Effect of inert species on the laminar burning velocity of hydrogen and ethylene

The maximum laminar burning velocity (LBV) of a fuel-air mixture is an important input parameter to vapor cloud explosion (VCE) blast load prediction methods. In particular, the LBV value has a significant impact on the predicted blast loads for high reactivity fuels with the propensity to undergo a deflagration-to-detonation transition (DDT). Published data are available for the maximum LBV of many pure fuel-air mixtures. However, little test data are available for mixtures of fuels, particularly for mixtures of fuels and inert species. Such mixtures are common in the petroleum refining and chemical processing industries. It is therefore of interest to be able to calculate the maximum LBV of a fuel/inert mixture based on the mixture composition and maximum LBV of each component.This paper presents measured test data for the maximum LBV of H₂/inert and C₂H₄/inert mixtures, with both nitrogen and carbon dioxide as the inert species. The LBV values were determined using a constant-volume vessel and the pressure rise method. This paper also provides a comparison of the measured LBV values with simplified LBV prediction methods.     

Experiment-based investigations of magnesium dust explosion characteristics

An experimental investigation was carried out on magnesium dust explosions. Tests of explosion severity, flammability limit and solid inerting were conducted thanks to the Siwek 20 L vessel and influences of dust concentration, particle size, ignition energy, initial pressure and added inertant were taken into account. That magnesium dust is more of an explosion hazard than coal dust is confirmed and quantified by contrastive investigation. The Chinese procedure GB/T 16425 is overly conservative for LEL determination while EN 14034-3 yields realistic LEL data. It is also suggested that 2000-5000 J is the most appropriate ignition energy to use in the LEL determination of magnesium dusts, using the 20 L vessel. It is essential to point out that the overdriving phenomenon usually occurs for carbonaceous and less volatile metal materials is not notable for magnesium dusts. Trends of faster burning velocity and more efficient and adiabatic flame propagation are associated with fuel-rich dust clouds, smaller particles and hyperbaric conditions. Moreover, Inerting effectiveness of CaCO₃ appears to be higher than KCl values on thermodynamics, whereas KCl represents higher effectiveness upon kinetics. Finer inertant shows better inerting effectiveness.     

CO2-超细水雾对瓦斯/煤尘爆炸抑制特性研究

为了解CO2-超细水雾对瓦斯/煤尘爆炸抑制特性，用自行搭建的实验系统，从超压、火焰传播速度和火焰结构3个方面研究了CO2-超细水雾形成的气液两相介质对9.5%瓦斯/煤尘复合体系爆炸的抑爆效果、影响因素与原因。研究结果表明:随着CO2体积分数和超细水雾质量浓度的增加，爆炸火焰最大传播速度、爆炸超压峰值均出现明显下降，火焰到达泄爆口时间显著延迟；尤其当CO2体积分数达到14%与超细水雾的共同抑爆效果凸显，瓦斯/煤尘复合体系爆炸超压的“震荡平台”消失，同时火焰结构呈现“整体孔隙化”。所得结论为煤矿井下高效防爆抑爆技术进行了完善和增强。     

含NaCl超细水雾抑制甲烷爆炸实验研究

为研究含NaCl添加剂超细水雾对甲烷爆炸的影响,在自制的半封闭透明管道内,进行含NaCl添加剂超细水雾抑制甲烷爆炸试验,通过检测和分析在不同NaCl浓度情况下超细水雾的粒径和甲烷爆炸的平均火焰传播速度、爆炸超压以及平均升压速率,探究NaCl浓度对超细水雾粒径及其对抑制甲烷爆炸有效性的影响。研究结果表明:NaCl浓度对超细水雾粒径影响较小;对于体积分数为9.5%的甲烷,相比于纯甲烷爆炸,其平均火焰传播速率、最大爆炸超压以及平均升压速率分别下降了53.7%,63.4%和60.7%,相比于超细纯水雾,其平均火焰传播速率、最大爆炸超压以及平均升压速率分别下降了38.6%,58%,56%;在通雾量相同的条件下,浓度为2.5%NaCl超细水雾对体积分数为9.5%的甲烷爆炸抑制性能最佳;含NaCl添加剂超细水雾的物理化学共同作用可以有效抑爆甲烷。     

A numerical modelling of an influence of CH4, N2, CO2 and steam on a laminar burning velocity of hydrogen in air

The influence of additives of various chemical natures (CH₄, N₂, CO₂, and steam) at a laminar burning velocity S_u of hydrogen in air has been studied by numerical modelling of a flat flame propagation in a gaseous mixture. It was found that the additives of methane to hydrogen–air mixtures cause as a rule monotonic reduction in the S_u value with the exception of very lean mixtures (fuel equivalence ratio ? = 0.4), for which a dependence of the laminar burning velocity on the additive's concentration has a maximum. In the case of the chemically inert additives (N₂, CO₂, H₂O) the laminar burning velocity of rich near-limit hydrogen–air flames drops monotonically with an increase in the additive's content, but no more than 1.5 times, and the adiabatic flame temperature changes slowly in this case. In the case of methane as the additive, the laminar burning velocity is diminished approximately 5 times with an increase in the adiabatic flame temperature from 1200 to 2100 K. Deviations from the known empirical rule of the approximate constancy of the laminar burning velocity for near-limit flames are shown.     

Minimum ignition temperatures and explosion characteristics of micron-sized aluminium powder

To forestall, control, and mitigate the detrimental effects of aluminium dust, a 20-L near-spherical dust explosion experimental system and an HY16429 type dust-cloud ignition temperature test device were employed to explore the explosion characteristics of micron-sized aluminium powder under different ignition energies, dust particle sizes, and dust cloud concentration (C_dust) values; the minimum ignition temperature (MIT) values of aluminium powder under different dust particle sizes and C_dust were also examined. Flame images at different times were photographed by a high-speed camera. Results revealed that under similar dust-cloud concentrations and with dust particle size increasing from 42.89 to 141.70 μm, the MIT of aluminium powder increased. Under various C_dust values, the MIT of aluminium dust clouds attained peak value when concentrations enhanced. Furthermore, the increase of ignition energy contributed to the increase of the explosion pressure (P_ex) and the rate of explosion pressure rise [(dP/dt)_ex]. When dust particle size was augmented gradually, the P_ex and (dP/dt)_ex attenuated. Decreasing particle size lowered both the most violent explosion concentration and explosive limits.     

Experimental research on methane/air explosion inhibition using ultrafine water mist containing methane oxidizing bacteria

In order to deeply understand the inhibitory effect of ultrafine water mist containing methane-oxidizing bacteria on methane explosion, a small-sized semi-closed visual experimental platform was built. Five different application mist amounts (0.7 mL, 2.1 mL, 3.5 mL, 4.9 mL, 6.3 mL) of ultrafine water mist containing methane-oxidizing bacteria on 9.5% methane explosion were studied experimentally. Ultrafine water mist was generated by the ultrasonic atomization generator, and mist size was measured by a winner319 laser particle size analyzer. During the methane explosion, a high-frequency pressure sensor collected pressure change data, and a high-speed camera recorded the flame development process. The results indicated that the maximum explosion overpressure (ΔP_max) decreased with time, and the arrival time of the maximum explosion overpressure (ΔP_max) delayed. The appearance time of the “tulip” shaped flame delayed, and the flame propagation speed decreased. The ultrafine water mist and deposition can effectively inhibit the methane explosion. The explosion suppression effect of the second step spraying water mist was better. The improvement of the explosion suppression effect of the ultrafine water mist containing methane-oxidizing bacteria was attributed to the degradation effect of the methane-oxidizing bacteria. Under long-term degradation, methane-oxidizing bacteria in water mist play a role in inhibiting methane explosion.     

The explosion of non-nano iron dust suspension in the 20-l spherical bomb

The understanding of dust explosion is still incomplete because of the lack of reliable data and accurate models accounting for all the physic-chemical aspects. Besides, most of the experimental data available in the current literature has been accumulated on the 20-l spherical bomb tests, which gives coarse results for the pressure history that cannot be easily converted into fundamental combustion parameters. Nevertheless, the large amount of experimental data available in the spherical bomb is attractive. In this work, the explosion of non-nano iron dust in the standard spherical vessel is analyzed, aiming at evaluating the burning velocity from the theoretical point of view and the simple experiments performed by the standard explosion tests. The choice of iron is of relevance because its adiabatic flame temperature is below the boiling temperature of both the reactants and oxidized gaseous, liquid, or solid (intermediate and final) products and for the negligible particle porosity, which instead is typical of organic dust. Therefore, a non-nano iron dust explosion can be reconducted to a reduced mechanism since heterogeneous (surface) combustion may be determinant, and the diffusion mechanism for oxygen is the only relevant. The laminar burning velocity is strongly dependant on the particle diameter, whereas little effects are due to the dust concentration. The reported final value was found in agreement with typical limiting laminar burning velocity, adopted for the estimation of flammability limits.     

Explosibility parameters for mixtures of pulverized fuel and ash

This paper reports the results of experiments done to examine the explosibility of pulverized fuel dusts (two different coals and petroleum coke) and their waste products (bottom and fly ashes). Tests were conducted for the fuel dusts alone and also blended with other fuels and ashes. The explosion parameters of interest were maximum pressure and rate of pressure rise, and ignition energy and temperature. Petroleum coke was found to present a lesser explosion hazard than the coals. Admixture of either coal or petroleum coke with fly ash resulted in explosible mixtures at volatile contents dependent on the composition of the mixture components and their particle sizes.     

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.     

The influence of the charge-to-mass ratio of the charged water mist on a methane explosion

To study the influence of the charge-to-mass ratio of a charged water mist on a methane explosion, the induction charging method was used to induce charge on a normal water mist; a mesh target method was employed to test the charge-to-mass ratio of its droplets. The propagation images, propagation average velocities, and overpressures of a methane explosion suppressed by charged water mist were analysed. The influence of the charge-to-mass ratio of the suppressant water mist on a methane explosion was studied. Results show that the explosion temperature, propagation average velocity, and peak overpressure deceased more obviously with charged water mist than ordinary water mist. With increasing charge-to-mass ratio, the suppression effect of the charged water mist underwent a significant increase. Under experimental conditions, compared with ordinary water mist, when the charge-to-mass ratio was 0.445 mC/kg and the mist flux was 4 L, the minimum flame propagation average velocity was 3.456 m/s, with a drop of 2.37 m/s (40.68%), and the peak overpressure of the methane explosion was 10.892 kPa, with a drop of 10.798 kPa (49.78%). The suppression effect is considered from the changes of the physico-chemical properties of the water mist as affected by the applied charge-to-mass ratio.     

障碍物对油气-空气混合气体泄压爆炸火焰传播特性影响

为了研究障碍物对油气泄压爆炸火焰传播特性的影响规律,进行了不同数量障碍物工况下的对比实验,并利用纹影仪和高速摄影仪记录了火焰传播过程,针对障碍物对火焰形态、火焰锋面位置及火焰传播速度的影响规律进行了研究,结果表明:圆柱体障碍物会导致油气泄压爆炸火焰形态产生褶皱和弯曲变形,诱导层流火焰向湍流火焰转变,加速火焰的传播,对油气泄压爆炸火焰的初始传播形态有显著影响;随着障碍物数量的增多,火焰锋面传播距离点火端的最大距离增大,但到达最远距离的时间减少;障碍物能够增强火焰的传播速度,尤其对障碍物下游火焰影响最为显著,随着障碍物数量的增多,火焰传播的最大速度也随之增大,但达到最大火焰传播速度的时间却随之减少;障碍物的存在增大了油气泄压爆炸过程外部爆炸压力,并且随着障碍物数量的增多,外部爆炸压力峰值增长幅度增大。     

A quantitative risk assessment tool for the external safety of industrial plants with a dust explosion hazard

A quantitative risk assessment (QRA) tool has been developed by TNO for the external safety of industrial plants with a dust explosion hazard. As a first step an industrial plant is divided into groups of modules, defined by their size, shape, and constructional properties. Then the relevant explosion scenarios are determined, together with their frequency of occurrence. These include scenarios in which one module participates, as well as domino scenarios. The frequency is partly based on casuistry.

A typical burning velocity is determined depending on the ignition type, the dust properties and the local conditions for flame acceleration. The resulting pressure development is predicted with the ‘thin flame model’. Module failure occurs when the explosion load exceeds thresholds, which are derived from single degree of freedom (SDOF) calculations for various types of modules. A model has been developed to predict the process of pressure venting after module failure and the related motion of launched module parts.

The blast effects of the primary explosion are based on results from calculations with BLAST3D. The blast and flame effects of the secondary external explosion due to venting are calculated using existing models. The throw of fragments and debris is quantified with a recently developed model. This model is based on trajectory calculations and gives the impact densities, velocities, and angles as output. Furthermore the outflow of bulk material is taken into account. The consequences for external objects and human beings are calculated using existing models. Finally the risk contours and the Societal risk (FN curve) are calculated, which can be compared to regulations.     

Fast and tiny: A model for the flame propagation of nanopowders

To avoid the influence of external parameters, such as the vessel volume or the initial turbulence, the explosion severity should be determined from intrinsic properties of the fuel-air mixture. Therefore, the flame propagation of gaseous mixtures is often studied in order to estimate their laminar burning velocity, which is both independent of external factors and a useful input for CFD simulation. Experimentally, this parameter is difficult to evaluate when it comes to dust explosion, due to the inherent turbulence during the dispersion of the cloud. However, the low inertia of nanoparticles allows performing tests at very low turbulence without sedimentation. Knowledge on flame propagation concerning nanoparticles may then be modelled and, under certain conditions, extrapolated to microparticles, for which an experimental measurement is a delicate task. This work focuses on a nanocellulose with primary fiber dimensions of 3 nm width and 70 nm length. A one-dimensional model was developed to estimate the flame velocity of a nanocellulose explosion, based on an existing model already validated for hybrid mixtures of gas and carbonaceous nanopowders similar to soot. Assuming the fast devolatilization of organic nanopowders, the chemical reactions considered are limited to the combustion of the pyrolysis gases. The finite volume method was used to solve the mass and energy balances equations and mass reactions rates constituting the numerical system. Finally, the radiative heat transfer was also considered, highlighting the influence of the total surface area of the particles on the thermal radiation. Flame velocities of nanocellulose from 17.5 to 20.8 cm/s were obtained numerically depending on the radiative heat transfer, which proves a good agreement with the values around 21 cm/s measured experimentally by flame visualization and allows the validation of the model for nanoparticles.     

Study on the characteristics of gas explosion affected by induction charged water mist in confined space

To investigate the suppression effect of charged water mist on gas explosion, a small charged water mist generator and a gas explosion simulation device were designed based on the principle of electrostatic induction. Experiments for testing characteristics of the gas explosion in a confined space under different charged polarities, charged voltages and mist fluxes were carried out. Experimental results indicated that, compared with the normal water mist, the explosion peak overpressure and the flame propagation speed could be more effectively reduced by the charged water mist. And this suppression effect could be promoted by increasing the charged voltage. To visualize the effect of the charged water mist's polarity on gas explosion, comparative experiments were conducted. The results showed that the explosion peak overpressure, the overpressure rising rate, and the propagation speed of the flame were reduced by 64.7%, 33.0% and 19.4%, respectively, when a +8 kV charged voltage was applied. In situation where a -8 kV charged voltage was applied, 64.1%, 26.5% and 16.0% reductions were achieved for the explosion peak overpressure, the overpressure rising rate, and the flame propagation speed respectively. Comparison of this data leads to the conclusion that the gas explosion could be more efficiently suppressed by the positively charged water mist.     

Ignition temperatures and flame velocities of metallic nanomaterials

The production of materials with dimensions in the nanometre range has continued to increase in recent years. In order to ensure safety when handling these products, the hazard potential of such innovative materials must be known. While several studies have already investigated the effects of explosions (such as maximum explosion pressure and maximum pressure rise) of powders with primary particles in the nanometre range, little is known about the ignition temperatures and flame velocities. Therefore, the minimum ignition temperature (MIT) of metallic nano powders (aluminium, iron, copper and zinc) was determined experimentally in a so called Godbert-Greenwald (GG) oven. Furthermore, the flame velocities were determined in a vertical tube. In order to better classify the test results, the tested samples were characterised in detail and the lower explosion limits of the tested dust samples were determined. Values for the burning velocity of aluminium nano powders are higher compared to values of micrometre powders (from literature). While MIT of nanometre aluminium powders is within the range of micrometre samples, MIT of zinc and copper nano powders is lower than values reported in literature for respective micrometre samples.     

On the determination of the laminar burning velocity from closed vessel gas explosions

A methodology to determine the laminar burning velocity from closed vessel gas explosions is explored. Unlike other methods which have been used to measure burning velocities from closed vessel explosions, this approach belongs to the category which does not involve observation of a rapidly moving flame front. Only the pressure–time curve is required as experimental input. To verify the methodology, initially quiescent methane–air mixtures were ignited in a 20-l explosion sphere and the equivalence ratio was varied from 0.67 to 1.36. The behavior of the pressure in the vessel was measured as a function of time and two integral balance models, namely, the thin-flame and the three-zone model, were fitted to determine the laminar burning velocity. Data on the laminar burning velocity as a function of equivalence ratio, pressure and temperature, measured by a variety of other methods have been collected from the literature to enable a comparison. Empirical correlations for the effect of pressure and temperature on the laminar burning velocity have been reviewed and two were selected to be used in conjunction with the thin-flame model. For the three-zone model, a set of coupled correlations has been derived to describe the effect of pressure and temperature on the laminar burning velocity and the laminar flame thickness. Our laminar burning velocities are seen to fall within the band of data from the period 1953–2003. A comparison with recent data from the period 1994–2003 shows that our results are 5–10% higher than the laminar burning velocities which are currently believed to be the correct ones for methane–air mixtures. Based on this observation it is concluded that the methodology described in this work should only be used under circumstances where more accurate methods can not be applied.     

Study of the influence of melamine polyphosphate and aluminum hydroxide on the flame propagation and explosion overpressure of aluminum magnesium alloy dust

When aluminum magnesium alloy dust floats in the air, a certain ignition energy can easily cause an accidental explosion. To prevent and control the occurrence of accidental explosions and reduce the severity of accidents, it is necessary to carry out research on the explosion suppression of aluminum magnesium alloy dust. This paper uses a vertical glass tube experimental device and a 20 L spherical explosive experimental device to carry out experimental studies on the suppression of the flame propagation and explosion overpressure of aluminum magnesium alloy dust with melamine polyphosphate (MPP) and Al(OH)₃. With increasing MPP and Al(OH)₃ concentrations, the flame brightness darkened, the flame velocity and propagation distance gradually decreased, and P_max and (dp/dt)_max decreased significantly. When the amount of MPP added reached 60%, the flame propagation distance decreased to 188 mm, which is a decrease of 68%, and the explosion overpressure decreased to 0.014 MPa, effectively suppressing the explosion of aluminum magnesium alloy dust. The experimental results showed that MPP was more effective than Al(OH)₃ in inhibiting the flame propagation and explosion overpressure of the aluminum magnesium alloy dust. Finally, the inhibitory mechanisms of the MPP and Al(OH)₃ were further investigated. The MPP and Al(OH)₃ endothermic decomposition produced an inert gas, diluted the oxygen concentration and trapped active radicals to terminate the combustion chain reaction.     

Effect of initial temperature on the explosion pressure of various liquid fuels and their blends

In this work, the effect of initial temperature on the explosion pressure, P_ex, of various liquid fuels (isooctane, toluene and methanol) and their blends (isooctane-toluene and methanol-toluene, with three different fuel-fuel ratios) was investigated by performing experiments in a 20-l sphere at different concentrations of vaporized fuel in air. The initial temperature was varied from 333 K to 413 K.Results show that, as the fuel-air equivalence ratio, Φ, is increased, a transition occurs from a “thermodynamics-driven” explosion regime to a “radiant heat losses-driven” explosion regime. The maximum pressure, P_max, is found in the former regime (Φ < 3), which is characterized by a trend of decreasing P_ex with increasing initial temperature. This trend has been explained by thermodynamics. In the latter regime (Φ > 3), P_ex increases with increasing initial temperature. This trend has been addressed to the decrease in emissivity (and, thus, radiant heat losses) with the increase in temperature.     

Investigation on the suppression effect of high momentum C6F12O (Novec-1230) flow on hydrogen jet flame

This work investigates the suppression effect of Novec-1230 on H₂ jet flame. The suppressants are motivated by N₂ flow to get higher momentum and approach the reaction kernel at flame base. The flame area with Novec-1230 is always smaller than that with water mist at the same condition. Novec-1230 exhibits better suppression effect on reaction kernel. The higher-momentum jet flame is more difficult to be suppressed. This is because that the higher-momentum flame makes the suppressant approach the reaction kernel more difficult. In addition, the high N₂ flow rate containing suppressant could destroy flame temperature structure and decrease it. Results inferred that the temperature of flame with Novec-1230 is higher than that with water mist. Moreover, the lower minimum extinguishing time indicates that the suppression efficiency of Novec-1230 is better than that of water mist. The jet flame is extinguished only when H₂ flow rate is low and N₂ flow rate is high. There are two reasons: one is that the higher-momentum jet flame prevents suppressants to enter flame core. The other one is that the burner nozzle is heated to as igniting source during suppression progress. Furthermore, the burning velocity, adiabatic flame temperature, heat production and free radicals are investigated theoretically at Φ = 1.6, 1.0, 0.8 and 0.6. Results indicate that the burning velocity with Novec-1230 is much lower than that with water mist. The adiabatic flame temperature, heat production and free radicals increase firstly and then decrease with Novec-1230 addition at lean flame.