Flame Front Velocity Research Articles

The experimental study on deflagration dynamics of premixed flame in T-type tube

ABSTRACT To further investigate the flame dynamics inside the special tubes, experiments were carried out in a T-type tube to investigate the effect of the equivalence ratio and the opening ratio on the deflagration of the methane-air premixed mixture. The changes of flame structure, propagation velocity and overpressure during flame propagation are analyzed. The experimental results shown when equivalence ratio was 1.0 and the outlets was fully open, the mixture burned completely, the flame propagation velocity was the highest, it taken only 51 ms for the flame to spread to the exits at both ends and the flame shape was more symmetrical. Under the same equivalence ratio, the increase of the opening ratio would cause the flame propagation velocity to increase, but the overpressure would decrease. When the opening ratio was 0.25, the resistance encountered during the flame propagation process was the largest, and the turbulence at the fork also increased, the interaction between the two caused the maximum overpressure of 0.58 bar at this time. The coupling effect of overpressure and flame front velocity could also clearly show that the two were positively correlated, and the velocity of the flame propagating to the left and right ends of the horizontal tube was symmetrical. When the equivalence ratio was 1.0, as the opening ratio increases, the time to reach the maximum overpressure peak shown a linear downward trend. These findings add to our knowledge of the deflagration dynamics of special tubes.

Experimental study of the dissociation of a double gas hydrate with a change in the initial height of the layer

In this work, experimental studies of the dissociation of methane-ethane hydrate with and without combustion are carried out with a change in the initial height of the layer. Gas hydrate powder with an initial height of 3 and 15 mm was used. In the samples, the initial concentration of methane is 64% and ethane is 36%. An increase in the initial height of the powder layer led to a 1.3-fold increase in the flame front velocity. Due to the increase in the initial height of the layer, the dissociation rate decreases by 7.2 times before combustion and by 4.1 times during combustion.

Влияние пористой полиуретановой перегородки на гидравлические характеристики потока и на распространение фронта пламени в открытом канале

The purpose of the research was to experimentally study the influence of a porous polyurethane partition on the flow parameters and on the flame front propagation in front of the partition upon initiation at the open end of the channel. The flame front velocities were determined in a hydrogen-air mixture at atmospheric pressure in a channel open on both sides with an inner diameter of 20 mm. The ratio between hydrogen and air was varied, so that the molar excess of hydrogen varied from 0.3 (ultra-poor mixture) to 1.0 (stoichiometric composition). The pore size varied within 0.3--2.5 mm, which corresponded to the number of pores per inch in the range of 10--80. The coefficients of hydraulic resistance of a polyurethane partition with an open pore type, depending on the size of the pores and the thickness of the partition, were calculated from the pressure loss when the air flow passes through the porous element. The flame front velocity was determined using the shadow method and high-speed survey. Findings of the research show that the optimal value of the thickness of the porous partition, after which the polyurethane partition begins to show filtration properties, in most cases is approximately equal to twice the pore size and the drag coefficient depends linearly on the total thickness of the partition. In this case, the deficit of the flow velocity with increasing thickness increases according to a power law. A qualitative correspondence between the flame front velocity in front of the porous partition and the flow velocity deficit is shown

Effect of coal dust parameters on gas-coal dust explosions in pipe networks

To study the effects of coal dust particle size and mass of coal dust on gas-coal dust hybrid explosions, we conducted sedimentary gas-coal dust explosion experiments using a pipe network system that included multiple components and individual interactions, which were independently designed and constructed. Experimental conclusions were then theoretically verified based on factor response surface theory. The results demonstrated that when the particle size of coal dust was constant, the pressure peak, flame front velocity peak, and flame front temperature peak of the explosion initially increased and then decreased with increasing mass. When the mass of coal dust was constant, the pressure peak, flame front velocity peak, and flame front temperature peak of the explosion also initially increased and then decreased with increasing particle size. The mass and particle size values of the coal dust were 25 g and 48–52 μm, respectively, and under these conditions, the explosion was the most violent. The effects of mass on the pressure peak and flame front temperature peak were greater than the particle size of the coal dust, and the effect of particle size on the flame front velocity was greater than the mass of the coal dust.

Effects of cloud size and built-in obstacles on hydrogen cloud explosion using large eddy simulation

The purpose of this work is to evaluate hydrogen cloud explosion using large eddy simulation. The LES model combined with acoustical theory is firstly established and validated. Then the effects of cloud scale on flame dynamics and explosion pressure are explored. Finally, the effects of obstacle-induced turbulence on hydrogen cloud explosion are analyzed. The research works indicated that the LES model combined with acoustical theory could be able to reproduce flame acceleration and explosion pressure evolution. Maximum explosion pressure and maximum flame front velocity continue to increase with increasing cloud scale. The deviation values of flame radius of four cloud scales are 0.25 m, 0.5 m and 0.75 m, once beyond the critical value, the flame front velocity of larger cloud scale is lower than that of smaller cloud scale. The increase rate of flame front velocity and explosion pressure, not their peak value, will be certainly increased by built-in obstacles, which could be attributed to the couplings of turbulent vortex and moving flame.

Influence of polyurethane foam on flame front propagation of hydrogen-air and acetylene-air mixtures in an open channel

The motivation for this work is to ensure explosion safety when working with flammable gases. One of the passive methods for suppressing combustion of gases, consisting of the use of porous materials, is considered in this work. The propagation of a flame front of hydrogen-air and acetylene-air mixtures when passing through polyurethane foam was experimentally studied. To reduce the velocity of the unburned mixture ahead of the flame front, the initiation was carried out at the open end of the channel. In this event, the foam skeleton has a general effect on the propagation of the flame and not on the flow of the unburned mixture. The flame front velocity was determined with a high-speed camera using the shadow method. It was shown that the flame front does not pass through polyurethane foam containing 45 and 80 pores per inch (ppi). However, the use of foam with 10 and 30 ppi resulted in an acceleration of the flame front, up to 200 m/s for acetylene-air and 550 m/s for hydrogen-air mixtures. It was shown that the relative increase in the velocity of the flame front varied exponentially depending on the length of the foam. The exponent was determined for both mixtures.

Experimental study of explosion dynamics of syngas flames in the narrow channel

This article introduced the experimental study of the propagation of a syngas premixed flame in a narrow channel. The structural evolution, flame front position and velocity characteristics of lean and rich premixed flames were investigated at different hydrogen volume fractions as the flame was ignited at the open end of the pipe and propagated to the closed end. The comparative study of the syngas fuel characteristics, flame oscillation frequency and overpressure oscillation frequency prove that the syngas explosion flame oscillation in the narrow passage has a strong coupling relationship with overpressure and fuel heat release rate. The results was shown that the flame structure was strongly influenced by the hydrogen volume fraction of the syngas and the fuel concentration. The distorted tulip flame only appears in lean mixture. At 30% of hydrogen volume fraction, the flame exhibits intense and unstable propagation, manifested as the reciprocating and alternating movement of the flame front. As the volume fraction of hydrogen increased, the velocity of flame propagation and the frequency of oscillation increased. When the hydrogen volume fraction γ ≥ 0.4 at the equivalence ratio of Φ = 0.8, the pressure oscillation amplitude gradually increases and reaching the peak after 200–320 ms. Significantly, when γ = 0.3, the pressure peak increases abnormally. This work can provide support for the safe use of syngas in industry by experimental study of various explosion parameters in the narrow channel.

Retracted : Effect of coal dust parameters on gas‐coal dust explosions in pipe networks

To study the effects of particle size of coal dust and mass concentration of coal dust on gas-coal dust explosions, we conducted sedimentary gas-coal dust explosion experiments using a pipe network system that included multiple components and individual interactions, which were independently designed and constructed. Experimental conclusions were then theoretically verified based on factor response surface theory. When the particle size of coal dust was constant, the maximum explosion pressure, flame propagation velocity, and flame temperature of the explosion initially increased and then decreased with increasing mass concentration. When the mass concentration of coal dust was constant, the maximum explosion pressure, flame propagation velocity, and flame temperature of the explosion also initially increased and then decreased with increasing particle size. The mass concentration and particle size values of the coal dust were 100 g/m3 and 50 μm, respectively, and under these conditions, the explosion was the most violent. The effects of mass concentration on the maximum explosion pressure and flame temperature were greater than the particle size of the coal dust, and the effect of particle size on the flame front velocity was greater than the mass concentration of the coal dust. The conclusions obtained in this study provide a generalized theoretical basis for the effects of coal dust on hybrid gas-coal dust explosions.

Experimental investigation on the effect of obstacle position on the explosion behaviors of the non-uniform methane/air mixture

Gas explosions in the underground coal mines where the obstacles exist can cause significant damage. A current knowledge gap in methane safety research concerns the effect of obstacle position on the explosion process of non-uniform mixtures. In this work, the effect of obstacle position on the explosion behavior of non-uniform methane/air mixture is investigated experimentally. The results show that the triple flame structure appears during the combustion of the non-uniform mixture, which appears delayed with increasing obstacle position. Then, broken flocculent turbulent flames occur when the obstacle is set at 400 and 600 mm, with the strongest turbulent disturbance response at 400 mm case. Moreover, the results show that the maximum flame front velocities and the maximum values of overpressures are obtained at the obstacle position of 400 mm in both diffusion times. Additionally, two obvious pressure peaks are found, which appear at increasing intervals as the distance between the obstacle and the ignition source increases. Meanwhile, the maximum overpressure values ranks as Pmax(400 mm) > Pmax(200 mm) > Pmax(600 mm). The study of gas explosion flame characteristics that meet the actual working conditions of coal mines plays an important role in the prevention of explosion accidents.

The role of flow confinement on turbulent kinetic energy transfer across premixed flames

The interaction between turbulent flows and premixed flames remains a significant topic of interest for improved modeling efforts and the advancement of clean and efficient combustion systems. Due to the challenging nature of the problem, the majority of studies related to turbulence-flame interactions have been conducted with unconfined numerical simulations or unconfined experimental burners. This paper describes the effects of flow confinement on the dynamics of turbulent reacting flows. An experimental premixed bluff-body combustion facility is operated under confined and unconfined conditions to understand the subsequent impact on the evolution of the turbulent reacting flow field across the flame. High-speed particle image velocimetry and chemiluminescence diagnostics are simultaneously employed to identify the flame front and capture turbulent velocity fields. The results show that the unconfined flame follows the classical description of turbulence evolution across the flame, in which turbulent fluctuations are dampened on the product side of the flame. In contrast, the opposite trend is observed for the confined configuration, and turbulent fluctuations are augmented by the reaction. The difference between the two cases is attributed to flow expansion and mean pressure gradients, which alters the balance between flame-generated turbulence and viscous dampening effects. Ultimately, the results provide a new perspective of premixed turbulent reacting flows and simultaneously highlight the importance of acquiring data in environments that better replicate practical combustion systems.

Experimental analysis of suppression effects of crosswise arranged porous iron-nickel materials on the natural gas explosion in utility tunnels

Natural gas pipeline in utility tunnels has been leading to great concern because of potential hazards characterized by flammability and explosives, which could pose a grave threat to the safe operation of utility tunnels and even surrounding facilities. Therefore, revealing the dynamic process of natural gas explosion in utility tunnels is quite important for the consequence analysis and risk assessment, especially in such complex scenarios with various facilities. In this study, a kind of porous material with low cost (porous iron-nickel materials) was selected and crosswise arranged to investigate its explosion-proof effects on the natural gas explosion in utility tunnels. The results demonstrate that porous iron-nickel materials have obvious suppression effects on the gas explosion process. The maximum overpressure decreases from 0.3898 Mpa to 0.2778 Mpa and the maximum flame front velocity decreases from 66.67 m/s to 37.51 m/s. It is also found that the inhibition effect of porous materials on explosion overpressure and fame front velocity increase with the increase of the number of holes in porous materials. This study is helpful to consequence assessment and risk analysis of natural gas explosion accidents in utility tunnels.

Numerical study of premixed gas explosion in a 1-m channel partly filled with 18650 cell-like cylinders with experiments

Abused and defective Li-ion cells can cause a catastrophic failure of a Li-ion battery (LIB), leading to severe fires and explosions. In recent years, several numerical and experimental studies have been conducted on the explosion hazard related to the vented combustible gases from failed Li-ion cells. Experimentally quantifying fundamental properties for failing LIBs is essential for understanding safety issues; however, it can be costly, time-consuming, and can be partly incomplete. Computational fluid dynamic (CFD) simulations have been an essential tool for studying the risk and consequences in the process industry. In this study, the prediction accuracy of the open-source CFD combustion model/solver XiFoam was evaluated by comparing numerical simulations and experiments of premixed gas explosions in a 1-m explosion channel partly filled with 18650 cell-like cylinders. The prediction accuracy was determined by calculating the mean geometric bias and variance for the temporal pressure evolution, maximum pressure peak, positive impulse, spatial flame front velocity for two different channel geometries, in addition to two gas compositions at several fuel-air equivalence ratios. From this method, the XiFoam model/solver gave an overall acceptable model performance for both geometries and gas composition.

Promoting of foam copper with chaotic distributed pores on the deflagration of premixed hydrogen-air flame in the tube

The deflagration flame of stoichiometric hydrogen-air mixtures is studied in this paper. Combined with the foam copper structure, the deflagration characteristic is analyzed in this paper. The experimental results show that the foam copper can improve the flow instability at the flame front and the degree of turbulence in the propagation process, the flame propagates more rapidly in turbulence state, and the number of stages of flame morphology during the propagation process in-creases. The classic ?tulip? flame can be transformed into a distorted ?tulip? flame and a fractal ?tulip? flame before it collapses. When the flame passes through the foam copper, the flame front velocity increases as the number of structural layers increases. The flame front velocity propagates at supersonic speed through the accumulation of three layers of foam copper. The instability of overpressure in the propagation process will cause oscillation. More layers of the structure, the oscillation frequency, and the amplitude of the overpressure are increased significantly. Foam copper structure has a reverse action on over-pressure. When the overpressure value is low in the early stage, the structure promotes the propagation, but the overpressure value is large in the later stage, the structure has a blocking effect.

Effects of Fe/Si Stoichiometry on Formation of Fe3Si/FeSi-Al2O3 Composites by Aluminothermic Combustion Synthesis

Aluminothermic combustion synthesis was conducted with Fe2O3–Al–Fe–Si reaction systems under Fe/Si stoichiometry from Fe-20 to Fe-50 at. % Si to investigate the formation Fe3Si/FeSi–Al2O3 composites. The solid-state combustion was sufficiently exothermic to sustain the overall reaction in the mode of self-propagating high-temperature synthesis (SHS). Dependence of iron silicide phases formed from SHS on Fe/Si stoichiometry was examined. Experimental evidence indicated that combustion exothermicity and flame-front velocity were affected by the Si percentage. According to the X-ray diffraction (XRD) analysis, Fe3Si–Al2O3 composites were synthesized from the reaction systems with Fe-20 and Fe-25 at.% Si. The increase of Si content led to the formation of both Fe3Si and FeSi in the final products of Fe-33.3 and Fe-40 at.% Si reaction systems, and the content of FeSi increased with Si percentage. Further increase of Si to Fe-50 at.% Si produced the FeSi–Al2O3 composite. Scanning electron microscopy (SEM) images revealed that the fracture surface morphology of the products featured micron-sized and nearly spherical Fe3Si and FeSi particles distributing over the dense and connecting substrate formed by Al2O3.

Simulation of a premixed explosion of gas vented during Li-ion battery failure

Several fires and explosions caused by Lithium-ion batteries (LIBs) have been reported in the last decades. If a LIB is thermally, electrically, or mechanically abused, it can cause a catastrophic failure of the LIB. Flammable gases such as hydrogen, carbon monoxide, and methane can be released from the LIB during a catastrophic failure. When these gases mix with air, they form a combustible mixture, which may cause explosions and fires. This study presents a computational fluid dynamic (CFD) method for simulating an explosion from gases vented from failing LIBs using only open-source software. The code mech2Foam was used to generate the required CFD input parameters for a Li-ion vent gas composition, which was then used in several CFD simulations of an explosion in a 1-m rectangular channel. The simulation results were compared with experimental results as a validation of the method and the CFD model. The CFD simulation results agreed well with the experimentally measured flame front position and the maximum explosion pressure. However, discrepancies between experiments and simulations were observed in the temporal evolution of the pressure and flame front velocity.

Dissociation and Combustion of a Layer of Methane Hydrate Powder: Ways to Increase the Efficiency of Combustion and Degassing

The interest in natural gas hydrates is due both to huge natural reserves and to the strengthened role of environmentally friendly energy sources conditioned by the deterioration of the global environmental situation. The combustion efficiency increase is associated with the development of understanding of both the processes of dissociation and combustion of gas hydrates. To date, the problems of dissociation and combustion have, as a rule, been considered separately, despite their close interrelation. Usually, during combustion, there is a predetermined methane flow from the powder surface. In the present paper, the combustion of methane hydrate is simulated taking into account the non-stationary dissociation process in the powder layer. Experimental studies on the methane hydrate dissociation at negative temperatures have been carried out. It is shown that due to the increase in the layer temperature and changes in the porosity of the layer over time, i.e., coalescence of particles, the thermal conductivity of the layer can change significantly, which affects the heat flux and the dissociation rate. The flame front velocity was measured at different external air velocities. The air velocity and the vapor concentration in the combustion zone are shown to strongly affect the combustion temperature, flame stability and the flame front velocity. The obtained results may be applied to increase the efficiency of burning of a layer of methane hydrate powder, as well as for technologies of degassing the combustible gases and their application in the energy sector.

Elucidating the dominant mechanisms in burn rate increase of thermite nanolaminates incorporating nanoparticle inclusions

It was experimentally found that silica and gold particles can modify the combustion properties of nanothermites but the exact role of the thermal properties of these additives on the propagating combustion front relative to other potential contributions remains unknown. Gold and silica particles of different sizes and volume loadings were added into aluminum/copper oxide thermites. Their effects on the flame front dynamics were investigated experimentally using microscopic dynamic imaging techniques and theoretically via a reaction model coupling mass and heat diffusion processes. A detailed theoretical analysis of the local temperature and thermal gradients at the vicinity of these two additives shows that highly conductive inclusions do not accelerate the combustion front while poor conductive inclusions result in the distortion of the flame front (corrugation), and therefore produce high thermal gradients (up to 1010 K.m−1) at the inclusion/host material interface. This results in an overall slowing down of the combustion front. These theoretical findings contradict the experimental observations in which a net increase of the flame front velocity was found when Au and SiO2 particles are added into the thermite. This leads to the conclusion that the faster burn rate observed experimentally cannot be fully associated with thermal effects only, but rather on chemical (catalytic) and/or mechanical mechanisms: formation of highly-stressed zones around the inclusion promoting the reactant mixing. One additional experiment in which physical SiO2 particles were replaced by voids (filled with Ar during experiment) to cancel the potential mechanical effects while preserving the thermal inhomogeneity in the thermite structure confirms the hypothesis that instead of pure thermal conduction, it is the mechanical mechanisms that dominate the propagation velocity in our specific Al/CuO multilayered films.

Influence of porous walls on flame acceleration and flame front perturbations in methane-air and acetylene-air mixtures

The patterns of flame front propagation in methane-air and acetylene-air gas mixtures in a half-open channel with a porous inner coating were studied. Polyurethane with open pores was used as a porous coating on the upper and lower internal walls of the channel. The molar excess of methane in the mixture was 0.8 and 1.0, and the molar excess of acetylene in the mixture was 0.6, 0.8 and 1.0. Pore sizes ranged from 0.3 mm to 2.5 mm. Using shadow photographs, the sizes of disturbances λ1 and λ2 generated at the flame front were determined. The sizes were divided into two groups, λ1 ~ 5–20 mm and λ2 ~ 1–6 mm. A comparison of the disturbances with the estimated values of the Darrieus-Landau instability was carried out. The flame front velocities were determined depending on the composition of the mixture and the pore size of the coating: 5–100 m/s for methane-air mixtures and 10–1200 m/s for acetylene-air mixtures. An estimate of the Reynolds numbers in a channel with smooth walls and a porous coating was carried out. Based on the readings of the piezoelectric pressure sensors, the pressures generated by the accelerating flame front were determined: 0.7–7 kPa for methane-air mixtures and 0.007–1.5 MPa for acetylene-air mixtures.

The Effect of Initial Conditions on the Laminar Flame Front Velocity in Gas Mixtures

In this paper, we consider the scatter of the laminar flame front velocities caused both by an error in the composition of the combustible mixture and by artificial initial disturbances. It is shown how the configuration of the initial perturbations of an initially flat front of a laminar flame in a gas mixture with the constant composition affects the velocity spread of expanding spherical flames. The effect of the error in the composition of the combustible mixture on the parameters that determine the flame front velocity was analyzed from the literature data on the spread of the flame propagation velocity. These parameters were recalculated for the possible variation in the mixture composition obtained based on the data on the accuracy of the equipment used in the experiments.

Evaluation of Steam-Exploded Wood Pellets Storage and Handling Safety in a Coal-Designed Power Plant

Solid biofuels are currently used in increasing volumes to replace fossil equivalents. Next to the traditional wood-derived fuels, the advancement of biomass upgrading technologies like steam explosion (SE), followed by pelleting, further broadens the variety of materials on the market. This thermochemical and mechanical upgrading improves greatly the general properties of such fuels (homogeneity, energy density, water resistance, biological decomposition, grindability, pneumatic transport, etc.) and at the same time offers the possibility for co-production of green chemicals/biorefinery processing. However, these alternative SE-material pellets have different properties from the fossil counterparts with regard to dust formation, dielectric, ignitability, and explosive characteristics, hence posing an increased risk of fire and explosion in handling, transport, and storage of the biosolid fuels. These divergent properties need to be properly addressed before their large implementation in the industry. To shed more light on the variability of such material properties, several batches of SE pellets varying in bulk density and subjected to different handling and storage conditions were evaluated. This was done using a number of standardized equipment and following industrial standard measurement methodologies to compare steam-exploded material with standard white wood pellets and reference coals. The main body of the work focused on the explosivity of dust generated in the handling of the pellets, studied using a Hartmann tube, coupled with a high-speed camera to evaluate the sensitivity [minimum ignition energy (MIE) and minimum explosive concentration (MEC)] and the severity maximum flame front velocity (FFVmax) of explosions of the generated SE-pellet dust. Thermogravimetric analyses (TGA) was used to measure and compare the ignitability of dust layers of SE-pellet dust vs coal and as a function of co-mixture. Finally, the SE-pellet self-heating behavior was addressed using the standard “basket method” and compared with that of a commercial coal sample. The results show that the SE pellets produce very low levels of dust even after extensive weathering, hence limiting the explosion potential. The MIE and MEC of SE-pellet dusts exhibited similar values compared to those of the parent raw biomass material; however, the FFVmax revealed lower values when compared to that of the parent raw biomass. Also, the self-heating propensity is very low as the pellets are basically immune to biological degradation, while the spontaneous ignition temperature is high and in the range of nonhazardous bulk materials.