Flame Propagation Mechanism Research Articles

Study on the explosion mechanism of two-phase organic dust — Based on polypropylene/propylene hybrid explosions as an example

A two-phase organic dust explosion is more complex than a single dust or gas explosion and poses a more significant threat to industrial safety production. To elucidate the explosion mechanism of two-phase organic dust, this study investigated the characteristics of flame propagation in PP/propylene hybrid explosions and revealed the flame propagation mechanism. The research findings indicate that adding propylene makes the flame brighter and more continuous, and velocity and temperature increase significantly. The introduction of propylene shifts the control of flame propagation from a process dominated by pyrolysis explosion kinetics to one governed by premixed gas explosion kinetics. Dimensionless characteristic numbers Bi and Da were introduced further to analyze the control mechanism of the dust explosion process. It was found that the explosion process was mainly diffusion flame combustion and premixed gas phase combustion.

Flame propagation mechanism of methanol fuel spray explosion in a square closed vessel

Methanol, as an important chemical raw material and clean energy source, easily forms explosive droplet clouds during its production, processing, and usage. Therefore, understanding the evolution and propagation mechanisms of methanol spray explosion flames is crucial for its widespread use and the design of safety measures. This paper studies the flame propagation behavior of methanol spray explosions using a 16.2-L visualized enclosed vessel. The results indicate that the flame structure of methanol spray explosions is similar to that of premixed gas explosions but significantly differs from traditional fossil fuels. It demonstrates homogeneous combustion characteristics primarily governed by the kinetics-controlled regime. As the methanol spray concentration increases, both the maximum flame propagation speed and the maximum explosion pressure initially increase and then decrease, reaching their peak at a concentration of 224.68 g/m3, with values of 9.96 m/s and 0.72 MPa, respectively. The heat losses during combustion exhibit a trend opposite to that of explosion pressure. Numerical simulation results indicate that the concentrations of key radicals such as O, H, and OH vary significantly between lean and rich combustion states. The increase in H radical concentration enhances the elementary reactions that suppress flame temperature and promotes chain-terminating reactions.

Effect of grain boundary on the ignition and flame propagation mechanisms of TC11 alloy in oxygen-enriched environment

Grain boundaries (GBs) were known as a kind of common defects, but their impact on metal combustion was unclear. This study discovered that when the TC11 alloy's grain size reduced, the critical temperature and oxygen pressure for ignition dropped and the burning velocity accelerated. A modified Frank-Kamenetskii model demonstrated a well match to the relationship between grain size and ignition temperature, and the GBs showed a lower activation energy for ignition than the intragrain. The primary processes via which GBs influenced the kinetics of combustion were the peritectic reaction between Ti and O and the intergranular diffusion of alloying elements.

Investigation of hydrogen/air co-flow jet flame propagation mechanism in supersonic crossflow

The design of fuel injection schemes is crucial for improving the combustion performance of high-Mach number scramjet. To clarify the feasibility of the coaxial jet injection scheme, high fidelity Large Eddy Simulation of the supersonic coaxial jet flame is conducted. The simulations are in good agreement with the experimental results in terms of time-averaged velocity, temperature, and species distribution. Auto-ignition phenomenon and the characteristics of partially premixed flame are well captured. The introduction of co-flow air increases the vorticity magnitude close to the injection port and downstream near-wall region, which results in a 400 K rise in the time-averaged temperature on the downstream near-wall region and a 4% increase in the proportion of premixed combustion near the injection port. Moreover, the instantaneous distribution of hydroxy radical indicates that the spanwise width of the windward reaction shear layer is reduced utilizing the coaxial jet scheme. Chemical kinetic analysis is applied to reveal the propagation mechanism of partially premixed flames. Thermal explosion is the chemical explosion mode for all coaxial jet flame front, which are dominated by a high-temperature reaction path. The low-temperature reaction path mainly exists in the transverse jet injection port, downstream near-wall region of the single transverse jet and co-flow lifted flame base. These significant findings provide valuable insights for the potential engineering application of the coaxial jet injection scheme to a high Mach number scramjet.

Flame propagation dynamic characteristics and mechanisms of methane–syngas–air mixtures in a semi-enclosed duct

The composition of syngas is complex, often leading to the generation of methane, which impacts the explosive characteristics of syngas. The analysis focused on the kinetic characteristics and mechanisms of flame propagation in methane–syngas–air mixtures in a semi-enclosed duct under various equivalence ratios (φ = 0.8–1.4) and methane volume ratios (R = 0 %–70 %) under ambient temperature and pressure. The results indicated that the flame speed and pressure propagation of methane–syngas–air mixtures could be divided into three stages: a stable stage, an oscillatory increase stage, and an oscillatory decrease stage. In the stable stage, as the methane proportion in the premixed gas increased, the oscillations in the flame and pressure decreased. Turbulent flow of the mixed gas was observed due to restrictions and obstructions within the duct. This led to an increase in the peak pressure and flame speed of the mixed gas, thereby resulting in an oscillatory increase stage. Distorted tulip-shaped flames were also formed in the semi-enclosed duct. The flame of the pure syngas/air mixture exhibited a hemispherical shape and a finger-shaped and slope-shaped propagation structure. When 30 %, 50 %, or 70 % of the methane in the methane–syngas–air mixtures was added, the flame exhibited a propagation structure characterized by a hemispherical shape, finger shape, slope shape, flat shape, tulip shape, or distorted tulip shape. The flame front evolved with time from a linear growth stage to a nonlinear growth stage. The flame front speed of the methane–syngas–air mixtures was closely related to the pressure and flame structure. With the increase in the methane proportion in the methane–syngas–air mixtures, the concentration of key radicals and the rate of OH* generation and consumption gradually decreased. Sensitivity analysis revealed that with increasing methane proportion in the premixed gas, the most important reaction dominating OH* consumption changed from R15 (H + O2(+M) <=> HO2(+M)) to R138 (CH4 + OH <=> CH3 + H2O) and then to R112 (2CH3(+M) <=> C2H6(+M)); moreover, the most important reaction dominating OH* generation remained R1 (H + O2 <=> O + OH). The addition of methane interrupted the hydrogen–oxygen chain reaction of the methane–syngas–air explosion process.

Experimental Investigation and Theoretical Analysis of Flame Spread Dynamics over Discrete Thermally Thin Fuels with Various Inclination Angles and Gap Sizes

Flame spread over discrete fuels is a typical phenomenon in fire scenes. Experimental and theoretical research on flame spread over discrete thermally thin fuels separated by air gaps with different inclination angles was conducted in the present study. Experiments with six inclination angles ranging from 0° to 85° and various fuel coverage rates from 0.421 to 1 were designed. The flame spread behavior, the characteristic flame size, and the flame spread rate were analyzed. The results show that the flow pattern, stability, and flame size exhibit different characteristics with different inclination angles and gap sizes. As the inclination angle increases, particularly with smaller gaps, turbulent and oscillating flames are observed, while larger gap sizes promote flame stability. The mechanism of flame propagation across the gap depends on the interplay between the flame jump effect and heat transfer, which evolves with gap size. Average flame height, average flame width, and flame spread rate initially increase and then decline with the increase in fuel coverage, peaking at fuel coverage rates between 0.93 and 0.571 for different inclination angles. A theoretical model is proposed to predict the flame spread rate and the variation in the flame spread rate with inclination angle and fuel coverage. Furthermore, the map determined by inclination angle and fuel coverage is partitioned into distinct regions, comprising the accelerated flame spread region, the flame spread weakening region, and the failed flame spread region. These findings provide valuable insights into flame spread dynamics over discrete thermally thin fuels under diverse conditions.

Study on flame propagation characteristic and mechanism of polypropylene dust explosion

Polypropylene dust explosion poses a significant threat to the safe production of synthetic resin. To elucidate the flame propagation mechanism of polypropylene dust explosions, the flame propagation characteristics of polypropylene dust were experimentally studied. The minimum explosible concentration (MEC) for polypropylene particles with diameter < 75 μm, 75–100 μm, and 100–212 μm was 50 g/m3,100 g/m3,150 g/m3, respectively. Obvious fluctuations were observed in the flame propagation velocity of small particles, with smaller particles exhibiting more pronounced fluctuations, primarily caused by thermal feedback between the particles in the preheating zone and the reaction zone. While for large particles, the flame temperature exhibited multiple peaks, resulted from the discrete flame floating in front of the flame. The combustion process of polypropylene dust comprised the diffusion combustion of large particles and the gas-phase combustion of combustible gases pyrolyzed by small particles.

Effects of methane concentration on flame propagation mechanisms and dynamic characteristics of methane/coal dust explosions

The study explores the flame propagation and explosion dynamic behavior of gas/powder two-phase compound explosions, taking methane and coal dust mixture as the research object. The influence of different methane concentrations on the compound explosion characteristics was researched. The vertical pipeline, high-speed camera, ion current probe, and pressure sensor be used to test the propagation behaviors, microstructure characteristics, and explosion dynamics of compound explosion flames. The function of methane concentration on the propagation behaviors, combustion reaction zone thickness, flame temperature, propagation velocity, and explosion pressure of compound flame was revealed, and the propagation mechanisms of gas/powder compound explosion flames were explained. The experimental research discovered that the compound flame of methane/coal dust explosion presents a white luminous zone with uniform distribution and appears to have an approximate homogeneous combustion state. The explosion dynamics parameters all showed a trend of first increased and then decreased as the methane concentration increased, but the reaction zone thickness of the combustion first reduced and then increased. In addition, the dominant position of homogeneous or heterogeneous combustion in the combustion reaction zone will also switch with the change in methane concentration. The compound flame is similar to premixed gas combustion under optimal methane concentration, the combustion zone thickness (δ) and preheating zone thickness (β) are the smallest, and the compound explosion shows powerful performance. In addition, the flame propagation mechanisms of methane/coal dust compound explosion were elucidated based on the theory of heat.

An acceleration model for low-speed flames in curved channels

This study investigates the initial acceleration process of low-speed flames in curved channels. It also examines how the curvature and width of the channels influence flame propagation patterns and mechanisms. Previous research has highlighted the advantages of curved channels in facilitating flame acceleration. Subsequent studies have explored the interaction between pressure waves and flames, particularly in the later stages of flame propagation, leading to a high-speed flame phase. However, the effects of curvature and channel width on the initial acceleration phase of low-speed flames remain unclear. To address this gap, the present study utilizes theoretical methods to analyze the early acceleration phase of flames in curved channels with fixed curvature and width, from the closed end to the open end. Semi-analytical theoretical solutions are derived to determine the shape of the unburned gas flow field, flame front, and flame acceleration rate. The results demonstrate that the flow field shape deviates from axial symmetry to conserve angular momentum, favoring the inner wall of the curved channel and causing the flame front to adhere to it. In channels with small curvature, increasing the width leads to a decrease in the flame acceleration rate. Conversely, in channels with large curvature, widening the channel results in an increased flame acceleration rate. These findings provide theoretical insights for optimizing the design of the front-stage structure of curved detonation channels.

Effect of stearic acid coating on the flame propagation and reaction mechanism of AlH3

AlH3 is considered to be the most promising high-energy additive and stearic acid (SA) is a common passivation coating material to enhance stability. To explore the effect of coating of SA on AlH3, flame propagation and reaction mechanisms are compared through ignition experiments and ReaxFF molecular dynamics, respectively. The flame propagation of AlH3/SA slowed down for ∼20 ms and the flame duration is longer for ∼50 ms, indicating that the combustion intensity is reduced. The combustion reaction of AlH3 is initiated through the release of •H by H-Al breaking and further oxidizing to form H2O and Al-O clusters. The presence of SA results in more O and C atoms contained in the system and more reaction paths. The addition of C atoms and O atoms makes the maximum cluster molecular weight of AlH3/SA increase by 0.1 × 10^4 than AlH3. The results of flame propagation and reaction mechanisms verify that the stability of AlH3 is improved by the coating of SA.

Experimental investigation on ignition and cross-flame performance of dual-annular combustor

The ignition and co-flame process of the dual annular combustor(DAC) was studied using high speed camera in order to investigate the mechanism of flame stabilization and propagation in the combustor. Particle Laser Velocimetry (PIV) combined with high speed camera were used to measure the key properties of ignition kernels, including orientation, growth direction, movement velocity, flame propagation speed. Results showed the flow field structure obtained by PIV includes the primary air jet, the dilution air jet, the inner/outer recirculation zones, the central body jet, and the middle area of the central body. The ignition process could be categorized into the four stages: kernel generation at the end of the igniter, kernel movement at the recirculation zone, kernel growth at the head, and burner-scale flame establishment at the combustor, the flame had significantly different performance in each stage. The time of each stage decreases with the increase in the inlet Reynolds number, which significantly reduced the ignition delay time. Additionally, as the excess air coefficient increased, the ignition of pilot stage become more complex and the ignition delay and the cross-flame time increased. The relative location of the flame center and the centroid confidence interval of the flame path was used to judge ignition success. The flame path analysis showed that the transverse propagation mode of flame in the head is the determining stage of ignition. The splitting direction and the flame statement were related to the flame's position and shape in the flow field, the transverse flame splitting played a vital role in the ignition process. Two typical co-flame mechanisms existed in the cross-flame process of the DAC as follows: the flame tongue co-flame mechanism under small working conditions and the heat reflux combined small flame co-flame mechanism under large working conditions.

Inerting characteristics of ultrafine Mg(OH)2 on starch dust explosion flame propagation

Experiments on the flame propagation of starch dust explosion with the participation of ultrafine Mg(OH)2 in a vertical duct were conducted to reveal the inerting evolution of explosion processes. Combining the dynamic behaviors of flame propagation, the formation law of gaseous combustion products, and the heat dissipation features of solid inert particles, the inerting mechanism of explosion flame propagation is discussed. Results indicate that the ultrafine of Mg(OH)2 powders can cause the agglomeration of suspended dust clouds, which makes the flame combustion reaction zone fragmented and forms multiple small flame regions. The flame reaction zone presents non-homogeneous insufficient combustion, which leads to the obstruction of the explosion flame propagation process and the obvious pulsation propagation phenomenon. As the proportion of ultrafine Mg(OH)2 increases, flame speed, flame luminescence intensity, flame temperature and deflagration pressure all show different degrees of inerting behavior. The addition of ultrafine Mg(OH)2 not only causes partial inerting on the explosion flame, but also the heat dissipation of solid inert particles affects the acceleration of its propagation. The explosion flame propagation is inhibited by the synergistic effect of inert gas-solid phase, which attenuates the risk of starch explosion. The gas-solid synergistic inerting mechanism of starch explosion flame propagation by ultrafine Mg(OH)2 is further revealed.

Influence of vent size and vent burst pressure on vented ethylene-air explosion: Experimental and numerical study

This paper describes the numerical simulation of the vented explosion of ethylene-air mixtures. The influence of concentration, vent size, and vent burst pressure on the vented ethylene-air explosion is investigated by comparing simulation results and experiments. The different phases of external flame propagation are concluded by comparing the experimental photos and the simulation results. The external explosion process is captured by the high-speed camera and simulation. It is found that the unburned mixture expanded to the external atmosphere, which was ignited by the flame. The flow velocity, velocity vector, and turbulence kinetic energy are applied to analyze the mechanism of external flame propagation. The velocity vector is deflected to both sides of the flame front and the reverse flow velocity is captured in the external flow field. The change of ethylene concentration and the venting flow velocity is used to explain the relationship between the pressure rise, chemical reaction rate, and venting flow. Results show that the internal pressure for AV= 0.18 m2 is significantly greater than the internal pressure for AV= 0.55 m2 in this case. The smaller vent size results in higher flow velocity and turbulence kinetic energy inside of the vessel. The vent burst pressure growth promotes the acceleration of the flame front inside of the vessel. The variation curve of ethylene concentration and venting flow velocity has a similar law with different vent burst pressures. The experimental and simulated results can provide technical and theoretical support for the design of gas explosion prevention in the field of process safety and risk engineering.

Particle size influence on the aluminum combustion dynamics in 1-m&lt;sup&gt;3&lt;/sup&gt; chamber

Introduction. The results of a standard study of the explosibility of aluminum air suspensions (AAS) can contribute to the development of AAS combustion physics. In particular, a complex of information about the polydispersity and of the AAS low explosion limit values in a 1-m3 chamber made it possible to determine the maximum particle size of the explosive fraction of a polydisperse sample d*m,t ≈ 40–50 µm (Poletaev, 2014). In the present work, a relationship is established between the AAS combustion dynamics in a 1-m3 chamber and persion. The dispersity of sample particles is described by the mass-average particle size of its explosive fraction (d*50), in contrast to the works of other researchers who use the mass-average size of all particles (d50).Initial data. Known information about the dispersity and explosion parameters of 15 aluminum samples studied in a 1-m3 chamber was used. The continuous particle size distribution functions necessary for calculating d*50 were represented by the Rosin – Rammler distributions filling the gaps between the discrete data of the sieve analysis of the samples.Combustion dynamics. The dynamics of AAS turbulent combustion in a 1-m3 chamber is represented by the maximum air suspension burn-up rate Ub. Ub was calculated using the formula (Kumar, 1992) intended for gas-air mixtures by substituting the AAS explosion parameters into this formula.Results and its discussion. A plot of the d*50 Ub complex versus d*50 is shown. The average value of the complex (≈ 33 µm·m/s) is constant in the range 10 ≤ d*50 ≤ 35 µm. The latter is typical for the product of the particle size and the normal velocity of the laminar flame in AAS (Ben Moussa, 2017) and indicates the similarity of the effect of particle dispersion on the dynamics of turbulent and laminar combustion of AAS.Conclusions. The dispersion of an explosive polydisperse aluminum sample is determined by the average particle size of the explosive fraction of the sample d*50. The similarity of the combustion patterns indicates a relationship between the mechanisms of laminar and turbulent flame propagation in AAS.

Numerical analysis of relight in an annular spray-flame combustor with preheated walls

Successful relight of aero-engine combustors is subjected to certification specifications and must be ensured under various scenarios which can involve substantially different combustor wall temperatures. Unlike restart from windmilling, quick relight is likely to be performed with hot combustor walls, affecting the resulting ignition time. Few published studies exist which address wall temperature effects on ignition: for example, experiments in the annular spray-flame combustor MICCA with preheated walls have revealed that the ignition time—also referred to as light-round duration—decreases compared to cold combustor walls for otherwise same operating conditions. Numerical simulations of ignition have been carried out as well, but usually rely on approximations for the imposed wall temperature boundary condition (mostly adiabatic, iso-thermal at best) due to the lack of detailed experimental data. Therefore, the present work aims at studying the effect of preheated walls on light-round in more detail. Conjugate Heat Transfer simulations are first carried out to compute more realistic wall temperature profiles under steady reacting operating conditions in MICCA-Spray. These temperature profiles are then used in non-reacting Large-Eddy Simulations (LES) to establish preheated initial conditions. Finally, LES of light-round ignition with preheated walls are carried out. The predicted light-round duration is compared to experimental measurements, and effects on the governing flame propagation mechanisms as well as the liquid phase are examined. Differences with regard to light-round with ambient temperature walls are pointed out and the importance of an appropriate choice of boundary conditions is briefly discussed.

Effect of aluminum foam on pressure oscillation of premixed methane-air deflagrations

The effect of aluminum foam on the pressure oscillation of premixed methane-air deflagrations was investigated using experiments performed in an empty duct and a duct with aluminum foam attached to the upper and lower walls. For a fair comparison, the passage areas of the two ducts are identical when considering a 10 mm thickness of the porous medium. The flame propagation, pressure oscillation, and interaction mechanism between the acoustic wave and flame front are discussed by comparing the flame structure and pressure waveform during premixed methane-air mixture deflagration. The results demonstrate that for the empty duct configuration, the flame propagates as a cellular structure and then evolves into an oscillating flame when approaching the closed end of the duct. The coupling effect of the flame and acoustic waves induces the flame to oscillate violently as the distance from the ignition source to the closed end of the duct (i.e., the ignition position) increases to a threshold value. The magnitude of pressure oscillation increases as the distance between the ignition source and the closed end of the duct increases and reaches a maximum when the ignition position is 700 mm from the closed end. Additionally, the pressure oscillation is consistent with the flame oscillation. This oscillation results in a large deflagration pressure and a high rate of pressure rise. By contrast, the aluminum foam-laden duct completely suppresses the oscillation of the premixed methane-air deflagrations for all ignition positions. The aluminum foam inhibits the coupling effect and consequently restrains the prominent deformation and oscillation of the flame front. Flame oscillation vanishes as the acoustic damping coefficient increases. In this case, there is no oscillation and the leading position of the left flame exhibits a linear relationship with time. Whether the foam material has an inhibitive effect on the deflagration pressure depends on the ignition position. The aluminum foam enhances the deflagration pressure when the ignition position is 100 mm from the closed end, with a 29% increase in the maximum overpressure. By contrast, the aluminum foam attenuates the deflagration pressure for other ignition positions, with a maximum drop ratio of 61.7% at the ignition position being 700 mm from the closed end.

Explosion behaviors of vapor–liquid propylene oxide/air mixture under high-temperature source ignition

Propylene oxide (PO) is widely used in fuel–air explosive (FAE) and pulse detonation engines (PDE). The explosion process of PO/air mixture in a 20 L spherical container under high-temperature source ignition was studied through experiment and numerical simulation. A good agreement was observed between the experimental and simulation results. The ignition temperature (1100–2500 K) and initial droplet size (10–200 μm) had no obvious effect on the maximum explosion pressure (Pmax) at a nominal concentration of 130 g/m3. Explosion time (te) and reaction time (tr) of PO aerosol explosion increased with the increase in initial droplet size. The flame propagation process in the direction of 90° was not the same as that of 45°. Explosion flame propagated from the center to the surrounding area, then sagged along wall facing the center and presented a U-shape. Explosion time at ignition temperature of 1600 K was 29.63 ms, which was the shortest compared with the temperature of 2000 K, 2500 K, and 1100 K. The influence of ignition temperature on te was more obvious than that of initial droplet size. The flame propagation mechanism of droplet sizes of 10 μm and 50 μm was different from that of 100 μm and 200 μm. The time to reach the maximum flame temperature(tf) increased with the increase in droplet size; the flame propagation speed decreased with the increasing droplet size, and the maximum flame velocity can reach 6.07 m/s.

Flexible Polyurethane Foam에서 화염확산에 대한 발화위치와 샘플 두께의 영향

Fire behavior of flexible polyurethane foam (FPUF) at different sample thicknesses and ignition positioning was investigated. Effects on flame height, mass loss rate and other parameters were tested, and the flame propagation mechanism was analyzed. A method for predicting equivalent combustion diameter (D) values of the dynamic change of liquid pool at different positions is proposed. Combined with data of sample mass loss rate, flame height can be predicted. Based on a transition state model, a method for predicting the fire risk of FPUF in late stage combustion by calculating the generation time of polyols is proposed. With edge ignition, FPUF burning produces an inclined surface during the combustion process which enhances the length of the preheating zone by means of heat conduction and heat radiation. Flame spread rate (FSV) in FPUF with edge ignition was greater than with center point ignition.

Experimental investigation on the ignition dynamics of an annular combustor with multiple centrally staged swirling burners

The ignition behaviors of an annular combustor consisting of 16 centrally staged swirling burners are experimentally investigated in this work. This research is mainly focused on the light-round mechanism of burner-burner flame propagation. The swirling flow structure of the staged burner and the flow interaction between multiple burners in the annular combustor are well measured via the particle image velocimetry method. Two high speed cameras are applied to analyze the light-round process from the side view and the top view. The light-round time, ignition and extinction limits, flame propagating pattern, and dynamics of flame leading point are analyzed. Increasing the equivalence ratio, the light-round time decreases gradually. A more complicated “sawtooth” pattern of flame propagation is discovered during the burner to burner flame propagation, compared to that with non-staged burners. The trajectories of the flame leading points are moving in a “zigzag” pattern during the light-round process. The trajectories of the anti-clockwise leading point are near the inside wall, while the trajectories of the clockwise one are closer to the outside wall. For various equivalence ratios and airflow rates, the circumferential flame speeds of the clockwise flame front are constantly faster than the anti-clockwise one. In addition, the two flame speeds and their differences increase with larger equivalence ratio. These characteristics are very different from those in an annular combustor with non-staged burners.

Mechanism of self-ignition and flame propagation during high-pressure hydrogen release through a rectangular tube

The dynamic mechanisms of self-ignition and flame propagation during high-pressure hydrogen release through a rectangular tube were experimentally investigated using pressure records, flame detection and high-speed photographs. Experimental results show that the minimum burst pressure for self-ignition decreases with an increase in axial distance to the diaphragm and then remains at an almost constant value. The self-ignition onset at the same location of the tube exhibits a certain randomness even if the intensity of the shock wave produced in the tube is similar. Multiple ignitions were observed at the early stage of hydrogen release. They usually had difficulty to sustainably develop and were extinguished owing to oxygen deficiency. At a subsequent stage, the ignition kernel appears again and grows rapidly in the axial and radial directions, finally converging to a complete flame across the tube width. It was found that the radial growth rate of the flame was lower than the axial growth rate.