Experimental investigation of the lower flammability limit of volatile liquid fuel-aluminum powder mixtures in air

Aluminum powder was always chosen as an additive to improve the explosive performance. In this work, experiments were performed to investigate the lower flammability limit (LFL) of volatile liquid fuel-aluminum powder mixtures using a 20 L closed spherical stainless steel vessel at a temperature of 20 °C (293 K) and 40 °C (313 K). The volatile liquid fuels tested in the work were diethyl ether (DEE), epoxypropane (PO), n-pentane and n-hexane. DEE, PO and n-pentane are in the liquid phase at room temperature and can easily transition to the gas phase at 40 °C (313 K). Through a series of experiments carried out, it was found that the change in phase would affect the interaction between the components. Aluminum powder always has an inhibitory effect on the flammability of the mixtures when it is mixed with gas-phase fuels. The inhibition effect was most obvious when the aluminum powder concentration reached 200 g/m³. While the interaction between aluminum powder and liquid-phase volatile fuels was promotion and was influenced by the component proportion and the type of the volatile fuels.     

Flow and flame visualization near the upper flammability limits of methane/air and propane/air mixtures at elevated pressures

In this study, direct visualization of flow and flame from the ignition of methane/air and propane/air mixtures near the UFL at elevated pressures of up to 2.0 MPa were obtained with a test cell comprised of double-sided plexiglass and a containment vessel with double-sided glass. These visualizations allowed direct observations of ignition and flame near UFL at elevated pressures. Two distinctive features were observed in ignition at elevated pressures that differ from those under ambient pressure: the hot igniter formed a convective plume, rather than a convection cell; and the flame initiated from the top of the test cell and propagated downwards, rather than directly from the igniter. Both these distinctive features are characteristics of convection at high Rayleigh number accompanied with increased gas density at elevated pressures. Our study also shows that visualization of the formation of planar flame provides the most objective criterion for defining flammability limits at elevated pressures.     

An investigation of flammability limits of gaseous mixtures at reduced pressures

An experimental investigation of flammability limits of hydrogen, methane and propane in air and oxygen at reduced pressures was carried out. A slow influence of sizes of an experimental vessel of a diameter higher than 125 mm on the flammability limits was revealed, but an influence of a type of an oxidizer (air or oxygen) and an ignition energy is significant. Critical values of an initial pressure for a possibility of a flame propagation were determined. The limiting values of the ignition energy were determined, for which an elevation of this parameter does not influence the critical pressure and the flammability region. A qualitative interpretation of obtained experimental results is given, which is based on a peculiarities of a flame initiation.     

A correlation of the lower flammability limit for hybrid mixtures

Hybrid mixtures are widely encountered in industries such as coal mines, paint factories, pharmaceutical industries, or grain elevators. Hybrid mixtures explosions involving dust and gas can cause great loss of lives and properties. The lower flammability limit (LFL) is a critical parameter when conducting a hazard assessment or developing mitigation methods for processes involving hybrid mixtures. Unlike unitary dust or gas explosions, which have been widely studied in past decades, only minimal research focuses on hybrid mixtures, and data concerning hybrid mixtures can rarely be found. Although methods to predict the LFL have been developed by using either Le Chatelier's Law, which was initially proposed for homogeneous gas mixtures, or the Bartknecht curve, which was adopted for only certain hybrid mixtures, significant deviations still remain. A more accurate correlation to predict an LFL for a hybrid mixtures explosion is necessary for risk assessment. This work focuses on the study of hybrid mixtures explosions in a 36 L dust explosion apparatus including mixtures of methane/niacin, methane/cornstarch, ethane/niacin and ethylene/niacin in air. By utilizing basic characteristics of unitary dust or gas explosions, a new formula is proposed to improve the prediction of the LFL of the mixture. The new formula is consistent with Le Chatelier's Law.     

Validation of a new formula for predicting the lower flammability limit of hybrid mixtures

A correlation of the lower flammability limit for hybrid mixtures was recently proposed by us. The experimental conditions including ignition energy and turbulence which play a primary role in a gas or dust explosion were at fixed values. The sensitivity of such experimental conditions to the accuracy of the proposed formula was not thoroughly discussed in the previous work. Therefore, this work studied the effect of varying the ignition energy and turbulence intensity to the formula proposed in our previous paper. For ignition energy effect, results from methane/niacin mixture demonstrated that the MEC and LFL will not be affected by changing ignition energy. There is no distinguishable difference among gas explosion index (K_G) and dust explosion index (K_St) derived from tests with every ignition energy (2.5 kJ, 5 kJ and 10 kJ) in a 36 L vessel. The proposed formula is independent of ignition energy. For turbulence effect, the proposed formula can have a good prediction of the explosion and non-explosion zone if the ignition delay time is within a certain range. The formula prediction is good as the ignition delay time increases up to 100 ms in this work. Propane/niacin and propane/cornstarch mixtures are also tested to validate the proposed formula. It has been confirmed that the proposed formula predicts the explosion and non-explosion zone boundary of such mixtures.     

Experimental investigation on the lower flammability limits of diethyl ether/ n-pentane/epoxypropane-air mixtures

Diethyl ether (DEE), epoxypropane (PO) and n-pentane have excellent ignition and combustion performance; hence, they have a wide variety of applications in industry and advanced aviation propulsion systems. As these fuels are flammable at normal temperature and pressure, their explosive characteristics need to be explored. In this study, the lower flammability limits (LFLs) of vapor mixtures of DEE/PO/n-pentane in air were measured in 20 L, closed, stainless steel spherical vessels. Experimental results were obtained at ambient atmospheric pressure and an initial temperature of 40 °C. The experimental results show that the LFLs of DEE-air, n-pentane -air, and PO-air are 1.81 vol%, 1.41 vol% and 2.44 vol%, respectively. The LFLs of binary/ternary fuel mixtures under different compositions were tested, and the experimental results are compared with the classical Le Chatelier's formula. The results show that, for the binary fuels (i.e., DEE/PO, DEE/n-pentane, PO/n-pentane)-air mixtures, the maximum difference of the LFLs between Le Chatelier's formula and the experimental results is 6.10%. For the ternary fuels (i.e., DEE/PO/n-pentane)-air mixtures, the maximum difference of the LFLs between the two results is 6.33%. Due to the adiabatic flame temperature of each single fuel mixture being close, the Le Chatelier's formula is applicable for an estimation of the LFL for DEE/PO/n-pentane-air mixtures.     

Theoretical evaluation of lower explosion limit of hybrid mixtures

Lower explosion limits of hybrid fuel mixtures are usually determined through time consuming and expensive experiments. Although, mathematical expressions like Le-Chatelier's Law and Bartknecht curve have been used by many researchers to predict the LEL of hybrid mixtures, significant deviations remain unexplained. This research work, presents a more sophisticated and general approach for the determination of LEL of hybrid mixtures.Assuming that the combustion kinetics of pure species are independent and unchanged by the presence of other combustible species, complete conversion of the reactants and no heat losses, a simple mathematical model has been derived from the enthalpy balance of the whole system. For the experimental validation of the modelled values, modified version of 20L sphere has been employed, following the European standard (EN 14034-3: 2011) as experimental protocol. Hybrid mixtures of three dusts with two gases were selected for the scope of this publication. By analyzing the modelled as well as the experimental values, it can be concluded that the LEL values of the individual components in the hybrid mixture set the upper and lower limit for the LEL of the hybrid mixture provided the total amount of fuel in the system is considered as the concentration of the hybrid mixture. Moreover, the amount of dust or gas required to render the hybrid mixture flammable mainly depends on the energy contribution upon combustion of the individual species to raise the temperature of the whole system from ambient to the flame temperature.Le-Chatelier's Law and Bartknecht curve are empirical relations, which might hold true for a first-order approximation of LEL of hybrid mixtures, but do not represent the most conservative values of LEL reported in literature. This implies that there is a non-zero probability of occurrence of an explosible mixture in the non-explosible concentrations ranges defined by these relations. Considering these arguments, the authors suggest to employ the model presented in this paper – which presents reasonably conservative values of LEL of hybrid mixtures – for theoretical calculation of LEL of hybrid mixtures, when no precise experimental data is available.     

The explosion parameters of methanol under variable pressures and 423 K

The high-temperature and high-pressure methanol one-step oxidation has been the primary process for the mass production of dimethoxymethane. However, the risk of explosion for this process is still not properly defined. This paper presents new results from the experimental study on the explosion characteristics, including the explosion pressure and the explosion limits for methanol/air mixtures with a variable oxygen level, under an initial pressure between 0.3 MPa and 0.75 MPa and at the initial temperature of 423 K. The upper explosive limits were found to increase along with the initial pressure. If the limits for normal air are known, the oxygen effect on flammability is predictable from the thermal balance method. With a correlation for the pressure effect and a method for the oxygen effect, we can have the flammable range predictable.     

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.     

Distinction between the upper explosion limit and the lower cool flame limit in determination of flammability limit at elevated conditions

Previous research showed that at certain conditions, close to the flammability range exists a regime where cool flame may develop either due to elevated temperature or it may be initiated by an ignition source. Propagation of the cool flame in a closed test vessel may double the initial pressure. Such pressure increase exceeds recommended ignition criteria for explosion limit determination that are based on 5 or 7% of pressure rise leading to inaccurate classification of the oxidation phenomena, i.e. cool flame propagation may be classified as hot flame propagation.Two mixtures were tested: n-butane-oxygen (extensively) and C1–C2–oxygen (in limited range), which represent a typical composition in ethylene oxide production, at elevated conditions at their upper explosion limits. Flame development was analysed by flame emission spectroscopy and the post-oxidation mixture was analysed by gas chromatography (GC) to characterise the oxidation mechanism of the flame. Additionally explosion pressure rise, flame temperature, and maximum rate of pressure rise were measured. In all experiments with the pressure rise ratio below two the low temperature oxidation mechanism assisted the flame propagation.     

The lower explosion point—A good measure for explosion prevention: Experiment and calculation for pure compounds and some mixtures

It is well known that explosive mixtures can exist at temperatures below the flash point (FP). Experiments show that the difference between the FP and the lower explosion point (LEP) may be up to 15 K and in some special cases even more. Consequently, an industrial process even running a few Kelvin below the FP may operate in an explosive vapour/air-atmosphere. Operating at temperatures below the LEP gives sufficient safety. Calculated LEPs are discussed and compared with experimental results.     

Effect of initial pressure on the lower explosion limit of nicotinic acid/acetone mixture

In the work presented in this paper, the effect of initial pressure on the lower explosion limit (LEL) of the hybrid nicotinic acid/acetone mixture was investigated through standard explosion tests carried out in the 20 L sphere. From experimental results, the flammability diagram was built in the plane (concentration/minimum explosive concentration) of nicotinic acid versus (concentration/LEL) of acetone. Interestingly, it has been found that, in going from low pressures (P < 1 atm) to high pressures (P > 1 atm), the extension of the flammability region increases. This behavior has been attributed to the fact that the turbulence kinetic energy (and thus the energy dissipation) decreases with increasing initial pressure. Bartknecht's correlation for LEL of hybrid mixtures was modified to take into account the effect of pressure, and two correlations were obtained able to give satisfactory predictions of experimental data at both low pressures and high pressures.     

Effects of N2 and CO2 on the flammability of 2,3,3,3-tetrafluoropropene at elevated temperatures

The flammability characteristics of refrigerants are affected by environmental factors, making them prone to flammability and explosion accidents in cooling systems. In this paper, the flammability characteristics of R1234yf–air mixtures with N₂ and CO₂ were investigated comparatively at temperatures between 20 and 50 °C at 80% relative humidity. The lower and upper flammability limits of R1234yf were measured. The limiting oxygen concentration (LOC), critical flammable ratio (CFR), and critical flammable concentration (CFC) of the R1234yf–air mixtures with inert gases were investigated. The paper developed a linear formula between the flammability limit of R1234yf and the temperature. The changes in CFC with different temperatures were negligible for R1234yf. Furthermore, the mixed refrigerant had both non-flammability and the lowest vapor pressure when the CFR of the R1234yf/CO₂ mixture was 2.9. The experimental results were used to propose a new prediction model to estimate the flammability limits of R1234yf. Finally, molecular simulation explained the effect of inert gases on the flammability of R1234yf from a microscopic point of view. The research aimed to provide valid evidence and data for preventing flammable and explosive refrigerant incidents.     

三氧化钼与有机蒙脱土复配体系在膨胀阻燃环氧树脂中的应用

为了提高膨胀阻燃环氧体系的阻燃和抑烟效率,利用聚磷酸铵-季戊四醇-三聚氰胺为膨胀阻燃剂、有机蒙脱土(OMMT)和MoO3为复配协效剂制备了膨胀阻燃环氧树脂,通过极限氧指数(LOI)、UL94、锥形量热仪和烟密度试验研究了膨胀阻燃环氧树脂的阻燃和抑烟性能。结果表明,单独添加OMMT或MoO3均能有效提高膨胀阻燃环氧树脂的LOI并降低燃烧过程中的热释放和生烟量,将二者复配使用还表现出较好的协效作用。添加质量分数1.5%OMMT和1.5%MoO3时,膨胀阻燃氧树脂的LOI达到27.8%,UL94达到V-0级,总释放热(THR)和总产烟量(TSR)相比未添加协效剂的膨胀阻燃环氧树脂分别下降了49.5%和57.8%。热重分析表明,单独添加OMMT或MoO3均能有效提高膨胀阻燃EP的热稳定性和成炭率,二者复配使用则表现出更高的初始分解温度并形成更多的残炭量。扫描电镜和红外光谱分析发现,OMMT和MoO3复配使用能促进膨胀阻燃氧树脂在燃烧过程中形成更多的交联结构以增强炭层的致密性和隔热性能,达到协效阻燃和抑烟作用。     

On the explosion and flammability behavior of mixtures of combustible dusts

In the work presented in this paper, the explosion and flammability behavior of combustible dust mixtures was studied. Lycopodium, Nicotinic acid and Ascorbic acid were used as sample dusts.In the case of mixtures of two dusts, the minimum explosive concentration is reproduced well by a Le Chatelier's rule-like formula, whereas the minimum ignition energy is a linear combination of the ignition energies of the pure dusts.An unexpected behavior has been found in relation to the explosion behavior and the reactivity. When mixing Lycopodium and Nicotinic acid or Ascorbic acid, the rate of pressure rise of the mixture is much higher than the rate of pressure rise obtained by linearly averaging the values of the pure dusts (according to their weight proportions), thus suggesting that strong synergistic effects arise; but it is comparable to that of the most reactive dust in the mixture.The observed behavior seems to be linked to the presence of minerals in the Lycopodium particles which catalyze oxidation reactions of Nicotinic acid and Ascorbic acid, as suggested by TG analysis.In the case of mixtures of three dusts, a similar behavior is observed when the concentration of Lycopodium is twice that of the other two dusts.     

Acceleration sensitivity of piezoelectric pressure sensors and the influence on the measurement of explosion pressures

To measure the explosion pressure inside an enclosure, it is common to install a piezoelectric pressure sensor in the enclosure wall. The pressure wave of the internal explosion inevitably leads to vibrations of the enclosure walls. This unwanted but naturally occurring motion is also transmitted to the pressure sensor mounted in the enclosure wall and results in inertial forces affecting the piezoelectric element. During the measurement of the explosion pressure, this affects the output signal of the pressure sensor since an undesired signal due to the acceleration of the pressure sensor is superimposed on the desired pressure signal. This behaviour of the sensor is described as acceleration sensitivity. The level of acceleration sensitivity depends on the type and construction design of the pressure sensor. Even though this sensor behaviour is basically not a new phenomenon, the evaluation of an international comparison between Ex testing laboratories in the field of flameproof enclosures has shown that the consideration of this issue is a major challenge in daily practice concerning the measurement of explosion pressures and is even often completely neglected.This work evaluates the behaviour of various piezoelectric pressure sensors with respect to the influence of acceleration and investigates the specific impact on the explosion pressure measurement in the field of flameproof enclosures. For this purpose, explosions from typically used explosive mixtures such as hydrogen, propane and ethyne in air are examined. These investigations involve simple model enclosures with various specifications as well as a commercially available equipment for hazardous areas. By using blind holes and specially designed adapters, a practical method is applied to be able to detect the effect of acceleration on the sensor signal separately from the pressure signal. For this purpose, both the discrete-time pressure curves and the frequency components are analysed using Fast Fourier Transform. The use of signal filters as a practical and fast approach to address these unwanted signal components is discussed and evaluated.This paper provides guidelines for typical end-users in the field of flameproof enclosures how to handle acceleration of piezoelectric pressure sensors and the influence on the measurement of explosion pressures correctly.     

乙醇汽油自燃温度及爆炸极限测试

为评估不同标号乙醇汽油(E10)的燃爆危险特性,补充完善乙醇汽油的技术指标,首次采用AIT551自燃温度测试仪和FRTA爆炸极限测试仪测试了E10的自燃温度和爆炸极限,并分析了温度对乙醇汽油爆炸极限的影响规律。结果表明:90号、93号、97号E10自燃温度分别为373℃、339℃、373℃,对应着火延迟时间为8 s、9 s、8s;90号、93号、97号E10的爆炸范围分别为1.223%~8.292%、1.343%~8.893%、1.294%~8.546%;温度从20℃升高至120℃,93号E10的爆炸范围从1.491%~8.765%变宽至1.318%~9.103%,即E10蒸气爆炸极限范围随温度升高而变宽;推导了测量E10蒸气爆炸极限时待测样品量预估计的公式。     

Influences of coal dust components on the explosibility of hybrid mixtures of methane and coal dust

In order to study the influences of coal dust components on the explosibility of hybrid mixture of methane and coal dust, four kinds of coal dust with different components were selected in this study. Using the standard 20 L sphere, the maximum explosion pressure, explosion index and lower explosion limits of methane/coal dust mixtures were measured. The results show that the addition of methane to different kinds of coal dust can all clearly increase their maximum explosion pressure and explosion index and decrease their minimum explosion concentration. However, the increase in the maximum explosion pressure and explosion index is more significant for coal dust with lower volatile content, while the decrease in the minimum explosion concentration is more significant for coal dust with higher volatile content. It is concluded that the influence of methane on the explosion severity is more pronounced for coal dust with lower volatile content, but on ignition sensitivity it is more pronounced for coal dust with higher volatile content. Bartknecht model for predicting the lower explosion limits of methane/coal dust mixture has better applicability than Le Chatelier model and Jiang model. Especially, it is more suitable for hybrid mixtures of methane and high volatile coal dust.     

高温环境下18650型锂离子电池热失控过程的数值分析

采用ANSYS对高温条件下铜棒代替锂离子电池的空白试验进行数值模拟,获得拟合的陶瓷化纤棉毯的比热容;然后对18650型锂离子电池的热响应进行模拟,通过模拟结果与试验数据的比较分析,获得锂离子电池内部的反应放热量;最后应用得出的陶瓷化纤棉毯比热容和化学反应热对高温环境下18650型锂离子电池的热失控进行模拟,研究18650型锂离子电池热失控的变化规律.结果表明:20W加热条件下,锂离子电池的放热反应热为30 kJ;锂离子电池在加热1 287 s后发生热失控,热失控持续113 s后锂离子电池温度达最高,之后开始缓慢减小;锂离子电池热失控温度为500 K,热失控前温度几乎是线性增加,之后热失控导致温度迅速增加(呈指数倍增长);锂离子电池保温材料陶瓷化纤棉毯的温度变化是非线性的.