Triple Flame Research Articles

Effects of CO addition on the propagation characteristics of laminar CH4 triple flame

This study proposes a numerical and experimental investigation of the propagation characteristics of CO-added CH4 flames in a confined quartz tube. The transient propagation of a CH4/CO–air flames were modeled using GRI-Mech 3.0, and the propagation characteristics are discussed based on the calculated results. This study characterizes the formation of the reaction zone, the transformation of the flame base structure, the ignition of fuel, and the propagation phenomena of the leading point of the flame base. The leading point of methane flame with a large amount of added CO is found to be difficult to define unambiguously. During flame propagation, a complex combination of chemical reactions coupled with the fluid dynamics between the stoichiometric line and the preferred equivalence ratio line occurs. The results suggest that the leading point of the propagating flame is still dominated by the redirection effect, while the effect of the intrinsic chemical properties of the fuel mixture on a propagating flame has finite thickness cannot be neglected.

Numerical Parametric Studies of Laminar Flame Structures in Opposed Jets of Partially Premixed Methane-Air Streams

Interactions of fuel-rich and fuel-lean mixtures and formation of interlinked multiple flame zones are observed in gas turbines and industrial furnaces. For fundamentally understanding such flames, numerical investigation of heat and mass transport, and chemical reaction processes, in laminar, counter flowing partially premixed rich and lean streams of methane and air mixtures, is presented. An axisymmetric numerical reactive flow model, with C2 detailed mechanism for describing methane oxidation in air and an optically thin radiation sub-model, is used in simulations. The numerical results are validated against the experimental results from literature. The equivalence ratios of counter flowing rich and lean reactant streams and the resulting strain rates have been varied. The effect of these parameters on the flame structure is presented. For a given rich and lean side equivalence ratios, by varying the strain rates, triple, double and single flame zones are obtained.

Flame stabilization in a supersonic combustor with hydrogen injection upstream of cavity flame holders: experiments and simulations

Experimental observations and numerical simulation were conducted to study the flame characteristics and flame stabilization mechanism in a supersonic combustor with hydrogen injection upstream of cavity flame holders. OH radical distribution of the reacting flowfield was obtained using OH spontaneous emission and OH-PLIF (planar laser-induced fluorescence). The supersonic combustion flowfield with L/ D = 7 cavity was calculated by large eddy simulation. The turbulence–combustion interaction model was based on a partially premixed flamelet model with a level-set approach. The results showed that hydrogen fuels were transported into the cavity shear; lean mixture and rich mixture were produced in the internal cavity and the declining jet, respectively. An approximately steady partially premixed flame front exists in the cavity shear layer. The flame front propagates and extends to the region around the fuel jet due to the interaction of counter-rotating vortices induced by the jet with the cavity shear layer. The flame front sustained in the shear layer likely penetrates the jet core and ignites the whole jet. Behind the flame front, most of the jet beam is burned as diffusion flames. The physical process of the flame stabilization demonstrated the similarity with triple flame characteristics, which indicted that triple flame theory might be the basic flame stabilization mechanism of the cavity flame holders.

Capabilities and limitations of multi-regime flamelet combustion models

Flamelet combustion models typically assume that burning occurs in either a fully premixed or a fully non-premixed mode. These assumptions tend to limit the applicability of the models to single-regime combustors. Efforts aimed at reducing this limitation have introduced multi-regime approaches that account for different types of mixing and chemistry interactions. In this study a multi-regime model is applied to two laminar n-heptane flames in an effort to characterize the capabilities and limitations of the approach. Both a 2-D laminar triple flame and a 2-D laminar counter-flow diffusion flame are numerically simulated using the multi-regime model. Data for comparison is generated by additionally simulating the flames using finite rate chemistry, a purely premixed flamelet model, and a purely non-premixed flamelet model. Simulations demonstrate that the multi-regime approach functions as desired, and tends to access flamelets from the appropriate regime under both non-premixed and premixed conditions. Some important differences between the flamelet solutions and finite rate solution are observed, however. These differences are caused by the finite rate solution deviating away from the assumed flamelet manifolds, rather than by inadequate regime predictions. In the analyses of these simulations, an emphasis is placed on understanding the formation of the pollutant species NO. It is shown that even when the local combustion regime is correctly predicted, small deviations from an assumed flamelet manifold can lead to changes in the NO production rate. The simulation results confirm that multi-regime flamelet models are applicable to a wide variety of reacting flows, but the results also help to characterize the limitations of these models.

Exploratory studies of modeling approaches for hydrogen triple flames

A review of triple flame modeling is first presented, which demonstrates the need for additional work in this area. Building on previous methods described in the literature, a hybrid model that uses a weighted average of one-dimensional premixed and diffusion flamelet reaction rates has been proposed and evaluated for a hydrogen triple flame. Results indicated that some type of progress variable is needed for application of the diffusion flamelet contribution. Weighting the premixed flamelet reaction rate contribution at 100%, it is shown that peak temperatures between the model and a case employing detailed chemistry vary 7.5%, while heat release rate, flame speed, and mass fraction contours agree well. A second model, based on a library of reaction rates built from numerical studies which directly resolve the propagating triple flame has also been tested. Computational time for the baseline case is shown to be reduced by a factor of 3 ½ in comparison to use of detailed chemistry. The role of scalar dissipation rate as a necessary independent variable to the library has also been investigated using simulations with variable mixing layer thicknesses. Overall, it is found that large changes in local mixture fraction gradient cause rather small changes in propagation speed and total heat release rate of the hydrogen triple flame. This implies that such a model may be useful for CFD simulations that do not employ spatial resolution capable of resolving the triple flame itself.

화염 곡률과 스칼라 소산율에 따른 층류부상화염의 화염전파속도에 관한 연구

삼지화염의 화염안정화 메커니즘 중 중요한 한 가지는 화염전파속도이다. 화염전파속도의 정량적인 규명을 위해 Bilger는 층류 유동이론에 근거하여 혼합분율 기울기에 비선형적으로 연관된 삼지화염전파속도를 제시하였다. 그러나 지금까지의 연구에서는 화염의 곡률 반경과 스칼라소산율 및 삼지화염의 화염전파속도에 관한 직접적인 관계에 관하여 제시된 바가 없었다. 본 논문은 실험과 수치해석에 따른 수치해석 결과를 검증하고, 수치해석을 통해 스칼라소산율에 따른 화염전파속도를 확인하였다. 그리고 화염스트레치 분석을 통하여 화염전파속도의 곡률반경 및 스칼라소산율에 따른 의존도를 명확히 하였다. Flame propagation velocity is the one ofmainmechanismof the stabilization of triple flame. To quantify the triple flame propagation velocity, Bilger presents the triple flame propagation velocity depending on the mixture fraction gradient, based on the laminar jet flow theory. However, in spite of these many analyses, there was not presented any relation of these variables, triple flame propagation velocity, radius of flame curvature and scalar dissipation rate indirectly. In the present research, we have checked the results of numerical simulation with experiment and numerical analysis and verified the flame propagation velocity with a scalar dissipation rate proposed by Bilger through the numerical simulation. Also we have clarified that flame propagation velocity was depended on the radius of flame curvature and scalar dissipation rate.

Structure of n-heptane/air triple flames in partially-premixed mixing layers

Results of a detailed numerical analysis of an n-heptane/air edge flame are presented. The equations of a low-Mach number reacting flow are solved in a two-dimensional domain using detailed models for species transport and chemical reactions. The reaction mechanism involves 560 species and 2538 reversible reactions. We consider an edge flame that is established in a mixing layer with a uniform velocity field. The mixing layer spans the equivalence ratios between pure air and 3.5. The detailed model enables us to analyze the chemical structure of the n-heptane edge flame. We identify major species profiles, discuss reactions causing the heat-release, and exploit Computational Singular Perturbation (CSP) to discuss the main fuel-consumption pathways and the structure of explosive modes in the edge flame. This analysis is performed for several regions in the edge flame to discuss the different processes at work in the premixed branches and the trailing diffusion flame. We compare different cuts through the 2D edge flame to canonical 1D premixed and diffusion flames. We also analyze the accuracy of a skeletal mechanism which was previously developed using CSP from homogeneous ignition calculations of n-heptane and show that a significant reduction in size of the mechanism can be achieved without a significant decrease in accuracy of the edge flame computation. This skeletal mechanism is then used to study the effects of increasing the equivalence ratio in the partially-premixed fuel stream.

층류제트 화염의 노즐직경에 따른 안정화 메커니즘과 화염형상에 관한 연구

Flame stability is the one of the main mechanism of laminar lifted flame and flame propagation velocity becomes a yardstick to measure the flame stability. Bilge has presented the flame propagation velocity of the triple flame and the flame stability mechanism related the flame configuration and mixture fraction. However, there was not able to observe all process of flame ignition and extinction for small nozzle diameter. In this paper, we have subdivided the flame configuration and stability mechanism and classified the flame behavior with a nozzle diameter. Also we have subdivided the `triple flame propagation opened` and the `triple flame propagation closed` from the triple flame propagation of triple flame criterion.

An analytical study of the effects of vaporization of twodimensional laminar droplets on a triple flame

The structure of triple flame propagation in combustion systems, containing uniformly distributed volatile fuel droplet was analyzed. The analysis was established for a one-step irreversible reaction with an asymptotic limit, where the value of the Zeldovich Number is large. Here, using unit Lewis number, the analytical results for the triple flame temperature were obtained considering two sections. In the first section, a non-vaporizing fuel stream was studied and in the second section, a volatile droplet fuel stream was taken into account. It is presumed that the fuel droplets vaporize to yield a gaseous fuel of known chemical structure, which is subsequently oxidized in the gaseous phase. Here two different cases are studied. In the first case, only the velocity parallel to the reactant flow was considered; while for the latter one, the vertical velocity was considered in addition. The energy equations were solved and the temperature field equations are presented. The results are first presented for a non-vaporizing fuel and compared to the experiment results. In addition, some other results of the temperature field for a vaporizing fuel stream are demonstrated within the comparison between the abovementioned cases which revealed the effect of the considering the vertical velocity component on the flame temperature field.

Effects of gellant concentration on the burning and flame structure of organic gel propellant droplets

The increasing demand for higher energy density fuels and the ever-increasing concern for their safety have propelled research in the field of gel propellants. For studying the fundamental parameters without the interference of neighbouring droplets, an isolated droplet was chosen to investigate experimentally the combustion process of gel propellants under normal gravity conditions. Phase separation of the gel propellant components leading to bubble nucleation, vapor jetting and microexplosions were found to be the main phenomenon involved during the combustion period. Experiments were comprehensively carried out to study the effect of gellant concentration on the burning rate constant as well as flame structure. The burning rate constant was found to decrease with increase in the gellant concentration. Decrease in the calorific value of the increasing gellant concentrations was proposed as one of the reasons for this variation. The flame exhibited a triple flame structure for all the cases for both C 2 ∗ radicals as well luminous flames. The horizontal and vertical flame standoff distances were observed to decrease with gellant content. Decrease in the content of the base fuel was proposed as the reason for the same.

Burned Gas Characteristics of Laminar Lifted Flame at the Beginning of Stream Heating

This research experimentally investigates the exhaust gas characteristics of a methane-air lifted-jet flame with elevated stream temperatures up to 450 K. Emission indices of NOx and CO were measured by direct gas sampling using a gas detector tube. The index of NOx decreased or did not change with increasing stream temperature up to 350 K, and then increased with further temperature increases. The index of CO changed in an opposite manner to that of NOx with increasing stream temperature. These tendencies demonstrated that the generation of emission gases did not monotonically increase with stream temperature. These non-monotonic properties of the emission indices were caused by particular variations in the lift-off height of flame and flame length. In order to examine these flame characteristics, we measured the flame curvature with OH-PLIF and related it to the lift-off height of triple flame. These flame properties could explain the unique emission characteristics of a lifted-jet flame with stream heating.

초전형 삼파장 적외선식 불꽃감지기 신호회로 개발

This thesis studies development of a triple pyroelectric Infrared flame detector to have appropriate reaction characteristics with proper operational features in various and radiant energy characteristics of fire sources, and to contribute to minimize the losses of human lives and properties by detecting fire in very early stage to restrict the spread of fire.

The stabilization characteristics of turbulent lifted diffusion flames of CH4/CO blended fuels

The outstanding stabilization characteristics of lifted turbulent jet diffusion flames of CH4/CO blended fuels in still air were investigated experimentally by igniting the flame from downstream in the initial condition of a nonreacting fuel jet. In this study, we concentrated on the effects of the shift of outstanding reaction paths on the stabilization characteristics as the composition of the blended CH4/CO is varied. Systematic experiments were conducted using the nonintrusive diagnostics of shuttered laser particle image velocimetry (PIV), OH∗ chemiluminescence, and digital image processing to study the flame stabilization characteristics in terms of the key parameters such as the instantaneous flame-base location, lift-off height, and streamline velocity distribution in the vicinity of the edge flame leading point by varying the exit velocity and blended-fuel concentration. A conditional leading point from shutter PIV measurement is defined to calculate the instantaneous streamline velocity profile at the flame base. For the CO concentration in fuel blends lower than 80%, the stabilization is similar in characteristics to that proposed by the triple flame model. As the CO concentration is increased to 90%, the flame stabilizes on the fuel-rich side and the stabilization characteristics may be related to the maximum burning velocity at the rich equivalence ratio. This is confirmed by the location of the instantaneous flame stabilization point defined by the instantaneous leading point of the lifted flame with respect to the fuel concentration profiles.

Flame structure and flame spread rate over a solid fuel in partially premixed atmospheres

We have investigated the downward flame spread over a thin solid fuel. Hydrogen, methane, or propane, included in the gaseous product of pyrolysis reaction, is added in the ambient air. The fuel concentration is kept below the lean flammability limit to observe the partially premixing effect. Both experimental and numerical studies have been conducted. Results show that, in partially premixed atmospheres, both blue flame and luminous flame regions are enlarged, and the flame spread rate is increased. Based on the flame index, a so-called triple flame is observed. The heat release rate ahead of the original diffusion flame is increased by adding the fuel, and its profile is moved upstream. Here, we focus on the heat input by adding the fuel in the opposed air, which could be a direct factor to intensify the combustion reaction. The dependence of the flame spread rate on the heat input is almost the same for methane and propane/air mixtures, but larger effect is observed for hydrogen/air mixture. Since the deficient reactant in lean mixture is fuel, the larger effect of hydrogen could be explained based on the Lewis number consideration. That is, the combustion is surely intensified for all cases, but this effect is larger for lean hydrogen/air mixture (Le < 1), because more fuel diffuses toward the lean premixed flame ahead of the original diffusion flame. Resultantly, the pyrolysis reaction is promoted to support the higher flame spread rate.

LES-pdf simulation of a spark ignited turbulent methane jet

A spark ignited turbulent jet flame is simulated using Large Eddy Simulation (LES) in conjunction with the filtered probability density function (pdf) equation approach, which is solved using the Eulerian stochastic field method. The spark energy deposition is mimicked by a gaussian distributed source term in the enthalpy equation. A dynamic model for the sub-grid stresses together with a simple gradient diffusion approximation for the scalar fluxes is adopted. The chemistry is represented by a global 4-step chemistry involving seven species. The different stages of the ignition sequence, namely the flame kernel growth, the triple flame propagation against the flow and the stabilisation were in good agreement with the experimental data.

Lifted flame structure of coannular jet flames in a triple port burner

In this study, we investigated the combustion field in a triple port burner. The coannular burner consists of three concentric tubes, where air flows in both inner (central) and outer tubes, and fuel flows in the annulus between these air tubes. In order to investigate the combustion characteristics in the triple port burner, the liftoff height and concentrations of soot and NOx were experimentally examined. A numerical simulation was also conducted to discuss the flame structure in detail. Since two non-premixed flames are formed in the boundaries of fuel and air, there are four flame configurations, consisting of attached flames, inner attached/outer lifted flames, inner lifted/outer attached flames, and twin lifted flames. By increasing the external air flow velocity, unique flame behaviors are observed, including (1) inner flame re-attachment and (2) flip-flop between inner and outer flames. When the flame is lifted, the maximum soot concentration and NOx emission are decreased. Based on the flame index, the attached flame is mainly a diffusion flame. When the inner or outer flame is lifted, the so-called triple flame structure is observed. Interestingly, in the case of twin lifted flames, two rich premixed flames are merged each other.

Numerical study of hydrogen triple flame response to mixture stratification, ambient temperature, pressure, and water vapor concentration

Mixing layer simulations of spark-ignited H 2/air that detail the influence of mixing layer thickness, domain pressure and air temperature, and water vapor concentration have been carried out. The stabilization speed of the propagating flame front is shown to increase with increasing mixing layer thickness as a result of streamline divergence ahead of the triple flame structure. The pressure changes considered are small (1, 2 bar), and within this range the pressure has only a modest effect on propagation speeds, while air layer temperature effects (298 K, 750 K) are pronounced. Higher adiabatic flame temperatures produced by elevated air temperature have been shown to lead to increased formation of nitrogen oxides. This tendency can be counteracted by increasing inert concentration. Therefore, the influence of water vapor concentration on emissions has been considered in studies where air layer species concentrations are replaced with up to 50% water by mole. Resulting changes in flame structure, heat release rates, flame speed, and emission concentrations – estimated through use of the standard Zeldovich NO mechanism, are given. Data trends for all parameter effects have been related to expected qualitative predictions for direct-injection hydrogen internal combustion engine behavior.

Structure of hydrogen triple flames and premixed flames compared

Triple flames consisting of lean, stoichiometric, and rich reaction zones may be produced in stratified mixtures undergoing combustion. Such flames have unique characteristics that differ from premixed flames. The present work offers a direct comparison of the structure and propagation behavior between hydrogen/air triple and premixed flames through a numerical study. Important similarities and differences are highlighted. Premixed flames are generated by spark-igniting initially quiescent homogeneous mixtures of hydrogen and air in a two-dimensional domain. Triple flame results are also generated in a two-dimensional domain by spark-igniting initially quiescent hydrogen/air stratified layers. Detailed flame structure and chemical reactivity information is collected along isocontours of equivalence ratio 0.5, 1.0, and 3.0 in the triple flame for comparison with premixed flames at the same equivalence ratios. Full chemistry and effective binary diffusion coefficients are employed for all computations.

Concentration distributions of intermediate hydrocarbons and soot in propane–air triple flames

AbstractAn experimental investigation has been conducted to study the distribution of intermediate hydrocarbons (CH4, C2H2, C2H4 and C2H6) and soot in propane/air triple flames at different inner and outer equivalence ratios. The flames are established on an axisymmetric, coflowing burner. A rich mixture of propane and air flows through the inner port of the burner, whereas a lean mixture flows through the annular port surrounding the central jet. The concentrations of hydrocarbons at different locations in the flames are measured with gas chromatography and the soot volume fractions are measured using a thermocouple particle densitometry (TPD) method. The flame structures have been described using the visual images of the flames as well as with the distributions of fuel (C3H8) and CO2 concentrations and temperature across the flames. The peak concentrations of the intermediate hydrocarbons are observed at the inner premixed flame location, which decay down rapidly also at the inner flame. The intermediat...

OH-radical behavior of unsteady lifted flame based on instantaneous change of the equivalence ratio

In an earlier study, the current authors showed that an unsteady-state lifted flame generated by an equivalence ratio conversion system for a given fuel, was similar to a steady-state lifted flame in terms of the change characteristics from a premixed flame to a critical flame and then to a triple flame with a diffusion flame positioned in the middle according to the concentration difference. Therefore, this study used an OH-PLIF method to investigate the characteristics of a steady-state lifted flame and an unsteady-state lifted flame created under conditions identical to the flames in the preceding study. PLIF (Planar laser induced fluorescence) is practically effective for visualizing the concentration fields within a flame. The resulting OH-radical measurements showed that an unsteady-state lifted flame created under the specific conditions used in this study showed similar tendencies in terms of OH-radical distribution, fluorescence intensity, and liftoff height, to a steady-state lifted flame, thereby confirming that the behavior of an unsteady-state lifted flame can be effectively predicted based on the behavior of a steady-state lifted flame.