Triple Flame Research Articles

Liftoff and blowout characteristics of laminar syngas nonpremixed flames

Liftoff and blowout behavior of nonpremixed syngas flames is investigated using a time-accurate CFD code with a detailed description of transport and chemistry. Lifted flames are established in coflowing laminar jets using N2 dilution in the fuel jet. Results focus on the effects of syngas composition and temperature on the liftoff, stabilization, and the edge (triple) flame structure. For a given syngas mixture, as the N2 dilution exceeds certain value, the flame lifts off from the burner rim and propagates along the stoichiometric mixture fraction line, and its structure changes from diffusion to double flame. With further dilution, the flame liftoff height increases rapidly, the base structure transitions from double to triple flame, and its stabilization involves a balance between the triple flame speed and local flow velocity. The temporal evolution of propagating jet flame also exhibits a similar behavior. The transition from diffusion to double and then to triple flame is examined using state relationships in mixture fraction coordinate. As H2 fraction in syngas and/or temperature is increased, the N2 dilution required for flame liftoff and blowout increases. The ratio of the triple flame speed to the unstretched premixed flame speed also increases with the increase in H2 fraction. For H2 fraction above 30%, the flame liftoff and blowout become less sensitive to syngas composition and temperature.

Asymptotic analysis to the effect of temperature gradient on the propagation of triple flames

We study asymptotically in this paper the influence of the temperature gradient across the mixing layer on the propagation triple flames formed inside a porous wall channel. The study begins by formulating the problem mathematically using the thermo-diffusive model and then presents a thorough asymptotic analysis of the problem in the limit of large activation energy and thin flames. Analytical formulae for the local burning speed, the flame shape and the propagation speed in terms of the temperature gradient parameter have been derived. It was shown that varying the feed temperatures can significantly enhance the burning of the reactants up to a critical threshold, beyond which no solutions can be obtained. In addition, the study showed that increasing the temperature at the boundaries will modify the usual triple structure of the flame by inverting the upper premixed branch and extending it to the boundary, which may have great implications on the safety of the adopted combustion chambers.

Influence of Fuel Premixing on Partially Premixed Flames in Porous-Wall Channels

Abstract This paper examines the influence of mixing a fuel with an oxidizer-rich reactant on the existence and propagation of partially premixed flames formed in a porous-wall channel. The amount of the injected fuel has been taken as a parameter to generate different states of the combustion mixtures. The problem has been mathematically formulated in the framework of the thermodiffusive model under a single-step chemical reaction. Numerical simulations based on finite elements were then employed to solve the governing partial differential equations. The study has shown that the degree of fuel premixing in the combustion mixtures as well as the value of the Lewis number of fuel and oxidizer plays a great role on the existence and structure of triple flames. It is found also that flame extinguishes for larger values of the premixing parameter. Flame structures were also found to exhibit several changes such as the shift of the leading edge toward the fuel supply side and the formation of twin flames. 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 underlying combustion chamber. For nonunit Lewis numbers, fuel premixing is found to shift triple flames from the stoichiometric surface and attach the flame to the channel walls. It is predicted also that the maximum reaction rate reduces with increasing the Lewis numbers.

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.

Downstream interaction between SNG – Air premixed flames

Experimental and numerical studies were conducted to understand flame extinction in SNG (synthetic natural gas) – air premixed flames with a counterflow configuration. The capability of some detailed reaction mechanisms in predicting flame extinction limit was evaluated through comparing predicted lean extinction boundaries with experimental ones. The results showed that those were best-fitted to numerical ones when the mechanism of UC San Diego was used and predicted flame locations were reasonably in agreement with experimental ones. Flame stability map was presented with a functional dependency on fuel volume fractions from lower and upper nozzles by varying global strain rate in interacting SNG – air premixed flames. Flame characteristics were identified to symmetric and asymmetric lean (rich) – lean (rich) flames and triple flames. The mechanism of flame extinction in interacting SNG – air premixed flames was explained and discussed by emphasizing important roles of downstream chemical interaction (via H-consumption) and upstream thermal interaction (via conductive heat loss to the ambience in the side of the stronger flame). Important role of H-radical (shared in downstream region) on chemical interaction was also discussed.

Formation of Soot in Ethylene–Air Partially Premixed Flames Over a Wide Range of Premixedness

This paper reports an investigation of soot formation in ethylene–air partially premixed flames (PPFs) over a wide range of premixedness. An axisymmetric co-flow configuration is chosen to establish PPFs from the fully nonpremixed to fully premixed conditions. Reducing the fuel flow rate as a percentage of the maximum from the core stream and supplying the same to the annular stream leads to stratification of the reactant concentrations. The thermal power, overall equivalence ratio, and the average velocity in both the streams are maintained constant under all conditions. The soot volume fraction is estimated by light attenuation method, and laser-induced incandescence (LII) is performed to map the soot distribution in the flow field. The soot volume fraction is observed to exhibit an “S”-type trend as the conditions are traversed from near the premixed to the nonpremixed regimes. That is, when traversing from the nonpremixed to near-premixed regime, below 60% fuel flow rate in core, the soot volume fraction drops drastically. The onset of sooting in the PPFs is clearly seen to be at the tip of the rich-premixed flame (RPF) branch of their triple flame structure, which advances upstream toward the base of the flame as the premixing is reduced. The S-type variation is clearly the effect of partial premixing, more specifically due to the presence of the lean premixed flame (LPF) branch of the triple flame. LII intensities are insufficient to capture the upstream advance of the soot onset with decreased premixedness. So, a quick and inexpensive technique to isolate soot luminescence through flame imaging is presented in the paper involving quasi-simultaneous imaging with a 650 nm and a BG-3 filter using a normal color camera.

Combustion in microgravity: The French contribution

Microgravity (drop towers, parabolic flights, sounding rockets and space stations) are particularly relevant to combustion problems given that they show high-density gradients and in many cases weak forced convection. For some configurations where buoyancy forces result in complex flow fields, microgravity leads to ideal conditions that correspond closely to canonical problems, e.g., combustion of a spherical droplet in a far-field still atmosphere, Emmons' problem for flame spreading over a solid flat plate, deflagration waves, etc. A comprehensive chronological review on the many combustion studies in microgravity was written first by Law and Faeth (1994) and then by F.A. Williams (1995). Later on, new recommendations for research directions have been delivered. In France, research has been managed and supported by CNES and CNRS since the creation of the microgravity research group in 1992. At this time, microgravity research and future activities contemplated the following:–Droplets: the “D2 law” has been well verified and high-pressure behavior of droplet combustion has been assessed. The studies must be extended in two main directions: vaporization in mixtures near the critical line and collective effects in dense sprays.–Flame spread: experiments observed blue flames governed by diffusion that are in accordance with Emmons' theory. Convection-dominated flames showed significant departures from the theory. Some theoretical assumptions appeared controversial and it was noted that radiation effects must be considered, especially when regarding the role of soot production in quenching.–Heterogeneous flames: two studies are in progress, one in Poitiers and the other in Marseilles, about flame/suspension interactions.–Premixed and triple flames: the knowledge still needs to be complemented. Triple flames must continue to be studied and understanding of “flame balls” still needs to be addressed.

Initiation and evolution of triple flames subject to thermal expansion and gravity

We investigate the combined effects of thermal expansion and gravity on the initiation and evolution of triple flames. In particular, we provide a possible criterion for the thermal energy per unit depth required for triple flame ignition by a cylindrical ignition kernel. Further, we describe the transient evolution of triple flames after initiation. Steady, non-propagating, planar solutions representing “flame tubes” are determined. The flame tube solutions are unstable; in time-dependent simulations it is found that initial perturbations increasing the thermal energy of flame tubes lead to the propagation of a triple flame, while perturbations decreasing the thermal energy lead to extinction. Therefore it is concluded that the thermal energy of flame tubes may be used to define a possible ignition energy per unit depth for planar triple flames in the mixing layer, analogous to spherical flame balls for spherically expanding flames. This is the first paper to provide a detailed study of the ignition energy of planar triple flames. When gravity is not taken into account, flame tubes subject to thermal expansion are found not to induce a flow, so that the flame tube energy can be determined without having to solve the full Navier–Stokes equations.

Coupling of harmonic flow oscillations to combustion instability in premixed segments of triple flames

Combustion instability of a lifted triple flame downstream of the injector nozzle in an acoustic field is studied, with implication on the combustion instability in liquid rocket engines. The analysis is focused on the premixed flame segment, and is based on the linear equation for a perturbed flamefront. Assuming that the radius of the flamefront curvature is much larger than that of the internal flame structure, the premixed front is approximated as being quasi-planar. Both acoustical stabilization of the hydrodynamic flame instability and excitation of the parametric instability by sound waves are considered, and the limitation for stable/unstable regimes as a function of flame parameters and acoustic frequency/intensity is determined. The velocity-wise nature of the sound is scrutinized, namely, the acoustics are imitated by a pocket of harmonic flow oscillations. First, the Landau limit of infinitely thin flames is employed. Then the formulation is extended to account for the finite flame thickness, and it is demonstrated how unstable regimes are modified by the flame thickness.

Structure of spray in hot-diluted coflow flames under different coflow conditions: A numerical study

This work studies the influence of coflow conditions on the structure of the Delft Spray in Hot-diluted Coflow (DSHC) flames, using Large Eddy Simulation (LES) techniques. The developed modeling approach was first applied to three different experimental cases for validation. Major properties such as droplet velocity, SMD, gas phase velocity and temperature can be reproduced with good accuracy. Also, the trends of flame lift-off height and flame width with change of coflow conditions were properly captured. Then the study was extended to four more virtual cases, which differ from each other only by coflow temperature or oxygen concentration. The purpose of this step is to eliminate the simultaneous change of multiple parameters as was done in the experiment and to isolate the influences of coflow temperature and O2 concentration, which are important parameters for a MILD furnace design and operation. Two reaction regions (RRs) have been identified in the DSHC flame. The inner RR is created by the premixed reaction of ethanol under hot and fuel rich condition. The outer one is formed by the non-premixed reaction of intermediate fuels, e.g. CO and H2, which are produced in the inner RR. The coflow temperature (Tcf) has a significant influence on the DSHC flames. Increase of flame lift-off height by a factor of five was observed when the coflow temperature was decreased from 1400 K to 1200 K. The flame changed from having a two-RRs structure to a triple flame when the coflow temperature is reduced. The outer RR shifted from non-premixed combustion at high Tcf to premixed combustion at low Tcf. The oxygen mole fraction in the coflow (XO2,cf) alters the role of two RRs. The inner premixed RR is strengthened with increasing XO2,cf. The flame peak temperature is significantly increased in the case with highest XO2,cf. According to Cavaliere and de Joannon’s definition [1], all studied cases except the one that has the highest XO2,cf, falls into the MILD regime. But when a restriction of flame peak temperature (Tpeak < 1800 K) is also applied, only the case with the lowest XO2,cf and moderate Tcf can be strictly called MILD.

Classification of the Mixing Field of Partially Premixed Flames Using Regime Diagram

ABSTRACTPartially premixed combustion is one of the common combustion modes for many practical combustion systems where the mixture is classified as nonhomogeneous. The available experimental data show that the nature of mixing affects significantly the flame stability and structure of partially premixed flames at the same level of turbulence and local stoichiometry. Quantitative description of the mixing field is not yet available in order to discuss and analyze the effect of mixing on the flame structure and stability. The broad range of fluctuations in mixture fraction space within this mode requires proper classification into sub-ranges with common characteristics. This classification should be applicable for both laminar and turbulent flames. Accordingly a new diagram classifying the mixing field of partially premixed combustion into eight regimes within and partially within the flammability limits is proposed. The local mean and range of mixture fraction fluctuations are used to construct this diagram. One of those regimes covers the triple flames where the fluctuations lie within the flammability limits. The link between the stability and the proposed regimes is investigated using quantitative experimental data of turbulent flames. The data show promising correlation between stability and location of the flames within the diagram. The proposed regime diagram can be used as a platform for the classification of the mixing field of partially premixed flames.

삼파장 4채널 적외선식 불꽃감지기의 자가진단 성능 개선

There is needed the quard channel triple wave lengths pyroelectric infrared flame detector to recognize the unique characteristics of fire sources in various type. This system detects the triple wave lengths of infrared flame emitting maximum radient energy and scattering frequency of flame. The quard channel infrared flame detector detects the duplicate wave lengths of maximum radient energy to enhance the accuracy of detecting fire. Especially this paper focuss on development of the self diagnosis function system including contamination, temperature and input voltage. Therefore, the prevention and early suppression of fire is available.

Response of Triple Flame to Acoustic Oscillations

Response of Triple Flame to Acoustic Oscillations

Flame balls in non-uniform mixtures: existence and finite activation energy effects

The paper's broad motivation, shared by a recent theoretical investigation [Daou and Daou, “Flame balls in mixing layers,” Combustion and Flame, Vol. 161 (2014), pp. 2015–2024], is a fundamental but apparently untouched combustion question; specifically, ‘What are the critical conditions insuring the successful ignition of a diffusion flame by means of an external energy deposit (spark), after mixing of cold reactants has occurred in a mixing layer?’ The approach is based on a generalisation of the concept of Zeldovich flame balls, well known in premixed reactive mixtures, to non-uniform mixtures. This generalisation leads to a free boundary problem (FBP) for axisymmetric flame balls in a two-dimensional mixing layer in the distinguished limit β → ∞ with εL = O(1); here β is the Zeldovich number and εL is a non-dimensional measure of the stoichiometric premixed flame thickness. The existence of such flame balls is the main object of current investigation. Several original contributions are presented. First, an analytical contribution is made by carrying out the analysis of Daou and Daou (2014) in the asymptotic limit εL → 0 to higher order. The results capture, in particular, the dependence of the location of the flame ball centre (argued to represent the optimal ignition location which differs from the stoichiometric location) on εL. Second, two detailed numerical studies of the axisymmetric flame balls are presented for arbitrary values of εL. The first study addresses the infinite-β FBP and the second one the original finite-β problem based on the constant density reaction–diffusion equations. In particular, it is shown that solutions to the FBP exist for arbitrary values of εL while actual finite-β flame balls exist in a specific domain of the β–εL plane, namely for εL less than a maximum value proportional to ; this scaling is consistent with the existence of solutions to the FBP for arbitrary εL. In fact, the flame ball existence domain is found to have little dependence on the stoichiometry of the reaction and to coincide, to a good approximation, with the domain of existence of the positively-propagating two-dimensional triple flames in the mixing layer. Finally, we confirm that the flame balls are typically unstable, as one expects in the absence of heat losses.

Gas phase flame structure of solid propellant sandwiches with different reaction mechanisms

The gas phase flame structure of composite solid propellant sandwiches with two ammonium perchlorate (AP) laminae sandwiching a middle lamina containing a homogenized mixture of hydroxyl terminated poly-butadiene (HTPB) binder and fine AP particles is numerically simulated. Experimentally obtained surface profiles and burning rates are used to decouple the gas phase from the condensed phase. In the literature, three global gas phase reactions for (1) the AP mono-propellant flame, (2) the primary diffusion flame, and (3) the final diffusion flame between gaseous products of AP decomposition/deflagration and the binder have been considered. Alternatively, multi-step detailed/reduced reaction mechanisms have been adopted to represent all these flames together. In the present study, three different reaction mechanisms are evaluated: (1) global chemistry using symbolic species for the three flames, (2) global chemistry using a 12-species mechanism for the above three flames, and (3) a 72-step reaction mechanism with 39-species representing all the flames. The 12-species, 3-step global reaction mechanism provides a temperature field similar to the 72-step mechanism unlike the global reaction mechanism involving symbolic species. Further, the triple flame structure of the primary diffusion flame and the ‘canopy’ premixed flame over the middle lamina (for a self-deflagrating mixture of fine AP and binder) are well captured by the 12-species mechanism unlike the other two considered.

Rich–Lean Flame Interaction in a Lamella-Type Burner

ABSTRACTAn expanded version of a practical rich–lean burner consisting of lamella geometry with lean flames in the central part and surrounded by two slits of rich flames (to prevent peripheral lean flames from lifting) is evaluated in the present study. The central part of this burner was studied in the search for an optimal geometry that maximized the stability of a methane-air lean flame, especially with regard to peripheral rich–lean flame interaction. For the burner central part, a slit thickness of 2 mm was found to be the optimum value for maximizing the overall burner stability of simple lean flames if the slit interspace was higher than 2 mm. This lean stability limit was then increased by about 9 ± 2% when the burner was operated with peripheral rich–lean flames. Detailed experimental characterization of the rich–lean flame interaction zone was performed for a lean flame of ϕL = 0.65, revealing the structure of a triple flame. In this rich–lean interaction, the lean flame was contaminated by diffusion of heat and radicals forcing the laminar flame speed to increase (improving overall burner stability) and, from chemiluminiscence signals, a change in the lean equivalence ratio could be quantified. An analytical expression was derived that showed how the apparent equivalence ratio of contaminated lean flames is linearly dependent on the equivalence ratio and velocity gradients as the driving forces of the mechanism of triple flame interaction. Experimental results validated the analytic expression, giving support to a holistic view of rich–lean flame interaction.

Rich spray-flame propagating through a 2D­Lattice of alkane droplets in air

In a recent numerical paper (Nicoli et al., 2014) [1], a model of isobaric flame propagation in lean sprays has been proposed. The initial state of the monodisperse mists was schematized by a system of individual alkane droplets initially located at the nodes of a face-centred 2D-lattice, surrounded by a saturated mixture of alkane and air. In the present study, the previous model is complemented with an original chemical scheme that allows us to study the combustion of rich alkane/air mixtures.The main parameters of this configuration ares, the lattice spacing (in reactive-diffusive length units), φL, the liquid loading (or equivalence ratio relative to the fuel under liquid phase), and φG (with φG ≤ 0.8), the gaseous equivalence ratio (i.e. that corresponding to the saturated vapour pressure in the fresh spray). We presently focus on sprays, the overall equivalence ratio of which is within the range 1≤(φL+φG)≤1.85.For a large set of parameters, we retrieve a feature often observed on the rich side in the experiments: flame propagation in the presence of droplets can be faster than the pure premixed flames with the same overall equivalence ratio. This is mainly observed when the lattice spacing is sufficiently large. However, the study underlines the role played by the velocities of two particular single-phase premixed flames: the “initial vapour flame” that only burns (if any) the mixture due to the saturated vapour and the “all fuel flame” that propagates (if any) in a mixture where all fuel is vaporised and mixed. When the “initial vapour flame” is too slow (i.e. a feeble spray Peclet number), the spray-flame speed results from the competition between two mechanisms: a speed chemically enhanced due to some enrichment coming from vaporisation (possibly bounded by the “all fuel flame” speed) and a slowing down in flame velocity because the vaporisation time scale sets the pace on combustion. On the other hand, for large spray Peclet number, the upper flammability limit is found to be strongly enlarged, and the spray-flame propagates with the velocity of the “initial vapour flame”.Moreover, the flame structure deeply depends on lattice spacing: for a large lattice, the combustion stage mainly corresponds to a triple flame, with the diffusion flame that develops around the oxygen pocket located behind the lean wing of the flame front (i.e. far from the droplets). On the other hand, as s decreases, this diffusion flame tends to be more and more incorporated into the flame front.

LES–CMC study of an excited hydrogen flame

The large eddy simulation/conditional moment closure approach has been used to study excited, attached and lifted flames in a stream of hot co-flow air. The excitation is obtained by adding a flapping forcing term with amplitude of 15% of fuel velocity and frequency corresponding to Strouhal numbers St = 0.175, 0.25, 0.325 to an inlet velocity profile. It is shown that such a type of forcing dramatically changes the global flame shape, resulting in a flame occupying very large volume or in bifurcating flames or even triple flames. We observed that the flame which is initially circular may transform downstream into a quasi-plane flame. The analysed cases include configurations with a small and large co-flow velocity. In the former case, the flame shape strongly depends on forcing frequency while in the latter substantial differences are seen only in an internal part of the flame. Both the lift-off heights of the flames and their spreading angles were found to be a function of the forcing frequency. The computational results are validated based on the solutions obtained for non-excited flames for which experimental data are available. The results obtained for various co-flow temperatures and velocities show that both auto-ignition times and lift-off heights are predicted correctly and exhibit trends observed experimentally.

Autoignition-affected stabilization of laminar nonpremixed DME/air coflow flames

The structure and stabilization mechanism of laminar nonpremixed autoignitive DME/air coflow flames were investigated. Computations were performed at 30 atmospheres with uniform inlet velocities of 3.2m/s for both streams, and the coflow air boundary temperatures were 700, 800, 900, and 1100K. The heat release rate and species profiles were examined for each case. Further investigation with Chemical Explosive Mode Analysis (CEMA) and Lagrangian Flamelet Analysis (LFA) were performed to identify the controlling chemistry and elucidate the dominant combustion mode and stabilization mechanism. At 700–900K, autoignition was observed to be the dominant stabilization mechanism, and NTC chemistry determines the stabilization point in mixture fraction space. Conversely, at 1100K, the kinematic balance between the premixed flame propagation velocity and the incoming flow velocity becomes the dominant stabilization mechanism, and the classical triple flame structure was observed. Extended stabilization regimes, in terms of increasing boundary temperature, are therefore identified, including frozen flow, kinetically stabilized, autoignition–propagation-coupled stabilized, kinematically stabilized, and burner stabilized regimes.

Coupling between hydrodynamics, acoustics, and heat release in a self-excited unstable combustor

The unsteady gas dynamic field in a closed combustor is determined by the nonlinear interactions between chamber acoustics, hydrodynamics, and turbulent combustion that can energize these modes. These interactions are studied in detail using hybrid RANS/large eddy simulations (RANS = Reynolds Averaged Navier-Stokes) of a non-premixed, high-pressure laboratory combustor that produces self-excited longitudinal instabilities. The main variable in the study is the relative acoustic length between the combustion chamber and the tube that injects oxidizer into the combustor. Assuming a half-wave (closed-closed) combustion chamber, the tube lengths approximately correspond to quarter-, 3/8-, and half-wave resonators that serve to vary the phasing between the acoustic modes in the tube and the combustion chamber. The simulation correctly predicts the relatively stable behavior measured with the shortest tube and the very unstable behavior measured with the intermediate tube. Unstable behavior is also predicted for the longest tube, a case for which bifurcated stability behavior was measured in the experiment. In the first (stable) configuration, fuel flows into the combustor uninterrupted, and heat release is spatially continuous with a flame that remains attached to the back step. In the second (unstable) configuration, a cyclic process is apparent comprising a disruption in the fuel flow, subsequent detachment of the flame from the back step, and accumulation of fuel in the recirculation zone that ignites upon arrival of a compression wave reflected from the downstream boundary of the combustion chamber. The third case (mixed stable/unstable) shares features with both of the other cases. The major difference between the two cases predicted to be unstable is that, in the intermediate length tube, a pressure wave reflection inside the tube pushes unburnt fuel behind the back step radially outward, leading to a post-coupled reignition mechanism, while in the case of the longest tube, the reignition is promoted by vortex convection and combustor-wall interaction. Other flow details indicated by the simulation include the relative phase between flow resonances in the tube and the combustor, increased mixing due to baroclinic torque, and the presence of an unsteady triple flame.