of CO as a result of partial combustion. The maximum CO is found to be greater than that in a diffusion flame. The maximum CO concentration has been found for φ = 1.7, the upper ignition limit for CH4/air mixtures. OH measurements indicate that OH is mainly formed in the outer flame and is consumed there by reactions with CO and H2 diffusing from the inner flame.

Triple flame propagation and stabilization in a laminar axisymmetric jet

The propagation of a methane–air triple flame in a partially premixed jet is investigated experimentally and numerically. The flame is ignited with a Nd : YAG laser in a nonuniform jet-mixing layer downstream of the burner. The ignition and flame propagation processes are recorded using a high-speed video camera. The flamefront propagation velocity in laboratory coordinates is inferred from the video images. A comprehensive, time-dependent computational model is used to simulate the transient ignition and flame propagation phenomena. The model employs a detailed description of methane–air chemistry and transport properties. Following ignition, a well-defined triple flame is formed that propagates upstream towards the burner along the stoichiometric mixture fraction line. As the flame propagates upstream, the flame propagation speed, which is defined as the normal flamefront velocity with respect to the local gas velocity, decreases linearly. Near the burner wall, the flame curvature increases to two times the value of its downstream freely propagating counterpart. During the flame propagation process, the curvature-induced stretch dominates over the hydrodynamic stretch and the flame speed decreases with increasing stretch rate in accord with previous measurements. We also examine the dominant reaction rates to follow the transition from a triple flame to a double flame structure.

Combustion Structures in Lifted Ethanol Spray Flames

The development of a double flame structure in lifted ethanol spray flames is visualized using OH planar laser-induced fluorescence (PLIF). While the OH images indicate a single reaction zone exists without co-flow, the addition of low-speed co-flow facilitates the formation of a double flame structure that consists of two diverging flame fronts originating at the leading edge of the reaction zone. The outer reaction zone burns steadily in a diffusion mode, and the strained inner flame structure is characterized by both diffusion and partially premixed combustion exhibiting local extinction and re-ignition events.

Effects of leading edge entrainment on the double flame structure in lifted ethanol spray flames

OH planar laser-induced fluorescence and smoke visualization have been performed in the near field of a turbulent ethanol spray flame to investigate reaction zone structure and the effects of air entrainment on combustion. Annular air co-flow surrounds an axisymmetric spray injector utilizing a pressure-swirl atomizer to supply a hollow cone fuel spray. OH fluorescence demarcates reaction zone contours while smoke visualization indicates ambient air entrainment and turbulent mixing within the spray. A double flame structure is observed, appearing as two diverging flame fronts originating at the stabilization point, and consists of an outer diffusion flame with an inner structure that transitions from mixing controlled to partially premixed combustion downstream of the leading edge. Without co-flow, the inner branch of the double structure burns intermittently with large regions of local extinction often observed, resulting from a high droplet flux and possibly high strain/scalar dissipation rates. Addition of 0.29 m/s co-flow lifts the flame base enough to increase air entrainment and enhance inner zone combustion. The inner zone burns continuously, with no apparent local extinction, due to turbulent mixing between entrained oxidizer and fuel vapor generated by easily vaporized droplets present in the recirculations along the shear layer. The polydisperse spray distribution yields larger droplets which are able to cross the inner reaction zone and then vaporize in the hot region bounded by the double flame structure. This region serves as a fuel source to feed both the stable outer diffusion flame and the diffusive structures of the inner zone. In both cases, the flame leading edge stabilizes in the low-speed flow just outside the periphery of the spray cone, where flame propagation against the incoming flow is possible.

Tri-brachial flames around two interactive droplets in flows with fuel vapor

The flame structures around two equal-sized and interactive fuel droplets immersed in high-temperature convective flows are examined. When the flame is sustained between the two droplets while the flow is pure air, an anchor-shape flame structure, namely the tri-brachial flame, is clearly observed. When far-field temperature or ambient equivalence ratio is increased, the flame propagates upstream and the fuel-lean premixed flame withers. However, if the ambient equivalence ratio is high to some extent such as 0.3, two wings of the flame, viz. the fuel-lean and the fuel-rich premixed flames, extend outward from the stoichiometric point. Once the flame encircles the leading droplet, it evolves into a double-flame structure consisting of a front premixed flame and an aft diffusion flame. Accordingly, the impact of the preceding two environmental factors on the droplets' flame structures is outlined.

Effects of Reaction Mechanisms on Structure and Extinction of Partially Premixed Flames

The structure and extinction behavior of partially premixed flames in a counterflow configuration are investigated by using five different chemistry models. These include the C 1 and C 2 mechanisms of Peters and Rogg (Peters, N., and Rogg, B., Reduced Kinetic Mechanisms for Applications in Combustion Systems, Springer-Verlag, Berlin, 1993, pp. 8-12), a 12-step augmented reduced mechanism, and GRI-2.11 and GRI-3.0 mechanisms. Simulations focus on the comparison of these mechanisms in predicting the structure and extinction of methane-air partially premixed flames over a wide range of strain rates and equivalence ratios, including those corresponding to premixed and diffusion flames. Premixed flame speeds calculated using the C 2 and GRI-2.11 mechanisms are in good agreement with the experimental data, whereas those obtained using the C 1 mechanism show significant differences, especially for fuel-rich conditions. The predicted flammability limits (0.5 < O < 1.4) are found to be nearly identical for the three mechanisms. In addition, the diffusion flame structures computed using the three mechanisms are essentially the same, except for small differences in the peak temperature values. Results for partially premixed flames indicate that all five mechanisms qualitatively reproduce the double-flame structure associated with these flames. There are, however, notable quantitative differences between the predictions of C 1 , C 2 , and GRI-2.11 mechanisms. For low to moderate strain rates and high levels of air premixing (O < 2.0), the rich premixed reaction zone for the GRI-2.11 and GRI-3.0 mechanisms is located very close to the fuel nozzle. In addition, the physical separation between the two reactions zones for these mechanisms is significantly larger compared to that for C 1 and C 2 mechanisms. Important quantitative differences are also observed in the predictions of C 1 and C 2 mechanisms. Compared to the C 1 mechanism, the predictions using the C 2 mechanism indicate that 1) the methane consumption and heat release rates in the premixed zone are higher, 2) the flame structure exhibits higher sensitivity to the equivalence ratio, and 3) the two reaction zones merge at a lower equivalence ratio. The extinction strain rates for partially premixed flames are significantly higher using the C 2 and GRI-2.11 mechanisms compared to those using the C 1 mechanism. The effect of radiation heat transfer, computed using an optically thin model on the partially premixed flame structure, is relatively small. Also note that the premixed flame speed plays an important role in determining the stretch rate and, therefore, the structure of partially premixed flames.

A study of flame propagation mechanisms in lycopodium dust clouds based on dust particles' behavior

Abstract Following our earlier study on the behavior of lycopodium dust flames, further experiments using a particle image velocimetry system with a high-resolution video camera have been conducted to clarify the mechanisms of laminar dust flame propagation in a vertical duct. Lycopodium, a nearly equal-sized particle, has been recognized as being monodispersed, but it was found that an actual lycopodium dust cloud consisted of individual and agglomerated particles. Corresponding to the particle forms, the reaction zone showed a double flame structure, consisting of enveloped diffusion flames (spot flame) of individual particles and diffusion flames (independent flame) surrounding some particles. Due to the convective flow caused by a flame, part of the gravitational settling particles was shifted to the surrounding sides and the rest of the particles changed their movements to upwards in front of the flame. Such particle movement causes a dynamic variation in dust concentration ahead of the flame, which propagates at lower dust concentration rather than the mean concentration. Although the flame moved discontinuously on a micro scale, an overall constant flame velocity was found, presumably due to the dynamic variation in dust concentration and induced flow ahead of the flame. Judging from the above-mentioned movement of single particles in front of the flame, a residence time of the unburnt particle in the preheating zone is needed to form combustible gases close to the particle. This residence time depends on the preheating zone thickness, the particle velocity and the flame propagation velocity. The observation of the movement of a single particle suggested a flame propagation mechanism where an enveloped and diffusion lycopodium dust flame discontinuously propagates from one particle to those adjacent in a laminar suspension.

Partially Premixed Combustion in Lifted Turbulent Jets

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.

Behavior of flames propagating through lycopodium dust clouds in a vertical duct

Abstract 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.