Triple Flame Research Articles

DNS study of stabilization of turbulent triple flames by hot gases

Direct numerical simulations of turbulent flames stabilized by hot gases are presented and analysed with the aim of investigating the mechanisms which control turbulent flame stabilization. Even if the simulations are run on an idealized 2D configuration and use a simplified formulation, with single-step chemistry and Le = 1 assumptions, they are useful in giving insight in the dynamics of flame stabilization. The current flames are stabilized by instantaneous reignition events triggered by hot gas convected by recirculation, which ignites lean premixed pockets that eventually produce a new triple flame upstream of the otherwise downstream travelling unsteady flame. While the occurrence of these events depends on the level of turbulence imposed to the flame, away from these occasional events the instantaneous flame propagates at a speed given by laminar flame properties, independent of the level of turbulence. This could explain why in some experiments [A. Upatnieks, J.F. Driscoll, Ch.C. Rasmussen, S.L. Ceccio, Combust. Flame 138 (2004) 259–272.] no correlation is found between the local turbulent intensity and the flame propagation speed, but a correlation is found between the mean flame position and the turbulence intensity.

Behaviour of a confined fire located in an unventilated zone

The behaviour of a fire in an enclosure is studied for a configuration where the fuel source is located in the upper hot unventilated zone trapped by a soffit. The experimental study, undertaken in a laboratory-scale compartment with a fuel source above the level of a soffit, included the determination of the parameters (ventilation factor, rate of fuel supply) controlling the combustion or leading to extinction. Measurements (PIV, thermocouples, gas sampling and analysis) were performed to propose a hypothesis on the structure of the flame (flame stabilisation mechanisms, premixed or diffusion types). Video photography is used to determine the area covered by the flames. This information is used as a criterion to identify the combustion regimes. The results show that the gaseous fuel is diluted in the combustion products (CP) in the upper layer and that a recirculatory motion is formed, driven by buoyancy forces, which enhances the mixing of fuel and CP. These then travel horizontally towards the vent along the interface between the lower fresh air and upper zones, and are premixed with the convected air in the enclosure, before entering the reaction zone and being burnt. The flame stabilises at the interface between the upper hot and lower ventilated layers in the compartment. The observed “ghosting flame” is stabilised by a triple flame if the flame speed of the premixed flame is higher than the natural convection velocity induced in the compartment. The flame stability is quantified by a criterion based on the area of the horizontal flame. It has been observed that the combustion is controlled by the available mass fuel flux at the reaction zone if the ventilation is sufficient. This information is essential for the modelling of the phenomena involved in fires with such an underventilated fuel source.

Effect of the reversibility of the chemical reaction on triple flames

We examine a new aspect of triple flames, namely the effect of the reversibility of the chemical reaction on flame propagation. The study is carried out in the configuration of the two-dimensional strained mixing layer formed between two opposing streams of fuel and oxidiser. The chemical reaction is modelled as a single reversible reaction following an Arrhenius law in the forward and backward directions. The problem is formulated within the constant-density (thermo-diffusive) approximation, the main non-dimensional parameters relevant to this study being a reversibility parameter R (equal to zero in the irreversible case), a non-dimensional measure of the strain rate ϵ and a stoichiometric parameter S. We provide analytical (asymptotic) expressions for the local burning speed of the triple flame in terms of ϵ, S, and R. In particular we describe how the propagation speed of the front is decreased by an increase in R and how the location of its leading edge is affected by reversibility. For example, it is found that the leading edge shifts towards the fuel stream for S > 1 and towards the oxidiser if S < 1, as R is increased. A detailed numerical study is conducted covering all propagation regimes ranging from weakly stretched positively propagating (ignition) fronts to thick negatively propagating (extinction) fronts. In the weakly stretched cases we show that the numerics are in good agreement with the asymptotic findings. Furthermore, the results allow the determination of the domains of the distinct propagation regimes, mainly in the terms of R and ϵ. In line with our physical intuition, it is found that reversibility reduces the domain of ignition fronts and promotes extinction. The results provide a systematic investigation which can be considered as a first step when considering a more realistic chemistry, or poorly explored aspects (such as the existence of a temperature gradient in the unburnt mixture), when analyzing triple flames.

Burning velocity of triple flames with gentle concentration gradient

We investigated the local flame speed of a two-dimensional, methane–air triple flame in a rectangular burner. The velocity fields and the concentration profiles were measured with particle image velocimetry and the Rayleigh scattering method, respectively. There was a requisite combination of initial velocity and initial concentration gradient for consistency of the local concentration gradient at the leading edge of the flame. In these cases, the flame curvatures were also consistent. Accordingly, the burning velocity, defined as local flow velocity at the triple point, was determined by the flame curvature. The burning velocity increased with increasing flame curvature, when the curvature was near zero. After that, the burning velocity decreased with increasing curvature. The peak value thus exceeded the adiabatic one-dimensional laminar burning velocity. Comparing the effects of the measured flame stretch rate on the flow strain κ s and flame curvature κ c, κ s is larger and increases more rapidly than κ c for flame curvatures satisfying 1/ R f < 250 m −1 and then becomes constant while κ c still increases for 250 m −1 < 1/ R f, so that κ c becomes much larger than κ s. There is also a peak in burning velocity at roughly the transition in flame curvature specified above. Therefore, the burning velocity for a low concentration gradient correlates with the flame stretch rate.

Stabilization, propagation and instability of tribrachial triple flames

A tribrachial (or triple) flame is one kind of edge flame that can be encountered in nonpremixed mixing layers, consisting of a lean and a rich premixed flame wing together with a trailing diffusion flame all extending from a single point. The flame could play an important role on the characteristics of various flame behaviors including lifted flames in jets, flame propagation in two-dimensional mixing layers, and autoignition fronts. The structure of tribrachial flame suggests that the edge is located along the stoichiometric contour in a mixing layer due to the coexistence of all three different types of flames. Since the edge has a premixed nature, it has unique propagation characteristics. In this review, the propagation speed of tribrachial flames will be discussed for flames propagating in mixing layers, including the effects of concentration gradient, velocity gradient, and burnt gas expansion. Based on the tribrachial edge structure observed experimentally in laminar lifted flames in jets, the flame stabilization characteristics including liftoff height, reattachment, and blowout behaviors and their buoyancy-induced instability will be explained. Various effects on liftoff heights in both free and coflow jets including jet velocity, the Schmidt number of fuel, nozzle diameter, partial premixing of air to fuel, and inert dilution to fuel are discussed. Implications of edge flames in the modeling of turbulent nonpremixed flames and the stabilization of turbulent lifted flames in jets are covered.

Structure and stabilization mechanism of a microjet methane diffusion flame near extinction

The flame structure and stabilization mechanism of a microjet methane diffusion flame near extinction are numerically investigated using multi-component transport model coupled with GRI-Mech 3.0 chemical kinetic mechanisms. The small flame size and intensive heat loss to the burner wall suggest that the microjet flame may always operate in a severe condition near extinction with distinct stabilization feature. Of particular interest is the flame structure in the standoff region, as it would directly relate to the mixing, diffusion, and chemical kinetic processes as well as flame stabilization. Computed results show that near extinction the fuel burns in a diffusion flame, in contradiction to a simple jet flame prediction. Neither a double flame nor a triple flame is observed in the computed structures, suggesting that the standoff flame is stabilized by the hot zone that connects to the reaction kernel, through the formation of HO 2 layer and subsequent key radical reactions.

Flame structure in small-scale liquid film combustors

Recent advances in technology for commercial, medical, military, and governmental applications have increased the demand for portable power systems. One solution is to design a small-scale liquid-fueled combustion system, which takes advantage of volumetric reactions for quick energy release and a nonvolatile fuel that provides high specific energy. To make use of the high surface-to-volume ratios with decreasing size that generally increase heat loss, an innovative concept is to inject the fuel as a liquid film on the inner combustor surface, effectively reducing losses by redirecting heat towards film vaporization. This film is generated and stabilized in a tubular chamber by introducing swirling airflow, and investigation of the resulting flames has demonstrated that flame ignition and stability are dependent upon wall temperature and film location, and do not require recirculation or a physical flameholder. Attempts to investigate flame structures in a low thermal conductivity quartz chamber have revealed that poor wall heat transfer compromises the stability of the flame. Chemiluminescence measurements of the OH ∗, CH ∗, and C 2 ∗ intensities, and an Abel transform of the integrated images to obtain the radial distribution of these radical species within a sapphire chamber supports the theory that the fuel-film flame is composed of two structures: a double flame at the exit rim and a central triple flame within the chamber.

Instabilities of reverse smolder waves

We use numerical strategies to examine the stability of reverse smolder waves in the context of a model that can permit both fuel-rich and fuel-lean waves. The steady-state response for such waves, maximum temperature versus blowing rate, is characterized, for increasing blowing rate, by a fuel-rich branch of rising temperature followed by a fuel-lean branch of falling temperature, followed by quenching. The propagation speed at the quenching point is non-zero. For the parameters that we consider, the entire fuel-rich branch is unstable to two-dimensional disturbances, but the dynamic consequences are modest. An interval of the fuel-lean branch whose left boundary is at the point of stoichiometry is stable, but the remainder of the branch, all the way to the quenching point, is unstable. These instabilities are destructive, and the smolder wave becomes fragmented. Tribrachial fragments can emerge, analogous to the tribrachial or triple flames familiar from gaseous edge-flame studies. Their emergence is characterized by a sharp rise in the maximum temperature, a rise that could lead to a transition to flaming (gas-phase) combustion.

The propagation of tribrachial flames in a confined channel

A flame formulated in a mixing layer has a typical structure of a tribrachial (or triple) flame. The propagation velocity of a tribrachial flame is much higher than the laminar burning velocity of a stoichiometric premixed flame, and the propagation velocity usually decreases as the gradient of fuel concentration increases. A separate experimental study that used a jet flow in an open space reported that there is a maximum propagation velocity at a critical fuel concentration gradient coupled with the enhancement of a diffusion flame branch and that the critical concentration gradient can be varied even by the difference in velocity variation near the flame. This study investigates how a confined flow field affects the structure of a tribrachial flame. The mean velocity and the concentration gradient of fuel were controlled by a multislot burner. Laser diagnostic methods were used to measure the velocity variation, the OH radical, and the temperature variation. Even in a confined geometrical space, the existence of the maximum propagation velocity was confirmed. Moreover, the critical concentration gradients in a confined channel were larger than those in an open jet case; that is, the role of the diffusion flame at the maximum propagation velocity becomes more significant in a confined structure due to the enhanced convective diffusion. This result shows the importance of the diffusion branch in a confined (or squeezed) stream tube.

Propagation rates of nonpremixed edge flames

The propagation rates ( U edge ) of nonpremixed ignition (advancing) fronts and extinction (retreating) edge flames were measured as a function of global strain rate ( σ), jet spacing ( d), mixture strength, stoichiometric mixture fraction ( Z st ), and Lewis number (Le) using a counterflow slot-jet burner in which edge flames propagated along its long dimension. Electrical heaters at both ends of the slot “anchored” the flames, allowing conditions resulting in negative values of U edge to be studied by triggering local extinction of anchored flames with an N 2 jet. Results are presented in terms of the effects of a dimensionless flame thickness ( ε) related to σ and a dimensionless heat loss ( κ) on a scaled U edge . Propagation rates were markedly enhanced/retarded in mixtures with low/high Le. Propagation rates and extinction conditions were highly asymmetric with respect to Z st = 0.5 despite the symmetry of flame location; mixtures with Z st greater/less than 0.5 behaved like stronger/weaker mixtures, apparently due to the relative locations of the radical production zone and maximum temperature zone for varying Z st . Two extinction limits were identified, corresponding to a high- σ strain-induced limit that was strongly dependent on Le but nearly independent of κ and a low- σ heat-loss-induced limit that was strongly dependent on κ but not Le. Most experimental findings were in good agreement with theoretical predictions; however, unlike predictions, “tailless” triple flames were not observed; instead standard triple flames and “short length” edge flames were found, and only for a very narrow range of experimental conditions. This is proposed to be due to the difference between the volumetric heat loss presumed in the models and the conductive transfer to the jet exits that dominates heat loss in the experiments.

A numerical investigation of flame liftoff, stabilization, and blowout

The effects of fuel stream dilution on the liftoff, stabilization, and blowout characteristics of laminar nonpremixed flames (NPFs) and partially premixed flames (PPFs) are investigated. Lifted methane-air flames were established in axisymmetric coflowing jets. Because of their flame suppression characteristics, two predominantly inert agents, CO2 and N2, were used as diluents. A time-accurate, implicit algorithm that uses a detailed description of the chemistry and includes radiation effects is used for the simulations. The predictions are validated using measurements of the reaction zone topologies and liftoff heights of both NPF and PPF. While an undiluted PPF is stabilized at the burner rim, characterized by significant radical destruction and heat loss to the burner, the corresponding undiluted NPF is lifted and stabilized in a low-velocity region extending from the wake of the burner. Detailed comparison of diluted NPF with PPF reveals that the base structures of both the flames are similar and exhibit a double flame structure in the near-field region, where the flame stabilization depends on a balance between the reaction rate and the scalar dissipation rate, which could also be interpreted as a balance between the edge-flame speed undergoing its local scalar dissipation rate and the local flow velocity. As diluent concentration is increased, the flames become weaker, move downstream along the stoichiometric mixture fraction line, and stabilize at a location where they can find a local flow field that has a lower scalar dissipation rate. Further increase of the diluent concentration moves the flames further downstream into the far-field region, where both the NPF and PPF exhibit a triple flame structure, and the flame stabilization mechanism also involves a balance between the triple flame speed and local flow velocity. The PPFs, however, shift to a higher liftoff height and blow out at a lower diluent concentration compared to the NPF, which can withstand larger amounts of dilution. In addition, both NPF and PPF are stabilized at lower liftoff heights and blowout at a lower diluent concentration, when they are diluted with N2 compared to that with CO2. The observed effects of fuel stream dilution and partial premixing on flame liftoff and blowout can be explained using the existing flame stabilization theories.

The blowout mechanism of turbulent jet diffusion flames

The complicated flame stabilization mechanisms and flame/flow interactions in the blowout of turbulent nonpremixed jet flames are experimentally studied using phenomenological observation, 2D Rayleigh scattering, 2D laser-induced predissociative fluorescence (LIPF) images of OH, and particle image velocimetry (PIV) techniques. The blowout process may be categorized into four characteristic regions: pulsating, onset of receding, receding, and extinction. Based on experimental findings, a blowout mechanism is proposed. The maximum “waistline” point of the stoichiometric contour, defined as the point where the radial distance between the elliptic stoichiometric contour and the jet axis reaches a maximum value, can be regarded as the dividing point separating the unstable and stable regions for the lifted flame in the blowout process. If the flame base is pushed beyond the maximum “waistline” point, the flame will step into the pulsating region and become unstable, triggering the blowout process. The triple flame structure is identified and found to play an important role in flame stabilization within the stable liftoff and pulsating regions. In the pulsating region, the stabilization point of the triple flame moves along the stoichiometric contour, stabilizing the flame where the flame base is bounded by the contours of lean and rich limits. If the flame is pushed beyond the tip of the stoichiometric contour, the stabilization point and triple flame structure vanish and the flame becomes lean. The flame then recedes downstream continuously and finally extinguishes.

Identification of Triple Flame Based on Numerical Data for Laminar Lifted Flames

The flame base structures of laminar lifted flames are numerically investigated in order to develop a model of triple flame applicable to the flamelet model. The lifted flames formed in the downstream expanded duct developed by Kioni et al. (1) are calculated systematically in terms of the fuel concentration gradient at the inlet using a variant of the HSMAC method, modified so as to deal with variations of density. The triple flame is formed at the flame base of lifted nonpremixed flame for every case, even the case which does not initially have partial mixing. The diffusion flame appears to be supported by the enhancement of the surrounding premixed flames, resulting in a temperature rise. All scalar quantities along the premixed flames of the triple flame decline along a similar profile in mixture fraction space, and scalar quantities in the region surrounded by the premixed flames are almost completely conserved. On the basis of the results, we developed a model of triple flame that is applicable to the flamelet model. In the model, the region without reaction upstream of the flame base is referred to as the unburned region (frozen flow structure region), followed by the transition region outside the premixed flames, in which the flame changes from the frozen structure to the fully burning diffusion structure. The region surrounded by the premixed flames is referred to as the triple flame structure region. The region following the triple flame structure region is the fully burning diffusion flame structure region.

Numerical simulation and flight experiment on oscillating lifted flames in coflow jets with gravity level variation

Characteristics of oscillating lifted flames have been investigated numerically and experimentally by varying the gravity level in coflow jets with propane fuel highly diluted with nitrogen. The results showed that the oscillation amplitude and frequency increased with gravity level. As the gravity level decreased, the oscillation ceased and stationary lifted flames were stabilized when the gravity level became smaller than a critical value. A flame blowout occurred at high gravity levels. The reason for this limited range of oscillation has been analyzed by considering the local characteristics of the propagation speed of tribrachial (triple) flame and axial velocity at the edges of lifted flames. Considerations of the maximum and minimum values of these two components with gravity level during the flame edge oscillation could successfully explain the lower bounds of oscillation accounting for the influences of buoyancy and flame curvature. The blowout at high gravity levels can be explained by the effect of buoyancies on burnt gas and on propane fuel in such a way that the stoichiometric contour near the flame zone became detached from the contour near the nozzle. Finally, the experiments by varying gravity level through the parabolic flights of an aircraft substantiated the overall behavior of the oscillating lifted flames.

THE FLAMELET GENERATED MANIFOLD METHOD APPLIED TO STEADY PLANAR PARTIALLY PREMIXED COUNTERFLOW FLAMES

ABSTRACT The recently introduced Flamelet Generated Manifold (FGM) method has proved to be an accurate and efficient reduction method for the modelling of premixed flames. The FGM method uses a chemical library based on one-dimensional unstrained premixed flames to model the chemistry of a multi-dimensional flame. Recently, the method has also been applied successfully to a so-called triple flame configuration, which is partially premixed. In this configuration, the gradient of the mixture fraction was relatively small compared to the flame thickness. In this paper the applicability of FGM in partially premixed combustion systems is investigated further. The method is tested in a planar counterflow configuration, which enables the control of the mixture fraction gradient. The gradient of the mixture fraction is changed by modifying the applied strain in one case and the inlet mixture fraction in the other case. The results show that, even though the mixing and flow time scales are of the same order as the flame time scales from the database. FGM is still relatively accurate. This can be explained by the fact that in a large part of the flame, the chemistry is not far from equilibrium, which means that the chemistry is still much faster than the flow and mixing processes. It has already been shown that this holds for strain, but this paper shows that it is also true for the dissipation rate. For very high strain and dissipation rates (up to 2000 s−1), errors up to 10% are obtained.

A numerical study of laminar methane/air triple flames in two-dimensional mixing layers

Triple flames formed in methane/air mixing layers with three different mixture fraction gradients were investigated by numerical simulation. The primitive variable method, in which the fully elliptic governing equations were solved with detailed chemistry and complex thermal and transport properties, was used. Radiation heat transfer from CO 2, CO and H 2O was calculated using the discrete-ordinates method coupled to a statistical narrow band correlated-K based wide band model. The results show that with the increase of the mixture fraction gradient, the combustion intensity in the diffusion flame branch of a triple flame is enhanced. In the near-stoichiometric mixture fraction region, the local burning flux of a triple flame is reduced when the mixture fraction gradient is increased. However, when the mixture fraction is significantly different from the stoichiometric value, the local burning flux increases as the mixture fraction gradient is increased. The correlation of the burning speed versus stretch rate established from conventional homogeneous premixed flames cannot completely explain the phenomena observed in triple flames. The interaction between the diffusion and premixed flame branches significantly affects the local burning velocity in regions where the mixture fraction is far from the stoichiometric value in a triple flame. This interaction is caused by both conduction heat transfer and radical exchange. Radiation has negligible effect on local burning properties in a triple flame.

A numerical study on NO x formation in laminar counterflow CH 4/air triple flames

Formation of NO x in counterflow methane/air triple flames at atmospheric pressure was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. Results indicate that in a triple flame, the appearance of the diffusion flame branch and the interaction between the diffusion flame branch and the premixed flame branches can significantly affect the formation of NO x , compared to the corresponding premixed flames. A triple flame produces more NO and NO 2 than the corresponding premixed flames due to the appearance of the diffusion flame branch where NO is mainly produced by the thermal mechanism. The contribution of the N 2O intermediate route to the total NO production in a triple flame is much smaller than those of the thermal and prompt routes. The variation in the equivalence ratio of the lean or rich premixed mixture affects the amount of NO formation in a triple flame. The interaction between the diffusion and the premixed flame branches causes the NO and NO 2 formation in a triple flame to be higher than in the corresponding premixed flames, not only in the diffusion flame branch region but also in the premixed flame branch regions. However, this interaction reduces the N 2O formation in a triple flame to a certain extent. The interaction is caused by the heat transfer and the radical diffusion from the diffusion flame branch to the premixed flame branches. With the decrease in the distance between the diffusion flame branch and the premixed flame branches, the interaction is intensified.

Determination of schmidt number of mixed fuels by the characteristics of laminar lifted jet flames

A method to determine the Schmidt number of fuel is proposed from the behavior of laminar lifted jet flames. Based on the observation of a laminar lifted flame edge, the flame stabilization point is located along the stoichiometric contour in the mixing layer of fuel and air in laminar jets, since a tribrachial (triple) flame structure exists which is composed of a diffusion flame, a rich premixed flame and a lean premixed flame, all extending from a single location. For the flame edge to be stationary, the axial velocity at the edge should balance with the propagation speed of the tribrachial flame. Since the region between the flame stabilization point and the nozzle exit can be treated as a cold jet, the jet theory of momentum and species can be applied to obtain the correlation of liftoff height with jet velocity and nozzle diameter of H L ∝ d 2 u o ( 2 S c F − 1 ) / ( S c F − 1 ) . Using this relation, the mixture of fuels having Sc<1 and Sc>1 are tested. The dependence of liftoff height on jet velocity is curve-fitted to extract the effective Schmidt number of mixed fuels. Experimentally determined Schmidt numbers agree satisfactorily with the theoretical predictions from the kinetic theory.

Utilização de chama para controle de plantas daninhas emersas em ambiente aquático

Dois estudos foram conduzidos com o objetivo de avaliar os efeitos da aplicação de chama no controle de Eichhornia crassipes, Brachiaria subquadripara, Pistia stratiotes e Salvinia auriculata. No primeiro estudo foram utilizadas diferentes doses de chama, representadas pela quantidade de gás consumida durante a aplicação, e, no segundo, usaramse duas aplicações de chama em intervalo de 14 dias, uma aplicação seqüencial (intervalo de sete dias) e aplicação única. As diferentes doses, tanto no primeiro quanto no segundo, foram comparadas com plantas que não receberam nenhum tratamento térmico. Avaliações de injúria foram realizadas aos 1, 3, 7, 10, 14, 17, 21 e 30 após a aplicação dos tratamentos, sendo realizadas também avaliações das biomassas secas das plantas aquáticas remanescentes em cada tratamento, ao final dos ensaios. No primeiro estudo foram observadas reduções significativas na produção da biomassa seca das espécies E. crassipes, B. subquadripara e P. stratiotes tratadas com as maiores doses referentes aos consumos de gás. Doses menores não diferiram estatisticamente da testemunha quanto à produção de biomassa seca, exceção feita para a espécie P. stratiotes. Todas as aplicações seqüenciais proporcionaram redução acima de 90% na produção da biomassa seca de E. crassipes e B. subquadripara. As aplicações seqüenciais e únicas proporcionaram reduções abaixo de 37% na produção da biomassa seca de S. auriculata. Os resultados demonstraram que existe a possibilidade de se utilizar o controle físico através da aplicação de chama como alternativa no manejo de plantas daninhas em ambientes aquáticos.

FEM-SIMULATION OF LAMINAR FLAME PROPAGATION II: TWIN AND TRIPLE FLAMES IN COUNTERFLOW

ABSTRACT In a low-Mach-number counterflow geometry, steadily propagating twin and triple flames are investigated. A complete numerical model of such flames is presented including a Galilean-like transformation to achieve steadiness of the flames in a moving frame of reference. The model is based on a similarity transformation of the fundamental conservation Equations in primitive variables, which transforms the inherently three-dimentional problem to two space dimensions. Suitable boundary conditions are derived employing potential theory. The governing Equations are discretized by a finite element method on an unstructured triangulized grid. Employing a local adaptive mesh-refinement procedure, for H2‒O2 systems with both a global one-step model and a detailed mechanism of elementary reactions, numerical solutions have been obtained.