Cellular Flame Structure Research Articles

Effects of buoyancy on the spherical flame and explosion pressure of a CH4 mixture under dilution conditions

To determine the effects of buoyancy on the spherical flame and explosion pressure under dilution conditions, a series of explosion experiments were conducted in a 20-L spherical container. The schlieren technique was used to record CH4 flames to assess the buoyancy effect. The change in the explosion pressure was recorded and correlated with the flame shape to analyze the explosion characteristics. The methane explosion occurred in two stages: initial ignition and flame development. During initial ignition, the addition of CO2 caused the flame buoyancy to increase considerably. Flames with different buoyancies had different shapes, such as spherical, ellipsoidal, and “ω” shapes. The radius of the flame at 12 mm may serve as the threshold for the onset of buoyancy effects; once the radius exceeds this value, the flame could deform due to buoyancy effects. Increasing the added CO2 concentration caused the buoyancy effect to be enhanced and appear at a very low radius of the flame. The combined effects of buoyancy and dilution reduced the flame expansion speed. The difference between the horizontal and upward expansion speeds linearly increased with the CO2 concentration. At a high CO2 concentration, the flame stagnated at speeds below 0.5 m/s. During flame development, CO2 addition made the flame surface smoother. Suppressing the cellular flame structure delayed the maximum explosion pressure rise rate and reduced the pressure peak. A coupled analysis of the flame shape and the maximum explosion pressure rise rate showed a dense cellular structure for a high explosion pressure rise rate and a smooth flame surface for a low maximum explosion pressure rise rate. The size and number of cellular structures for different mixtures were similar at a fixed maximum explosion pressure rise rate.

Premixed flames in narrow heated channels of circular cross-section: Steady-state solutions, their linear stability analysis and the multiplicity of stable dynamic modes

Premixed flames in narrow heated circular channels subjected to a Poiseuille flow are investigated within the constant density model for various Lewis numbers using irreversible one-step Arrhenius kinetics. A global stability analysis of steady-state axisymmetric solutions is carried out, together with time-dependent direct numerical simulations. The analysis reveals the criteria for the appearance of oscillatory and three-dimensional cellular flame structures. The problem is also studied separately within the framework of the narrow-channel approximation.Among the results obtained, the following can be singled out as the main ones. First, the multiplicity of stable dynamic modes, oscillatory and steady-state, taking place for the same set of parameters for flames with Le<1 is demonstrated. The actual occurrence of one mode or another depends on the initial conditions. Second, the appearance of chaotic regimes is shown for flames with Le>1. The chaotic dynamics occurs in a narrow range of values of the flow rate, with Feigenbaum period-doubling cascades taking place both before and after this interval. The results of this study could be useful in the development and use of small-scale combustion devices.Novelty and significance statement: A systematic study of premixed flames in narrow heated circular channels in the presence of Poiseuille flow is carried out for various Lewis numbers using irreversible one-step Arrhenius kinetics. One of the novelties presented in the paper is the linear global stability analysis of steady state axisymmetric solutions of this problem, which has not been reported before. Also, for the first time, the existence of multiple stable dynamic modes, oscillatory and time-independent, which occurs at the same parameter values, is demonstrated for flames with Lewis number smaller than one. For flames with a Lewis number greater than one, cases with chaotic dynamics are found that manifest themselves in a narrow range of the flow rate. Finally, it is demonstrated that the Feigenbaum cycle-doubling cascade can appear before and after this interval of chaotic dynamics. Such analysis has not been reported before for this problem of flames in partially heated channels.

Numerical analysis and flamelet modeling of NO[formula omitted] formation in a thermodiffusively unstable hydrogen flame

In a thermodiffusively unstable premixed hydrogen flame, cellular flame structures exist due to the strong differential diffusion of hydrogen featuring wide ranges of curvature and strain rate. NOx formation pathways are directly affected by the complex cellular flame structures as different species have different sensitivities to differential diffusion effects and curvature. In this work, the NOx formation characteristics of a thermodiffusively unstable laminar premixed hydrogen flame are investigated. At first, the NOx formation characteristics in the tangential and normal directions of the flame front are analyzed, focusing on the effects of curvature. Then, reaction path analyses are conducted to identify the relevant pathways and elementary reactions that pre-dominantly contribute to the formation of NOx pollutants. To investigate the effects of curvature on NOx formation, the reaction fluxes and the elementary reaction rates are calculated by conditioning on the curvature value. Finally, the performance of a flamelet model in predicting the NOx species in the thermodiffusively unstable premixed hydrogen flame is evaluated through an a priori analysis. The results show that curvature directly affects NOx formation, particularly the dominant NNH and N2O reaction pathways while the contribution of the thermal-NO pathway is found to be overall negligible. The NOx species are mainly formed in positively-curved flame segments where the flamelet model gives good predictions, while discrepancies with respect to the model exist in the negatively-curved flame regions. The reason for this is clarified through a chemical timescale analysis and a conditional reaction path analysis.

Experimental study on the instability characteristics of CO2-diluted n-butane/air cellular flames on McKenna burner

Present experimental study reports on the effects of CO2 dilution on intrinsic instabilities of non-adiabatic n-butane/air cellular flames on McKenna burner at initial 300 K temperature and 0.1 MPa pressure. The Planar Laser Induced Fluorescence (OH-PLIF and CH2O-PLIF) optical diagnostic technology was utilized to probe the structure of cellular flames. The results show that the cellular flames can be obtained under the conditions with lean, stoichiometric and rich CO2 diluted n-butane/air mixtures. At lean conditions, hydrodynamic instability was dominant and the cellular flames were formed by connected wrinkles. However, at rich conditions, diffusive-thermal instability was dominant and the cellular flames were formed by separated cells. It confirmed that the CO2 dilution can enhance flame instability significantly, as well as the enhanced effect was positive correlation with CO2 dilution ratio. Furthermore, the enhanced effects of flame instability was stronger by CO2 dilution than by N2 dilution. The chemical and thermal effects by CO2 dilution were attributed to enhance flame instability.

Simultaneous stereo‐PIV and OH×CH2O PLIF measurements in turbulent ultra lean CH4/H2 swirling wall‐impinging flames

We present experimental results from turbulent low-swirl lean H2/CH4 flames impinging on an inclined, cooled iso-thermal wall, based on simultaneous stereo-PIV and OH×CH2O PLIF measurements. By increasing the H2 fraction in the fuel while keeping Karlovitz number (Ka) fixed in a first series of flames, a fuel dependent near-wall flame structure is identified. Although Ka is constant, flames with high H2 fraction exhibit significantly more broken reaction zones. In addition, these high H2 fraction flames interact significantly more with the wall, stabilizing through the inner shear layer and well inside the near-wall swirling flow due to a higher resistance to mean strain rate. This flame-wall interaction is argued to increase the effective local Ka due to heat loss to the wall, as similar flames with a (near adiabatic) ceramic wall instead of a cooled wall exhibit significantly less flame brokenness. A second series of leaner flames were investigated near blow-off limit and showed complete quenching in the inner shear layer, where the mean strain rate matches the extinction strain rate extracted from 1D flames. For pure CH4 flames (Ka ≈ 30), the reaction zone remains thin up to the quenching point, while conversely for the 70% H2 flames (Ka ≈ 1100), the reaction zone is highly fragmented. Remarkably, in all near blow-off cases with CH4 in the fuel, a large cloud of CH2O persists downstream the quenching point, suggesting incomplete combustion. Finally, ultra lean pure hydrogen flames were also studied for equivalence ratios as low as 0.22, and through OH imaging, exhibit a clear transition from a cellular flame structure to a highly fragmented flame structure near blow-off.

Numerical study on soot formation in outwardly propagating, iso-octane, rich, cellular flames

Soot formation in outwardly propagating iso-octane rich unstable cellular flames were investigated by means of a two-dimensional numerical simulation at mixture equivalence ratios of 1.4, 1.6, and 1.8. The formation of a cellular flame structure was observed as an indicator of flame instability. In addition, soot formation was found to be enhanced. The cusp area had a negative flame stretch rate; in contrast, the convex area had a positive flame stretch rate. This difference in stretch rate caused a lower flame front temperature and lower temperature gradient. It was also revealed that a difference in temperature in these areas might affect the soot surface growth, which was dependent on the C2H2 concentration in these areas.

Premixed flame propagation of CH4 and C3H8 in a narrow-gap disk burner using constant-volume processes at elevated-pressure

Premixed flame propagation within a narrow combustion space has been an important issue in combustion studies. Recently, it was found that flame-induced instabilities are very sensitive to the length-scale of the combustion space. However, experimental results obtained at higher pressures have not been sufficient. In this study, the propagation characteristics of methane and propane premixed flames were investigated using a new narrow-gap disk burner, of which the disk gap and the initial pressure could be varied precisely while the volume was kept constant. Quenching distances were measured during increase of the initial pressure to 4 bar. Flame propagation characteristics were compared with variation of the disk gap, the equivalence ratio, and the initial pressure. Significant changes were found in the trends of the flame propagation velocity and flame shape. Cellular flame structures were observed when the disk gap was slightly larger than the quenching distance regardless of the Lewis number. In contrast, smooth flames could form when the disk gap was sufficiently larger than the quenching distance. The elevated pressure enhanced flame oscillation and cellular structures (if they could be generated) regardless of Lewis number of mixtures. The relationship between the pressure and the flame propagation was evaluated, and a mechanism for the flame instability is discussed herein.

Cellular flame structures and thermal characteristics of axi-symmetric ceiling fires: An experimental study and scaling analysis

The present study provides an experimental characterization of cellular flame structures and thermal characteristics of axi-symmetric ceiling fires. Experiments were carried out employing circular gaseous burners of various diameters at the ceiling facing downward using methane and propane as fuel. For this configuration, a hot buoyant diffusion flame burning beneath the cold fuel gas issuing from the ceiling above was naturally resulted in. A cellular flame structure, due to the inherent “Rayleigh-Taylor’’ instability of this buoyant flow configuration, was formed beneath the ceiling. The flame length (radius), temperature and heat flux distribution along the ceiling as well as characteristic inner cell size evolution of this special cellular flame were quantified experimentally for such ceiling fire of various conditions. Scaling analysis was performed to analyze the obtained parameters of the cellular flame structure and thermal characteristics of the ceiling fire. Results showed that the overall length (radius) of the cellular flame has a 2/5 power dependence on heat release rate and is independent of burner sizes and fuel types. Temperature profile and heat flux distribution for various conditions can be universally expressed as a function of L/Rf, the distance from the burner center normalized by the flame length. The characteristic cell size of the cellular flame was related to temperature, which can be well represented by a 2.33 power function based on the “Rayleigh-Taylor’’ instability scaling analysis. The cell size spatial evolution inside the cellular flame was self-similar corresponding to the dependence of temperature on relative position, and a non-dimensional function was obtained. This study provides fundamental data and a scaling analysis into the basic physics of the cellular flame behavior and thermal characteristics of ceiling fires.

Experimental study of flame stability enhancement on lean premixed combustion of a synthetic natural gas in Thailand

An experimental study was done of the intrinsic instability of flames and flame stability enhancement burning two types of Thai natural gas. They were studied using a McKenna flat burner and a ceramic porous burner. The effects of the equivalence ratio and firing rate using various combinations of fuels and burners on the flames were studied. Gas combustion using a McKenna flat burner revealed that the eastern natural gas had a more stable flame with a smaller cell size and a wider cellular flame flammability limit than the western natural gas. In contrast, the ceramic porous burner produced two distinct flame types depending on the equivalence ratio. These were a surface stabilized flame with strong thermal radiation emitted by the burner surface to the surroundings at relatively rich combustion, 0.9 ≤ Φ ≤ 1.0 and a conventional cellular free flame at relatively lean combustion 0.75≤ Φ ≤ 0.8, where the ceramic porous burner transforms the surface stabilized flame into conventional cellular flame structures. The ceramic porous burner can extend the flame stability limit of the western natural gas up to Φ = 0.75 with no flame flashback and flame blow-off. This can make a ceramic porous burner suitable for household cooking despite the lower fuel quality of western natural gas.

Experimental Study on Cellular Premixed Propane Flames in a Narrow Gap between Parallel Plates

ABSTRACTExperimental results on laminar propane flames propagating in a narrow gap between parallel glass plates are presented. Development and evolution of cellular flame structure is demonstrated by video and long-exposure photo recordings for stoichiometric and fuel-rich mixtures with equivalence ratio of 1.0:1.25. It is shown that in narrow gaps the lateral cell sizes by far exceed the gap width. Average flame propagation speed obtained for gap widths between 2.5 and 4.4 mm is shown to depend significantly on the gap width due to the effect of heat losses on the normal flame speed, as well as due to cooling of products behind the flame. Effect of gap width on the cellular structure is demonstrated, and statistical characteristics of the cells are obtained for two gap widths. Flame propagation patterns are also obtained for non-central ignition performed at one or simultaneously at two points.

Unsteady propagation of premixed methane/propane flames in a mesoscale disk burner of variable-gaps

Unsteady flame propagation of air-premixed methane and propane flames was investigated in a new mesoscale disk burner, of which disk-gap could be precisely varied. To begin with, the quenching disk-gaps on the flammability limits were measured. In most cases, with the slight increase of the disk-gap, cellular flame structures could be generated. The initiation of such cellular structures could be explained by the thermally induced hydrodynamic instability, and it could be enhanced if the Lewis number was sufficiently small. When the disk-gap was sufficiently larger than a critical value (approximately 1.5 times the quenching distance), the cellular flame structure changed into a smooth one in the azimuthal direction. With a further increase of the disk-gap, the flame propagation velocities approached to constant values. These values were comparable to the laminar burning velocities except for the propane-rich conditions, in which much larger propagation velocities were observed. The flame stretch effects (coupled with Le-effects) within a narrow space were suspected as the reason. The structural transition of the premixed flame could be investigated successfully through various disk-gaps, from the smallest quenching-scale to the ordinary large scales via whole mesoscales including Hele–Shaw scales.

Measurement and simulation of partially-premixed cellular tubular flames

Experimental and numerical simulation results are reported of partially-premixed cellular tubular flames. Parametric measurements across stretch rate and equivalence ratio are taken by chemiluminescent imaging and are presented for the first time. Select hybrid cases with both cellular and non-cellular flame structures are examined with laser-induced spontaneous Raman scattering. Results are spatially resolved in two dimensions and radial interpolations of reaction and extinction zones are compared to numerical simulations using multicomponent transport and detailed chemical kinetics. Experimental cell structures and extinction zones are well predicted by numerical simulation, with discrepancies of temperature and H2O and temperature primarily observed in locations with moderate and high mole fractions of CO2. A novel cellular structure, denoted as a “split-cell” flame, is reported for the first time with both chemiluminescent imaging and Raman scattering. Results indicate that partially-premixed flames are valuable as experimental and numerical benchmarks to advance fundamental combustion research.

Experimental study on the Markstein length and laminar burning velocity of CH4/RP-3 mixture

Experimental studies were performed to investigate the effects of methane addition on the combustion characteristics of Chinese RP-3 kerosene/air mixture. Experiments were in a combustion vessel and the influences of methane addition on the flame propagation characteristic, Markstein length and laminar burning velocity of CH4/RP-3/air mixture at initial temperature of 420 K, initial pressure of 0.1 MPa, equivalence ratios of 0.8-1.3 and methane additions of 0.0-0.8 were investigated. Results show that with the increase of equivalence ratio, the flame front of RP-3/air mixture becomes unstable and a cellular flame structure is observed when the equivalence ratio increases to 1.3, but at the equivalence ratio of 1.3, the increased methane addition can make the flame front stable significantly. With the increase of equivalence ratio from 0.8 to 1.3, laminar burning velocities of CH4/RP-3/air mixture first increase and then decrease; the effect of methane addition on laminar burning velocity is not evident at the equivalence ratios of 0.9-1.1, but when the equivalence ratio is larger than 1.1, laminar burning velocities with methane addition of 0.8 are always the slowest. The Markstein length decreases with the increase of equivalence rati; at equivalence ratio of 0.8 the Markstein length decreases with the increase of methane addition, and with the increase of equivalence ratio the influence of methane addition on Markstein length becomes less significant and irregular.

Effects of pressure on cellular flame structure of high hydrogen content lean premixed syngas spherical flames: A DNS study

The structure and propagation of turbulent lean premixed high hydrogen content syngas flames at pressure values of 1 bar, 2 bar and 4 bar are studied using three-dimensional direct numerical simulations. Simulations for each pressure considered were performed for three different initial turbulence levels at the turbulent Reynolds numbers of 50, 100 and 150. The DNS of expanding spherical flames has taken into account detailed chemical kinetics and preferential diffusion effects. The mixture-averaged transport model has been employed. Effects of pressure on flame structures, heat release rate and radical species distributions under low to high initial turbulent conditions are examined. The results show that elevated pressures greatly influence the cellular flame structure of syngas spherical flames under different initial turbulent conditions. At the atmospheric pressure, the flame shows weakly wrinkling structures due to the initial turbulence level imposed in the simulation. At elevated pressures under turbulence conditions, the flame develops cellular burning structures, superimposed by flame wrinkles due to turbulence. It is shown that highly wrinkled cells that develop over the surface of a spherical flame increase its area and thereby the global propagation speed at elevated pressures. Results show noticeable increases in the local heat release rate with increasing pressure. The analysis also reveals the formation of cusped structures with low heat release rate values in areas of negative curvatures at elevated pressures, in agreement with previous studies. The joint probability density functions of OH radical mass fraction and temperature show broadening OH values in the high temperature zone at elevated pressures compared to its distribution at the atmospheric pressure. Comparisons of mean species distributions of OH and HO2 over temperature between laminar and turbulent flames show that pressure elevation has a major influence on the flame structure in the composition space.

High hydrogen content syngas fuel burning in lean premixed spherical flames at elevated pressures: Effects of preferential diffusion

This study addresses the effects of preferential diffusion on flame structure and propagation of high hydrogen content (HHC) turbulent lean premixed hydrogen-carbon monoxide syngas flames at elevated pressures. The direct numerical simulations with detailed chemistry were performed in three-dimensional domain for expanding spherical flame configuration in a constant pressure combustion chamber. To identify the role of preferential diffusion on flame structure and propagation under low and high turbulence levels at elevated pressure, simulations were performed at an initial turbulent Reynolds number of 15 and 150 at a pressure value of 4 bar. The results demonstrate that the thermo-diffusive instability greatly influences the lean premixed syngas cellular flame structure due to strong preferential diffusion effects under low turbulence level at elevated pressure. In contrast, the results reveal that the thermo-diffusive effects are destabilising and preferential diffusion is overwhelmed by turbulent mixing under high turbulence level at elevated pressure. This finding suggests that the development of cellular flame structure is dominated by turbulence with little or no contribution from the thermo-diffusive instability for the lean premixed syngas flame which operates under conditions of high turbulence and elevated pressures. However, results demonstrate that the flame acceleration and species diffusive flux are still influenced by the preferential diffusion for the lean premixed syngas flame which operates under conditions of high turbulence and elevated pressures.

Numerical simulation of premixed H2–air cellular tubular flames

The detailed flame structure of laminar premixed cellular flames in the tubular domain is simulated in 2D using a fully-implicit primitive variable finite difference formulation that includes multicomponent transport and detailed chemical kinetics. Numerical results for H2/air flames are presented and compared against spatially resolved experimental measurements of temperature and chemical species including atomic H and OH. The experimental results compare well for flame structure and cell number, despite the numerical model under-predicting the peak temperature by 200 K. Numerical experiments were performed to assess the ability for cellular tubular flames to impact experimental and numerical investigations of practical flames. The cellular flame structure is found to provide a highly sensitive geometry that is useful for validating diffusive transport modelling approximations. This capability is exemplified through the development of a simple and accurate approximation for thermal diffusion (i.e. the Soret effect) that is suitable for practical combustion codes.

Cellular instabilities of non-adiabatic laminar flat methane/hydrogen oxy-fuel flames highly diluted with CO2

Cellular instability of non-adiabatic laminar premixed flat methane/hydrogen oxy-fuel flames highly diluted with CO2 was studied systematically. The CH4/H2/CO2/O2 flames were stabilized on a McKenna burner at atmospheric pressure. The CH4/H2/CO2/O2 flames with varied equivalence ratios and hydrogen mole fractions were investigated. Quantitative structure parameters of the cellular flame fronts were obtained with OH-PLIF technique. On the basis of experimental results, theoretical analysis was made to clarify the effect of mixture properties and heat loss on flame cellular instability. Results showed that cellular flame structure appeared near the lean limit combustion condition. Cell number of flame and its hydrodynamic factor, ωDL, present a positive correlation. The average stand-off distance, L, is well identified to characterize the cellular flames. Average cell amplitude, A, and wavelength, λ, increased with the increase of L. A good indicator of instability level, A·λ, was defined. The RS model was used in a linear stability analysis, and the onset of cellular flames was found to be governed by both hydrodynamic and thermal-diffusive instability mechanisms, while the heat loss made the flame more stable. The RS model is not an appropriate model to predict critical wavenumber for the non-adiabatic flame.

The differential diffusion effect of the intermediate species on the stability of premixed flames propagating in microchannels

The propagation of premixed flames in adiabatic and non-catalytic planar microchannels subject to an assisted or opposed Poiseuille flow is considered. The diffusive–thermal model and the well-known two-step chain-branching kinetics are used in order to investigate the role of the differential diffusion of the intermediate species on the spatial and temporal flame stability. This numerical study successfully compares steady-state and time-dependent computations to the linear stability analysis of the problem. Results show that for fuel Lewis numbers less than unity, LeF < 1, and at sufficiently large values of the opposed Poiseuille flow rate, symmetry-breaking bifurcation arises. It is seen that small values of the radical Lewis number, LeZ, stabilise the flame to symmetric shape solutions, but result in earlier flashback. For very lean flames, the effect of the radical on the flame stabilisation becomes less important due to the small radical concentration typically found in the reaction zone. Cellular flame structures were also identified in this regime. For LeF > 1, flames propagating in adiabatic channels suffer from oscillatory instabilities. The Poiseuille flow stabilises the flame and the effect of LeZ is opposite to that found for LeF < 1. Small values of LeZ further destabilise the flame to oscillating or pulsating instabilities.

Cellular combustion at the transition of a spherical flame front to a flat front at the initiated ignition of methane–air, methane–oxygen and n-pentane–air mixtures

High-speed cinematography was used to study the transition of a spherical flame front to a flat front in n -pentane–air and methane–air mixtures initiated by a spark discharge. Cellular flame structures were observed at the transition. Modeling based on compressible reactive Navier–Stokes equations at a low Mach number showed a qualitative agreement with experimental data.

Simulation of nitrogen emissions in a premixed hydrogen flame stabilized on a low swirl burner

There is considerable interest in developing fuel-flexible, low emissions turbines for power generation. One approach is based on burning a variety of lean premixed fuels with relatively low flame temperatures. Such flames can be stabilized in a low swirl burner configuration, for example, using a variety of fuels such as pure hydrogen and hydrogen-seeded hydrocarbon mixtures. However, many hydrogen-rich fuels are thermodiffusively unstable and burn in cellular flame structures, which can have a significant impact on the local nitrogen chemistry. These cellular burning patterns are characterized by a local enhancement of fuel concentration and a corresponding increase in local flame temperature just downstream. In turn, these regions become sites for enhanced thermal NOx production. The structure of these cells, and their impact on the net emissions of a flame is influenced by the global flame stabilization mechanisms and by local turbulence properties. Here we investigate the role of thermodiffusive instabilities on NOx emissions in the context of a laboratory-scale low swirl burner fueled with a lean hydrogen–air mixture at atmospheric pressure. The simulations show how the cellular burning structures characteristic of lean premixed hydrogen combustion lead to local and global enhancements in the NOx emissions. We quantify the chemical pathways that lead to the formation of NO and N2O, and how they are enhanced within local regions of intense burning.