Experimental study on the quenching process of methane/air deflagration flame with porous media

This article reports experimental investigation of deflagration flame quenching behavior by porous media. In this study, a semi-vented deflagration chamber with a porous media plate was constructed, taking account of effects of obstacles and porous media materials on the flame quenching process. A high speed video camera was used to image the process and behavior of flame propagation, meanwhile, the gas-phase temperatures and ion currents, upstream, within, and downstream of the porous media, were measured using micro-thermocouples and ion probes, respectively. Results show that methane/air deflagration flame can be quenched by the Al₂O₃ porous media with thickness of 20 mm and pore density of 10 ppi. However, the presence of obstacles along the flame path may lead to significant increase of flame speed, thereby both the decreases of gas-phase temperature and ion current when the flame passes through the porous medium in the case with continuous obstacles are less, eventually the unburnt gases downstream the porous media may be reignited. Compared to Al₂O₃, Al porous media shows superior flame quenching performance because this metallic material has higher thermal conductivity, which makes combusting flame release more heat to the pore walls and adjoining structures of the porous media.     

Experimental study on methane explosion characteristics with different types of porous media

Porous media has a significant effect on flame and overpressure of methane explosion. In this paper, the pore diameter and thickness of porous media are studied. Nine experimental combinations of different pore diameter and thickness on the propagation of flame and overpressure of methane explosion in a tube are analyzed. The results show that the porous media not only can suppress the explosive flame propagation, but the porous media with large pore diameter can cause deflagration and accelerate the transition of flame from laminar to turbulent. The pore diameter of the porous media mainly determines the quenching of the flame. Simply increasing the thickness of porous media may cause the flame to temporarily stop propagating, but the flame is not completely extinguished for larger pore diameter. However, the deflagration propagation speed of flame is affected by the thickness. The attenuation of overpressure by porous media is mainly reflected in reducing the duration of overpressure and the peak value of overpressure. The smaller the pore diameter, the greater the thickness, and the more remarkable the reduction in overpressure duration and peak value. Suitable pore diameter and thickness of porous media can effectively suppress flame propagation and reduce the maximum value and duration of overpressure.     

Effects of hydrogen addition on propagation characteristics of premixed methane/air flames

An experimental study has been conducted to investigate the effects of hydrogen addition on the fundamental propagation characteristics of methane/air premixed flames at different equivalence ratios in a venting duct. The hydrogen fraction in the methane–hydrogen mixture was varied from 0 to 1 at equivalence ratios of 0.8, 1.0 and 1.2. The results indicate that the tendency towards flame instability increased with the fraction of hydrogen, and the premixed hydrogen/methane flame underwent a complex shape change with the increasing hydrogen fraction. The tulip flame only formed when the fraction of hydrogen ranged from 0 to 50% at an equivalence ratio of 0.8. It was also found that the flame front speed and the overpressure increased significantly with the hydrogen fraction. For all equivalence ratios, the stoichiometric flame (Φ = 1.0) has the shortest time of flame propagation and the maximum overpressure.     

Synergistic suppression effects of flame retardant,porous minerals and nitrogen on premixed methane/air explosion

A novel composite inhibitor based on porous mineral materials and conventional flame retardant of ammonium polyphosphate (APP) is prepared to suppress the premixed methane/air explosion. Taking advantages of gas and powder inhibitor, N₂ and the prepared composite inhibitor are combined to use. The suppression performance of N₂-composite inhibitor on methane explosion is investigated on a 20-L spherical experimental explosion apparatus and the characteristic pressure data are obtained. The combined inhibition effects of N₂ and the prepared composite inhibitor are greater than either acting alone. Thermal decomposition behavior and gaseous products of composite inhibitor are analyzed with thermogravimetric analysis and thermogravimetric-mass spectrometry, respectively. Based on physical and chemical actions, the inhibition mechanisms of N₂-composite inhibitor system are proposed. This work provides a reference to prepare high-performance gas explosion inhibitor based on the synergism of binary or multiple components.     

Effects of premixed methane concentration on distribution of flame region and hazard effects in a tube and a tunnel gas explosion

Study of flame distribution laws and the hazard effects in a tunnel gas explosion accident is of great importance for safety issue. However, it has not yet been fully explored. The object of present work is mainly to study the effects of premixed gas concentration on the distribution law of the flame region and the hazard effects involving methane-air explosion in a tube and a tunnel based on experimental and numerical results. The experiments were conducted in a tube with one end closed and the other open. The tube was partially filled with premixed methane-air mixture with six different premixed methane concentrations. Major simulation works were performed in a full-scale tunnel with a length of 1000 m. The first 56 m of the tunnel were occupied by methane–air mixture. Results show that the flame region is always longer than the original gas region in any case. Concentration has significant effects on the flame region distribution and the explosion behaviors. In the tube, peak overpressures and maximum rates of overpressure rise (dp/dt)_max for mixtures with lower and higher concentrations are great lower than that for mixtures close to stoichiometric concentration. Due to the gas diffusion effect, not the stoichiometric mixture but the mixture with a slightly higher concentration of 11% gets the highest peak overpressure and the shock wave speed along the tube. In the full-scale tunnel, for fuel lean and stoichiometric mixture, the maximum peak combustion rates is achieved before arriving at the boundary of the original methane accumulation region, while for fuel rich mixture, the maximum value appears beyond the region. It is also found that the flame region for the case of stoichiometric mixture is the shortest as 72 m since the higher explosion intensity shortens the gas diffusion time. The case for concentration of 13% can reach up to a longest value of 128 m for longer diffusion time and the abundant fuel. The “serious injury and death” zone caused by shock wave may reach up to 3–8 times of the length of the original methane occupied region, which is the widest damage region.     

渐变型多孔介质中低浓度瓦斯燃烧特性数值模拟

利用模拟软件Fluent6.3建立二维模型并进行合理设置,对多孔介质中的煤矿低浓度瓦斯燃烧进行数值模拟,研究了均匀型和渐变型燃烧器中的温度分布和NOx生成情况,以及三种不同渐变型多孔介质组合燃烧器中的温度分布和NOx生成情况。结果表明,渐变型多孔介质燃烧器的温度分布比均匀型燃烧器更加均匀;同一瓦斯当量比和流速下,渐变型多孔介质燃烧器的NOx产生量比均匀型燃烧器低很多,有利于减少污染物排放;三种不同渐变型多孔介质组合的温度分布和NOx生成有着不同特点,燃烧器设计时,可根据不同需求选取不同组合形式,综合考虑各因素,组合一燃烧特性最佳。     

Large eddy simulation and experimental study of the effect of wire mesh on flame behaviours of methane/air explosions in a semi-confined pipe

Preventing the propagation of flames in a pipeline is an effective measure for avoiding gas explosion accidents and reducing losses. To evaluate the effect of wire mesh, acting as a porous media, experimental and simulation studies are conducted to determine the influence of the wire mesh on the dynamics of premixed methane/air flame propagation in a semi-closed pipe. Four different kinds of wire mesh with different numbers of layers are chosen in the experiments and simulation, and the mechanism of wire mesh quenching of the flame is investigated. The experimental and simulation results are consistent. Flames are quenched when 4 layers of 40-mesh or 3 layers of 60-mesh wire mesh are used; however, once the flame propagates through the wire mesh, the risk of methane combustion may increase. The wire mesh becomes the key factor causing flame folds and acceleration, and the greater the number of layers or the larger the mesh size is, the more obvious the folds after the flame passes through the wire mesh. Moreover, the combination of heat absorption and disruption of the continuous flame surface by the mesh causes flame quenching. Wire mesh can effectively attenuate the flame temperature during premixed flame propagation in a pipe, and the attenuated maximum rate reaches approximately 79% in the case of adding 3 layers of 60-mesh wire mesh.     

Enhancement effects of methane/air explosion caused by water spraying in a sealed vessel

Experiments about the influence of ultrafine water mist on the methane/air explosion were carried out in a fully sealed visual vessel with methane concentrations of 8%, 9.5%, 11% and 12.5%. Water mists were generated by two nozzles and the droplets' Sauter Mean Diameters (SMD) were 28.2 μm and 43.3 μm respectively which were measured by Phase Doppler Particle Anemometer (PDPA). A high speed camera was used to record the flame propagation processes. The results show that the maximum explosion overpressure, pressure rising rate and flame propagation velocity of methane explosions in various concentrations increased significantly after spraying. Furthermore, the brightness of explosion flame got much higher after spraying. Besides, the mist with a larger diameter had a stronger turbulent effect and could lead to a more violent explosion reaction.     

Effects of vent layout and vent area on dynamic characteristics of premixed methane-air explosion in a tube with two side-vented ducts

To further elucidate the influence mechanism of side vents on the dynamic characteristics of gas explosions in tubes is helpful to design more reasonable vent layouts. In this paper, 9.5% methane-air explosion experiments were conducted in a tube with two side-vented ducts, and the effects of vent layouts and vent areas on the dynamic characteristics of explosion overpressure and flame propagation speed were investigated. The results demonstrate that under the same condition with a single vent area of 100 mm × 100 mm, when only the end vent is open, the maximum explosion overpressure and the maximum flame propagation speed are the highest among the five vent layouts. When the side vents 1 and 2 and the end vent are open, the maximum explosion overpressure is the lowest, and an unusual discovery is that the flame front changes into a hemispherical shape, finger shape, quasi-plane shape, tulip shape and wrinkled structure. When only side vent 1 is open, a unique Helmholtz oscillation occurs, and a new discovery is that there is a consistent oscillation relationship among the overpressure, flame propagation speed and flame structure. Helmholtz oscillation occurs only when a single vent area is 100 mm × 100 mm–60 mm × 60 mm, and the oscillation degree decreases with decreasing vent area. During the vent failure stage, the maximum explosion overpressure is generated, the flame front begins to appear irregular shape, and the flame propagation speed shows a prominent characteristic peak. After the vent failure stage, the driving effect of the end vent on the flame is higher than that of the side vent on the flame. Furthermore, the correlation equations of the mathematical relationships among the maximum explosion overpressure P_red, the static activation pressure P_stat and the vent coefficient K_v under four vent layouts are established, respectively.     

Effects of hydrogen addition on the confined and vented explosion behavior of methane in air

Multi-component gas mixture explosion accidents occur and recur frequently, while the safety issues of multi-component gas mixture explosion for hydrogen–methane mixtures have rarely been addressed.Numerical simulation study on the confined and vented explosion characteristics of methane-hydrogen mixture in stoichiometric air was conducted both in the 5 L vessel and the 64 m³ chamber, involving different mixture compositions and initial pressures. Based on the results and analysis, it is shown that the addition of hydrogen has a negative effect on the explosion pressure of methane-hydrogen mixture at adiabatic condition. While in the vented explosion, the addition of the hydrogen has a significant positive effect on the explosion hazard degree. Additionally, the addition of hydrogen can induce a faster reactivity and enhance the sensitivity of the mixture by reducing the explosion time and increasing the rate of pressure rise both in confined and vented explosion. Both the maximum pressure and the maximum rate of pressure rise increase with initial pressure as a linear function, and also rise with the increase of hydrogen content in fuel. The increase in the maximum rate of pressure rise is slight when hydrogen ratio is lower than 0.5, however, it become significant when hydrogen ratio is higher than 0.5. The maximum rate of pressure rise for stoichiometric hydrogen-air is about 10 times the one of stoichiometric methane-air.Furthermore, the vent plays an important role to relief pressure, causing the decrease in explosion pressure and rate of pressure rise, while it can greatly enhance the flame speed, which will extend the hazard range and induce secondary fire damages. Additionally it appears that the addition of hydrogen has a significant increasing effect on the flame speed. The propagation of flame speed in confined explosion can be divided into two stages, increase stage and decrease stage, higher hydrogen content, higher slope. But in the vented explosion, the flame speed keeps increasing with the distance from the ignition point.     

Effects of ignition energy on fire and explosion characteristics of dilute hybrid fuel in ventilation air methane

Deflagration explosions of coal dust clouds and flammable gases are a major safety concern in coal mining industry. Accidental fire and explosion caused by coal dust cloud can impose substantial losses and damages to people and properties in underground coal mines. Hybrid mixtures of methane and coal dust have the potential to reduce the minimum activation energy of a combustion reaction. In this study the Minimum Explosion Concentration (MEC), Over Pressure Rise (OPR), deflagration index for gas and dust hybrid mixtures (K_st) and explosive region of hybrid fuel mixtures present in Ventilation Air Methane (VAM) were investigated. Experiments were carried out according to the ASTM E1226-12 guideline utilising a 20 L spherical shape apparatus specifically designed for this purpose.Resultsobtained from this study have shown that the presence of methane significantly affects explosion characteristics of coal dust clouds. Dilute concentrations of methane, 0.75–1.25%, resulted in coal dust clouds OPR increasing from 0.3 bar to 2.2 bar and boosting the K_st value from 10 bar m s⁻¹ to 25 bar m s⁻¹. The explosion characteristics were also affected by the ignitors’ energy; for instance, for a coal dust cloud concentration of 50 g m⁻³ the OPR recorded was 0.09 bar when a 1 kJ chemical ignitor was used, while, 0.75 bar (OPR) was recorded when a 10 kJ chemical ignitor was used.For the first time, new explosion regions were identified for diluted methane-coal dust cloud mixtures when using 1, 5 and 10 kJ ignitors. Finally, the Le-Chatelier mixing rule was modified to predict the lower explosion limit of methane-coal dust cloud hybrid mixtures considering the energy of the ignitors.     

Effects of a carbon monoxide-dominant gas mixture on the explosion and flame propagation behaviors of methane in air

This work aimed to experimentally evaluate the effects of a carbon monoxide-dominant gas mixture on the explosion characteristics of methane in air and report the results of an experimental study on explosion pressure measurement in closed vessel deflagration for a carbon monoxide-dominant gas mixture over its entire flammable range. Experiments were performed in a 20-L spherical explosion tank with a quartz glass window 110 mm in diameter using an electric spark (1 J) as the ignition source. All experiments were conducted at room temperature and at ambient pressure, with a relative humidity ranging from 52 to 73%. The peak explosion pressure (P_max), maximum pressure rise rate ((dp/dt)_max), and gas deflagration index (K_G) were observed and analyzed. The flame propagation behavior in the initial stage was recorded using a high-speed camera. The spherical outward flame front was determined on the basis of a canny method, from which the maximum flame propagation speed (S_n) was calculated. The results indicated that the existence of the mixture had a significant effect on the flame propagation of CH₄-air and increased its explosion risk. As the volume fraction of the mixed gas increases, the P_max, (dp/dt)_max, K_G and S_n of the fuel-lean CH₄-air mixture (7% CH₄-air mixture) increase nonlinearly. In contrast, addition of the mixed gas negatively affected the fuel-rich mixture (11% CH₄-air mixture), exhibiting a decreasing trend. Under stoichiometric conditions (9.5% CH₄-air mixture), the mixed gas slightly lowered P_max, (dp/dt)_max, K_G, and S_n. The P_max of CH₄-air mixtures at volume fractions of 7%, 9.5%, and 11% were 5.4, 6.9, and 6.8 bar, respectively. The S_n of CH₄-air mixtures at volume fractions of 7%, 9.5%, and 11% were 1.2 m/s, 2.0 m/s, and 1.8 m/s, respectively. The outcome of the study is comprehensive data that quantify the dependency of explosion severity parameters on the gas concentration. In the storage and transportation of flammable gases, the information is required to quantify the potential severity of an explosion, design vessels able to withstand an explosion and design explosion safety measures for installations handling this gas.