Experimental studies on the ignition behavior of pure silane released into air

Silane is a well known pyrophoric gas which normally ignites upon contact with air. However, a silane release from a pressure source may not always lead to prompt ignition and frequently the ignition occurs when the release is shutoff. In a confined space, significant quantities of silane can accumulate prior to autoignition leading to an explosion, causing significant damage. To date, the mechanism and condition of pure silane ignition upon release into air has not been completely explained. Thus there is a need for additional experimental investigations covering a wide range of release conditions to enable a full understanding of silane ignition and explosion characteristics.This work presents a series of tests that aims to uncover the precise condition for pure silane ignition upon release into air. Tests were performed for releases at controlled and steady velocities. Steady flow of silane to a burn box and nitrogen to the desired vent stub are first established through mass flow controllers. An electrically controlled four-way switching valve is used to switch the silane and nitrogen flow such that steady silane flow without acceleration to the vent stub can be established. With careful control of vent conditions, we are able to find a reproducible critical exit velocity for prompt ignition of steady silane releases for different vent diameters. If the releases are reduced to below the critical exit velocity, prompt ignition of silane release is ensured. Above this critical exit velocity, silane can be released indefinitely into air without any ignition. The critical exit velocity is found to vary with the vent diameter. This relationship between the critical exit velocity and the vent diameter suggests that the silane release without prompt ignition was most likely caused by quench of the reactive kernel from the scalar dissipation between the release gas and the ambient air. Analysis of locations where prompt ignition occurred from the clips from high speed video camera found that the most reactive ignition kernel occurs in the range when the ratio of volumetric flow rate of entrained air to the silane flow reaches 0.322 ± 0.076, which is equivalent to the most reactive silane concentration of 75.6% in air. The implications from these results are discussed with emphasis on the safety of silane supply systems and operation practices.     

Numerical study of the effect of obstacles on the spontaneous ignition of high-pressure hydrogen

A numerical simulation of the spontaneous ignition of high-pressure hydrogen in a duct with two obstacles on the walls is conducted to explore the spontaneous ignition mechanisms. Two-dimensional rectangular ducts are adopted, and the Navier–Stokes equations with a detailed chemical kinetic mechanism are solved by using direct numerical simulations. In this study, we focus on the effects of the initial pressure of hydrogen and the position of the obstacles on the ignition mechanisms. Our results demonstrate that the presence of obstacles significantly changes the spontaneous ignition mechanisms producing three distinct ignition mechanisms. In addition, the position of the obstacles drastically changes the interaction of shock waves with the contact surface, and spontaneous ignition may take place at a relatively low pressure in some obstacle positions, which is attributed to the propagation direction and interaction timing of two reflected shock waves.     

3D simulation of hydrogen ignition in a rapid compression machine

Low-temperature (at T < 900–950 K) ignition delays of hydrogen–air mixtures are mainly measured in rapid compression machines (RCM). This communication is aimed at numerical simulation of ignition delays of hydrogen–air mixtures in the RCM by means of a coupled three-dimensional (3D) Unsteady Reynolds-Averaged Navier–Stokes (URANS) – Particle Method (PM) simulation of RCM operation capable of catching turbulence–chemistry interaction. The study indicates that the time history of piston motion in an RCM affects the final state of a test mixture at the end of compression stroke and therefore influences the phenomena relevant to test mixture ignition. More specifically, the calculations show that different laws of piston motion at a fixed average piston velocity (i.e., fixed piston displacement and fixed compression time) and fixed compression ratio result in different evolution of mean pressure, temperature and velocity fields in the RCM test section leading to different ignition behavior. The reasons for the arising differences lie in the fact that the local instantaneous piston velocity determines the roll-up vortex structure, strength and turbulence dissipation in it, heat transfer in test-section walls, and mass leakage through piston rings.     

Numerical study on the spontaneous ignition of pressurized hydrogen release through a tube into air

Spontaneous ignition of pressurized hydrogen release through a tube into air is investigated using a modified version of the KIVA-3V CFD code. A mixture-averaged multi-component approach is used for accurate calculation of molecular transport. Autoignition and combustion chemistry is accounted for using a 21 step kinetic scheme. Ultra fine meshes are employed along with the Arbitrary Lagrangia–Eulerian (ALE) method to reduce false numerical diffusion. The study has demonstrated a possible mechanism for spontaneous ignition through molecular diffusion.

In the simulated scenario, the tube provided additional time to achieve a combustible mixture at the hydrogen–air contact surface. When the tube was sufficiently long under certain release pressure, autoignition would initiate inside the tube at the contact surface due to mass and energy exchange between low temperature hydrogen and shock-heated air through molecular diffusion. Following further development of the hydrogen jet downstream, the contact surface became distorted. Turbulence plays an important role for hydrogen/air mixing in the immediate vicinity of this distorted contact surface and led the initial laminar flame to transit into a stable turbulent flame.     

Numerical study on the spontaneous ignition of pressurized hydrogen during its sudden release into the tube with varying lengths and diameters

Fuel cell vehicles (FCV) and other hydrogen systems with pressurized hydrogen has a safety hazard of spontaneous ignition during its sudden release into the tube. Tube parameter is a key factor affecting the spontaneous ignition of pressurized hydrogen. In this paper, a numerical study on the spontaneous ignition of pressurized hydrogen during its sudden release into the tube with varying lengths and diameters is conducted. The models of Large Eddy Simulation (LES), Eddy Dissipation Concept (EDC), Renormalization Group (RNG), 10-step like opening process of burst disk and 18-step detailed hydrogen combustion mechanism are employed. 6 cases are simulated based on the previous experiments. Numerical results show that the possibility of spontaneous ignition of pressurized hydrogen increases inside the longer and thinner tubes, which agrees with the experimental results. The increasing of tube length has little influence on the shock wave formation and propagation inside the tube. However, there exists critical tube lengths for the generation of Mach disk and the normal shock wave: the maximum and minimum distances for the generation of the Mach disk in 10 mm diameter tube are 7.8 and 6.7 mm, respectively. As for the normal shock wave, these critical values are 22.1 and 19.4 mm, respectively. In addition, the formation times and initial positions of Mach disk and normal shock wave are delayed inside the thicker tube. Due to the shock-affected time increases with the increasing of tube length, the temperature could rise to the critical ignition temperature and triggers the spontaneous ignition due to the sufficient tube length even though the less hydrogen/air mixture and the contact surface with lower temperature is produced inside the thicker tube. Finally, a simple time scale analysis is conducted.     

自然老化木材燃烧过程热行为特性研究

为提供古建筑火灾数值模拟及风险评估的基础理论,从自然老化角度入手,选取经过长久自然老化的建筑服役木材作为研究对象,通过扫描电镜(SEM)及热物性试验对比分析自然老化木材与参照木材的外观形态及热传导特征;采用差示扫描量热DSC法,研究自然老化木材在燃烧过程中热释放的阶段特性,分析不同升温速率对老化木材热行为影响特征;基于...     

Explosion hazards related to hydrogen releases in nuclear facilities

Hydrogen explosion risk needs to be carefully assessed and evaluated in nuclear facilities because of the potential catastrophic consequences: breakdown of safety equipments, failure of containment, dissemination of radioactive materials in the environment.When studying an indoor release, one possible simplification is to assume a perfect gas mixing inside the room. This assumption is effectively often used to evaluate toxic risks in the environment outside a building (Mastellone, Ponte, & Arena, 2003). However, perfect gas mixing assumption is only a rough approximation, as indoor concentrations can largely differ from mean values, due to buoyancy, recirculation zones or obstacles for example.In order to better evaluate the risk of explosion in case of an accidental release of hydrogen, IRSN conducted a numerical study using FLACS CFD software. Several parameters have been studied to identify dangerous situations and draw a representative picture of the risk: room size, position and direction of hydrogen leak, ventilation characteristics. Hydrogen release flow rates used for numerical simulations have been chosen as the highest leak rate which, by applying the assumption of perfect mixing, produces an average concentration in the room equal to hydrogen lower flammability limit (LFL).Simulation results indicate that in some particular configurations, especially for impinging hydrogen jets, hydrogen concentrations can locally be above LFL and then create explosive atmospheres with significant volumes.     

Thermal evaluation of junction and connection boxes in explosion protection

To reliably avoid potential ignition sources and thus ignition of the potentially explosive atmosphere in junction and connection boxes of type of protection Increased Safety ‘e', the self-heating shall not exceed a specified level depending on the temperature class. For the conformity assessment of such products, complex thermal tests are necessary due to the great variety of mounting types and arrangements of terminal blocks in the enclosure, depending on the enclosure size. These tests are very time-consuming for the manufacturer of junction and connection boxes or for the testing laboratory and are therefore associated with considerable costs. To reduce this effort and to ensure a uniform assessment in the conformity evaluation by a certification body, it is therefore essential to create fitting charts depending on the enclosure size.This work introduces a calculation tool by means of systematic investigations on different enclosure sizes and with different assemblies, which enables the calculation of fitting charts with justifiable effort. For this purpose, the maximum temperature in the enclosure was determined as a function of the current and the assembly at 20 ± 5 °C ambient temperature. From this, electrical and enclosure-specific constants such as the maximum permissible current per conductor and a conductor specific factor were determined and combined with an exponential dependence of the power dissipation. It is shown that this relationship is valid for an overtemperature of 40 K for compliance with temperature class T6 up to an ambient temperature of +40 °C. Finally, to verify the reliability of the calculation tool, the results are compared with the enclosure-specific rated value of the maximum power dissipation according to IEC 60079-7, 5.7.     

球形压力容器中甲烷-氢气-空气爆炸过程数值模拟及实验研究*

为研究受限空间内甲烷-氢气-空气混合气体爆炸特性参数分布规律,在20 L球形压力容器装置内开展甲烷-氢气-空气混合气体爆炸实验,探究掺氢比变化对当量比为1的甲烷-氢气-空气混合气体爆炸过程的影响;运用Fluent数值模拟软件,采用标准k-ε湍流模型,结合层流有限速率燃烧模型,探究混合气体爆炸过程中燃烧特性（爆炸温度、压力、密度等）与反应时间的变化规律。研究结果表明:爆炸过程中,添加一定氢气时爆炸压力峰值、爆炸压力上升速率峰值增大,而到达峰值时间缩短;反应初期,中心点火处密度下降,反应釜各处密度持续上升;距离点火点越远,密度变化越大,反应釜中压力分布基本相同。研究结果可为甲烷-氢气-空气混合燃料的安全使用提供相关参考。     

粉尘密度对20 L球罐内粉尘分散规律影响

为分析不同粉尘因密度的差异对20 L球形爆炸装置球罐内粉尘分散过程流场变量变化和点火延迟时间的影响,利用CFD数值模拟的方法,研究了3种不同密度的粉尘在球罐分散过程中湍流动能、流场速度、粉尘浓度3种流场变量在球心处的变化规律。研究结果表明:在其他条件一致的情况下,粉尘密度越小,湍流动能的峰值越小,粉尘云浓度和流场速度的峰值则越大;粉尘密度对湍流动能的增值速率没有影响,而粉尘密度越小,流场速度和粉尘浓度的增值速率越快,粉尘浓度衰减至稳定值的时间也越短。表明粉尘密度越小,点火延迟时间也越小,因此,建议铝粉点火延迟时间在50~60 ms之间,锆粉和锌粉在60~80 ms之间。     

烟气输运的大涡模拟研究

应用大涡模拟方法，并结合Smagorinsky亚格子尺度模型，数值研究了烟气输运问题。为了有效地模拟烟气输过过程，文中采用低马赫数近似下的三维可压缩Navier-Stokes方程进行求解，并采用有限差分法数值计算了滤波后的控制方程。对于燃烧过程中所产生的热释放，文中采用Lagrangian粒子携带热量（即热元模型）的近似方法来模拟，同时这些运动粒子进一步用来标识烟气的输运过程。文中数值计算了燃烧热源所形成的热气流和烟气输运过程，分析讨论了相关的湍流特性，以及温度场和速度场，并探讨了不同功率的热源对流动特性的影响。     

A numerical approach to investigate the maximum permissible nozzle diameter in explosion by hot turbulent jets

The ignition of a combustible environment by hot jets is a safety concern in many industries. In explosion protection concepts, for a protection of the type “flameproof enclosures” a maximum permissible gap is of major importance. In this work a numerical framework is described to investigate the ignition processes by a hot turbulent jet which flows out from such gaps. A Probability Density Function (PDF) method in conjunction with a reaction-diffusion manifold (REDIM) technique is used to model the turbulent reactive flow. In this paper the ignition of a stoichiometric mixture of hydrogen/air gas by a hot exhaust turbulent jet is examined. The impact of the nozzle diameter on the ignition delay time is investigated, too. The method is used to explore the maximum nozzle diameter for specific boundary conditions for which there is no ignition.     

Shock-induced dust ignition in curved pipeline with steady flow

The ultimate objective of the research outlined in this paper is to determine the conditions governing shock-induced ignition of dusty flows in curved pipelines using analytical and computational approaches. The results of numerical simulation indicate that ignition of two-phase flows in curved channels is mainly conditioned by the shock-induced flow and is not very sensitive to the flow structure in front of the shock wave. The calculations of nonreactive shock propagation in the quasi-steady two-phase flow and in the uniform quiescent dust suspension revealed significant differences in the postshock flow structure downstream the channel corner. Nevertheless, ignition occurred in the region where the predominant role was played by reflected shock waves, i.e., in the vicinity to the channel corner. The results call for further studies dealing with shock-induced ignition and explosion build-up in curved channels.     

Consequence prediction for dust explosions involving interconnected vessels using computational fluid dynamics modeling

Combustible dust explosions continue to present a significant threat toward operating personnel and pneumatic conveyance equipment in a wide variety of processing industries. Following ignition of suspended fuel within a primary enclosure volume, propagation of flame and pressure fronts toward upstream or downstream interconnected enclosures can result in devastating secondary explosions if not impeded through an appropriate isolation mechanism. In such occurrences, an accelerated flame front may result in flame jet ignition within the secondary vessel, greatly increasing the overall explosion severity. Unlike an isolated deflagration event with quantifiable reduced pressures (vent sizing according to NFPA 68 guidance), oscillation of pressure between primary and secondary process vessels leads to uncertain overpressure effects. Dependent on details of the application such as relative enclosure volumes, relief area, fuel type, suspended concentration, duct size, and duct length, the maximum system pressure in both interconnected vessels can be unpredictable. This study proposes the use of FLame ACceleration Simulator (FLACS) computational fluid dynamics (CFD) modeling to provide reliable consequence predictions for specific case scenarios of dust deflagrations involving interconnected equipment. Required minimum supplement to the originally calculated relief area (A_v) was determined through iterative simulation, allowing for reduced explosion pressures (P_red) to be maintained below theoretical enclosure design strengths (P_es).     

Application of CFD on the sensitivity analyses of some parameters of the modified Hartmann tube

The aim of this work is to determine the influence of operating parameters such as the dispersion pressure, the ignition delay and height on the dust flammability. A Computational Fluid Dynamics (CFD) simulation, based on an Euler–Lagrange approach, was developed with Ansys Fluent™ and validated experimentally. Such analysis will facilitate the choice of the most conservative conditions for a flammability test. This paper is focused on a case study performed on wheat starch with the modified Hartmann tube. The dispersion process of the powder was studied with granulometric analyses performed in situ and high speed videos. Tests were performed with injections at gas pressure ranging from 3 to 6 bars and the evolution of the particle size distribution (PSD) was recorded at different ignition heights (5, 10 and 15 cm over the dispersion nozzle). The observations highlighted the presence of agglomeration/deagglomeration processes and dust segregation. Besides, a CFD simulation analysis was aimed at evaluating the impact of a set of parameters on the PSD and the local turbulence, which are closely linked to some flammability parameters. For this computational analysis, the CFD simulation was coupled with a collision treatment based on a Discrete Element Method (DEM) in order to consider the cohesive behavior of the combustible dust. Thus the results suggest performing the injection of the gases at approximately 5 bars for the flammability tests of wheat starch in order to obtain the finest PSD at a given ignition height. It is also shown that the finest PSD are obtained at 5 cm over the dispersion nozzle. However, the local instabilities and turbulence levels are so high during the first stages of the dispersion that the flame growth can be disturbed for short ignition delays. Moreover, the stabilization of the bulk of the dust cloud requires longer periods of time when the ignition sources are located at 15 cm. As a result, the recommended height to perform a flammability test is 10 cm in this case. Finally, this study proposes some tools that might improve the procedure of dust flammability testing.     

Numerical simulation of LNG dispersion under two-phase release conditions

The use of LNG (liquefied natural gas) as fuel brings up issues regarding safety and acceptable risk. The potential hazards associated with an accidental LNG spill should be evaluated, and a useful tool in LNG safety assessment is computational fluid dynamics (CFD) simulation. In this paper, the ADREA-HF code has been applied to simulate LNG dispersion in open-obstructed environment based on Falcon Series Experiments. During these experiments LNG was released and dispersed over water surface. The spill area is confined with a billboard upwind of the water pond. FA1 trial was chosen to be simulated, because its release and weather conditions (high total spill volume and release rate, low wind speed) allow the gravitational force to influence the cold, dense vapor cloud and can be considered as a benchmark for LNG dispersion in fenced area. The source was modeled with two different approaches: as vapor pool and as two phase jet and the predicted methane concentration at sensors' location was compared with the experimental one. It is verified that the source model affect to a great extent the LNG dispersion and the best case was the one modeling the source as two phase jet. However, the numerical results in the case of two phase jet source underestimate the methane concentration for most of the sensors. Finally, the paper discusses the effect of neglecting the ?9.3° experimental wind direction, which leads to the symmetry assumption with respect to wind and therefore less computational costs. It was found that this effect is small in case of a jet source but large in the case of a pool source.     

Locating the gas leakage source in the atmosphere using the dispersion wave method

A new and simple method for locating emission source was proposed in this work based on gas dynamic dispersion information. The simulation of the unsteady state dispersion of leakage gas emission from the geosequestration project showed that the transportation process of emission gases in the atmosphere is similar to wave propagation, and the time parameter of the dispersion wave is linearly related to the downwind distance. Therefore, monitoring the dispersion wave at different downwind positions can be used to estimate the leakage source position. An estimation formula for locating emission sources was derived. First, an estimation formula for locating emission sources was derived under some initial assumptions. Then, the deviation of the location formula was investigated using a computational fluid dynamics (CFD) model and analytic solution to get the offset distance under different conditions. The results showed that the average distance is stable for a certain atmosphere and terrestrial conditions. This method needs no more than 3 sensors’ dynamic information to locate the emission source, and hence it is highly useful for conditions with limited sensors. A numerical test demonstrated that the absolute error of the source estimation is within the range of 1–30 m. Finally, experimental tests were conducted to verify the feasibility of the source location with dispersion waves. Therefore, the dispersion wave monitor is a potentially simple and feasible way to estimate the source location for gas emission event management with limited sensors in the process industries.     

Worst-case identification of gas dispersion for gas detector mapping using dispersion modeling

To quickly and accurately quantify the material release in process units, gas detectors may be placed according to the results of gas dispersion modeling. DNV's PHAST software is one of the most useful and reliable tools for material dispersion modeling. In this software, fluid dispersion is modeled based on the process conditions, the weather conditions and the specifications of the material release point. However, varying weather conditions throughout the year and the exact determination of the release point on the plot plan and the release elevation are problematic; these issues cause the results to be non-exact and non-integrated. Choosing the most appropriate conditions is challenging. In this paper, a scheme was provided to select the most appropriate conditions for gas dispersion modeling. This scheme approaches modeling based on the worst-case scenario (the situation in which the dispersed gas reaches the detector later in comparison to the other cases). Therefore, different weather conditions, release elevations and release points on the plot plan were modeled for an absorber tower of the Gonbadli Dehydration Unit of the Khangiran Refinery. The worst case of each release condition was then chosen. Finally, gas detectors were placed using the gas dispersion modeling results based on the worst-case scenario.     

基于极限学习机的炭化可燃物着火时间预测

机器学习技术近年来在许多传统科学领域取得了应用,针对火灾中炭化可燃物着火时间与物性参数及环境参数之间关系复杂的特点,提出了一种基于极限学习机的预测方法以实现不同物性及环境参数时着火时间的快速准确预测,为防治及扑救火灾提供参考。首先建立炭化可燃物热解数值模型,考虑了可燃物热解过程中的含水率以及热解反应、气体流动等复杂物理化学反应过程,然后搭建极限学习机,以数值模拟数据为基础进行训练及验证工作。结果表明基于极限学习机的预测方法能够有效实现炭化可燃物着火时间的快速准确预测,平均相对误差小于3%。     

Comparison of explosion parameters for Methane–Air mixture in different cylindrical flameproof enclosures

Flameproof enclosures having internal electrical components are generally used in classified hazardous areas such as underground coalmines, refineries and places where explosive gas atmosphere may be formed. Flameproof enclosure can withstand the pressure developed during an internal explosion of an explosive mixture due to electrical arc, spark or hot surface of internal electrical components. The internal electrical component of a flameproof enclosure can form ignition source and also work as an obstacle in the explosion wave propagation. The ignition source position and obstacle in a flameproof enclosure have significant effect on explosion pressure development and rate of explosion pressure rise. To study this effect three cylindrical flameproof enclosures with different diameters and heights are chosen to perform the experiment. The explosive mixture used for the experiment is stoichiometric composition of methane in air at normal atmospheric pressure and temperature.It is observed that the development of maximum explosion pressure (P_max) and maximum rate of explosion pressure rise (dp/dt)_ex in a cylindrical flameproof enclosure are influenced by the position of ignition source, presence of internal metal or non-metal obstacles (component). The severity index, K_G is also calculated for the cylindrical enclosures and found that it is influenced by position of ignition source as well as blockage ratios (BR) of the obstacles in the enclosures.