Flame structures and behaviors of opposed flow non-premixed flames in mesoscale channels

An opposed flow non-premixed flame (OFNPF) in a narrow channel was chosen as a model of a non-premixed flame in a mesoscale combustion space or micro-combustor. The stabilization limits and behaviors of methane-air flames and propane-air flames were compared for various experimental parameters such as flow velocity, nozzle distance, nozzle width, channel gap, and fuel dilution. Flames could be stabilized in a wide range of strain rates (0.9–150 s⁻¹) and dilution ratios (∼80% nitrogen at the fuel side). The flame extinction limits were classified into three types and their mechanisms were investigated: higher-strain-rate (HSR) extinction limit determined by the flame stretch, lower-strain-rate (LSR) extinction limit determined by the conductive or convective heat loss from the flame, and fuel-dilution-ratio (FDR) extinction limit determined by the decrease in the heat release rate from the flames. The HSR extinction limits in mesoscale channels could be explained with a modified strain rate, and the LSR extinction limits could be explained by employing a premixed quenching theory in which the heat loss through the dead space near the wall was considered as a major extinction mechanism. Finally, the variation of the extinction limits with the FDR in both the HSR and the LSR conditions could be explained with a modified global reaction rate in which the variations in flame temperature and species concentrations were reflected. This study provides an essential model for the stabilization and extinction of non-premixed flames in mesoscale combustion spaces.     

Extinction and near-extinction instability of non-premixed tubular flames

Tubular non-premixed flames are formed by an opposed tubular burner, a new tool to study the effects of curvature on extinction and flame instability of non-premixed flames. Extinction of the opposed tubular flames generated by burning diluted H₂, CH₄ or C₃H₈ with air is investigated for both concave and convex curvature. To examine the effects of curvature on extinction, the critical fuel dilution ratios at extinction are measured at various stretch rates, initial mixture strengths and flame curvature for fuels diluted in N₂, He, Ar or CO₂. In addition, the onset conditions of the cellular instability are mapped as a function of stretch rates, initial mixture strengths, and flame curvature. For fuel mixtures with Lewis numbers much less than unity, such as H₂/N₂, concave flame curvature towards the fuel suppresses cellular instabilities.     

Effect of hydrogen percentage and air jet Reynolds number on fuel lean flame stability of LPG-fired inverse diffusion flame with hydrogen enrichment

The flame type studied in this paper is a circumferential-fuel – jet inverse diffusion flame, and the fuel is liquefied petroleum gas enriched with hydrogen gas. Fuel lean flame stability limit regarding to the volumetric percentage of hydrogen and the air jet Reynolds number was investigated. There were three flame stable-related limits examined: local extinction limit, restore limit, and complete extinction limit. Global Energy Consumption Rate of fuel, fuel jet velocity, and overall equivalence ratio of the air/fuel mixture at the three stable-related limits were presented. Experimental results indicate that with hydrogen addition, the inverse diffusion flame can sustain burning with a lower global energy than without it. The most significant stabilization effect was obtained with 30% hydrogen addition for complete extinction limit and 30%–90% for local extinction limit. The corresponding fuel jet velocity at complete extinction limit also decreases with hydrogen addition. However, fuel jet velocities at local extinction limit and restore limit increase significantly, when hydrogen percentage is larger than 70%. Air jet Reynolds number does not show notable influence on Global Energy Consumption Rate or fuel jet velocity at the three stability limits. In addition, overall equivalence ratio, which is an important parameter of inverse diffusion flame combustion dropping dramatically with air jet Reynolds number when it is less than 2000.     

A survey of recent studies on flame extinction

A survey of the existing achievements in the domain of problems of extinction and flammability limits is presented. It is shown that the fundamental geometrical proportions between flame thickness and preheat zone for any flame, limit flames being included, are constant and independent of the mixture composition. The behaviour of limit flames under various severe physical conditions of the experiment is described and the influence of various factors on the extinction process is discussed. The conditions of flame quenching by the wall are analysed. Two particular mechanisms of flame extinction are discussed: for a flame moving in a tube in the direction of acceleration and in the opposite direction. The principal aim of the present paper was to describe the physical mechanisms of flame extinction under various initial and boundary conditions.     

Experimental study of limit lean methane/air flame in a standard flammability tube using particle image velocimetry method

Lean limit methane/air flame propagating upward in a standard 50 mm diameter and 1.8 m length tube was studied experimentally using particle image velocimetry method. Local stretch rate along the flame front was determined by measured gas velocity distributions. It was found that local stretch rate is maximum at the flame leading point, which is in agreement with earlier theoretical results. Similar to earlier observations, extinction of upward propagating limit flame was observed to start from the flame top. It is stated that the observed behavior of the extinction of the lean limit methane/air flame can not be explained in terms of the coupled effect of flame stretch and preferential diffusion. To qualitatively explain the observed extinction behavior, it is suggested that the positive strain-induced flame stretch increases local radiation heat losses from the flame front. An experimental methodology for PIV measurements in a round tube is described.     

Analytical and experimental study of premixed methane–air flame propagation in narrow channels

This study investigates analytically and experimentally the influence of preheat temperature on flame propagation and extinction of premixed methane–air flame in single quartz tubes with inner tube diameters of 3.9, 3, 2 and 1 mm respectively. The effects of preheat temperature, tube diameter, equivalence ratio and mixture flow rate on the flame speed and extinction conditions are determined. The analytical results show that high preheat temperature of the mixture can effectively suppress flame quenching, and the occurrence of stable solution in the slow flame branch extends the flammability limit leading to possible flame propagation in mini channels. Experimental results confirm that the flame speed increases and the flammability limit shifts toward the fuel lean direction either through increasing the preheat temperature or decreasing the mixture flow rate, or both. Decrease of propagating flame speed is observed before the stoichiometric equivalence ratio at high preheat temperatures. The analytical model provides insights into how propagating flame in mini channels can be sustained; however, the model is only good at predicting flame speed near the fuel lean branch. Influence of Cu²⁺ ions exchanged zeolite 13X catalyst on flame speed is also addressed. It is noted that the zeolite based catalyst can lower the preheat temperature requirement in order to sustain the flame propagation in narrow channels.     

The effect of stretch on cellular formation in non-premixed opposed-flow tubular flames

Cellular formation in non-premixed flames is experimentally studied in an opposed-flow tubular burner. This burner allows independent variation of the global stretch rate and overall flame curvature. In opposed-flow flames formed by 21.7% hydrogen diluted in carbon dioxide versus air, cells are formed near extinction with a low fuel Lewis number and a low initial mixture strength. Using an intensified CCD camera, the flame chemiluminescence is imaged to study cellular formation from the onset of cells to near extinction conditions. The experimental onset of cellular instability is found to be at or at a slightly lower Damköhler number than the numerically determined extinction limit based on a two-point boundary value solution of the tubular flame. For fuel Lewis numbers less than unity, concave curvature towards the fuel retards combustion and weakens the flame and convex curvature towards the fuel promotes combustion and strengthens the flame. In the cell formation process, the locally concave flame cell midsection is weakened and the locally convex flame cell ends are strengthened. With increasing stretch rate, the flame breaks into cells and the cell formation process continues until near-circular cells are formed with no concave midsection. Further increase in the stretch rate leads to cell extinction. With increasing stretch rate, the flame thickness at the cell midsection decreases similar to a planar opposed-flow flame while the flame thickness at the cell edges is unchanged and can even increase due to the strengthening effect of convex curvature at the flame edges toward the low Lewis number fuel. The results show the existence of cellular flames well beyond the two-point boundary value extinction limit and the importance of local flame curvature in the formation of flame cells.     

Experimental study on turbulent structure of humid air flame in a bluff-body burner

The main objective of the present experimental study is to analyze the turbulent structure in humid air non-premixed flame, and determine the effect of humidity on the flow field and the flame stability limit in turbulent non-premixed flame. Particle Image Velocimetry (PIV) is used to capture the instantaneous appearance of vortex structures and obtain the quantitative velocity field. The distributions of Reynolds shear stress, mean and root-mean squared fluctuating (rms) velocities are examined to get insight into the effect of fuel-to-air velocity ratio on velocity flow field. The results show that with steam addition, the air-driven vortex in the bluff-body wake is thinner; the biggest peaks of rms velocity and Reynolds shear stress are lower; the distance between the peaks of rms velocity on the sides of centerline reduces. Besides these, the flame stability is affected. Both central fuel penetration limit and partially quenching limit reduce with steam addition.     

Extinction limits and associated uncertainties of nonpremixed counterflow flames of methane,ethylene, propylene and n-butane in air

Fundamental flame characteristics derived from counterflow flames are routinely used in chemical kinetic model optimization and validation. This paper reports an experimental and computational investigation aimed at understanding and quantifying the source of uncertainties associated with such characterization of extinction limits of fuel–air mixtures, ranging from low extinction strain rate methane–air flames to high-extinction strain rate ethylene–air flames. In the experiments, two pairs of convergent nozzles with exit diameters of 7.9 mm and 14.5 mm were used to introduce opposed jets of nonpremixed fuel and air to establish a planar flame in the counterflow mixing region. Velocity profiles and extinction data were measured using both LV and PIV setups. Experiments were conducted at various nozzle separation distances to investigate potential differences in axial velocity profiles along the axial and radial directions and the corresponding local extinction strain rates. The slope of axial velocity in the axial and radial directions at the air outlet boundary was found to increase with decreasing nozzle separation distance. The variation of local extinction strain rate with changes in separation distance was within the uncertainty of experimental data. Using a C1–C4 chemical kinetic model, quasi one-dimensional computations have been performed to quantify the experimentally determined boundary condition effects on the predicted extinction strain rate of counterflow flames.     

Experimental study on premixed CH4/air mixture combustion in micro Swiss-roll combustors

Excess enthalpy combustion is a promising approach to stabilize flame in micro-combustors. Using a Swiss-roll combustor configuration, excess enthalpy combustion can be conveniently achieved. In this work, three types of Swiss-roll combustors with double spiral-shaped channels were designed and fabricated. The combustors were tested using methane/air mixtures of various equivalence ratios. Both temperature distributions and extinction limits were determined for each combustor configuration at different methane mass flow rates. Results indicate that the Swiss-roll combustors developed in the current study greatly enhance combustion stability in center regions of the combustors. At the same time, excess enthalpy combustors of the Swiss-roll configuration significantly extend the extinction limits of methane/air mixtures. In addition, the effects of combustor configurations and thermal insulation arrangements on temperature distributions and extinction limits were evaluated. With heat losses to the environment being significant, the use of thermal insulations further enhances the flame stability in center regions of the Swiss-roll combustors and extends flammable ranges.     

The structure of partially premixed methane flames in high-intensity turbulent flows

Direct numerical simulations (DNS) are conducted to study the structure of partially premixed and non-premixed methane flames in high-intensity two-dimensional isotropic turbulent flows. The results obtained via “flame normal analysis” show local extinction and reignition for both non-premixed and partially premixed flames. Dynamical analysis of the flame with a Lagrangian method indicates that the time integrated strain rate characterizes the finite-rate chemistry effects and the flame extinction better than the strain rate. It is observed that the flame behavior is affected by the “pressure-dilatation” and “viscous-dissipation” in addition to strain rate. Consistent with previous studies, high vorticity values are detected close to the reaction zone, where the vorticity generation by the “baroclinic torque” was found to be significant. The influences of (initial) Reynolds and Damköhler numbers, and various air-fuel premixing levels on flame and turbulence variables are also studied. It is observed that the flame extinction occurs similarly in flames with different fuel-air premixing. Our simulations also indicate that the CO emission increases as the partial premixing of the fuel with air increases. Higher values of the temperature, the OH mass fraction and the CO mass fraction are observed within the flame zone at higher Reynolds numbers.     

Flame spread over thin fuels in actual and simulated microgravity conditions

Most previous research on flame spread over solid surfaces has involved flames in open areas. In this study, the flame spreads in a narrow gap, as occurs in fires behind walls or inside electronic equipment. This geometry leads to interesting flame behaviors not typically seen in open flame spread, and also reproduces some of the conditions experienced by microgravity flames.Two sets of experiments are described, one involving flame spread in a Narrow Channel Apparatus (NCA) in normal gravity, and the others taking place in actual microgravity. Three primary variables are considered: flow velocity, oxygen concentration, and gap size (or effect of heat loss). When the oxidizer flow is reduced at either gravity level, the initially uniform flame front becomes corrugated and breaks into separate flamelets. This breakup behavior allows the flame to keep propagating below standard extinction limits by increasing the oxidizer transport to the flame, but has not been observed in other microgravity experiments due to the narrow samples employed. Breakup cannot be studied in typical (i.e., “open”) normal gravity test facilities due to buoyancy-induced opposed flow velocities that are larger than the forced velocities in the flamelet regime.Flammability maps are constructed that delineate the uniform regime, the flamelet regime, and extinction limits for thin cellulose samples. Good agreement is found between flame and flamelet spread rate and flamelet size between the two facilities. Supporting calculations using FLUENT suggest that for small gaps buoyancy is suppressed and exerts a negligible influence on the flow pattern for inlet velocities ?5 cm/s. The experiments show that in normal gravity the flamelets are a fire hazard since they can persist in small gaps where they are hard to detect. The results also indicate that the NCA quantitatively captures the essential features of the microgravity tests for thin fuels in opposed flow.     

Extinction limit extension of unsteady counterflow diffusion flames affected by velocity change

The unsteady extinction limit of (CH₄ + N₂)/air diffusion flames was investigated in terms of the time history of the strain rate and initial strain rates. A spatially locked flame in an opposed-jet counterflow burner was perturbed using linear velocity variation, and time-dependent flame luminosity and unsteady extinction limits were measured with a high-speed intensified CCD (ICCD) camera. In addition, the transient maximum flame temperature and hydroxyl (OH) radical were measured as a function of time using Rayleigh scattering and OH laser-induced fluorescence, respectively. In this experiment, unsteady flames survive at strain rates that are much higher than the extinction limit of steady flames and unsteady extinction limits increase as the slope of the strain rate increases or as the initial strain rate decreases. We found that the equivalent strain rate represents well the unsteady behavior in the outer convective-diffusive layer of the flame. By using the equivalent strain rate, we were able to accurately estimate the contribution of the unsteady effect in the outer convective-diffusive layer to the extinction limit extension, and we also identified the unsteady effect in the inner diffusive-reactive layer of the flame. Consequently, the extension of unsteady extinction limits results from the unsteady effects of both the convective-diffusive layer and the diffusive-reactive layer. The former effect is dominant at the beginning of the velocity change, and the latter effect is dominant near the extinction limit.     

A study on flame interaction between methane/air and nitrogen-diluted hydrogen–air premixed flames

Numerical and experimental studies are conducted to grasp downstream interactions between premixed flames stratified with two different kinds of fuel mixture. The selected fuel mixtures are methane and a nitrogen-diluted hydrogen with composition of 30% H₂ + 70% N₂. Extinction limits are determined for methane/air and (30% H₂ + 70% N₂)/air over the entire range of mixture concentrations. These extinction limits are shown to be significantly modified due to the interaction such that a mixture much beyond the flammability limit can burn with the help of a stronger flame. The lean extinction limit shows both the slanted segments of lower and upper branches due to the strong interaction with Lewis numbers of deficient reactant less than unity, while the rich extinction limit has a square shape due to the weak interaction with Lewis numbers of deficient reactant larger than unity. The regimes of negative flame speed show an asymmetric aspect with a single wing shape. The negative flame always appears only when methane is weak. The extent of interaction depends on the separation distance between the flames, which are the functions of the mixtures’ concentrations, the strain rate, the Lewis numbers, and the preferential diffusions of the penetrated hydrogen from the nitrogen-diluted hydrogen flame. The important role of preferential diffusion effects of hydrogen in the flame interaction is also discussed.     

Tubular premixed and diffusion flames: Effect of stretch and curvature

Tubular flames are ideal for the study of stretch and curvature effects on flame structure, extinction, and instabilities. Tubular flames have uniform stretch and curvature and each parameter can be varied independently. Curvature strengthens or weakens preferential diffusion effects on the tubular flame and the strengthening or weakening is proportional to the ratio of the flame thickness to the flame radius. Premixed flames can be studied in the standard tubular burner where a single premixed gas stream flows radially inward to the cylindrical flame surface and products exit as opposed jets. Premixed, diffusion and partially premixed flames can be studied in the opposed tubular flame where opposed radial flows meet at a cylindrical stagnation surface and products exit as opposed jets. The tubular flame flow configurations can be mathematically reduced to a two-point boundary value solution along the single radial coordinate. Non-intrusive measurements of temperature and major species concentrations have been made with laser-induced Raman scattering in an optically accessible tubular burner for both premixed and diffusion flames. The laser measurements of the flame structure are in good agreement with numerical simulations of the tubular flame. Due to the strong enhancement of preferential diffusion effects in tubular flames, the theory-data comparison can be very sensitive to the molecular transport model and the chemical kinetic mechanism. The strengthening or weakening of the tubular flame with curvature can increase or decrease the extinction strain rate of tubular flames. For lean H₂-air mixtures, the tubular flame can have an extinction strain rate many times higher than the corresponding opposed jet flame. More complex cellular tubular flames with highly curved flame cells surrounded by local extinction can be formed under both premixed and non-premixed conditions. In the hydrogen fueled premixed tubular flames, thermal-diffusive flame instabilities result in the formation of a uniform symmetric petal flames far from extinction. In opposed-flow tubular diffusion flames, thermal-diffusive flame instabilities result in cellular flames very close to extinction. Both of these flames are candidates for further study of flame curvature and extinction.     

Experimental study of cellular instability and extinction of non-premixed opposed-flow tubular flames

An experimental study of cellular instability of non-premixed opposed-flow tubular flames was conducted burning H₂ diluted with CO₂ flowing against air. The transitions to cellularity, cellular structures, and extinction conditions were determined as a function of the initial mixture strength, stretch rate, and curvature. The progression of cellular structures from the onset of cells through extinction was analyzed by flame imaging using an intensified CCD camera. Three different procedures of decreasing the Damköhler number (forward process), as well as using those same procedures in the opposite progression of increasing the Damköhler number (backward process) were completed. Significant flame hysteresis was seen and the forward transition occurred at a lower Damköhler number than the backward transition. Mechanical perturbations were conducted to show that the onset of cellularity could be realized at a higher Damköhler number than without perturbations. Once cellular instability was induced, it was possible to perturb the flame into multiple stable cellular states and extinguish the flame at a much higher Damköhler number than without perturbations. Images are shown of rotating cellular flames and a cellular instability regime at an initial mixture strength greater than unity and away from extinction conditions. A qualitative explanation of flame rotation and a general categorizing of three distinct flame regimes is given.     

Suppression limits of low strain rate non-premixed methane flames

The suppression of low strain rate non-premixed flames was investigated experimentally in a counterflow configuration for laminar flames with minimal conductive heat losses. This was accomplished by varying the velocity ratio of fuel to oxidizer to adjust the flame position such that conductive losses to the burner were reduced and was confirmed by temperature measurements using thermocouples near the reactant ducts. Thin filament pyrometry was used to measure the flame temperature field for a curved diluted methane-air flame near extinction at a global strain rate of 20 s⁻¹. The maximum flame temperature did not change as a function of position along the curved flame surface, suggesting that the local agent concentration required for suppression will not differ significantly along the flame sheet. The concentration of N₂, CO_2, and CF₃Br added to the fuel and the oxidizer streams required to obtain extinction was measured as a function of the global strain rate. In agreement with previous measurements performed under microgravity conditions, limiting non-premixed flame extinction behavior in which the agent concentration obtained a value that insures suppression for all global strain rates was observed. A series of extinction measurements varying the air:fuel velocity ratio showed that the critical N₂ concentration was invariant with this ratio, unless conductive losses were present. In terms of fire safety, the measurements demonstrate the existence of a fundamental limit for suppressant requirements in normal gravity flames, analogous to agent flammability limits in premixed flames. The critical agent volume fraction in the methane fuel stream assuring suppression for all global strain rates was measured to be 0.841 ± 0.01 for N₂, 0.773 ± 0.009 for CO₂, and 0.437 ± 0.005 for CF₃Br. The critical agent volume fraction in the oxidizer stream assuring suppression for all global strain rates was measured as 0.299 ± 0.004 for N₂, 0.187 ± 0.002 for CO₂, and 0.043 ± 0.001 for CF₃Br.     

LIMIT OF EQUIVALENCE RATIO ON MIXING ENHANCEMENT NEAR THE TUBE EXIT IN A TONE EXCITED RICH FLAME

An experimental investigation has been made with the objective of studying the limit of equivalent ratio (ϕ) on mixing enhancement in a tone excited jet rich flame. The jet is pulsed by means of a loudspeaker-driven cavity and experiments are limited to very rich flames (ϕ>1⋅5). The excitation frequency is chosen for the resonant frequency identified as a pipe resonance due to acoustic excitation. Methane, propane and butane are used to examine the effect of mixture property on the limit of equivalence ratio. Mixing is always enhanced in a methane/air flame as the excitation intensity increases. In the case of propane/air and butane/air flames, mixing enhancement can be obtained only when the equivalence ratio lies in the range from a certain value (the equivalence ratio limit) to infinity (non-premixed flame), irrespective of mean mixture velocities. It is also found that the equivalence ratio limit is related to flame instability; the lower the Lewis number, the higher the equivalence ratio limit. As the excitation intensity increases, flame separation occurs below the equivalence ratio limit; an inner (premixed) flame is transformed into a cellular flame which then moves upstream, but the height of an outer (non-premixed) flame is not decreased. Acoustic pressure measurements using a microphone are made to quantify the oscillating velocity. The oscillating velocity amplitude at the cellular flame position is proportional only to mean mixture velocity regardless of fuel type. © 1997 by John Wiley & Sons, Ltd.     

Study on stretch extinction limits of CH4/CO2 versus high temperature O2/CO2 counterflow non-premixed flames

Extinction limits of counterflow non-premixed flames with normal and high temperature oxidizers were studied experimentally and numerically for development of new-type oxygen-enriched mild combustion furnace. Extinction stretch rates of CH₄/CO₂ (at 300 K) versus O₂/CO₂ flames at oxygen mole fractions of 0.35 and 0.40 and oxidizer temperatures of 300 K, 500 K, 700 K and 1000 K were obtained. Investigation was also conducted for CH₄/N₂ (at 300 K) versus air (O₂/N₂) flames at the same oxidizer temperatures. An effect of radiative heat loss on stretch extinction limits of oxygen-enriched flames and air flames was investigated by computations with optical thin model (OTM) and adiabatic flame model (ADI). The results show influence of radiative heat loss on stretch extinction limits was not significant in relative high fuel mole fraction regions. The extinction curve of the oxygen-enriched flames with oxygen mole fraction of 0.35 was close to that of the air flames at the oxidizer temperature of 300 K. However, the extinction curve of air flames with high temperature oxidizer was comparable with that of oxygen-enriched flames with oxygen mole fraction of 0.40. Scaling analysis based on asymptotic solution of stretch extinction was applied and it was found that stretch extinction limits can be expressed by two terms. The first term is total enthalpy flux of fuel stream based on thermo-physical parameters. The second term is a kinetic term which reflects an effect of the chemical reaction rate on stretch extinction limits. OH radicals which play important roles in chain propagating and main endothermic reactions were used to represent the kinetic term of both oxygen-enriched and air flames. The global rates of OH formation in these two cases were compared to understand the contribution of kinetic term to stretch extinction limits. Variation of extinction curves of oxygen-enriched flames and air flames was well explained by the present scaling analysis. This offers an effective approach to estimate stretch extinction limits of oxygen-enriched flames based on those of air flames at the same oxidizer temperature.     

The Particle Image Velocimetry Experimental Study of Flame Extinction in Standard Square Flammability Column Resulting From Stretching

The main goal of this work is to determine experimentally local stretching rate distribution along the limits of methane/air and propane/air flames, using particle image velocimetry (PIV). This method allows obtaining necessary moving flame velocity fields in a standard flammability column and also recognition of the flame structures. For this purpose each mixture was seeded with MgO particles (of known size) before entering the tube (column), using a special system. The amount of seeds in the mixture, their dispersion system, and the laser power producing a sheet of light penetrating the column were carefully chosen (so as not to disrupt the combustion or flame propagation in it). After a learning process, this finally it allowed us obtain good-quality velocity field images in the region of concern, images acceptable for further processing. The methodology developed for these experiments proved to be reliable and able to supply analyses with repeatable data. On the basis of performed experiments it was possible to derive the flame stretching rate that causes its extinction in both mixtures.