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.     

Laminar burning velocities of hydrogen–air mixtures from closed vessel gas explosions

The laminar burning velocity of hydrogen–air mixtures was determined from pressure variations in a windowless explosion vessel. Initially, quiescent hydrogen–air mixtures of an equivalence ratio of 0.5–3.0 were ignited to deflagration in a 169 ml cylindrical vessel at initial conditions of 1 bar and 293 K. The behavior of the pressure was measured as a function of time and this information was subsequently exploited by fitting an integral balance model to it. The resulting laminar burning velocities are seen to fall within the band of experimental data reported by previous researchers and to be close to values computed with a detailed kinetics model. With mixtures of an equivalence ratio larger than 0.75, it was observed that more advanced methods that take flame stretch effects into account have no significant advantage over the methodology followed in the present work. At an equivalence ratio of less than 0.75, the laminar burning velocity obtained by the latter was found to be higher than that produced by the former, but at the same time close enough to the unstretched laminar burning velocity to be considered as an acceptable conservative estimate for purposes related to fire and explosion safety. It was furthermore observed that the experimental pressure–time curves of deflagrating hydrogen–air mixtures contained pressure oscillations of a magnitude in the order of 0.25 bar. This phenomenon is explained by considering the velocity of the burnt mixture induced by the expansion of combusting fluid layers adjacent to the wall.     

Prediction of flammability limits of fuel-air and fuel-air-inert mixtures from explosivity parameters in closed vessels

Flammability limits of fuel-air and fuel-air-inert gaseous mixtures, especially at non-atmospheric conditions, are essential properties required for establishing safety operating conditions for handling and processing flammable gases. For pure fuels, an important data pool exists, formed by the flammability limits of fuel-air and fuel-air-inert gaseous mixtures at ambient initial conditions measured by standard methods. Such methods can be used for experimental determination of flammability limits for multi-fuels mixed with air, with or without additives, under non-atmospheric conditions. Their use is however a time- and material-consuming process; in addition, the flammability limits obtained by various standard methods may be scattered as a result of different choices in the operating parameters, for each standard method. It appears that a preliminary estimation of the flammability limits for fuel-air and fuel-air-inert gaseous mixtures can minimize the effort of measuring them in specific initial conditions.The present paper describes a new method for estimating the flammability range of fuel-oxidizer gaseous mixtures based on measurements of explosivity properties e.g. the peak explosion pressure and maximum rate of pressure rise recorded during closed vessel laminar explosions of fuel-oxidizer mixtures far from limits. Data obtained for several hydrocarbon-air gaseous mixtures with or without inert gas addition are used to examine the accuracy of estimated flammability limits (LFL – the lower and UFL – the upper flammability limit) as well as of the Limiting Oxygen Concentration (LOC) and the Minimum Inert Concentration (MIC). The predictive ability of the proposed method is examined against the predictive ability of other recently described methods.     

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.     

Effect of the concentration distribution on the gaseous deflagration propagation in the case of H2/O2 mixture

The investigation of flame propagation accompanying the explosions of unconfined gaseous reactive clouds which are diluted in atmosphere ambient is a fundamental interest in the analysis of industrial risk assessment.Following the previous work [Sochet, I., Guelon, F., Gillard, P. (2002). Deflagrations of non-uniform mixtures: A first experimental approach, Journal of physics, 12, 7–273, 7–280], an experimental study is conducted on a deflagration of a hydrogen/oxygen gaseous cloud which is released in air. The burning velocity is directly or indirectly measured. The flammability limits of the non homogeneous cloud has been as well investigated.     

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.     

Combustion hazard of mixing ammonia with nitric oxide

Premixed ammonia/nitric oxide flame was simulated using the Lindstedt 1994 and Miller–Bowman 1989 reaction mechanisms in CHEMKIN. The predicted laminar burning velocities compared well with limited measured values in the literature. The effects of unburnt mixture temperature and pressure on laminar burning velocity, flammability limits, adiabatic flame temperature and species profiles were studied. The unburnt mixture temperature had a positive impact on both the laminar burning velocity and the adiabatic flame temperature, and it extended the ammonia-rich flammability limit. The pressure had a marginally negative influence on the laminar burning velocity, while it had a slightly positive effect on the adiabatic flame temperature.     

Simulation of dust explosions in complex geometries with experimental input from standardized tests

The present paper describes the development of a new CFD-code (DESC) for the assessment of accidental hazards arising from dust explosions in complex geometries. The approach followed entails the estimation of the laminar burning velocity of dust clouds from standardized laboratory-scale tests, and its subsequent use as input to the combustion model incorporated in DESC. The methodology used to obtain the laminar burning velocities is demonstrated by igniting turbulent propane-air mixtures to deflagration in a standard 20-litre USBM-vessel, and extracting the laminar burning velocity from the pressure–time curves; the results are compared with literature data. Laminar burning velocities for clouds of maize starch dust in air were estimated following the same procedure, and the resulting empirical model was used to simulate dust explosions in a 236-m³ silo.     

Estimation of turbulent flame speed during DME/air premixed gaseous explosions

DME is thought to be a good alternative fuel due to its cleanliness and more excellent fuel economy. Although the prediction and loss prevention of flammability hazard is very important for safety of DME installations, the evaluation method with sufficient accuracy has not been established. In this study, a numerical combustion model is constructed and a 3-dimensional computational fluid dynamics (CFD) simulation of a premixed DME/air explosion in a large-scale domain is conducted. The main feature of the numerical model is the solution of a transport equation for the reaction progress variable using a function for turbulent flame velocity which characterizes the turbulent regime of propagation of free flames derived by introducing the fractal theory. The model enables the calculation of premixed gaseous explosion without using fine mesh of the order of micrometer, which would be necessary to resolve the details of all instability mechanisms. The value of the empirical constant contained in the function for turbulent flame velocity is evaluated by analyzing the experimental data of LPG/air and DME/air premixed explosions. The comparison of flame behavior between the experimental result and numerical simulation shows good agreement.     

Effect of self-inhibition at a flame propagation in rich gaseous mixtures of combustible–air–diluent

An analysis of published experimental data characterizing the influence of diluents of various chemical nature (both halogenated and chemically inert) on upper flammability limits of flammable gases in air (methane and hydrogen have been considered as examples) has been done. Dependences of critical oxygen concentration and critical fuel equivalence ratio _cr for mixtures combustible–air–diluent at upper flammability limits on diluent concentration were determined. The obtained data were interpreted on the basis of the concept of self-inhibition at combustion of rich mixtures of organic combustibles in air. A method for evaluation of relative effectiveness of various inhibitors and for determination of availability of self-inhibitive properties of a combustible gas has been proposed.     

非线性拉伸对甲烷-空气预混火焰燃烧特性的影响

使用定容燃烧弹与高速纹影照相系统研究了不同当量比下甲烷-空气预混气体的层流火焰燃烧特性。实验数据同时应用传统线性模型和非线性模型分析了不同当量比对球形扩展火焰的传播速率和马克斯坦长度的影响。结果显示:随着当量比的增加,层流燃烧速率先增大后减小,直到当量比为1.1时,火焰速率达到最大值。马克斯坦长度始终为正值,且随着当量比的增大而增大。在所有当量比条件下,线性和非线性方法计算的火焰速率大致相同,差值小于0.01 m/s;线性方法得到的马克斯坦长度均大于非线性模型计算的结果,并随着当量比的增大,两种方法得到的马克斯坦长度的差值更加显著。     

Numerical simulation of flame propagation and localized preflame autoignition in enclosures

A novel computational approach based on the coupled 3D Flame-Tracking–Particle (FTP) method is used for numerical simulation of confined explosions caused by preflame autoignition. The Flame-Tracking (FT) technique implies continuous tracing of the mean flame surface and application of the laminar/turbulent flame velocity concepts. The Particle method is based on the joint velocity–scalar probability density function approach for simulating reactive mixture autoignition in the preflame zone. The coupled algorithm is supplemented with the database of tabulated laminar flame velocities as well as with reaction rates of hydrocarbon fuel oxidation in wide ranges of initial temperature, pressure, and equivalence ratio. The main advantage of the FTP method is that it covers both possible modes of premixed combustion, namely, frontal and volumetric. As examples, combustion of premixed hydrogen–air, propane–air, and n-heptane–air mixtures in enclosures of different geometry is considered. At certain conditions, volumetric hot spots ahead of the propagating flame are identified. These hot spots transform to localized exothermic centers giving birth to spontaneous ignition waves traversing the preflame zone at very high apparent velocities, i.e., nearly homogeneous preflame explosion occurs. The abrupt pressure rise results in the formation of shock waves producing high overpressure peaks after reflections from enclosure walls.     

Numerical studies on minimum ignition energies in methane/air and iso-octane/air mixtures

In this study, the dependence of minimum ignition energies (MIE) on ignition geometry, ignition source radius and mixture composition is investigated numerically for methane/air and iso-octane/air mixtures. Methane and iso-octane are both important hydrocarbon fuels, but differ strongly with respect to their Lewis numbers. Lean iso-octane air mixtures have particularly large Lewis numbers. The results show that within the flammability limits, the MIE for both mixtures stays almost constant, and increases rapidly at the limits. The MIEs for both fuels are also similar within the flammability limits. Furthermore, the MIEs of iso-octane/air mixtures with a small spherical ignition source increase rapidly for lean mixtures. Here the Lewis number is above unity, and thus, the flame may quench because of flame curvature effects. The observations show a distinct difference between ignition and flame propagation for iso-octane. The minimum energy required for initiating a successful flame propagation can be considerably higher than that required for initiating an ignition in the ignition volume. For iso-octane with a small spherical ignition source, this effect was observed at all equivalence ratios. For iso-octane with cylindrical ignition sources, the phenomenon appeared at lower equivalence ratios only, where the mixture's Lewis number is large. For methane fuel, the effect was negligible. The results highlight the significance of molecular transport properties on the decision whether or not an ignitable mixture can evolve into a propagating flame.     

A sneak peek into the phenomenology of fuel mist explosions: The key role of vapor fractions

The modern world depends greatly on hydrocarbons, which are ubiquitous, indispensable fuels used in nearly every existing industry. Although important, their use may trigger dangerous incidents, whether in their production, handling, storage, or transporting phase, especially when aerosolized. In light of proposing a standard procedure to assess the flammability and explosivity of fuel mists, a new test method was established based on the EN 14034 standards series. For the previous purposes, a gravity-fed mist generation system was designed and employed in a modified 20 L explosion vessel. This test method allowed the determination of the ignition sensitivity of several fuels. In addition, their explosion severity was represented by the explosion overpressure P_ex, and the rate of pressure rise dP/dt_ex, two thermo-kinetic parameters determined with a specifically developed control system and custom software. Nonetheless, a noticeable difference in the ignition sensitivity and the explosion severity was perceived when changing suppliers or petroleum cuts of some fuels. Moreover, sensitivity studies showed that both the droplet size distribution and the temperature of the droplets play a significant role in fuel mist explosion. These parameters can be directly related to the vapor fraction surrounding a droplet during its ignition. Consequently, this study focuses on the influence of varying the composition of three well-known and abundantly used fuels. Different petroleum cuts were introduced in different fractions into isooctane, Jet A1 aviation fuel, and diesel fuel mixtures, which were then aerosolized into a uniformly distributed turbulent mist cloud and ignited using spark ignitors of 100 J. Subsequently, complementary tests were executed in a vertical flame propagation tube coupled with a high-speed video camera allowing the visualization of the flame and the determination of the spatial flame velocity, and a tentative estimation of the laminar burning velocity. The latter was also estimated from the pressure-time evolution in the 20 L sphere using existing correlations. Indeed, the determination of the laminar burning velocity can be useful in modeling such accidents. Finally, highlighting the essential role of the mist and vapor fraction during their ignition has led to a better understanding of their explosion mechanisms.     

Explosion behavior of hydrogen–methane/air mixtures

The effects of enriching natural gas with hydrogen on local flame extinction, combustion instabilities and power output have been widely studied for both stationary and mobile systems. On the contrary, the issues of explosion safety for hydrogen–methane mixtures are still under investigation.In this work, experimental tests were performed in a 5 L closed cylindrical vessel for explosions of hydrogen–methane mixtures in stoichiometric air. Different compositions of hydrogen–methane were tested (from pure methane to pure hydrogen) at varying initial pressures (1, 3 and 6 bar).Results have allowed the quantification of the combined effects of both mixture composition (i.e., hydrogen content in the fuel) and initial pressure on maximum pressure, maximum rate of pressure rise and burning velocity. The measured burning velocities were also correlated by means of a Le Chatelier’s Rule-like formula. Good predictions have been obtained (at any initial pressure), except for mixtures with hydrogen molar content in the fuel higher than 50%.     

Determination of turbulent burning velocities of dust air mixtures with the open tube method

Correlating turbulent burning velocity to turbulence intensity and basic flame parameters-like laminar burning velocity for dust air mixtures is not only a scientific challenge but also of practical importance for the modelling of dust flame propagation in industrial facilities and choice of adequate safety strategy. The open tube method has been implemented to measure laminar and turbulent burning velocities at laboratory scale for turbulence intensities in the range of a few m/s. Special care has been given to the experimental technique so that a direct access to the desired parameters was possible minimising interpretation difficulties. In particular, the flame is propagating freely, the flame velocity is directly accessible by visualisation and the turbulence intensity is measured at the flame front during flame propagation with special aerodynamic probes. In the present paper, those achievements are briefly recalled. In addition, a complete set of experiments for diametrically opposed dusts, starch and aluminium, has been performed and is presented hereafter. The experimental data, measured for potato dust air mixtures seem to be in accordance with the Bray Gülder model in the range of 1.5 m/s<u′<3.5 m/s. For a further confirmation, the measurement range has been extended to lower levels of turbulence of u′<1.5 m/s. This could be achieved by changing the mode of preparation of the dust air mixture. In former tests, the particles have been injected into the tube from a pressurised dust reservoir; for the lower turbulence range, the particles have been inserted into the tube from above by means of a sieve–riddler system, and the turbulence generated from the pressurised gas reservoir as before. For higher levels of turbulence, aluminium air mixtures have been investigated using the particle injection mode with pressurised dust reservoir. Due to high burning rates much higher flame speeds than for potato dusts of up to 23 m/s have been obtained.     

Flammability of kerosene in civil and military aviation

The investigation of the ignition conditions of kerosene vapors in the air contained in an aircraft fuel tank contributes to the definition of onboard safety requirements. Civil and military kerosene are characterized by specification. The specification of civil aviation kerosene is based upon usage requirements and property limits. while military kerosene is primarily controlled by specific chemical composition. Characterization of the flammability properties is a first step for the establishment of aircraft safety conditions. Flash point, vapor pressure, gas chromatography analysis, and flammability properties of the kerosene used by the French Military aviation (F-34 and F-35 kerosene) are compared with the flammability properties of civil kerosene. The empirical law established by the Federal Aviation Administration (FAA) in 1998, expressing the ignition energy in terms of fuel, temperature, flash point and altitude is modified and expressed in terms of fuel temperature, flash point and pressure.     

2,5-二甲基呋喃-空气燃烧过程研究

利用球形发展火焰研究了常温常压下不同当量比,不同相态时2,5-二甲基呋喃-空气的层流燃烧速度和马克斯坦长度,分析了火焰拉伸对火焰传播速度的影响。研究结果表明:随着当量比的增加,2,5-二甲基呋喃-空气混合气的马克斯坦长度减少,火焰的稳定性减弱。并且分别计算出当量比为1.25和1.5的层流燃烧速度,分别为:1.189m/s,1.135m/s.。对于同一当量比1.5的情况下,不同相态的2,5-二甲基呋喃-空气混合物,在相同时刻的气液两相混合物的火焰半径已经拉伸火焰传播速度远远大于纯气相的混合物。     

An analytical model for predicting counterflow flame propagation through premixed dust micro particles with radiative heat loss

In this paper, an analytical model has been performed to scrutinize the structure of the flame propagation in counterflow configuration where the mixture of solid fuel particles and air are injected as opposed streams. The structure of counterflow premixed flame in a symmetric configuration, containing uniformly distributed volatile fuel particles, with nonunity Lewis number is examined with considering radiative heat loss effect in counterflow configuration with strain rate issue. The flame structure governing equations, required boundary conditions, and matching conditions are applied for each zone in order to solve the differential equations. The flame position is determined, mass fraction of solid particles and gaseous phases, effect of Lewis number change on the gaseous and solid fuel mass fraction distribution, and the role of strain rate, and different particle diameters are investigated with and without considering thermal radiation effect. In addition, the effect of equivalence ratio on the flame temperature, mixture temperate and non-dimensional flame position is investigated in counterflow flame propagation. According to our finding, the burning velocity of counterflow flame remarkably increases as a function of vaporization Damköhler number as well as non-dimensional vaporization temperature with considering thermal radiation effect in counterflow domain.     

水平燃料填充床反向阴燃传播及其熄灭分析

根据单步反应机理(仅包括燃料的氧化),建立了一维非稳态燃料填充床反向阴燃的数学模型。通过简化模型参数及大活化能渐近分析,得出了定性描述燃料反向阴燃传播的两个方程。结果表明:随着空气流量的增大,阴燃温度是不断上升的,但由于受到反向空气风流的影响,阴燃温度的增长幅度是逐渐变小的;阴燃传播速度却呈现出先增大后减小直至熄灭的变化趋势。这种变化趋势与前人的实验结果相一致。通过定性分析得出:在气体流量为零的情况下,燃料仍然可以发生阴燃,而维持阴燃不断传播所需要的氧气量源于反应区域周围气体的扩散。此外,也分析了燃料特性参数(如密度、孔隙率、比热、导热系数及活化能)对燃料阴燃温度和阴燃速度传播的影响。