Flame Propagation Phenomena Research Articles

Decoupling pyrolysis and combustion of organic powders to determine the laminar flame speed

AbstractDetermining the laminar flame speed of dusts is far from straightforward. A strong dependency on the experimental setup and the data treatment's high complexity makes it a true challenge. This work compares three complementary experimental setups to measure the laminar flame speed of organic dust (here, cellulose): a modified Hartmann tube, a 20 L sphere, and a micro‐fluidized bed (MFB) burner. The first two consider the flame propagation phenomenon in its globality, which means that numerous steps are involved simultaneously (particle heating, pyrolysis, oxidation, radiative transfer, flame stretching), while the third one decouples pyrolysis and combustion, to focus mainly on the oxidation rate. An MFB was conceived to generate pyrolysis products and burn them in a laminar flame. Unstretched flame velocities determined with the first two setups were consistent and equal to 22.0 and 26.6 cm ∙ s−1, respectively. Using Silvestrini's equation, values ranging between 14.0 and 33.4 cm ∙ s−1 were obtained according to the dust concentration. With the MFB burner, the flame speed was much higher (135–155 cm ∙ s−1), due to the higher temperature of the fresh mixture and the fact that only the oxidation of the pyrolysis gases is considered. A numerical simulation (Chemkin) confirmed these results since the range 135 to 231 cm ∙ s−1 was obtained for equivalence ratios of 0.6 and 1.2, respectively. The discrepancy between the laminar flame speed determined in the sphere or in the tube and that obtained in the MFB highlights the significant influence of particle heating and pyrolysis during a dust explosion.

Numerical investigation on turbulent flame jet characteristics of a pre-chamber fueled with CH4/O2 and CH4/NH3/O2 mixture under different initial pressures

Ammonia is a very potential carbon-free alternative fuel with extremely low reactivity. The application of ammonia in internal combustion engines faces the risks of ignition failure, low burning rate, and poor combustion stability. Pre-chamber turbulent jet ignition (TJI), which can provide much more ignition energy, is a promising method to ignite ammonia fuel. However, the transient propagation process of pre-chamber jet flame has not been fully investigated yet. In this paper, the propagation characteristic of methane jet flame was numerically studied based on a constant volume combustion system. The supersonic jet flame propagation and Mach disk phenomenon were further analyzed with numerical methods. And the propagation characteristic of methane/ammonia mixed jet flame was investigated. According to the results, the formation and propagation process of methane jet flame has different characteristics at different stages. In High-speed development stage (Stage II) and Low-speed oscillation development stage (Stage III), the penetration distance of flame jet tip is almost proportional to the penetration time. However, the penetration speed of the two stages is significantly different. The change trend of the jet velocity at the outlet of orifice hole can also be divided into four stages: slow increasing, rapid increasing, gradual decreasing, and periodic oscillation. The periodic oscillation of the jet tip velocity and the jet velocity at the outlet of orifice hole is caused by the effect of forward and reverse shock waves. The calculation results of CH4/NH3/O2 mixture jet flame show that the addition of ammonia has significant effects on the flame propagation process in PC, the thermodynamic state of PC, and the propagation characteristics of jet flame. With the increase of ammonia proportion, the maximum pressure of PC gradually decreases and the pressure rise rate slows down. Meanwhile, the two-stage (the high-speed development stage and the low-speed development stage) characteristic of jet propagation process gradually disappears. And the addition of ammonia is more likely to destroy the two-stage characteristic under low initial pressure.

Numerical simulation of a continuously released ethanol spill fire in an inclined tunnel with longitudinal ventilation

Under the influences of the chimney effect and longitudinal ventilation, the risks of continuous spill fires in inclined tunnels are catastrophic with great complexity and uncertainty. In this paper, the spread and combustion behaviour of a continuous spill fire under the effect of longitudinal ventilation in an inclined tunnel is studied and compared with that in a horizontal tunnel through fire dynamics simulator (FDS) software. The critical time shrink point (SP) to reach the maximum combustion area (MCA), the change rates of the dimensionless combustion area A* and stable combustion area (SCA), the flame propagation speed, and the dimensionless burning rate W* values and heat transfer mechanisms of the combustion fuel layer, tunnel wall and flame smoke are analysed; the prediction equation between the dimensionless burning rate W* and the dimensionless wind speed V* of the upstream and downstream spill fire are identified. The results show that longitudinal ventilation leads to a nonlinear increase in the MCA, and the longitudinal slope leads to a great decrease in the critical time SP. In the stable combustion stage, the SCA in the downstream spill fire is greater than that in the upstream spill fire. Additionally, there are significant differences in the flame propagation phenomenon of spill fires. On the windward side, the maximum flame propagation speed of the upstream spill fire is 3.8 times that of the downstream spill fire; however, on the leeward side, the maximum flame propagation speed of the upstream spill fire is only 18% of that of the downstream spill fire. According to the prediction equation, with the increase in the dimensionless wind speed V*, the dimensionless burning rate W* decreases gradually; the dimensionless burning rate W* of the upstream spill fire is always greater than that of the downstream spill fire. The increase in thermal feedback caused by the inclined flame is the main reason for the significant difference in the spread of the spill fire in the ventilated inclined tunnel, causing the flame propagation speed of the −10% slope on the leeward side to gradually approach that of the −5% slope.

Experimental observation of turbulent jet ignition of pre-chamber with scavenging system for carbon dioxide diluted mixtures

The exhaust gas recirculation (EGR) method can suppress knock and improve the thermal efficiency of engines. But it will also deteriorate the combustion stability and engine power. Turbulent jet ignition (TJI) is a reliable ignition resource for improving ignition stability and burning rate. However, the residual productions in the pre-chamber will worsen the performance of the TJI. To this end, a self-designed pre-chamber with a scavenging system has been proposed. In this study, the ignition process and flame propagation phenomena under different EGR dilution ratios for H2/N2/O2 and CH4/N2/O2 mixtures were conducted in a constant-volume combustion chamber. The results suggested that the increase in EGR dilution weakens the influence of cellular instability and causes buoyancy instability, the latter of which could be mitigated by the passive TJI method. For the passive TJI mode, the exit time of the hot jet was delayed, and the turbulent flame speed decreased with the increase of EGR dilution ratio. Four ignition phenomena, namely jet re-ignition, flame buoyancy, re-ignition failure, and misfire, were distinctly identified. However, EGR tolerance cannot be extended by passive pre-chambers. Therefore, the pre-chamber with a scavenging system that can effectively extend the lean combustion tolerance with EGR dilution compared to SI and passive TJI was proposed. The effects of air and fuel injection quantities on ignition and flame propagation were investigated. The flame propagation velocity was positively related to the air injection mass, whereas an optimum fuel mass was required to achieve fast flame propagation. The EGR limit based on dual injections in the pre-chamber was obviously extended. Moreover, under near EGR tolerance conditions, a leaner fuel injection in the pre-chamber was required to realize successful ignition in the main chamber, as strong turbulence could cause high heat transfer loss with the cool unburnt mixture and suppress the occurrence of re-ignition.

Experimental investigation on the inhibition of flame retardants on the flammability of R1234ze(E)

Owing to environmental friendliness, R1234ze(E) is considered as an alternative refrigerant, but the flammability restricts its use and promotion. To reduce the combustible risk, nonflammable refrigerants can be blended, while there is a lack of research on mixture flammability. In this paper, the inhibition effects of three typical refrigerants R744, R1336mzz(E) and R1233zd(E) on the flammability of R1234ze(E) were experimentally investigated under the condition of 23 °C and 50% relative humidity. The flammable limits at different concentrations were measured and the critical suppression concentrations of the three retardants were determined as 0.41, 0.2 and 0.4 respectively, by which the parameters of fitting equations to their flammable limits were obtained. The inhibition mechanisms were briefly analyzed in terms of both physical and chemical effects, and typical flame propagation phenomena were compared with further analyses. A model for estimating critical suppression concentrations of ternary blends was established. The critical suppression lines of ternary blends R744/R1234ze(E)/R1336mzz(E) and R744/R1234ze(E)/R1233zd(E) were obtained, by which the nonflammable zones of the two ternary blends were preliminarily determined. The results have provided complements for previous studies using R744/R1234ze(E) at flammable concentrations and will contribute to utilize R1234ze(E) and its blends in safety for both scientific researches and industrial applications.

Evolution characteristics of vented gaseous ethanol-gasoline air explosions in a small cuboid channel

Vented explosion experiments with commercial E10 (10 vol% ethanol - 90 vol% gasoline) were carried out in an optically accessible vented 10 L, 100 × 100 mm2 cross-section channel. Liquid E10 was added to a channel preheated to 60 °C, and a plastic diaphragm initially covered the vent area located on the top wall near the end wall, at the opposite end of the spark plug. The measured peak overpressure and flame propagation phenomena were analyzed for different E10 liquid volumes and vent areas. As the volume of E10 added increased, both the inner and external peak pressure increased, but then decreased for larger amounts of E10 added. For a given amount of fuel, the pressure time-history was relatively smooth for the smallest vents, and highlighted by strong pressure oscillations for the largest vent openings. The peak channel pressure decreased monotonically with increased vent area because more of the fuel discharged and burned as a flame jet out the vent opening before it could combust inside the channel. The peak pressure and flame acceleration generated for gasoline only, measured for a single vent area, was only slightly lower than that produced with E10. From a process, storage and handling safety standpoint, this implies E10 can be treated similar to gasoline at 60 °C.

Numerical Investigation on the Performance of a 4-Stroke Engine with Different Passive Pre-Chamber Geometries Using a Detailed Chemistry Solver

Pre-chamber turbulent jet ignition represents one of the most promising techniques to improve spark ignition engines efficiency and reduce pollutant emissions. This technique consists of igniting the air-fuel mixture in the main combustion chamber by means of several hot turbulent flame jets exiting a pre-chamber. In the present study, the combustion process of a 4-stroke, gasoline SI, PFI engine equipped with a passive pre-chamber has been investigated through three-dimensional CFD (Computational Fluid Dynamics) analysis. A detailed chemistry solver with a reduced reaction mechanism was employed to investigate ignition and flame propagation phenomena. Firstly, the combustion model was validated against experimental data for the baseline engine configuration (i.e., without pre-chamber). Eventually, the validated numerical model allowed for predictive simulations of the pre-chamber-equipped engine. By varying the shape of the pre-chamber body and the size of pre-chamber orifices, different pre-chamber configurations were studied. The influence of the geometrical features on the duration of the combustion process and the pressure trends inside both the pre-chamber and main chamber was assessed and discussed. Since the use of a pre-chamber can extend the air-fuel mixture ignition limits, an additional sensitivity on the air-fuel ratio was carried out, in order to investigate engine performance at lean conditions.

Oscillating transient flame propagation of biochar dust cloud considering thermal losses and particles porosity

This research investigates the transient flame propagation and oscillation phenomenon in the flame speed of porous biochar dust cloud. Time-dependent mass, momentum and energy equations are solved in the spherical coordinate. The gas phase reaction includes the chemical reactions, thermodynamic properties, and multi-element transition properties. To account for the porosity effects of particles, the biochar dusts are modeled as spherical particles with unlimited number of pores (semi sphere) on the surface. The particle trajectory is governed by the equation of motion. The thermophoretic, gravitational, buoyancy and drag forces are employed in this model. In the energy equation, the absorption and radiation emissions by particles is considered. The results reveal that the inertia differences between the particles and gas causes a difference in the velocities of these two phases at the flame front—which is more evident at the early stages of flame propagation when there is a significant change in the density of dust particles. Moreover, the oscillation is further intensified by enhancing the oxygen concentration due to a higher reaction rate, and, as a result, higher velocity difference between the two phases.

Turbulent flame propagation of polymethylmethacrylate particle clouds in an O2/N2 atmosphere

The combustion of solid particle clouds is extensively used in many engineering areas. However, experimental data describing the turbulent flame propagation behavior and the combustion mechanism of solid particle clouds have remained limited. In this work, the combustion of polymethylmethacrylate (PMMA) particle clouds was studied by employing a unique fan-stirred constant-volume chamber. For the quasi-monodispersed particle clouds, the flame propagation velocity increased with the increase in the turbulence intensity and the decrease in the quasi-monodispersed particle size. However, the particle concentration had little effect on the flame propagation velocity, which is unique in a turbulent flow field. The consistency of the results between the current study of PMMA particle clouds and the previous study for coal particle clouds showed that the heterogeneous combustion of char particles had little effect on the turbulent flame propagation velocity of the solid particle clouds. Further, two types of quasi-monodispersed particles were mixed to study how the interactions between small and large (polydispersed) particles affect turbulent flame propagation. We found that the turbulent flame propagation velocity had a nonlinear relationship with the mass ratio of small particles (J-shaped curve). The turbulent flame propagation velocity slightly increased with low mass ratio of small particles, while it sharply increased with high mass ratio of small particles. Increasing the turbulence intensity and decreasing the primary particle (large particle) size can advance the starting point of the sharp increase. These unique features were explained by a mechanism considering the polydispersed interparticle interaction proposed by the authors. In the combustion of turbulent polydispersed particle clouds, the particle–particle agglomeration and the agglomeration break-up in the turbulent flow field affect turbulent flame propagation. To the best of our knowledge, this is the first report on the fundamental spherical turbulent flame propagation phenomenon and the mechanism of solid particle cloud combustion considering the polydispersed interparticle interactions.

Effect of ammonia/oxygen/nitrogen equivalence ratio on spherical turbulent flame propagation of pulverized coal/ammonia co-combustion

Because ammonia is one of the most promising candidates for energy carrier in the future, various applications of ammonia as a fuel are currently considered. One medium for utilizing ammonia is by introducing it to coal-fired boilers. To the best of our knowledge, this paper is the first to report the fundamental mechanism of the flame propagation phenomenon for pulverized coal/ammonia co-combustion. The effects of the equivalence ratio of the ammonia-oxidizer mixture on the flame propagation velocity of pulverized coal/ammonia co-combustion in turbulent fields were clarified by the experiments employing a unique fan-stirred constant volume chamber. The flame propagation velocities of pulverized coal/ammonia co-combustion, pure ammonia combustion, and pure pulverized coal combustion were compared. As expected, the flame propagation velocity of pulverized coal/ammonia was higher than that of the pure pulverized coal combustion for all conditions. However, the comparison of the flame propagation velocities of pulverized coal/ammonia co-combustion and that of the pure ammonia combustion, revealed that whether the flame propagation of the pulverized coal/ammonia was higher than that of the pure ammonia combustion was dependent on the equivalence ratio of the ammonia-oxidizer. This unique feature was explained by a mechanism including three competing effects proposed by the authors. In the ammonia lean condition, the positive effects, which are the strong radiation from the luminous flame and the increment of local equivalence ratio by the addition of volatile matter, are larger than the negative effect, which is the heat absorption by coal particles in preheat zone. In the ammonia rich condition, the effect of an increment of the local equivalence ratio by the addition of volatile matter turns into a negative effect. Consequently, the negative effects overcome the positive effect in the ammonia rich condition resulting in a lower flame propagation velocity of pulverized coal/ammonia co-combustion.

Flame propagation of corn starch in a modified Hartmann tube with branch structure

Branch structure is used widely in network of dust extraction systems for dust processing and collection. However, flame propagation mechanisms of dust explosion in branch pipes are rarely investigated. In this study, the Hartmann tube was modified with four branch configurations, and both the dust cloud motion behavior and corn starch flame propagation were studied. A high-speed camera was used and an image processing procedure was developed based on the YCbCr color model, by which the luminous intensity distribution was observed and the flame speed was calculated. The results indicate that only a small amount of dust enters the branch pipe during the dispersion process. A variety of flame propagation phenomena were observed, including vortex groups, smooth flame front, increased luminance area behind the front and backward flows, implying complex flame behaviors owing to the effects of branch configuration and primary pipe deflagration. The fluctuation of flame speed in the branch region implies that the deceleration of dust deflagration is reduced by the internal combustion in the branch pipes. The discrepancy of the average flame speed for four configurations indicates that the energy losses in the primary pipes result in the flame speed decline; meanwhile, according to the correlations of flame speed peaks between those in the branch pipes and the primary pipes, the flame propagation in the branch region is influenced not only by the internal combustion but also by the deflagration in the primary pipes.

A safe and clean way to produce H2O2 from H2 and O2 within the explosion limit range

The direct synthesis of hydrogen peroxide (DSH) from hydrogen and oxygen is an attractive production route due to its green nature. However, it faces multiple technical challenges, the biggest being the explosion risk of the flammable gas mixture. Herein we have used microreactors to perform the reaction in an inherently safer way which allows the hydrogen concentration to fall within the explosion limit range. For the first time, we have studied the flame propagation phenomena inside a microreactor to determine the optimum channel dimension for DSH. A mechanism of “fast synthesis and slow destruction” has been proposed via investigation on the influence of channel length and liquid flow rate. Besides, a variety of reaction parameters including gas flow rate, oxygen: hydrogen ratio, catalyst composition and gas pressure have been studied carefully. The successful employment of a microreactor in this case has indicated the potential of using microreactors to inhibit the explosion risks of hazardous processes.

Combustion Visualization of Partially Premixed and Non Premixed Diesel Fuel on Single Cylinder Optical Engine

Ignition and flame propagation phenomenon of diesel fuel combustion was investigated in an optical single cylinder engine. Conventional non-premixed diesel combustion and partially premixed combustion types were investigated. Experiments were performed on an optical single cylinder, heavy duty, four-stroke compression ignition engine with a quartz elongated piston bowl and 45° mirror was used for visualization as a measurement technique. . Self-ignition, flame propagation, heat rejection and effects on number of pilot injections on combustion visualization are investigated. In conventional diesel with closed and far pilot and partially premixed charge compression ignition (PCCI) tests were performed.

Parameters influencing the burning rate of laminar flames propagating into a reacting mixture

The laminar flame speed is an important property of a reacting mixture and it is used extensively for the characterization of the combustion process in practical devices. However, under engine-relevant conditions, considerable reactivity may be present in the unburned mixture, introducing thus challenges due to couplings of auto-ignition and flame propagation phenomena. In this study, the propagation of transient, one-dimensional laminar flames into a reacting unburned mixture was investigated numerically in order to identify the parameters influencing the flame burning rate in the conduction-reaction controlled regime at constant pressure. It was found that the fuel chemical classification significantly influences the burning rate. More specifically, for hydrogen flames, the “evolution” of the burning rate does not depend on the initial unburned mixture temperature. On the other hand, for n-heptane flames that exhibit low temperature chemistry, the burning rate depends on the instantaneous temperature and composition of the unburned mixture in a coupled way. A new approach was developed allowing for the decoupling the flame chemistry from the ignition dynamics as well as for the decoupling of parameters influencing the burning rate, so that meaningful sensitivity analysis could be performed. It was determined that the burning rate is not directly affected by fuel specific reactions even in the presence of low temperature chemistry whose effect is indirect through the modification of the reactants composition entering the flame. The controlling parameters include but not limited to mixture conductivity, enthalpy, and the species composition evolution in the unburned mixture.

Explosion characteristics of mildly flammable refrigerants ignited with high-energy ignition sources in closed systems

For evaluation of explosion scenarios in closed systems involving the mildly flammable refrigerants R1234yf, R1234ze and R32 dependent on the ignition energy, ignitions were carried out in a closed autoclave. A newly developed ignition system was used, which allows generating electric arcs with defined energies in a range between 3 J and 1000 J. The lower explosion limit of R32 decreases with increasing ignition energy. R32-explosions can be more severe than explosions involving highly flammable substances. However, in case of R1234yf and R1234ze, the ignition energy had to be increased to more than 100 J and more than 500 J to detect explosions in the closed system at all, although flame Propagation phenomena can already be observed if these substances are ignited with much weaker ignition sources in open glass tubes. The explosions were very mild with these substances.

Studies on accidental gas and dust explosions

Fires and explosions are the disasters caused by uncontrolled combustion phenomena. The flame propagation during gas and dust explosions is much faster than the flame spread during fire. Therefore, the serious damages by accidental explosions often expand widely in a very short time. In this paper, the gas and dust explosions are described from a viewpoint of flame propagation phenomena. The understanding of flame propagation phenomena is indispensable to perform the consequent analysis of accidental explosions. Basic matters and recent research results on the flame propagation during gas and dust explosions are explained.

Outward propagation velocity and acceleration characteristics in hydrogen-air deflagration

Propagation characteristics of hydrogen-air deflagration need to be understood for an accurate risk assessment. Especially, flame propagation velocity is one of the most important factors. Propagation velocity of outwardly propagating flame has been estimated from burning velocity of a flat flame considering influence of thermal expansion at a flame front; however, this conventional method is not enough to estimate an actual propagation velocity because flame propagation is accelerated owing to cellular flame front caused by intrinsic instability in hydrogen-air deflagration. Therefore, it is important to understand the dynamic propagation characteristics of hydrogen-air deflagration. We performed explosion tests in a closed chamber which has 300 mm diameter windows and observed flame propagation phenomena by using Schlieren photography. In the explosion experiments, hydrogen-air mixtures were ignited at atmospheric pressure and room temperature and in the range of equivalence ratio from 0.2 to 1.0. Analyzing the obtained Schlieren images, flame radius and flame propagation velocity were measured. As the result, cellular flame fronts formed and flame propagations of hydrogen–air mixture were accelerated at the all equivalence ratios. In the case of equivalent ratio φ = 0.2, a flame floated up and could not propagate downward because the influence of buoyancy exceeded a laminar burning velocity. Based upon these propagation characteristics, a favorable estimation method of flame propagation velocity including influence of flame acceleration was proposed. Moreover, the influence of intrinsic instability on propagation characteristics was elucidated.

Laminar flame speeds of nano-aluminum/methane hybrid mixtures

An existing flame speed bomb was utilized to study the fundamental phenomena of flame propagation through a uniformly dispersed aerosol with a relatively small mass of metal nanoparticles. The aerosol was dispersed with a burst of gas and then allowed to settle for at least 45s, to ensure that the conditions inside the test chamber were quiescent and uniform. Extinction of the light from a HeNe laser was used so that the mass of suspended nano-particles with a fundamental size of 100nm could be determined as a function of time prior to combustion, and the suspended mass of aluminum (up to 90mg) was measured in situ during an experiment. A particle size distribution was measured as well, resulting in an average pre-combustion agglomeration size of 446.1nm. Two series of experiments were performed with CH4 fuel, both at stoichiometric conditions: one with the mixture in air and the second with the mixture in a 70/30 N2/O2 mix. The results herein show a maximum decrease in flame speed, 5–7% for the baseline mixture, when nano-aluminum was introduced at the suspended masses utilized in the present study. For the 70/30 N2/O2 mixture, the aluminum resulted in a maximum decrease of 5cm/s from the baseline value of 80.5cm/s; and in the air mixture, a 2cm/s maximum decrease from 35.3cm/s was observed. Simple modeling of the process extremes indicates that the observed decrease in flame speed is somewhere between the limits of simple particle heating with no particle reaction and the kinetics limit where the Al reacts in the gas phase. Interestingly, the trends qualitatively matched those of the kinetics model applied herein. It was also found that the addition of nanoparticles caused the flame to become unstable much sooner when compared to the baseline mixture of CH4/air.

Ethylene–air mixtures under flowing conditions: a model-based approach to explosion conditions

Propylene oxide, ethylene oxide (EO), methanol, and phthalic anhydride are examples of versatile, widely applied chemical intermediates, produced at elevated temperature and pressure conditions, demanding rigorous safety considerations. Well-known industrial applications in which ethylene at the vapour phase is oxidized with oxygen are the manufactures of vinyl acetate and of EO. Partial oxidation of ethylene is usually performed at elevated temperature and pressure in multitubular cooled reactors where the application of explosive limits experimentally obtained under stagnant conditions could entail a not justified economical handicap. Bearing in mind these considerations, in this paper, we developed a novel physical–mathematical model to predict the ignition and flame-propagation phenomena in the presence of gaseous explosive mixtures. The explicit formulae for the ignition condition and the transition from local reaction to the fully developed explosion were obtained by exploring a broad range of operative conditions. A fairly good agreement was found between the predictions in this study of the oxygen critical concentration corresponding to the explosion point and the results of previous experimental studies performed by different researchers.

Ignition, flame propagation and extinction in the supersonic mixing layer flow

The supersonic mixing layer flow, consisting of a relatively cold, slow diluted hydrogen stream and a hot, faster air stream, is numerically simulated with detailed transport properties and chemical reaction mechanisms. The evolution of the combustion process in the supersonic reacting mixing layer is observed and unsteady phenomena of ignition, flame propagation and extinction are successfully captured. The ignition usually takes place at the air stream side of braid regions between two vortexes due to much higher temperature of premixed gases. After ignition, the flame propagates towards two vortexes respectively located on the upstream and downstream of the ignition position. The apparent flame speed is 1569.97 m/s, which is much higher than the laminar flame speed, resulting from the effects of expansion, turbulence, vortex stretching and consecutive ignition. After the flame arrives at the former vortex, the flame propagates along the outer region of the vortex in two branches. Then the upper flame branch close to fuel streamside distinguishes gradually due to too fuel-riched premixed mixtures in the front of the flame and the strong cooling effect of the adjacent cool fuel flow, while the lower flame branch continues to propagate in the vortex.