Flame Propagation Velocity Research Articles

Effect of hydrogen addition on the dynamics of premixed C1–C4 alkane-air flames in a microchannel with a wall temperature gradient

The effect of hydrogen (H2) addition on the flame dynamics of premixed C1–C4 alkane/air mixtures in a microchannel is investigated using a detailed-chemistry model through two-dimensional numerical computations. A detailed computational study have been performed in a 2 mm diameter tube with 120 mm length and a wall temperature gradient along the axial direction of the channel. The numerical simulations are carried out for various stoichiometric hydrocarbon (HC)/H2 mixtures at 0.15 m/s mixture inlet velocity. Flame repetitive extinction and ignition (FREI) flame pattern has been identified for all the fuel mixtures at these channel wall and mixture flow conditions. CH4/air mixture shows a higher HRR than C3–C4 alkane/air mixtures. Flame residence time in microchannel increases with increase in hydrogen addition percentage for all the three hydrocarbon/air mixtures considered in the present study. A non-monotonic behavior of FREI frequency is identified for CH4/air mixture, whereas it decreases monotonically for C3H8/air and C4H10/air mixtures with H2 addition. The amount of HRR and flame propagation velocity decreases with increase in H2 addition for lower-alkanes/air mixtures. The flame bifurcation effect is observed for CH4/air mixture, which disappears due to H2 addition in the mixture. The bifurcation effect is not present for other hydrocarbon/air mixtures investigated in the present study. The addition of H2 in the mixture enhances the flame stability of hydrocarbon/air mixtures in the microchannel.

Explosion suppression flame and mechanism of energetic dust with distinct morphologies: Aluminum-containing metal as a typical

A series of explosion suppression experiments on aluminum powder with diverse morphologies were carried out in vertical pipelines. By combining the flame propagation images and flame temperatures, the physical and chemical effects of inhibitors on the flame propagation of spherical aluminum powder (S Al ) and flaky aluminum powder (F Al ) were explored. The results demonstrated that the amount of inhibitor required for F Al was greater than S Al , and the chemical effects of piperazine pyrophosphate (PPAP) were more effective in inhibiting S Al , whereas the physical effects of nano-SiO 2 were more significant in suppressing F Al . PPAP dramatically reduced the flame propagation velocity, flame temperature, and flame propagation maximum height of S Al as the inhibitor mass fraction increased. The flame temperature, ignition delay time, and flame illumination of F Al could even be considerably changed by nano-SiO 2 . The microscopic and macroscopic suppression mechanisms of PPAP and nano-SiO 2 on different morphology of aluminum powder were proposed. • PPAP displays superior performance in inhibiting spherical aluminum powder. • Flake aluminum powder explosion suppression is more difficult than spherical powder. • Nano-SiO 2 can significantly affects the explosion of flake aluminum powder. • Microscopic and macroscopic suppression mechanisms of PPAP and nano-SiO 2 are studied.

Effect and mechanism of Aluminium hydroxide and Magnesium hydroxide powder on flame suppression of flour explosion

ABSTRACT In order to systematically study the suppression effect of Aluminum hydroxide[Al(OH)3] and Magnesium hydroxide[Mg(OH)2] on the explosion of flour, Hartmann experiment system was used to carry out the explosion flame propagation experiment, and the influence of adding different proportions of Al(OH)3 and Mg(OH)2 powder on the morphology and propagation speed of flour combustion flame was studied. Combined with the thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) curves of Al(OH)3 and Mg(OH)2 powder, the inhibition mechanism of Al(OH)3 and Mg(OH)2 powder on flour explosion was analyzed. The results showed that with the increase of Al(OH)3 and Mg(OH)2, the flame propagation speed gradually decreases. After adding 40%wt Mg(OH)2, when t = 182 ms, the flame reaches the top of the glass tube. When 60 wt% Mg(OH)2 and 60 wt% Al(OH)3 powder were added, the maximum flame propagation velocity of flour decreased from 13.86 m/s to 5.78 m/s and 1.62 m/s, respectively, and Al(OH)3 was superior to Mg(OH)2 in inhibiting flame propagation. Both inhibitors can inhibit flour combustion by adsorption of H and OH radicals.

Optimized two-step (OTS) chemistry model for the description of partially premixed combustion

Once properly optimized, single-step chemistry models are able to recover essential characteristics of flames such as burned gases temperature, flame propagation velocity and thickness. Ignition delay values can be reproduced as well. However, an improved description of either the premixed flame inner structure or the ignition process requires the consideration of the multi-step nature of chemical kinetics. Indeed, single-step chemistry models can neither describe properly the internal structure of laminar premixed flames nor the development of ignition processes. These limitations arise because single-step chemistry does not take into account intermediate species, the importance of which is essential. Thus, the present work is aimed at extending the recently-proposed optimized single-step (OSS) framework to multi-step chemistry. To this purpose an unbranched chain reaction with only two consecutive steps relevant to chain initiation and chain termination is considered. The corresponding kinetics model does involve two fictive species, the characteristics of which are optimized together with the two-step chemical scheme parameters, e.g., pre-exponential factors and activation energies. One of this fictive species is relevant to combustion products while the other represents the whole pool of radicals. The chemistry optimization procedure makes use of a well-defined in-house genetic algorithm and the resulting optimized two-step (OTS) model is assessed through comparisons made with data obtained from detailed chemistry computations used as reference. For similar (and even lower) CPU costs, the computations performed with the OTS model put into evidence significant improvements compared to the results obtained with the OSS description.

A comparative study on aluminum dust explosion suppression by powder inhibitors

ABSTRACT An aluminum dust explosion is a major safety hazard that cannot be ignored. It is of great practical significance to investigate explosion flame propagation characteristics and powder inhibition mechanisms of aluminum dust. Based on a self-designed dust explosion experimental system, explosion flame propagation characteristics such as flame shape, temperature and velocity were captured. The effect of aluminum particle size on the flame propagation was explored, and inhibition performances and mechanisms of NaHCO3, CaCO3 and Na2HPO4 on aluminum dust explosion were experimentally and theoretically revealed. The proportion of white bright areas decreased from 17.9% to 7.9%, 8.0% and 8.8% respectively, and the temperature and velocity decreased obviously with powder inhibitors. Especially, NaHCO3 can reduce the maximum flame temperature and maximum flame velocity to 1411°C and 14.27 m/s respectively, and its inhibition performance was optimal. CaCO3 had better performance than Na2HPO4 in suppressing the flame propagation velocity and in delaying the ignition time, and Na2HPO4 was more advantageous in suppressing the explosion flame temperature. It is explained by the mechanism that substances containing Na and P can play an important role in the suppression of gas-phase reactions. The Na cycle (NaO ↔ Na) is effective in scavenging free radicals.

Explosion inhibition of coal dust clouds under coal gasification atmosphere by talc powder

During coal gasification, the explosion intensity of coal dust was enhanced and more difficult to inhibit due to the presence of CO and H2. To mitigate the risk of dust explosion under a coal gasification atmosphere, the inhibition performance of talc powder (TP) on coal dust explosion was investigated by using a 20 L spherical explosive vessel. The results showed that the explosion intensity of dust clouds was attenuated with the inhibition of TP. As the particle size of TP decreased, the explosion pressure gradually was reduced and the explosion time was extended. The maximum explosion pressure of dust clouds decreased from 0.845 to 0.270 MPa after the addition of 8000 mesh TP, representing a reduction of 68 %. And the flame propagation velocity was consequently dropped from 1.22 to 0.17 m/s. Moreover, the explosion of higher rank coal dust was more easily inhibited by TP. XPS analysis demonstrated that the C–C and C–O–C in coal dust were effectively protected under the effect of TP. And the inhibition mechanism of TP on coal dust explosion was elaborated thoroughly in combination with the analysis of explosion products.

Research on quenching performance and multi-factor influence law of hydrogen crimped-ribbon flame arrester using response surface methodology

The effects of initial conditions, structure parameters, and expansion chamber structures on the flame quenching performance of hydrogen crimped-ribbon flame arrester were experimentally studied based on the self-built experimental apparatus. The influence law of different working conditions on flame propagation velocity (Vf) and explosion pressure (Pf) entering the flame arrester and the corresponding flame quenching results were obtained. The results indicated that the Vf and Pf showed a trend of first increasing and then decreasing with increasing the hydrogen concentration (C0 = 10%-50%). The Vf and Pf were gradually increased with increasing the initial pressure and ignition energy, resulting in the obvious increase of flame quenching failure probability. However, the decrease in porosity and the increase in element thickness resulted in an obvious increase in the flame quenching success probability. Meanwhile, the expansion chamber, as an important influencing factor, could obviously affect the flame quenching results by affecting the flame propagation velocity and explosion pressure entering the narrow channels. By comparing the effects of three expansion chambers on flame quenching performance, the influence extents of different expansion chamber structures were determined (the extended type expansion chamber > the baffle type expansion chamber > the conventional type expansion chamber). Then the multi-factor prediction models of Vf and Pf under different initial conditions and structure parameters were established based on the Response Surface Methodology (RSM) respectively, and the extents of each influencing factor and their interactions on Vf and Pf were determined. Besides, the multi-factor prediction models of the critical flame propagation velocity (Vc) and explosion pressure (Pc) under different structure parameters were also established respectively. It was found that Vc and Pc were only related to the flame arrester structure parameters, and the critical flame quenching criterion was proposed.

Flame Propagation Characteristics of Hybrid Explosion of Ethylene and Polyethylene Mixture under Pressure Accumulation

In order to study the flame propagation characteristics of a ethylene/polyethylene hybrid explosion under pressure accumulation, a visual pressure-bearing gas/power hybrid-explosion experimental platform was built. The flame propagation characteristics of polyethylene and ethylene/polyethylene hybrid explosions in the closed vessel were analyzed. The results show that the flame brightness, flame front continuity and average flame propagation velocity of polyethylene dust explosion in the closed vessel increased first and then decreased when the polyethylene dust concentration increased. The curve of the flame propagation velocity with time had obvious pulsation characteristics. Adding 1% and 3% ethylene to different concentrations of polyethylene dust significantly improved its explosion flame brightness, flame front continuity and average flame propagation velocity. Moreover, it also improved the fluctuation amplitude of the explosion flame propagation velocity with time curve. The explosion flame of the polyethylene dust and ethylene/polyethylene hybrid mixture included four zones during the propagation process, which were denoted as the unburned zone, preheated zone, premixed flame zone and dust flame zone. The addition of ethylene to polyethylene dust can significantly increase its thickness of premixed flame zone and preheated zone, and the thickness increased when the ethylene concentration increased.

Flame inhibition of coal dust with different particle sizes by Al(H2PO2)3 and Al2O3

To effectively mitigate coal dust/syngas deflagration accidents caused by leakage in a coal gasification scenario, inhibition experiments of flame propagation were conducted in a vertical combustion tube equipped with a high speed camera. And the inhibition performance of Al2O3 and Al(H2PO2)3 on deflagration of coal dust with different particle sizes was further investigated. The results showed that as the particle size of coal dust decreased, the flame intensity was enhanced first and then weakened. The flame propagation velocity and average degree of pulsation were the largest for 300 mesh coal dust, which were 23.90 and 7.32 m/s, respectively. After adding 80% Al(H2PO2)3 and Al2O3 to 200 mesh coal dust, the flame peak velocity decreased by 50% and 25%. And the flame was quenched and cannot propagate continuously under the inhibition of Al(H2PO2)3. The product analysis indicated that Al(H2PO2)3 generated PH3 and PO2 to dilute combustible substances in the reaction center and released heat-resistant phosphates to form an inorganic coating layer. Furthermore, the excellent inhibition performance of Al(H2PO2)3 was attributed to the scavenging of highly reactive radicals needed for combustion by P-containing radicals.

Study on the influence of obstacle on the flame behaviors of methane/air in a semi-confined pipe with wire mesh installed

Studying the coupling effect of obstacle and fire-resistant structure on the propagation of premixed flame is an effective measure to improve the flame quenching theory and the prevention measures for gas explosion. To evaluate the coupling effect of the obstacle and the fire-resistant structure, this article focuses on the influence of the characteristics of the methane/air flame in a semi-confined pipe when the wire mesh and the obstacle are coexisting. In this study, an experimental system and a geometric model with multi-holes obstacle and wire mesh were constructed, and the methods of experiment and numerical simulation were adopted. The results show that the installation of obstacle will promote the flame propagation process, and the wire mesh will be ineffective under severe combustion conditions. When the obstacle is installed, the flame front position will be raised, and the velocity of flame propagation will increase. In the process of the flame propagating to the bottom of the obstacle, the reflux stage and the “turbulent vortex” flame in this stage vanish under the influence of the obstacle. In addition, the changes of flow field in the pipe are mainly influenced by the upward propagation of unburned gas and reflected waves, and the turbulent vortexes in the flow field are symmetrical in cross section. In the process of flame passing through obstacle, under the coupling effects of Rayleigh-Taylor instability, entrainment and wrinkled flame, the flow field becomes turbulent and the process of flame propagation ends earlier.

Study on the Influence of Vent Shape and Blockage Ratio on the Premixed Gas Explosion in the Chamber with a Small Aspect Ratio.

Given the current situation of deflagration caused by gas leakage for domestic use and the study of the single shape of the vent, the differential change law of vent parameters on the small aspect ratio of the chamber explosion flame evolution and explosion overpressure and other effects were experimentally investigated. Based on the theory of geometric similarity, the experimental platform of a small length–diameter ratio explosion chamber was designed and built, and the premixed gas explosion experiment was carried out by changing the shape (square and rectangle) and blockage ratio (0.1, 0.3, 0.5, 0.7, and 0.9). The explosion flame structure, flame front position, flame propagation speed, explosion pressure waveform, overpressure peak value, and so on were tested and analyzed. The results showed that the blockage ratio had the most significant effect on flame propagation and explosion flame evolution. With the increase of blockage ratio, the stretching degree of the flame became more and more obvious, the flame front became sharper and sharper, and the sharp flame changed from the lower part to the upper part. The position of the flame front increased rapidly, and the steepness and peak value of explosion overpressure became larger. The oscillation of the flame propagation velocity was more violent after the turn, and the speed of propagation to the outside of the chamber gradually accelerated. In the same blockage ratio, there was a difference in the time point at which the flame propagation velocity turned under the rectangular and square shape of the vent. In blockage ratios of 0.1, 0.3, 0.5, and 0.9, the peak overpressure reduction under the shape of the blast square relative to the rectangle was 41.3, 47.9, 1.03, and 27.6%. This indicated that the explosion relief effect of the square was better than that of a rectangle. The research results can provide a reference and basis for the reasonable deployment of explosion venting and explosion decompression work.

The effects of built-in obstacles on methane-air explosion with concentration gradients: An experimental research

To investigate the influence of different opening ratios of the circular-shaped obstacles in the presence of concentration gradient, experimental investigations on explosion characteristics of premixed methane-air mixtures were carried out in a square explosion pipeline. During the combustion process, the acceleration of the flame propagation velocity was accelerated. The combined effect of obstacles and concentration gradient enhanced the instability of the flame, resulting in the fluctuation of pressure peak time. The presence of obstacles affected the flame propagation velocity more significantly than the presence of concentration gradient. Affected by the concentration interface and heat transfer efficiency, explosion characteristic parameters of flame propagation velocity and explosion overpressure measured at the opening ratio of 40% decreased with the increase of concentration gradient, while those measured at 50% and 60% first increased and then decreased, and the maximum explosion parameters were reached with the concentration gradient of 1.0%, and the trend is similar to that under the circumstance of various concentration gradients without obstacles.

Experimental research on hydrogen/air explosion inhibition by the ultrafine water mist

This experimental study focused on the inhibition of ultrafine water mist on hydrogen explosion inside the closed vessel. The inhibition law and mechanism were studied through changes of explosion intensity, flame propagation velocity and temperature under different mist concentrations. Results indicate that flame propagation and pressure rise inside the closed vessel were corresponding. Explosion intensity was reduced after adding mist, which was mainly manifested in the reductions of explosion pressure and flame propagation velocity. Flame was accelerated to extinguish and the inhibition effect was enhanced with increasing mist concentration. However, the explosion prussure did not present obvious reduction as the mist concentration reached a certain value. Besides, it indicates that the absoption heat effect of ultrafine water mist was an important factor on hydrogen explosion inhibition by the reductions of flame temperature and propagation velocity. The inhibition effect was mainly attributed to the combination effect of physical and chemical inhibitions.

Numerical simulation of methane-hydrogen-air premixed combustion in turbulence

3D modeling is based on an experimental platform named turbulence constant volume combustion chamber (CVCC). The turbulence model applies the realizable k-ε model. The combustion model applies the premixed combustion model. Simulation of methane-hydrogen-air turbulent premixed combustion is realized using the model. The initial condition is T0 °C = 298 K, P0 = 1 bar and the gas equivalence ratio is φ = 1. The characteristics of flame are analyzed by modifying the turbulence intensity (uRMS = 0 m/s, 0.7 m/s, 1.3 m/s, 2.0 m/s)and hydrogen fraction (R (H2) = 0%, 10%, 30%, 50%). It is calculated that in the process of spherical flame combustion and propagation, the pressure increase rate increases first and then decreases. Turbulence intensity and hydrogen fraction have similar effect on the flame propagation process. If the turbulence intensity is used as an independent variable, the flame propagation rate increases with its increase. When the flame radius is 30 mm, the flame propagation velocity of 2.0 m/s turbulence intensity is about 2.7 times that of laminar flow. Due to which both the maximum combustion pressure and the maximum pressure rise rate are monotonically increasing. As it increases, the combustion pressure rises and falls the fastest. As the R (H2) increases, the flame propagation rate can significantly increase. When R (H2) is 50%, the maximum velocity is 4.82 m/s, which is about 1.5 times that of the maximum of pure methane. When R (H2) is high, the greater the maximum pressure is, the shorter the time for the maximum pressure to occur will be. When R (H2) is 50%, the maximum pressure is about 9 bar, which is 2.25 times of the force of pure methane combustion.

Flame Propagation Characteristics of Syngas-Air in the Hele-Shaw Duct with Different Equivalence Ratios and Ignition Positions.

In this paper, the effects of different ignition positions and equivalence ratios on the explosion characteristics of syngas in a half-open Hele-Shaw duct were investigated. The ignition points are set at distances of 0 and 500 mm from the closed end. Moreover, the research range of equivalence ratio is 0.8–1.2. The experimental results indicate that different ignition positions and equivalence ratios influence the flame front structure and the dynamic characteristics of flame propagation. When the ignition position is at the closed end, the flame front undergoes several typical propagation stages before eventually reaching the open end of the duct. The time required by the flame to reach the open end decreases as the equivalence ratio increases. Meanwhile, when the ignition is in the middle of the duct, the flame simultaneously spreads to the open and closed ends. The time required to reach both sides decreases with the increase in the equivalence ratio. The flame front structure and pressure are primarily affected by the ignition position and the equivalence ratio. At the same ignition position, flame propagation velocity and maximum overpressure increase with the equivalence ratio. The pressure oscillation becomes more intense when the ignition position is close to the open end. At IP500, when the equivalence ratio is 0.8, multiple finger-shaped flame fronts emerge, accompanied by high-frequency flame oscillations. This study can provide guidance for the study of the flame propagation characteristics of syngas in millimeter-scale burners.

Analysis of steady and oscillating flames fueled by biomass particles and syngases considering two-step pyrolysis and heterogeneous and homogeneous reactions

Under unprecedented environmental crisis associated with greenhouse gas emission, biomass has attracted a great deal of attention due to renewable and carbon neutral nature. In this study, the premixed combustion of various types of wood and its derived syngases are examined for steady and oscillating sates. For this purpose, the poplar, birch, beech and pin sawdust woods and syngases composing of H2, CH4 and CO are considered. To model dust cloud combustion, a novel and comprehensive flame structure consisting of drying, two-step pyrolysis and homogeneous and heterogeneous reactions is proposed. Afterward, the governing equations and their appropriate boundary conditions are derived and solved analytically-numerically. The oscillating combustion is also modeled by exerting an external perturbation on the velocity field. The results indicate that due to the occurrence of heterogeneous reactions in wood combustion, the flame propagation velocity of wood is higher than that of syngases which contributes to high oscillations amplitude of syngases. When the mixture initial temperature changes between 300 and 550 K, the flame velocities of woods and syngases vary in the ranges of 0.4–0.7 m/s and 0.1–0.27 m/s, respectively. The maximum amplitude of temperature oscillation of syngases is approximately 8 times more than that of woods.

Flame propagation behaviours and explosion characteristics of Al[sbnd]Mg alloy dust based on thermodynamic analysis

The ignition sensitivity and explosion hazard of AlMg alloy dust were systematically studied in this paper. The flame propagation and explosion characteristics of AlMg alloy were studied experimentally. Then the activation energy of the reaction was studied and the explosive products were analysed. The experimental results show that. Smaller particle sizes of AlMg alloy dusts are more sensitive to ignition and are more likely to explosion. The explosion flame propagation velocity gradually decreases with increasing particle size, and the flame propagation acceleration of small particle size of AlMg alloy dust gradually increases. Smaller particle size AlMg alloy dusts have higher explosion pressure and higher explosion risk. Due to the smaller particle size the activation energy of the AlMg alloy dust is smaller. Therefore, AlMg alloy dust with small particle size has a faster chemical reaction rate, is more prone to explosive reactions and is more dangerous.

Effects of aluminum particle size distributions on the explosion behaviors of hydrogen/aluminum dust hybrid mixtures

The effects of aluminum particle size on the explosion behaviors of hydrogen/aluminum dust hybrid explosion were studied. The results show that due to the large specific surface area of small particles, the Pex and the (dP/dt)ex increased significantly with the decrease of aluminum particle size. The maximum value of the (dP/dt)ex can reach 239 MPa/s. When the particle size of aluminum dust decreased from 56.18 μm to 5.07 μm, the flame propagation displacement changed from double peaks to a single one. The actual burn time can be fitted as a function of initial hydrogen concentration. The maximum flame propagation velocities increased significantly with the decrease of aluminum particle size. When 56.18 μm and 30.82 μm aluminum dusts were burned in hydrogen/air mixtures, the expanded porous oxide layers appeared in the residues, while there was no porous oxide layer in the explosion residues of 5.07 μm aluminum dust.

Effect of hydrogen addition on the explosion characteristics of methane-hydrogen-air mixture in T-shaped bifurcation pipe

ABSTRACT Adding a certain proportion of hydrogen into the natural gas pipeline network is an effective way to solve the economical transportation problem of hydrogen. However, due to the special physical and chemical properties of hydrogen, there will be great potential safety hazards in the process of hydrogen transportation in the pipe network. In this paper, we studied the effect of adding hydrogen into T-tube on the explosion characteristics of methane hydrogen air premixed gas through experiments and numerical simulation. The results showed that hydrogen addition has an obvious effect on the explosion characteristics of the mixed gas. When the volume content of hydrogen exceeds 10%, the strengthening effect of the T-shaped bifurcation structure on the explosion reaction was more significant. The explosion characteristics in the upstream of the T-shaped bifurcation structure pipeline are consistent with those in the straight pipe, but it has a great impact on the downstream of the T-shaped bifurcation structure. When the flame passed through the bifurcation, the turbulence intensity in the pipe increased rapidly, which resulted the flame propagation velocity and explosion overpressure rising rapidly and exceeding the level before the bifurcation. The dynamic strain occurred on the thin wall of the pipe under the dynamic impact of gas cloud explosion, and the response of thin-wall strain had a good consistency with the explosion overpressure. In the experiment, the maximum strain at the bifurcation structure reached when the volume fraction of hydrogen was 20%.

Flame propagation and temperature distribution characteristics of magnesium dust clouds in an open space

In order to explore the flame propagation behaviors and temperature characteristics of magnesium dust clouds, the measurement system and the two-color pyrometer technique were used to study the spatial-temporal distribution of flame temperature fields and flame propagation behaviors with different particle sizes. The experiment results showed that the flame propagation characteristics largely depended on the particle size and the flame propagation velocity was higher with decreasing particle sizes. The flame temperatures of the leading edges were kept stable between 3100 and 3300 K, and the average flame temperature was related to the dust concentrations and particle sizes. The average flame temperature of −400 mesh magnesium with 830 g/m3 dust concentration was the highest, and the descent speed of the average flame temperature had a negative correlation with the particle size. The two-color pyrometer technique could evaluate the severity of magnesium dust clouds explosion, which provides a theoretical basis for the prevention.