Edge Flame Speed Research Articles

Revealing the role of limiting oxygen concentration test for a wick flame in characterizing the liquid fuel flammability

A two-dimensional numerical model was established to simulate the wick combustion system for determining the limiting oxygen concentration (LOC) of organic solvents, called the wick-LOC method. The ethanol wick flame under forced flow with decreased oxygen concentrations in normal gravity was studied numerically. Three simplified reaction mechanisms, 1-step, 2-step, and quasi-global mechanisms of ethanol were employed in the simulations for comparison with experimental results. The responses of flame height and edge flame standoff distance to the oxygen decrease were analyzed for validation. As the oxygen concentration was reduced to the stability limit of the full flame, the blow-off of the edge flame can be observed. By comparing the simulated LOCs and near-limit stabilized flame structures, the 2-step mechanism can reproduce the wick-LOC well, while the quasi-global mechanism can provide a more detailed flame structure. Further investigation into flame blow-off processes revealed that the reaction rate in fuel-lean conditions is more important for determining the wick-LOC, and the local flow velocities at the flame kernel are almost constant until blow-off occurs. By combining the flame stabilization theory with simulations, the physical meaning of the wick-LOC value can be explained as the oxygen concentration where the local flow velocity can be balanced with critical edge flame speed (more fundamentally, the critical laminar burning velocity) in a certain fuel-lean equivalence ratio. Further applications of the wick-LOC method are expected to examine the simplified reaction mechanisms of other liquid fuels.

Modeling of H2/air flame stabilization regime above coaxial dual swirl injectors

The prediction of the stabilization regime of partially premixed H2/air flames above an injector is a main subject of interest for the development of gas turbines powered by hydrogen. The way the flame is stabilized has an impact on NOx emissions, thermal stress on the injector, and combustion stability. A model based on the triple flame speed is revisited and improved to predict flame stabilization using information gathered at cold flow conditions. The model is called TFUP for Triple Flame Upstream Propagation. According to this model, the flame can anchor to the injector only if a zone with a flammable mixture and a sufficiently low local flow velocity exists continuously from the lifted flame to the injector lips. The TFUP model is applied to the case of a H2/air coaxial, dual swirl injector in which both the hydrogen and air streams are swirled. Fuel is injected through the central channel and oxidizer through the external channel. Particle image velocimetry and one dimensional Raman scattering measurements made at strategic locations are used to predict the edge flame speed for which a lifted flame should re-anchor to the injector. These predictions made in isothermal conditions are compared to observations of the flame stabilization regime. The central flow of hydrogen can be mixed with methane and helium and the external flow of air with nitrogen in order to prescribe different theoretical values for the triple flame speed. Predictions agree well with the observations made for all swirl levels conferred to the central fuel stream, fuel injection velocities, hydrogen contents in the fuel mixture injected in the central channel, and nitrogen contents in the oxidizer annular channel tested in the study. Validity limits are also discussed. The methodology presented in this study provides a simple framework to predict flame stabilization on coaxial injectors that can optionally be equipped with swirl vanes.

Stabilization criteria of laminar lifted propane flames and their transition to turbulent lifted flames under extended pressures

Recently, the mechanism of a laminar lifted flame (LLF) was clearly explained using an effective Schmidt number. It was found that a stable LLF can be formed within a triangular stabilization regime (TSR) in the domain of normalized lift-off height and Reynolds number. However, the experimental pressure range in the previous study was limited to three-times (1–3 bar) because there was an early transition to turbulent flow at elevated pressures. This study examined the effects of pressure on the LLF characteristics of propane, within an extended pressure range of 12.5-times (0.2–2.5 bar) with larger fuel tubes. This allowed more precise and abundant experimental results to be obtained for flame structures, and the general TSR for all LLFs was confirmed. In addition, variations in the effective Schmidt numbers and effective diffusivities were estimated for extended pressures, and the correlation between the fuel jet velocities and the lift-off heights was explained. Conclusively, a more general relationship between the edge flame speed (EFS) and the effective fuel concentration gradient (EFCG) was obtained under extended pressures. The existence of a minimum EFS at the maximum EFCG was confirmed. Furthermore, the non-monotonic variation in the lift-off height was explained, and the variations in the flame structure during the transition from an LLF to a turbulent lifted flame were investigated in more detail.

Stabilization criteria of laminar lifted flames in a non-premixed jet through experiments at elevated pressures

The stabilization mechanism of a laminar lifted flame in a non-premixed jet remains an essential issue in combustion theory. Recently, effective Schmidt numbers and effective diffusivities were evaluated from experimental results, and the relationship between the fuel jet velocities and lift-off heights could be converted to a theoretical relationship between the fuel concentration gradient and the flame propagation velocity. In this study, the lift-off heights of propane and butane flames were measured at elevated pressures, and the results were used to determine the effective Schmidt numbers and the effective diffusivities. Based on the experimental results, a triangle regime for stable laminar lifted flames was introduced, and its three limiting mechanisms were explained. First, the minimum lift-off height was determined using the momentum core. Second, the maximum lift-off height was determined by elimination of the stoichiometric condition. Third, the sudden extinction of a lifted flame was explained by the critical Reynolds number. Furthermore, when the tube diameter was larger and the pressure was higher, an additional flame stabilization mode having transient phenomena to a turbulent lifted flame was found. Within the flame stabilization triangle, an improved mechanism of a lifted flame was introduced. The effective Schmidt number increases from a smaller mass diffusion of fuel to a larger one of air as the lift-off height increases. In addition, the flow redirection at the flame base increased the effective Schmidt number even more to reach the local maximum value. A revised fuel concentration gradient that can be used even at elevated pressures was proposed, and the existence of maximum values was confirmed. Finally, the relationship between the revised fuel concentration gradient and the flame propagation velocity (or the edge flame speed) was revealed.

Stationary ethylene–air edge flames in a wedge-shaped region at low and high strain rates

ABSTRACT Edge flames are a canonical two-dimensional flame structure appearing in lifted jet flames and in the growth and repair of flame holes in nonpremixed turbulent combustion. Computational studies of edge flames with hydrodynamic-coupling at high strains are difficult owing to difficulties defining a stationary state. A wedge-shaped counterflow configuration is here used to provide control over the position of the edge flame, and allowing access to stationary, hydrodynamically-coupled retreating flames (at low and high strains). ethylene–air edge flames are established in the resulting non-uniformly strained counterflow, with combustion modeled using a skeletal reduction of the USC Mech II. The details of the ethylene–air edge flame are discussed, and comparisons are made between stoichiometric, fuel-lean, and fuel-rich compositions. Mixture-fraction-based coordinates local to the flame front are developed and analysed for a range of edge-flame speeds and compositions. A strain-rate parameter is described that admits a speed-strain relationship that is insensitive to the mixture stoichiometry, and a model for retreating flame speeds at high strain rates is discussed.

The localised forced ignition and early stages of flame development in a turbulent planar jet

The localised forced ignition and the early stages of the subsequent flame propagation in a planar turbulent methane/air jet in ambient air have been simulated using Direct Numerical Simulation (DNS) and a two-step chemical mechanism. Sixteen identical energy depositions events were simulated for four independent flow realisations at four different locations. The successful ignition and subsequent flame propagation have been found to be well correlated to the mean mixture fraction and flammability factor values of the energy deposition location. Furthermore, similarly to what has been observed in experiments, the early stages of flame development from the ignition kernel involved initial downstream convection of the kernel, followed by simultaneous radial expansion and downstream propagation and finally the upstream propagation of the flame base indicating the onset of flame stabilisation. The mixture composition and the scalar dissipation rate (SDR) values in the immediate vicinity of the ignitor have been identified to play key roles in determining the outcome of the external energy deposition, while the development of an edge flame structure propagating along the stoichiometric mixture fraction iso-surface was found to be necessary but not sufficient for the flame to propagate upstream. It has also been found that in the case of successful self-sustained burning, the edge flame was developing in low SDR regions, and that the most probable edge flame speed remains close to the theoretical laminar value irrespective of the flame development history. Finally, the mean flame speed of the edge flame elements propagating towards the nozzle exit has been found to be considerably greater than the unstrained laminar burning velocity. Thus, the edge flame, depending on its orientation with respect to the flow, is able to propagate upstream and initiate the onset of flame stabilisation.

Effects of diluents on the lifted flame characteristics in laminar nonpremixed coflow propane jets

The effects of diluents including CO2, N2, and He on the lifted flame characteristics of laminar coflow propane jets are experimentally investigated by measuring their liftoff heights, HL, and visualizing flow fields with oil mist. For lifted flames at a specified diluent mole fraction, XD, HL increases in the order of He, N2, CO2 when the diluent is added to the coflow stream. However, the order of HL becomes opposite when the diluent is added to the fuel stream. In addition, HL increases with increasing XD for all cases. From the visualization of nonreacting flow fields, it is found that the negative buoyancy represented by the density difference between the fuel jet and the coflow induces a stagnation flow near the fuel nozzle by decelerating the fuel jet. As such, the lifted flame is found to be stabilized further upstream with increasing negative buoyancy. In addition to the buoyancy effect, the effects of diluents on HL via the edge flame speed of the lifted flames, Se, are estimated by evaluating the laminar burning velocity, SL0. Among the dilution, thermal, and chemical effects, the dilution effect is found to be dominant in reducing SL0 for all cases, while the chemical effect is negligible. Finally, a correlation for HL is formulated using the ratios of the positive to negative buoyancy and the fuel jet velocity, U0, to SL0, which shows a satisfactory agreement with the experimental data.

Effects of Biogas Composition on the Edge Flame Propagation in Igniting Turbulent Mixing Layers

Three-dimensional compressible Direct Numerical Simulations have been used to investigate the localised forced ignition of statistically planar biogas/air mixing layers for different levels of turbulence intensity and biogas composition. The biogas is represented by a hbox {CH}_4/hbox {CO}_2 mixture and a two-step mechanism capturing the variation of the unstrained laminar flame speed with equivalence ratio and hbox {CO}_2 dilution was used. The mixture composition was found to significantly affect the flame kernel development which was reflected in the diminished growth rate of the burned gas volume with increasing hbox {CO}_2 dilution. A successful ignition of hbox {CH}_4/hbox {CO}_2/air mixing layer gives rise to a tribrachial flame structure involving fuel-rich and lean premixed branches on either side of the diffusion flame stabilised on the stoichiometric mixture fraction iso-surface. The most probable edge flame speed decreases in time and converges to a value that is at most equal to its laminar theoretical limit, and can even locally become negative for large values of the dilution and/or turbulence intensity. The decomposition of the edge flame speed showed a negligible or negative contribution of the mixture fraction surface displacement speed, while the displacement speed of the fuel mass fraction surface appeared as the dominant contributor. Finally, the edge flame speed dependences to the fuel mass fraction and mixture fraction gradients, fuel mass fraction iso-surface curvature and tangential strain rate have been analysed and found, within the dilution values considered, qualitatively similar to those of undiluted mixtures regardless of the amount of hbox {CO}_2, although quantitative differences were observed.

Structures of laminar lifted flames in a non-premixed jet and their relationship with similarity solutions

The concept of an edge flame structure has been adopted to explain the laminar lifted flames in a non-premixed jet. Two similarity solutions of velocity and fuel concentration have been used to explain flame stability. Recently, it was shown that the experimental results from studies of jet velocities and lift-off heights could be converted to a relationship between the edge flame speed and the fuel concentration gradient. However, this was not sufficient to explain actual structures and behaviors of lifted flames. In this study, related theories and their similarity solutions were improved to get more realistic results through an efficient process. Experiments were conducted with various parameters such as jet velocity, fuel type, tube diameter, air-premixing ratio, and fuel dilution. Effective mass diffusivities and effective Schmidt numbers were estimated based on the experimental results. Using these values, the theoretical basis for a new relationship between the edge flame speed and the fuel concentration gradient was obtained. It was conclusively found that non-monotonic variation in the edge flame speed occurs. From this, flame structures were classified into three regimes based on flame structures and heights: ordinary edge flame in a lower-regime, merged edge flame in a middle-regime, and premixed flame in a higher-regime. Flame speeds of the ordinary edge flames in the lower-regime were selected for comparison with the results of previous studies. From this work, the relationship between the similarity solutions and the experimental results can now be understood in greater detail, and the deviations due to the flame structure can be distinguished.

Edge flame propagation statistics in igniting monodisperse droplet-laden mixtures

The effects of droplet diameter, overall (i.e., liquid+gaseous phases) equivalence ratio, and turbulence intensity on the edge flame propagation statistics for localized forced ignition of uniformly dispersed n-heptane droplet-laden mixtures under homogeneous isotropic decaying turbulence have been analyzed based on direct numerical simulations data. It has been found that the edge flame structure becomes increasingly prominent for large overall equivalence ratios and droplet diameters. Although the mean edge flame speed has been found to be positive and its most probable value remains comparable to the theoretical value for laminar edge flames in purely gaseous mixtures, the mean values have been found to decrease and the probabilities of finding locally negative edge flame speeds have been found to increase with increasing turbulence intensity. The marginal probability density function and curvature and strain rate dependences of the edge flame speed have been found to be principally governed by the displacement speed of the fuel mass fraction isosurface intersecting the stoichiometric mixture fraction isosurface. The displacement speed of the stoichiometric mixture fraction isosurface has also been found to influence the local scalar gradient dependences of the edge flame speed in this configuration, especially for large droplets. The displacement speed of the fuel mass fraction isosurface Sd has been found to be principally governed by leading order contributions of the reaction and molecular diffusion components and the evaporation contribution remains weak in comparison to these leading order contributors. The local edge flame speed exhibits nonlinear curvature and strain rate dependences and its variation with the magnitudes of both fuel mass fraction and mixture fraction gradients has been found to be nonmonotonic for all cases considered here. The correlations of the edge flame speed with curvature, strain rate, and scalar gradient have been found to be qualitatively similar to the corresponding statistics reported in the existing literature for edge flames in purely gaseous mixtures. Additionally, the curvature and tangential strain rate dependences of the edge flame speed have been found to be dependent on the droplet size and overall equivalence ratio, and these dependences become weak for cases with large droplets.

Propagation speeds for interacting triple flames

Triple (or tribrachial) flames propagate through mixtures faster than the premixed laminar flame speed due to streamline divergence ahead of the flame base that decelerates the flow into the leading edge of the flame. When multiple triple flames are in close proximity, the bulk propagation speed of the structure can be even faster due to additional streamline divergence. Turbulent flames in partially-premixed conditions can encounter these situations, where multiple stoichiometric crossing are in close proximity, leading to multiple interacting triple flames being formed. Propagation speeds of the flame structure with respect the bulk flow for individual triple flames have been well characterized in previous studies, and the local flame speed of interacting triple flames have been reported; however, characterization of the propagation speed for the overall flame structure of interacting triple flame speeds has not been reported. The present work utilizes a laminar five slot burner, which allows both the concentration gradients and stoichiometric separation distance of two interacting triple flames to be varied. The bulk propagation speed of the multiple edge flames has been characterized as a function of the distance between the flame bases (or stoichiometric locations) and the local flame curvatures in order to better understand the conditions which lead to larger streamline divergence and faster propagation speeds. Interaction between multiple edge flames has been found to play an essential role in this propagation speed. Interacting edge flame speeds were modeled by modifying the relationship for single triple flame propagation speed with an added term for the interaction between the two flames to account for the increased effective divergence.

Direct estimation of edge flame speeds of lifted laminar jet flames and a modified stabilization mechanism

The flame stabilization mechanism of a lifted flame in a laminar fuel jet has been explained based on the edge flame concept. Previous studies have employed a similarity solution between velocity and fuel concentration, and showed that a lifted flame can be stabilized when the Schmidt number, Sc, is within a range of either Sc > 1 or Sc < 0.5. However, two unsolved problems remained, and they were mainly answered in this study. First, the edge flame speed could not be determined from the similarity solution using the experimental results of stable lifted flames. To resolve this, the experimental relationship between the fuel flow rate and the liftoff height was measured with a higher resolution, and a new method employing an effective Schmidt number was suggested. As a result, the relationship between the edge flame speed and the fuel concentration gradient could then be directly estimated from the simple experimental values for flow rate and the liftoff height. This new method was validated for various experimental parameters including the tube diameter, air-premixing ratio, and nitrogen-dilution ratio. Second, the reason why a stable lifted flame was not obtained when Sc < 0.5 could not be explained theoretically. Here, the existence of a unique criterion of Sc > 1, for a stable lifted flame was clarified theoretically. This study will advance understanding of the characteristics and stabilization mechanism of lifted edge flames in laminar non-premixed fuel jets.

Two-stage autoignition and edge flames in a high pressure turbulent jet

A three-dimensional direct numerical simulation is conducted for a temporally evolving planar jet of n-heptane at a pressure of 40 atmospheres and in a coflow of air at 1100 K. At these conditions, n-heptane exhibits a two-stage ignition due to low- and high-temperature chemistry, which is reproduced by the global chemical model used in this study. The results show that ignition occurs in several overlapping stages and multiple modes of combustion are present. Low-temperature chemistry precedes the formation of multiple spatially localised high-temperature chemistry autoignition events, referred to as ‘kernels’. These kernels form within the shear layer and core of the jet at compositions with short homogeneous ignition delay times and in locations experiencing low scalar dissipation rates. An analysis of the kernel histories shows that the ignition delay time is correlated with the mixing rate history and that the ignition kernels tend to form in vortically dominated regions of the domain, as corroborated by an analysis of the topology of the velocity gradient tensor. Once ignited, the kernels grow rapidly and establish edge flames where they envelop the stoichiometric isosurface. A combination of kernel formation (autoignition) and the growth of existing burning surface (via edge-flame propagation) contributes to the overall ignition process. An analysis of propagation speeds evaluated on the burning surface suggests that although the edge-flame speed is promoted by the autoignitive conditions due to an increase in the local laminar flame speed, edge-flame propagation of existing burning surfaces (triggered initially by isolated autoignition kernels) is the dominant ignition mode in the present configuration.

Spark ignition of a turbulent shear-less fuel–air mixing layer

A planar methane–air mixing layer with equal velocity in the two streams has been used to examine the ignition probability and the non-premixed edge flame speed following spark ignition. The mixing layer has approximately homogeneous turbulent intensity and lengthscale. Mean local mixture fraction has also been measured for the whole flow field. The ignition and subsequent flame propagation were visualized with a high-speed camera and the flame’s edges in the upstream, downstream and cross-stream directions have been identified. The average rate of flame evolution in these directions allowed an estimation of the average absolute flame speed. Ignition probability contour of the mixing layer takes a V-shape, which matches the shape of the lean and rich flammability limits with a little discrepancy in the rich side. By subtracting the uniform mean velocity resulted in estimates of the mean relative edge flame speed. This quantity was approximately 2.5SL, where SL is the laminar burning velocity of stoichiometric methane–air premixed flames. The results are consistent with DNS of turbulent edge flames.

A Study on Laminar Lifted Jet Flames for Diluted Methane in Co-flow Air

The laminar lifted jet flames for methane diluted with helium and nitrogen in co-flow air have been investigated experimentally. Such jet flames could be lifted in both buoyancy-dominated and jet momentum dominated regimes (even at nozzle exit velocities much higher than stoichiometric laminar flame speed) despite the Schmidt number less than unity. Chemiluminescence intensities of OH* radical (good indicators of heat release rate) and the radius of curvature for tri-brachial flame were measured using an intensified charge coupled device (ICCD) camera and digital video camera at various conditions. It was shown that, an increase in OH * concentration causes increase of edge flame speed via enhanced chemical reaction in buoyancy dominated regime. In jet momentum dominated regime, an increase in radius of curvature in addition to the increased OH * concentration stabilizes such lifted flames. Stabilization of such lifted flames is discussed based on the stabilization mechanism.

A flow pattern that sustains an edge flame in a straining mixing layer with finite thermal expansion

The role of thermal expansion in diffusion edge flames is investigated numerically in the strained mixing layer configuration. We are able to access both the positive as well as the negative edge flame speed regime when density variation, and therefore full hydrodynamic coupling, is present. The computational approach employs a homotopy method to gradually map the solutions from the computationally simpler constant-density flow to the more challenging variable-density case. Particular attention is paid to the role of boundary conditions and how they, in turn, can induce an undesirable streamwise pressure gradient in the trailing diffusion flame that affects the edge flame speed. A new approach is designed to eliminate this adverse pressure gradient. Previous studies observe that the ratio of the edge flame speed to the premixed stoichiometric laminar flame velocity scales approximately as the square root of the ratio of the cold stream density to the stoichiometric density (which is lower). At least for the small set of parameters investigated here, it is found that the speedup of the normalized edge flame velocity might be superlinear on the density ratio, a fact we attribute to the lack of pressure gradient behind the edge flame. This result is new and complements previous results, for different boundary conditions, which strongly suggest that the edge flame speed is a strong function of the particular hydrodynamic boundary conditions employed in the simulations.

Flame speed and tangential strain measurements in widely stratified partially premixed flames interacting with grid turbulence

Methane-air partially premixed flames subjected to grid-generated turbulence are stabilized in a two-slot burner with initial fuel concentration differences leading to stratification across the stoichiometric concentration. The fuel concentration gradient at the location corresponding to the flame base is measured using planar laser induced fluorescence (PLIF) of acetone in the non-reacting mixing field. Simultaneous PLIF of the OH radical and particle image velocimetry (PIV) measurements are performed to deduce the flow velocity and the flame front. These flames exhibit a convex premixed flame front and a trailing diffusion flame, with flow divergence upstream of the flame, as indicated by the instantaneous OH–PLIF, Mie scattering images, and PIV data. The mean streamwise velocity profile attains a global minimum just upstream of the flame front due to expansion of a gases caused by heat release. The flame speed measured just upstream of the flame leading edge is normalized with respect to the turbulent stoichiometric flame speed that takes into account variations in turbulent intensity and integral length scale. The turbulent edge flame speed exceeds the corresponding stoichiometric premixed flame speed and reaches a peak at a certain concentration gradient. The mean tangential strain at the flame leading edge locally peaks at the concentration gradient corresponding to the peak flame speed. The strain varies non-monotonically with the flame curvature unlike in a non-stratified curved premixed flame. The mechanism of peak flame speed is explained as the competition between availability of hot excess reactants from the premixed flame branches to the flame stretch induced due to flame curvature. The results suggest that the stabilization of lifted turbulent partially premixed flames occurs through an edge flame even at a relatively gentle concentration gradient. The strain is also evaluated along the flame front; it peaks at the flame leading edge and decreases gradually on either side of the leading edge. The present results also show qualitatively similar trends as those of laminar triple flames.

Spark ignition of lifted turbulent jet flames

This paper presents experiments on ignition and subsequent edge flame propagation in turbulent nonpremixed methane jets in air. The spark position, energy, duration, electrode diameter and gap, and the jet velocity and air premixing of the fuel stream are examined to study their effects on the ignition probability defined as successful flame establishment. The flame is visualized by a high-speed camera and planar laser-induced fluorescence of OH. It was found that after an initially spherical shape, the flame took a cylindrical shape with a propagating edge upstream. The probability of successful ignition increases with high spark energy, thin electrode diameter and wide gap, but decreases with increasing dilution of the jet with air. The flame kernel growth rate is high when the ignition probability is high for all parameters, except for jet velocity. Increasing the jet velocity decreases the ignition probability at all locations. The average flame position as a function of time from the spark was measured and the data were used to estimate a net propagation speed, which then resulted in an estimate of the average edge flame speed relative to the incoming flow. This was about 3 to 6 laminar burning velocities of a stoichiometric mixture. The measurements can assist theoretical models for the probability of ignition of nonpremixed flames and for edge flame propagation in turbulent inhomogeneous mixtures, both of which determine the success of ignition in practical combustion systems.

Transient dynamics of edge flames in a laminar nonpremixed hydrogen–air counterflow

Dynamics of edge flames encountered upon local quenching of nonpremixed flames is studied numerically considering the detailed chemistry of hydrogen–air combustion. The simulations are performed by superimposing two pairs of counter-rotating vortices on a steady counterflow nonpremixed flame. The density-weighted displacement speed and scalar dissipation rate are found to be appropriate parameters in characterizing the propagation speed of the edge flame with finite thickness and chemical structure, for both steady and transient conditions. It is also found that the edge flame speed strongly depends on the transient and history effects of the imposed flow field in addition to the instantaneous local scalar dissipation rate. A negative edge flame speed is observed during the early phase of the interaction due to the enthalpy loss to the transverse direction in the presence of a large strain. The effects of the fuel Lewis number on the edge flame speed are found to be consistent with the previous theoretical predictions based on the laminar flamelet theory. The results suggest that the transient and history aspects need to be carefully taken into account for an accurate description of the realistic turbulent nonpremixed combustion with partial quenching.