Triple Flame Research Articles

Doubly Conditional Source-term Estimation (DCSE) applied to a series of lifted turbulent jet flames in cold air

The objective of the present study is to evaluate the applicability of a new partially premixed combustion model, Doubly Conditional Source-term Estimation (DCSE), applied to a series of lifted jet flames using Reynolds Averaged Navier Stokes (RANS) calculations. The partially premixed implementation involves double conditioning on two variables, mixture fraction and progress variable. The present work represents the first application of DCSE for lifted turbulent flames. Conditionally averaged chemical source terms are closed by conditional averaged scalars which are obtained by inverting an integral equation. Three different jet conditions are simulated, over a range of Reynolds number between 7,000 and 19,500. Two-equation k–ε turbulence model is used to solve the flow field. The predicted mean centerline and radial profiles of methane concentration are in good agreement with the experimental values. The present DCSE calculations are successful in predicting the lift-off heights and reproducing many aspects of the flame structure in agreement with the experimental observations. The stabilization point is located on the fuel lean side, the mixture fraction isolines are deflected around the flame base, significant entrainment of air by the fuel jet upstream of the flame stabilization point is observed and the calculated flame width becomes larger with increasing Reynolds number. Some discussion is given related to triple flames. Future work is suggested related to the scalar dissipation rate modelling and DCSE implementation in Large Eddy Simulation (LES).

Temperature field measurement of an array of laminar premixed slot flame Jets using Mach-Zehnder interferometry

An experimental study was performed to investigate the influence of Reynolds number (Re) and non-dimensional jet-to-jet spacing (S/Dh) on flame shape, structure and temperature field of an array of laminar premixed slot flame jets. Mach-Zehnder interferometry technique is used to obtain an insight to the overall temperature field between single, twin and triple slot flame jets. The slot jets with large aspect ratio (L/W), length of L=60mm and width of W=6mm were used to eliminate the three-dimensional effect of temperature field. The effect of jet-to-jet spacing was investigated on flame characteristics under the test conditions of 200≤Re≤400 and equivalence ratio (φ) of unity. The present measurement reveals that the variation of maximum flame temperature with increment of Reynolds number is mainly due to heat transfer effects and is negligible while the flame height is increased. For the cases of twin and triple flame jets by increasing Reynolds number and decreasing non-dimensional jet-to-jet spacing (S/Dh), the interferences between the jets are increased and the jets attracted each other. Strong interference was observed at S/Dh=1.15. For the case of triple jets at this S/Dh, the central jet was suppressed while the side jets deflected towards the inner jet. The interference between jets was found to reduce the heat flux in the jet-to-jet interacting zone due to incomplete combustion. Also the optimum jet-to-jet spacing of triple flame jets is obtained at each Reynolds number to enhance the heat transfer performance of the jets.

Ignition, Liftoff, and Extinction of Gaseous Diffusion Flames

This review uses as a vehicular example the jet-flame configuration to examine some phenomena that emerge in nonpremixed gaseous combustion as a result of the interaction between the temperature-sensitive chemical reaction, typical of combustion, and the convective and diffusive transport. These include diffusion-controlled flames, edge flames and their role in flame attachment, triple flames and their role as ignition fronts, and strain-induced extinction, including flame-vortex interactions. The aim is to give an overall view of the fluid dynamics of nonpremixed combustion and to review the most relevant contributions.

音響振動がTriple Flameの曲率および浮き上がり高さに及ぼす影響

音響振動がTriple Flameの曲率および浮き上がり高さに及ぼす影響

Characterization of hydrogen triple flame propagation in vitiated laminar coaxial flow

This paper numerically analyzes the propagation characteristics of a hydrogen flame in coaxial vitiated flow in a confined quartz tube. The transient propagation of the flame is calculated using Li's Mechanism of hydrogen oxidation, and the propagation characteristics are discussed based on these calculations. The formation of the reaction zone, the ignition of fuel, the transformation of the flame's base structure, and the propagation behavior of the hydrogen jet flame base are characterized in the present study. The flame characteristics is analyzed based on calculated results, the information provide several insight into the flame propagation and ignition layer at the leading edge in vitiated situations. The results show that the leading point of a hydrogen flame in coaxial fresh air propagates along the preferred equivalence ratio isoline as the flame has a triple flame structure. On the contrary, in vitiated coaxial flow, the propagation of the hydrogen flame fits triple flame theory more precisely. The flame's kinetic properties show that hydrogen flame propagation in coaxial vitiated flow is still dominated by the triple flame. The results also suggest that the transformation of the flame during propagation is affected by the pool of radicals as well as the chemical reactions.

Premixed Combustion of Kapok (ceiba pentandra) seed oil on Perforated Burner

Availability of fossil fuels in the world decrease gradually due to excessive fuel exploitation. This situations push researcher to look for alternative fuels as a source of renewable energy, one of them is kapok (ceiba pentandra) seed oil. The aim this study was to know the behavior of laminar burning velocity, secondary Bunsen flame with open tip, cellular and triple flame. Premixed combustion of kapok seed oil was studied experimentally on perforated burner with equivalence ratio (φ) varied from 0.30 until 1.07. The results showed that combustion of glycerol requires a large amount of air so that laminar burning velocity (SL) is the highest at very lean mixture (φ =0.36) in the form of individual Bunsen flame on each of the perforated plate hole. Perforated and secondary Bunsen flame both reached maximum SL similar with that of ethanol and higher than that of hexadecane. Slight increase of φ decreases drastically SL of perforated and secondary Bunsen flame. When the mixture was enriched, secondary Bunsen and perforated flame disappears, and then the flame becomes Bunsen flame with open tip and triple flame (φ = 0.62 to 1.07). Flame was getting stable until the mixture above the stoichiometry. Being isolated from ambient air, the SL of perforated flame, as well as secondary Bunsen flame, becomes equal with non-isolated flame. This shows the decreasing trend of laminar burning velocity while φ is increasing. When the mixture was enriched island (φ = 0.44 to 0.48) and petal (φ = 0.53 to 0.62) cellular flame take place. Flame becomes more unstable when the mixture was changed toward stoichiometry.

Triple flame: Inherent asymmetries and pentasectional character

A two-dimensional triple-flame numerical model of a laminar combustion process reflects flame asymmetric structural features that other analytical models do not generate. It reveals the pentasectional character of the triple flame, composed of the central pure diffusion-flame branch and the fuel-rich and fuel-lean branches, each of which is divided into two sections: a near-stoichiometric section and a previously unreported near-flammability-limits section with combined diffusion and premixed character. Results include propagation velocity, fuel and oxidiser mass fractions, temperature and reaction rates. Realistic stoichiometric ratios and reaction orders match experimental planar flame characteristics. Constant density, a one-step reaction, and a mixture fraction gradient at the inlet as the simulation parameter are imposed. The upstream equivalence ratio or the upstream reactant mass fractions are linear or hyperbolic functions of the transverse coordinate. The use here of experimental kinetics data differs from previous analytical works and results in flame asymmetry and different flammability limits. Upstream mixture composition gradient affects propagation velocity, flame curvature, diffusion flame reaction rate, and flammability limits. Flammability limits extend beyond those of a planar flame due to transverse heat and mass diffusion causing the pentasectional character.

Pulverized coal combustion with opposing/cross-flow methane/air mixtures

Pulverized coal co-firing with opposing premixed methane/air flames in a cross-flow arrangement was found to extend the coal burning limits and reduce its NOx emissions by creating a high temperature NOx-reducing zone between the resulting multiple flame envelopes. By using a primary air stream, the stability limit reached a peak value of 25.1 m/s while the exhaust NOx mole fraction got values as low as 1.2 × 10–4 due to HCN reaction with NOx generated from both the coal primary flame envelope and the premixed flames. The NOx exhaust emissions were reduced to values below 60 ppm as the gas/coal heat input ratio increased to 1.5, where a nonunity Lewis number predicted the peak NOx concentrations across the annular region surrounding the flame. The effectiveness of NOx reduction increased by employing triple flames in place of the primary air since each NOx peak generated from methane combustion was followed by a region of NOx reduction upon consuming a significant amount of the HCN released from coal. Extending the coal flame stability limit to 32.3 m/s contributed to NOx reduction via decreasing the residence time for NOx formation across the combustor exit section. Decreasing the triple flame mixture fraction gradient and keeping a higher overall equivalence ratio reduced the NOx exhaust concentrations to 50 ppm and increased the percent of heat transfer by radiation to reach a peak of 48.8%. Decreasing the particle size to 135 µm and increasing the number of opposing jets to 16 pronounced favorable aerodynamic effects by minimizing the HC and NOx emissions, respectively, to values below 0.16% and 65 ppm. Increasing the opposing jets’ diameter to separation reduced the flame length by 26.7%, while maximizing the staging height kept the NOx emissions below 40 ppm at a strain rate of 2050 s–1.

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.

Lean Flame Dynamics Through a 2D Lattice of Alkane Droplets in Air

Flame propagation along a 1-D array or through a 2D-lattice of fuel droplets has long been suggested to schematize spray-flames spreading in a two-phase premixture. The present numerical work considers the fresh aerosol as a system of individual alkane droplets initially located at the nodes of a face-centered 2D-lattice, surrounded by a variable mixture of alkane and air, in which the droplets can move. The main parameters of the study are s, the lattice path, and ϕ L , the liquid loading, which are both varied, whereas ϕ T , the overall equivalence ratio, is maintained lean (ϕ T = 0.85). Main results are as follows: (a) For a large lattice path (or when the droplets are large enough), spreading occurs in two stages: a short time of combustion followed by a long time lag of vaporization and a classical triple flame (with a very short rich wing) spreads around the droplets; (b) spray-flame speed decreases as liquid loading increases; (c) an elementary model invoking both propagation stages allows us to interpret flame speed as a function of the sole parameter s × ϕ L ; (d) when the lattice path shortens, the spray-flame exhibits a pattern that continuously goes from this situation to the plane flame front.

Simulation of a high Reynolds number reactive transverse jet and the formation of a triple flame

This paper briefly describes a hybrid Eulerian–Lagrangian approach for the numerical simulation of turbulent combustion and its application to the study of transverse reactive jets. Because of their interesting mixing properties, transverse jets are important to a variety of industrial applications such as film cooling, primary or dilution jets in gas turbines, and flame stabilization in high speed combustion. To capture the jet complex structure and the associated reaction dynamics, we developed a fast, multiscale and parallel 3D code using a Lagrangian particle method to solve the vorticity transport equation and an Eulerian adaptive grid-based method to solve the reactive transport equations.Flame anchoring computations were done for a pure methane jet in a crossflow of air at a Reynolds number Re=1000 and at a velocity ratio of 5 between the jet and the crossflow velocity. The results show that the reactive jet structure strongly resembles that of the non-reactive case, although the reactive jet is slower to bend into the cross flow and it is wider in the spanwise direction. The flame is anchored downstream of the nozzle in a relatively low velocity region and shows a triple flame structure. This triple flame seats in a region of relatively low scalar dissipation rate. We describe the three dimensional topology of the triple flame, as well as the impact of the jet counter-rotating vortex pair (CVP) on the flame front.

삼파장 적외선식 불꽃감지기의 자가진단 회로 개발

There is needed the triple pyroelectric Infrared flame detector to really recognize problem, for the prevention and early suppression of fire. This system recognizes the characteristics of fire sources in various type and is communicated the message to the operators. Therefore, the prevention and early suppression of fire is available. Especially this paper focuss on development of the self diagnosis type flame detector for preventing malfunction comparing of basic and detected values.

Premixed Combustion of Coconut Oil on Perforated Burner

Coconut oil premixed combustion behavior has been studied experimentally on perforated burner with equivalence ratio (φ) varied from very lean until very rich. The results showed that burning of glycerol needs large number of air so that the laminar burning velocity (SL) is the highest at very lean mixture and the flame is in the form of individual Bunsen flame on each of the perforated plate hole. As φ is increased the SL decreases and the secondary Bunsen flame with open tip occurs from φ =0.54 at the downstream of perforated flame. The perforated flame disappears at φ = 0.66 while the secondary Bunsen flame still exist with SL increases following that of hexadecane flame trend and then extinct when the equivalence ratio reaches one or more. Surrounding ambient air intervention makes SL decreases, shifts lower flammability limit into richer mixture, and performs triple and cellular flames. The glycerol diffusion flame radiation burned fatty acids that perform cellular islands on perforated hole. Without glycerol, laminar flame velocity becomes higher and more stable as perforated flame at higher φ. At rich mixture the Bunsen flame becomes unstable and performs petal cellular around the cone flame front.

A comparative study of the port geometrical effects on sharp corners’ jet triple flames

The performance of sharp corners’ jet triple flames was modulated by compromising the increase in both the number of sharp corners between 3 and 16 and the overall perimeter up to 218% with the reduction in vertex angle to 30°. While the turbulent kinetic energy peak value correlated with the vertex angle and its average value depended on the number of corners, the entrainment rate gain was pronounced in accordance with the perimeter. By having each port stabilizing the inner flame wing such that the sharp corners lay on the diffusion flame envelope at its base, the flow strain due to corners increased the triple flame stability limit to a maximum of 9.2m/s and minimized the NOx emissions to 14ppm. Upon incorporating the swirl impact, the highest firing rate was reached at a jet velocity of 26.3m/s at which excessive entrainment of reactants into corners provided a compact flame with a firing intensity of 50.3MJ/m3. While the flame length was related to the extent of jet deformation from the circular shape, a flame shortening by 67% was verified by an enhancement of 192% over the circular port entrainment rate. Improved combustion efficiencies with respective decrease in CO and HC emissions to 61ppm and 0.04% were reported by maximizing the effect of sharp edged appendices. The swirl introduced a favorable advection transport, whereby the turbulence peaks at corners merged to provide a relative increase of 11.8 times in the average turbulence energy. By effectively combining the active and passive control techniques via providing swirl and modulating the port geometry, the lean operation limit extended by 19% from that of the circular port whereby NOx emissions for triple flames case II had a minimum of 4ppm.

The effect of gravity and thermal expansion on the propagation of a triple flame in a horizontal channel

We investigate the effect of thermal expansion and gravity on the propagation of a triple flame in a horizontal channel with porous walls, where the fuel and oxidiser concentrations are prescribed. The triple flame therefore propagates in a direction perpendicular to the direction of gravity, a configuration that does not seem to have received any dedicated investigation in the literature. In particular, we examine the effect of the non-dimensional flame-front thickness ∊ on the propagation speed of the triple flame for different values of the thermal expansion coefficient α and the Rayleigh number Ra. When gravity is not accounted for (Ra=0), and for small values of ∊, the numerically calculated propagation speed is found to agree with predictions made in previous studies based on scaling laws [1]. We show that the well known monotonic relationship between U and ∊, which exists in the constant density case when the Lewis numbers are of order unity or larger, persists for triple flames undergoing thermal expansion. Under strong enough gravitational effects (Ra≫1), however, the relationship is no longer found to be monotonic. For a fixed value of ∊, the relationship between the Rayleigh number and the propagation speed is shown to vary qualitatively depending on the value of ∊ chosen, exhibiting hysteresis if ∊ is small enough and displaying local maxima, local minima or monotonic behaviour for other values of ∊. All of the steady solutions presented in the paper have been found to be stable, except for those on the middle branches of the hysteresis curves.

Numerical simulation of the influence of partial premixing on the propagation of partially premixed flames

This paper examines the influence of a partial premixing between the reactants prior to the reaction in a mixing layer on the structure and propagation of triple flames formed in this configuration. The problem has been mathematically formulated in the framework of the thermo-diffusive model under a single-step irreversible chemical reaction. A computational modeling and numerical simulations based on finite elements were then employed to solve the governing equations. The study has shown that the flame structure and characteristics such as its speed and maximum temperature were significantly modified in the presence of premixing. The triple flame structure were found to exhibit several changes such as the shift of the leading edge toward the combustion mixture side, the weakening of the upper premixed branch, and the increase in the flame area. It has been predicted that the premixing can substantially enhance the reactivity of the combustion mixture, but on the other hand it tends to shift the flame to the boundary which may destroy the combustion chamber.

Swirl and Cross-Flow Effects on Vitiated Jet Flames

A burner was developed to accommodate extensive limits of firing for lean and rich fuel/air mixtures via efficient vitiation of the reactive streams by a pilot premixed flame jet under swirl and cross-flow effects. The stability and combustion characteristics of the corresponding normal and inverse triple flames were addressed for vitiating a double-swirled flow through a concentric vane swirler by a pilot stream swirled due to a tangential entry. Increasing the vitiating stream heat input ratio to 0.5 increased the stability limit to eight times the laminar flame speed as the inner swirl number increased to 2.0. Increasing the cross-flow air momentum flux ratio from 1.0 to 3.0 further increased the normalized stability limit to 8.5, while employing double coswirl and triple coswirl increased such limit to respective values of 10.2 and 11.9. Vitiating a double-swirled rich fuel/air mixture stream by a tangential entry-swirled lean fuel/air mixture stream enlarged the stoichiometric zone to pronounce a peak turbulent kinetic energy of 3.5 m2/s2. Increasing the fuel mass fraction gradient increased the stability limit only with a vitiating lean mixture, while increasing the swirled portion of the vitiating stream increased the normalized stability limit to 19.6. Allocating the counterswirl across the vitiated stream was more efficient to increase the normalized stability limit to 26.7 as the relative swirl angle reached 120°. Increasing the cross-flow opposing jet strain rate enhanced the H and H2 preferential diffusion, increased the temperature homogeneity, and reduced the flame length. There was a corresponding reduction in NOx emissions to a minimum of 38 ppm as the strain rate increased to 13000 s−1, while the normalized stability limit increased to 27.3. Decreasing the longitudinal spacing between the cross-flow stages from 12.0 to 4.0 cm decreased the NOx emissions further to 18 ppm and increased the normalized stability limit to 30.5.

Effect of Unsaturated Bond on NOx and PAH Formation in n-Heptane and 1-Heptene Triple Flames

Various engine and shock tube studies have observed increased NOx emissions from the combustion of biodiesels relative to regular diesel and linked them to the degree of unsaturation or the number of double bonds in the molecular structure of long-chain biodiesel fuels. We report herein a numerical investigation on the structure and emission characteristics of triple flames burning n-heptane and 1-heptene fuels, which represent, respectively, the hydrocarbon side chain of the saturated (methyl octanoate) and unsaturated (methyl octenoate) biodiesel surrogates. Our objective is to examine the effect of unsaturated (double) bond on NOx and soot emissions in a flame environment containing regions of lean premixed, rich premixed, and nonpremixed combustion. A validated detailed kinetic model with 198 species and 4932 reactions was used to simulate triple flames in a counterflow configuration with different levels of premixing and strain rates. Results indicate that although the global structures of n-heptane and 1-heptene triple flames are quite similar, there are significant differences with respect to NOx and polycyclic aromatic hydrocarbon (PAH) emissions from these flames. The NOx production rates in the rich premixed, lean premixed, and nonpremixed zones are higher in 1-heptene flames than in n-heptane flames, and the differences become more pronounced as the level of premixing is increased. The NOx formed through the prompt, thermal, N2O, and NNH mechanisms is also higher in 1-heptene flames. NOx formation in the rich premixed zone is primarily due to the prompt NO, that in the nonpremixed zone is through the thermal NO, and that in the lean premixed zone is due to the NNH and N2O routes. The PAH species are mainly formed in the rich premixed zone, and their emissions are significantly higher in 1-heptene flames than in n-heptane flames. The reaction pathway analysis indicated that the dominant path for benzene formation involves the recombination of two propargyl (C3H3) radicals, and the presence of the double bond in 1-heptene provides a significant route for its production through the formation of C3H5. This path is not favored in the oxidation of n-heptane, as it decomposes directly to smaller alkyl radicals. Whereas the NOx and PAH emissions decrease with the increase in strain rate, they are consistently higher in 1-heptne flames than in n-heptane flames, irrespective of the strain rate.

Large-Eddy Simulation of combustion instabilities in a variable-length combustor

This article presents a simulation of a model rocket combustor with continuously variable acoustic properties thanks to a variable-length injector tube. Fully compressible Large-Eddy Simulations are conducted using the AVBP code. An original flame stabilization mechanism is uncovered where the recirculation of hot gases in the corner recirculation zone creates a triple flame structure. An unstable operating point is then chosen to investigate the mechanism of the instability. The simulations are compared to experimental results in terms of frequency and mode structure. Two-dimensional axi-symmetric computations are compared to full 3D simulations in order to assess the validity of the axi-symmetry assumption for the prediction of mean and unsteady features of this flow. Despite the inaccuracies inherent to the 2D description of a turbulent flow, for this configuration and the particular operating point investigated, the axi-symmetric simulation qualitatively reproduces some features of the instability.

Early structure of LPG partially premixed conically stabilized flames

This paper presents experimental investigation of LPG partially premixed turbulent flames stabilized within a conical nozzle burner under constant degree of partial premixing. The stability limits and mean flame structure are presented based on the mean gas temperature and the concentration of CO, O2, NO, and HC at the flame early region of reaction. The investigation covered the influence of the nozzle cone angle, the jet exit velocity and the jet equivalence ratio. The stability results show that the flames with cone are more stable than those without cone. For conical stabilized flames, the stability results exhibit three different sensitivity regions between the jet velocity and equivalence ratio. The inflame measurements prove that the flame stability could be attributed to the triple flame structure at the flame leading edge. The data show that the triple flame structure is influenced by cone angle, the jet velocity and the equivalence ratio. The flame is believed to be controlled by the recirculation flow inside the cone. Increasing the cone angle induced higher air entrainment to the reaction zone as depicted by a higher O2 concentration within the flame leading edge. Increasing the jet velocity to a certain limit enhances the intensity of combustion at the flame leading edge, while excessive increase in jet velocity reduces this intensity. At a fixed jet velocity the higher the equivalence ratio, the higher the amount of fuel diffused and engulfed to the reaction zone, the more delay of the combustion completion and the higher the emission concentrations of the flame.