Experimental study on explosion characteristics of syngas with different ignition positions and hydrogen fraction

The influence of different ignition positions and hydrogen volume fractions on the explosion characteristics of syngas is studied in a rectangular half-open tube. Three ignition positions were set at the axis of the tube, which are 0 mm, 600 mm and 1100 mm away from the closed end, respectively. A range of hydrogen volume fraction (φ) from 10% to 90% were concerned. Experimental results show that different ignition positions and hydrogen volume fraction have important influence on flame propagation structure. When ignited at 600 mm from the closed end on the tube axis, distorted tulip flame forms when flame propagates to the closed end. The formations of the tulip flame and the distorted tulip flame are accompanied by a change in the direction of the flame front propagation. The flame propagation structure and pressure are largely affected by the ignition position and the hydrogen volume fraction. At the same ignition position, flame propagation speed increases with the growing of hydrogen volume fraction. And the pressure oscillates more severe as the ignition location is closer to the open end. And pressure oscillations bring two different forms. The first form is that the pressure has a periodic oscillation. The amplitude of the pressure oscillation gradually increases. It takes several cycles from the start of the oscillation to the peak. For the second form, the pressure reaches the peak of the oscillation in the first cycle of the start to the oscillation.     

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.     

Experimental study of premixed syngas/air flame deflagration in a closed duct

The propagation behaviour of a deflagration premixed syngas/air flame over a wide range of equivalence ratios is investigated experimentally in a closed rectangular duct using a high-speed camera and pressure transducer. The syngas hydrogen volume fraction, φ, ranges from 0.1 to 0.9. The flame propagation parameters such as flame structure, propagation time, velocity and overpressure are obtained from the experiment. The effects of the equivalence ratio and hydrogen fraction on flame propagation behaviour are examined. The results indicate that the hydrogen fraction in a syngas mixture greatly influences the flame propagation behaviour. When φ, the hydrogen fraction, is ≥0.5, the prominently distorted tulip flame can be formed in all equivalence ratios, and the minimum propagation time can be obtained at an equivalence ratio of 2.0. When φ < 0.5, the tulip flame distortion only occurs in a hydrogen fraction of φ = 0.3 with an equivalence ratio of 1.5 and above. The minimum flame propagation time can be acquired at an equivalence ratio of 1.5. The distortion occurs when the maximum flame propagation velocity is larger than 31.27 m s^?1. The observable oscillation and stepped rise in the overpressure trajectory indicate that the pressure wave plays an important role in the syngas/air deflagration. The initial tulip distortion time and the plane flame formation time share the same tendency in all equivalence ratios, and the time interval between them is nearly constant, 4.03 ms. This parameter is important for exploring the quantitative theory or models of distorted tulip flames.     

Large eddy simulation of premixed hydrogen/methane/air flame propagation in a closed duct

In this paper, large eddy simulation (LES) is performed to investigate the propagation characteristics of premixed hydrogen/methane/air flames in a closed duct. In LES, three stoichiometric hydrogen/methane/air mixtures with hydrogen fractions (volume fractions) of 0, 50% and 100% are used. The numerical results have been verified by comparison with experimental data. All stages of flame propagation that occurred in the experiment are reproduced qualitatively in LES. For fuel/air mixtures with hydrogen fractions of 0 and 50%, only four stages of “tulip” flame formation are observed, but when the hydrogen fraction is 100%, the distorted “tulip” flame appears after flame front inversion. In the acceleration stage, the LES and experimental flame speed and pressure dynamic coincide with each other, except for a hydrogen fraction of 0. After “tulip” flame formation, all LES and experimental flame propagation speeds and pressure dynamics exhibit the same trends for hydrogen fractions of 0 and 100%. However, when the hydrogen fraction is 50%, a slight periodic oscillation appears only in the experiment. In general, the different structures displayed in the flame front during flame propagation can be attributed to the interaction between the flame front, the vortex and the reverse flow formed in the unburned and burned zones.     

Effects of ignition location on premixed hydrogen/air flame propagation in a closed combustion tube

The dynamics of premixed hydrogen/air flame ignited at different locations in a finite-size closed tube is experimentally studied. The flame behaves differently in the experiments with different ignition positions. The ignition location exhibits an important impact on the flame behavior. When the flame is ignited at one of the tube ends, the heat losses to the end wall reduce the effective thermal expansion and moderate the flame propagation and acceleration. When the ignition source is at a short distance off one of the ends, the tulip flame dynamics closely agrees with that in the theory. And both the tulip and distorted tulip flames are more pronounced than those in the case with the ignition source placed at one of the ends. Besides, the flame–pressure wave coupling is quite strong and a second distorted tulip flame is generated. When the ignition source is in the tube center, the flame propagates in a much gentler way and the tulip flame can not be formed. The flame oscillations are weaker since the flame–pressure wave interaction is weaker.     

Experimental investigation on unconfined hydrogen explosion with different ignition height

Experimental research is performed to investigate the effects of ignition height on explosion characteristics in a 27 m³ hydrogen/air cloud. With the ignition height decreasing, the flame propagation velocity increases gradually. The flame travels in oscillating mode and the average oscillating frequency lies between 145Hz and 155Hz. An original parameter τ, which involves flame scale and flame propagation velocity, is proposed to measure the effect of buoyancy. The higher the value of τ, the more obvious the buoyancy effect. As the ignition height increases, the critical flame scale for flame deceleration increases. The middle ignition height in the gas cloud causes the highest overpressure peak, overpressure impulse, overpressure rising and decreasing rate. As the ignition point approaches the initial gas boundary, the explosion intensity would decrease gradually. For the open space outside the flame, overpressure peak for the lower space is higher, while, the middle space experiences higher overpressure impulse.     

Vented hydrogen–air deflagration in a small enclosed volume

Since the rapid development of hydrogen stationary and vehicle fuel cells the last decade, it is of importance to improve the prediction of overpressure generated during an accidental explosion which could occur in a confined part of the system. To this end, small-scale vented hydrogen–air explosions were performed in a transparent cubic enclosure with a volume of 3375 cm³. The flame propagation was followed with a high speed camera and the overpressure inside the enclosure was recorded using high frequency piezoelectric transmitters. The effects of vent area and ignition location on the amplitude of pressure peaks in the enclosed volume were investigated. Indeed, vented deflagration generates several pressures peaks according to the configuration and each peak can be the dominating pressure. The parametric study concerned three ignition locations and five square vent sizes.     

Suppression effects of ultrafine water mist on hydrogen/methane mixture explosion in an obstructed chamber

The mitigation effects of ultrafine water mist on hydrogen/methane mixture explosions with hydrogen fraction (ϕ) of the range from 0% to 60% were experimentally studied in a vented chamber with obstacles. The spraying time, droplets size of water mist and the volume ratio of hydrogen were varied in the tests, and the key parameters that reflect the explosion characteristics such as the flame propagation imagines, flame propagation velocity, and explosion overpressure were obtained. The results show that the ultrafine water mist presents a significant mitigation effect on hydrogen/methane mixture explosions. The flame propagation structures are similar under the condition of without and with ultrafine water mist while the flame temperature is declined by the physical and chemical inhibition by ultrafine water mist. In addition, the mitigation effect increases with the increase of water mist flux. As a result, the maximum flame speed and overpressure of ϕ = 30% hydrogen/methane mixture explosion are declined by 33.3% and 58.4% under the condition of spraying for 2 min with 15 μm ultrafine water mist, respectively. Besides, the mitigation effects of ultrafine water mist on ϕ = 30% hydrogen/methane mixture explosion descends evidently with the increase of the droplets size of the range from 6 μm to 25 μm, which due to the easier evaporation and the greater total droplets surface area of the smaller water mist. However, the explosion mitigation effect of ultrafine water mist on the hydrogen/methane mixture actually descends with the increase hydrogen fraction.     

An experimental study of premixed hydrogen/air flame propagation in a partially open duct

High-speed schlieren cinematography and pressure records are used to investigate the dynamics of premixed hydrogen/air flame propagation and pressure build up in a partially open duct with an opening located in the upper wall near the right end of the duct. This work provides basic understanding of flame behaviors and the effects of opening ratio on the combustion dynamics. The flame behaves differently under different opening conditions. The opening ratio has an important influence on the flame propagation and pressure dynamics. When the opening ratio α ≤ 0.075 a significant distorted tulip flame can be formed after the full formation of a classical tulip flame. The propagation speed of flame leading tip increases with the opening ratio. The coupling of flame front with the pressure wave is strong at low opening ratio. Both the pressure growth rate and oscillation amplitude inside the duct increases as the opening ratio decreases. The formation times of tulip and distorted tulip flames and the corresponding distances of flame front increase with the increase of the opening ratio.     

Experimental and numerical study on premixed hydrogen/air flame propagation in a horizontal rectangular closed duct

Hydrogen is a promising energy in the future, and it is desirable to characterize the combustion behavior of its blends with air. The premixed hydrogen/air flame microstructure and propagation in a horizontal rectangular closed duct were recorded using high-speed video and Schlieren device. Numerical simulation was also performed on Fluent CFD code to compare with the experimental result. A tulip flame is formed during the flame propagating, and then the tulip flame formation mechanism was proposed based on the analysis. The induced reverse flow and vortex motion were observed both in experiment and simulation. The interactions among the flame, reverse flow and vortices in the burned gas change the flame shape and ultimately it develops into a tulip flame. During the formation of the tulip flame, the tulip cusp slows down and stops moving after its slightly forward moving, and then, it starts to move backward and keeps on a longer time, after that, it moves forward again. The structure of the tulip flame is becoming less stable with its length decreasing in flame propagation direction. The flame thickness increases gradually which is due to turbulence combustion.     

Effect of nitrogen on deflagration characteristics of hydrogen / methane mixture

In order to study the influence of nitrogen on the deflagration characteristics of premixed hydrogen/methane, the explosion parameters of premixed hydrogen/methane within various volume ratios and different dilution ratios were studied by using a spherical flame method at room temperature and pressure. The results are as follows: The addition of nitrogen makes the upper limit of explosion of hydrogen/methane premixed gas drop, and the lower limit rises. For explosion hazard (F-number), hydrogen/methane premixed fuel with a hydrogen addition ratio of 10% has the lowest risk, and nitrogen has a greater impact on the dangerous degree of hydrogen and methane premixed gas whose hydrogen addition ratio does not exceed 30%. In terms of flame structure, the spherical flame was affected by buoyancy instability as the percentage of nitrogen dilution increased, but the buoyancy instability gradually decreased as the percentage of hydrogen addition increased. The addition of diluent gas reduces the spreading speed of the stretching flame and reduces the stretching rate in the initial stage of flame development. The laminar flame propagation velocity calculated by the experiment in this paper is consistent with the laminar flow velocity of the hydrogen/methane premixed gas calculated by GRI Mech 3.0. Considering the explosion parameters such as flammability limit, laminar combustion rate and deflagration index, when hydrogen is added to 70%, it is the turning point of hydrogen/methane premixed fuel.     

Effects of combined obstacles on deflagration characteristics of hydrogen-air premixed gas

To study the mechanism by which an increase in the number of obstacles affects the propagation of hydrogen-air premixed gas explosions under a constant overall volume of obstacles, a large eddy simulation method was used to carry out numerically simulate configurations with different distribution modes of combined obstacles. The study focused on the flame structure, evolution process of overpressure dynamics, and flame-flow coupling relationship. The results showed that the flame propagation velocity and flame front area are increased during the conversion of the combined obstacles from 1-30 mm to 4–7.5 mm, while the flame front area logarithmically depends on the number of obstacles. The flames gradually develop from “corrugated flamelets” to “thin reaction zones” in different distribution modes. In addition, the results showed that although increasing dispersion increases the explosion overpressure, a critical number of obstacles likely exist. Beyond the critical point, explosion overpressure peak no longer strongly varies with the number of obstacles. Furthermore, for working configurations with different numbers of obstacles, an increase in the overall number of obstacles before reaching the same number of obstacles weakly affects the flame shape and flow rate of the flame front. This study provides theoretical guidelines for safety designs to prevent hydrogen-air premixed gas explosion in obstructed spaces.     

An experimental study on the effect of the flaring rate on flame dynamics in open pipes

Under the condition that the gas composition constant equivalence ratio is Φ = 1, and the initial temperature and initial pressure are T₀ and P₀, respectively, the experimental study of the premixed gas flames with different hydrogen doping ratios (φ = 10%–40%) is different. The behavior and shape change of propagation in the flaring rate pipe (? = 1.0–0.25). The study found that the pre-mixed gas flame in the flared pipe has undergone more complicated shape changes than other studies. One of the outstanding findings is that the tulip flame appeared twice in this open pipe experiment. And through the high-speed camera and high-frequency pressure sensor to obtain the tulip flame picture and the pressure change in the combustion chamber, comprehensive analysis of the experimental results, and the results show that every appearance of the tulip flame is accompanied by the deceleration of the flame front and the increase of overpressure in the combustion chamber.     

Experimental and numerical study of the effect of initial temperature on the combustion characteristics of premixed syngas/air flame

The effects of different initial temperatures (T = 300–500 K) and different hydrogen volume fractions (5%–20%) on the combustion characteristics of premixed syngas/air flames in rectangular tubes were investigated experimentally. A high-speed camera and pressure sensor were used to obtain flame propagation images and overpressure dynamics. The CHEMKIN-PRO model and GRI Mech 3.0 mechanism were used for simulation. The results show that the flame propagation speed increases with the initial temperature before the flame touches the wall, while the opposite is true after the flame touches the wall. The increase in initial temperature leads to the increase in overpressure rise rate in the early flame propagation process, but the peak overpressure is reduced. The laminar burning velocity (LBV) and adiabatic flame temperature (AFT) increase with increasing initial temperature. The increase in initial temperature makes the peaks of H, O, and OH radicals increase.     

Investigating the explosion hazard of hydrogen produced by activated aluminum in a modified Hartmann tube

Metallic powders exposed to water are sources of hydrogen gas that may result in an explosion hazard in the process industries. In this paper, hydrogen production and flame propagation in a modified Hartmann tube were investigated using activated aluminum powder as fuel. A self-sustained reaction of activated aluminum with water was observed at cool water and room temperatures for all treatments. One gram of Al mixed with 5 wt% NaOH or CaO resulted in a rapid rate of hydrogen production and an almost 100% yield of hydrogen generation within 30 min. The flame structures and propagation velocity (FPV) of released hydrogen at different ignition delay times were determined using electric spark ignition. Flame structures of hydrogen were mainly dependent on hydrogen concentration and ignition delay time, likely due to different mechanisms of hydrogen generation and flame propagation. As expected, FPVs of hydrogen in the Hartmann tube increased with ignition delay time. However, the FPV of upward flame propagation was much larger than that of downward flame propagation due to the effect of spreading acceleration at the explosion vent. Once ignited, the FPV of upward flame propagation reached 31.3–162.5 m/s, a value far larger than the 7.5–30 m/s for downward flame propagation. Hydrogen explosion caused by the accumulation of wet metal dust can be far more dangerous than an ordinary hydrogen explosion.     

Under-expansion jet flame propagation characteristics of premixed H2/air in explosion venting

In premixed H₂/air explosion venting, an under-expansion jet may be caused by the pressure difference between the inside and outside of the explosion vent. Based upon the under-expansion jet, studying the structure of the under-expansion jet flame and the factors influencing its formation is essential to hydrogen safety in explosion venting. This study explored the basic characteristics of the under-expansion jet flame in premixed H₂/air explosion venting, and discussed the formation of two under-expansion structures (Mach disk and diamond shock wave) of such jet flames by conducting a premixed H₂/air explosion venting experiment. The influences of hydrogen fraction, explosion venting diameter, and duct length on the structure of under-expansion jet flames were evaluated. The results showed that after successful explosion venting, the under-expansion jet flame would be generated when the hydrogen fractions were 30–60 vol.%, and as the hydrogen fractions were 30–50 vol.%, the lengths of the venting duct were 30 and 50 cm. The duration of under-expansion jet flame was the longest when the hydrogen fraction was 40 vol.%. With the explosion venting diameter and hydrogen fraction increased, the spacing between under-expansion jet flame structures (S) increased. However, an increase in duct length led to the attenuation of the S. During the explosion venting, the under-expansion jet caused a pressure imbalance near the explosion vent and high-intensity convection forms on both sides of a jet, which can generate two or more explosions. Therefore, understanding the basic characteristics of under-expansion jet flame can aid the effective development of measures to prevent, mitigate, and protect against premixed H₂/air explosions.     

Experimental study on the premixed syngas-air explosion in duct with both ends open

Premixed flame of stoichiometric syngas-air mixture with various hydrogen volume fractions, 10% ≤ X (H₂) ≤ 90%, propagating in a duct with both ends open is experimentally investigated in this study. Two representative ignition locations, i.e., Ig-1, locating at the center of the duct, and Ig-2, locating at the right open end, are considered. Results show that the tulip flame is first attained in the duct with both ends open at 10% ≤ X (H₂) ≤ 50% as the flame is ignited at Ig-1. However, the flame maintains the convex shape with the cellular structure on the flame surface as the flame is ignited at Ig-2. The cellular structure results from Darrieus-Landau instability, but the Darrieus-Landau instability cannot invert the convex flame front. The flame tip and pressure dynamics have been examined. When the flame is ignited at Ig-1, the flame oscillates violently, and the overpressure profiles oscillate as a Helmholtz-type. When the flame is ignited at Ig-2, the left flame front propagates in an atmospheric pressure with a nearly constant speed. The prominent flame acceleration and oscillation are not observed at Ig-2 because of lacking flame acoustic interaction. What's more, the characteristic time of flame propagation has been compared. The time t_w is shorter while the time t_p is longer than the calculated value, and the time t_e has been delayed by both open ends. The flame propagation process is moderated as the flame propagates in the duct with both ends open.     

Flame characteristics of premixed H2-air mixtures explosion venting in a spherical container through a duct

The explosion venting duct can effectively reduce the hazard degree of a gas explosion and conduct the venting energy to the safe area. To investigate the flame quantitative propagation law of explosion venting with a duct, the effects of hydrogen fraction and explosion venting duct length on jet flame propagation characteristics of premixed H₂-air mixtures were analyzed through experiment and simulation. The experiment results under initial conditions of room temperature and 1 atm show that when hydrogen fraction was high enough, part of the unburned hydrogen was mixed with air again to reach an ignitable concentration, resulting in the secondary combustion was easier produced and the duration of the secondary flame increased. With the increase of venting duct length, the flame front distance and propagation velocity increased. Meanwhile, the spatial distribution of pressure field and temperature field, and the propagation process and mechanism of the flame venting with a duct were analyzed using FLUENT software. The variation of the pressure wave and the pressure reflection oscillation law in the explosion venting duct was captured. Therefore, in the industrial explosion venting design with a duct, the hazard caused by the coupling of venting pressure and venting flame under different fractions should be considered comprehensively.     

Propagation of premixed hydrogen-air flame initiated by a planar ignition in a closed tube

Numerical simulations were used to study the dynamics of premixed flames propagating after planar ignition in a closed tube filled with stoichiometric hydrogen-air mixture. The two-dimensional fully compressible reactive Navier–Stokes equations coupled to a calibrated chemical-diffusive model were solved using a high-order numerical method and adaptive mesh refinement. The results show that the flame evolves from an initially planar flame to a double-cusped tulip flame, subsequently to a multi-cusped tulip flame, and finally to a series of distorted tulip flames (DTFs). The DTF forms one after another until the end of combustion. The initial flame lips of the double-cusped tulip flame are produced due to the stretching effect of nonuniform flow caused by the wall friction. The multi-cusped tulip flame forms as secondary cusps are created on the leading flame tips near the sidewalls. The formation of DTFs here is thought to be closely connected to pressure waves generated in the flame propagation process. The first DTF is caused by the combined effects of the vortex motions and the Rayleigh–Taylor (RT) instability driven by pressure waves, while the subsequent DTFs form due to reverse flows and RT instability. Nevertheless, both the vortex motions and reverse flows are essentially induced by the interactions between pressure waves and flow fields. Furthermore, the numerical results were compared to that in the case with a semicircular ignition. It was found that although there are significant differences in the early flame acceleration and tulip formation stages between the two differently shaped ignitions, the dynamics of DTFs are substantially consistent.     

Effect of obstacle arrangement on premixed hydrogen flame: Eddy-dissipation concept model based numerical simulation

In this paper, computational fluid dynamics (CFD) numerical simulation is used to analyze and discuss the horizontal propagation process of premixed hydrogen flame with obstacles. A total of three different obstacle channel arrangements at the blocking ratio of 0.5, which will affect the explosion flame and pressure development. The results show that the premixed flame is affected by flow instabilities and vortices when propagating through the obstacle channel, thereby distorting the flame. The vortices outside the flame boundary are more conducive to the acceleration of the flame. The continuous acceleration and synergistic promotion of the flame is more prominent due to the existence of the channel in the central axis of flame propagation, and the maximum velocity even achieved 307.91  m/s. The degree of the wrinkle of flame increases with the number of obstacle channels. The flame propagation process is always accompanied by pressure variations, and the dynamic pressure builds up at the flame front and intensifies periodically. But the downstream pressure gradually increases as the number of obstacle channels increases. CFD simulation of the explosion process clearly reveals the changing trends and interactions of explosion characteristic factors.