Determination of the performance limits of flame arresters at increased oxygen concentrations

This investigation shows how an increased oxygen concentration influences the performance limits of crimped ribbon deflagration flame arresters at elevated pressures. An evaluation of the maximum experimental safe gap (MESG) as reliable criterion for describing the performance limits under non-atmospheric conditions is given. Measurements of MESGs and flame arrester performance tests were performed. Various fuel/oxygen/air mixtures containing ethylene and propane were used as testing gases. Former studies on the pressure dependence and the influence of oxygen on the MESG were initially confirmed. Furthermore, performance tests using a commercial deflagration flame arrester revealed that such a flame arrester may prevent flame transmission also at non-atmospheric conditions within a limited range. For various oxygen concentrations the performance limits were reached at the same MESG. Hence, it can be assumed that a flame arrester possesses a device- and fuel-specific maximum experimental safe gap for a specific gas mixture in different concentrations and at different pressures. This performance-related maximum safe gap can be used as a parameter for estimating and describing the performance limits of a flame arrester. It offers an attempt to simplify the testing and qualification of deflagration flame arresters for non-atmospheric conditions.     

Experimental investigation on flame and detonation quenching: applicability of static flame arresters

The present investigation was aimed at demonstrating the pertinent use of a flame/detonation arrester for the safety in the process industry. The experimental data provide information on the performance of a static flame arrester. The experiments were conducted in a 3-m long and 28-mm i.d. tube. The arrester, itself consists in 2-mm i.d. tubes arranged in a honeycomb type structure which is located in the middle of the test tube. This is aimed at promoting heat losses during the passage of the flame. The experiments were conducted with fuel-rich mixtures of methane–ethane–ethylene at initial pressures in the range of 0.1–0.6 MPa. These very rich mixtures were more or less diluted in inert, namely, N₂, as well as a mixture of CO₂ and Ar, in order to match with compositions that are currently used in the process industry. The experimental results show that this device is appropriate for flame quenching but, in most cases, detonations were not successfully arrested. These unsuccessful operations of the device occurred for flames at a high velocity, namely for Re numbers of the order of or greater than 15,000.     

An experimental study of flame acceleration and deflagration to detonation transition in representative process piping

The paper summarizes the results of experimental tests and accompanying analyses to investigate the factors that govern flame acceleration and potential transition to detonation in a relatively long unobstructed piping system. The overall aim of the work was to obtain sufficient experimental data so as to be able to develop and evaluate methodologies for classifying and predicting potential detonation flame acceleration and deflagration to detonation transition (DDT) hazard in industrial process pipes and mixtures. The present results show that the flame acceleration process in an unobstructed pipe exhibit three distinct phases: an initial establishment phase; a second rapid acceleration phase and a final transition to detonation phase. Test results with ethylene indicate that the acceleration process is not sensitive to initial pressure (all other parameters remaining constant) but can be sensitivity to initial pipe wall temperature or possibly mixture humidity. The presence of bends increases the local rate of turbulent combustion, an effect attributed to the additional turbulence generated downstream of the bend. For straight pipes, detonation was only observed to develop for hydrogen–air and ethylene–air mixtures. Detonation was not observed with methane, propane or acetone as fuel in the present piping apparatus.     

Gas flame acceleration in long ducts

In many practical situations, a flame may propagate along a pipe, accelerate and perhaps transform into a devastating detonation. This phenomenology has been known, more or less qualitatively, for a long time and mitigation techniques were proposed to try and avoid this occurrence (flame arresters, vents,...). A number of parameters need to be known and in particular the “distance to detonation” and more generally the flame acceleration characteristic scales. Very often, the ratio between the detonation run-up distance and the pipe diameter is used without any strong justification other that using a non-dimensional parameter (L/D). In this paper, novel experimental evidence is presented on the basis of relatively large scale experiments using 10 cm and 25 cm inner diameter duct with a length between 7 and 40 m. Homogeneous C₂H₄-air, CH₄-air, C₃H₈-air and H₂-air mixtures were used and different ignition sources. The interpretation suggests that the self-acceleration mechanism of the flame may be much better represented by flame instabilities than by turbulence build-up. One consequence would be that the maximum flame velocity and, following, the maximum explosion overpressure, would be rather linked with the run-up distance than with the L/D ratio.     

Decomposing detonation and deflagration properties of ozone/oxygen mixtures

In this study, the decomposing detonation and deflagration properties of ozone/oxygen mixtures of up to 20 vol.% of ozone in oxygen under high pressure of up to 1.0 MPa in a tube were experimentally investigated. The mixtures were ignited by an electric spark at the end of the tube. Flame propagation properties such as flame velocity and pressure were measured with thermocouples and piezo electric transducers mounted along the tube. Slow and constant flame propagation profiles were obtained. We also investigated the quenching ability of a wire gauze as well as the concentration limit for flame propagation. However, in spite of slow flame propagation velocity and easy flame quenching properties under these experimental conditions, direct initiation of detonation by the driver detonation of the stoichiometric oxy-hydrogen mixture was easily achieved at much lower concentrations than the limit of deflagration. The observed detonation properties, such as wave velocity and pressure, agreed fairly well with C–J calculated values. The detonation velocity (900–1200 m/s) and the pressure ratio to initial pressures (5–9.5) were not affected by the initial pressure of the mixtures. Near the detonation limit, typical spinning detonations with oscillatory pressure waves were observed.     

Structure and flame speed of dilute and dense layered coal-dust explosions

Multidimensional time-dependent simulations were performed to study the interaction of a stoichiometric methane–air detonation with layers of coal dust. The simulations solved equations representing a Eulerian kinetic-theory-based granular multiphase model applicable to dense and dilute particle volume fractions. These equations were solved using a high-order Godunov-based method for compressible fluid dynamics. Two dust layer concentrations were considered: loose with an initial volume fraction of 1%, and dense with an initial volume fraction of 47%. Each layer was simulated with two types of dust: reactive coal and inert ash. Burning of the coal particles results in a coupled complex consisting of an accelerating shock leading a coal-dust flame. The overall structure of the shock–flame complex resembles that of a premixed fast flame with length scales on the order of several meters. The large length scales are direct results of time needed to lift, mix, heat, and autoignite the particle. The flame speeds are large and much larger than the gas-phase velocity. Large spikes of flame speed are characteristic of the 47% case. These spikes and high flame speed are caused by pockets of coal dust autoigniting ahead of the flame. The flame is choked in the 1% case due to the gas-phase products exceeding the sonic velocity with respect to the flame. The 47% case is choked due to attenuation of pressure waves as they propagate through particles. Inert layers of dust substantially reduce the overpressure, impulse, and speed produced by propagating blast wave. The results also show that loose layers of dust are far more dangerous than dense layers. The shock and flame are more strongly coupled for loose layers, propagate at higher velocity, and produce large overpressures and impulses.     

Efficient simulation of flame acceleration and deflagration-to-detonation transition in smooth pipes

Evaluation of accident scenarios including flame acceleration and deflagration-to-detonation transition (DDT) in chemical plant piping systems increases the need for an efficient numerical simulation tool capable of dealing with this phenomenon. In this work, a hybrid pressure-density-based solver including deflagrative flame propagation as well as detonation propagation is presented. The initial incompressible acceleration stage is covered by the pressure-based solver until the flame velocity reaches the fast flame regime and transition to the density-based solver is done. The deflagration source term is formulated in terms of a turbulent flame speed closure model incorporating various physical effects crucial for flame acceleration at low turbulence conditions (Katzy and Sattelmayer, 2018). Modelling of the detonation source term is based on a quadratic heat release function (Hasslberger, 2017). The presented numerical approach is validated in terms of DDT locations and pressure data from Schildberg (2015) as well as recently completed flame tip position measurements. For this purpose, H₂/O₂/N₂ mixtures ranging from 25.6 vol-% H₂ to 29.56 vol-% H₂ in two different pipe geometries are considered. The focus of the current work is on predicting the DDT location correctly and good agreement is observed for the investigated cases.     

Limit of safety against flame transmission for sintered metal flame arrester elements in the case of flashback in fuel gas/oxygen mixtures

In autogenous welding, cutting and allied processes, so-called dry safety devices are used to protect the outlet terminals of gas distribution lines. To prevent flame transmission, these units are fitted with sintered metal flame arrester elements. At the Federal Institute for Materials Research and Testing (BAM), extensive investigations have been carried out with an experimental flame arrester to determine the limits of safety against flame transmission for sintered metal flame arrester elements in the case of flashback in fuel gas/oxygen mixtures. On the basis of the results of these investigations, it is possible to estimate the pore sizes up to which a sintered metal element can prevent any flame transmission with a given fuel gas.     

Numerical simulation of flame acceleration and deflagration-to-detonation transition of ethylene in channels

Ethylene (C₂H₄) is a hydrocarbon fuel and widely used in chemical industry, however, ethylene is highly flammable and therefore presents a serious fire and explosion hazard. This work is initiated by addressing the hazard assessment of ethylene mixtures in different scale channels (d = 5 mm, 10 mm and 20 mm) from the aspect of flame acceleration (FA) and deflagration-to-detonation transition (DDT) by using large eddy simulation (LES) method coupled with the artificially thickened flame (ATF) approach. The fifth order local characteristics based weighted essentially non-oscillatory (WENO) conservative finite difference scheme is employed to solve the governing equations. The numerical results confirm that flame velocity increase rapidly at the beginning stage in three channels, and the flame acceleration rate is slower in the subsequent stage, afterwards, the flame velocity has an abrupt increase, and the onset of detonation occurs. Due to the fact that wall effect is significant in the narrow channel (e.g.,5 mm), especially in the ignition stage of the flame, flames have different shapes in wider channels (10 mm and 20 mm) and narrow channel (5 mm). Both the pressure and temperature profiles confirm DDT run-up distances are 0.251 m, 0.203 m and 0.161 m in 20 mm, 10 mm and 5 mm channels, respectively, which indicates that a shorter run-up distance is required in narrower channel. The cellular detonation structures for the ethylene-air mixture in different channels indicate that multi-headed detonation structures can be found in 20 mm channel, as the channel width decreases to 10 mm, detonation has a single-headed spinning structure, as the width is further reduced to 5 mm, only large longitudinal oscillation of the pressure can be observed.     

The influence of inert gas on limiting experimental safe gap of fuel-air mixtures at various initial pressures

The Maximum Experimental Safe Gap (MESG) is an important criterion to assess the propagation of flames through small gaps. This safety-related parameter is used to classify the flammable gases and vapors in explosion groups, which are fundamental to constructional explosion protection. It is used both, for the safe design of flameproof encapsulated devices as well as for selecting flame arresters appropriate to the individual application. The MESG of a fuel is determined experimentally according to the standard ISO/IEC 80079-20-1:2017 at normal conditions (20 °C, 1.0 bar) with air as oxidizing gas. The aim of this work is to investigate the effect of inert gas addition on the MESG in order to assess the effectiveness of inertization in constructional explosion protection. The term limiting experimental safe gap (S_G) is used for the result of these measurements. The fuel-air mixtures (fuels: hydrogen, ethylene, propene, methane) used as representatives for the explosion groups in flame arrester testing were chosen and diluted with inert gas (nitrogen, carbon dioxide) before testing. The dependence of the limiting experimental safe gap on the total initial pressure, amount and nature of inert additive is discussed. The initial pressure was varied up to 2.0 bar to include increased pressure conditions used in flame arrester testing. Apart from the well-known reciprocal dependence on the initial pressure, the added inert gas results in an exponential increase of S_G. This effect depends on the inertizing potential of the gas and is therefore different with nitrogen and carbon dioxide. The ranking of the fuels is the same as with MESG. As a result, various mixtures of the same limiting experimental safe gap can now be chosen and tested with an individual flame arrester to prove the concept of a constant and device-related limiting safe gap. The work was funded by BG-RCI in Heidelberg (PTB grant number 37056).     

Detonation transition criteria from the interaction of supersonic shock-flame complexes with different shaped obstacles

The present study is an experimental investigation of the last stages of the deflagration-to-detonation transition. A fast flame following a lead shock was generated by passing a detonation wave through a perforated plate. The shock flame complex then interacts with an obstacle of different shape. We study the influence of the obstacle shape on the transition mechanism to a detonation. The obstacles studied are a single round or square obstacle, a flat plate, a C-shaped and an H-shaped obstacle. The experiments were performed in a thin transparent channel permitting high speed schlieren visualization. Stoichiometric propane-oxygen was investigated at sub-atmospheric conditions. For each obstacle configuration, the initial pressure was changed to modify the flame burning velocity and the Mach number of the leading shock. The burning velocity prior to the interaction was measured experimentally from the displacement velocity of the flame in the videos. This required estimating the speed of the gas ahead of the flame. A linear correction to the speed immediately behind the lead shock was applied using the shock change equations and the measured pressure gradient behind the lead shock in order to account for the non-steadiness of the lead shock and viscous losses to the walls. Three main findings were that the obstacle shape had a minimal influence on the critical flame strength required for transition, although obstacles with a forward facing cavity were able to suppress the transition by isolating the re-initiation event inside the cavity. The main transition mechanism for all geometries was the enhancement of the flame burning velocity through the flame interaction with the shock reflected on the obstacle leading to Richtmyer-Meshkov instability. Finally, it was found that the flame burning velocity of the initial flame required for transition was closely approximated by the Chapman-Jouguet burning velocity. Consistent with the visual observations, this supports the view that transition is favored when the flame is in phase with the acoustic waves, and strong internal pressure waves can be amplified.     

Effect of flame propagation regime on pressure evolution of nano and micron PMMA dust explosions

Experiments using an open space dust explosion apparatus and a standard 20 L explosion apparatus on nano and micron polymethyl methacrylate dust explosions were conducted to reveal the differences in flame and pressure evolutions. Then the effect of combustion and flame propagation regimes on the explosion overpressure characteristics was discussed. The results showed that the flame propagation behavior, flame temperature distribution and ion current distribution all demonstrated the different flame structures for nano and micron dust explosions. The combustion and flame propagation of 100 nm and 30 μm PMMA dust clouds were mainly controlled by the heat transfer efficiency between the particles and external heat sources. Compared with the cluster diffusion dominant combustion of 30 μm dust flame, the premixed-gas dominant combustion of 100 nm dust flame determined a quicker pyrolysis and combustion reaction rate, a faster flame propagation velocity, a stronger combustion reaction intensity, a quicker heat release rate and a higher amount of released reaction heat, which resulted in an earlier pressure rise, a larger maximum overpressure and a higher explosion hazard class. The complex combustion and propagation regime of agglomerated particles strongly influenced the nano flame propagation and explosion pressure evolution characteristics, and limited the maximum overpressure.     

惰性气体抑制瓦斯爆燃火焰传播特性实验研究*

为研究惰性气体抑制瓦斯爆燃火焰传播特性,在自行搭建的中尺度爆炸激波管道上,采用数据采集系统、压电式传感器、火焰传感器、同步控制系统和激光纹影测试系统,通过对比4种不同喷射压力（0.5,1.5,2.5,3.5 MPa）的实验工况,选用N2做为惰性介质时抑制火焰的传播特性与喷射压力密切相关,火焰传播速度随着喷射压力增加呈现先增加后减弱的趋势。研究结果表明:少量N2在管道中扩散,加剧了未反应预混气体的扰动状态,造成火焰阵面褶皱的卷吸能力增强,进而加速化学反应进程,促进预混气体燃烧;喷射压力为1.5 MPa时,火焰阵面拉升、变形最强,火焰传播速度提高,最高可达到250 m/s;喷射压力为3.5 MPa时,火焰阵面出现明显三维凹陷结构,运动发生明显滞后现象,火焰传播速度大幅度降低至5.4 m/s,惰性气体抑制火焰传播效果明显。     

气体爆轰波在弯曲管道中传播特性的实验研究

对丙烷 -空气爆轰波通过 90°弯管道时的传播特性作了实验研究 ,主要是气体爆轰波通过弯管道前后的火焰速度以及加速情况的研究 ,初步得出 ,爆轰波经过弯管道后单位距离上的火焰速度增量显著增加。这一研究结果证明 ,弯曲管道对于爆燃与爆轰波火焰有明显的加速作用     

Crimped metal ribbon flame arrestors for the protection of gas measurement systems

This paper summarises the results of extensive research to determine the limit of safety against flame transmission for flame arrestors of relatively small size fitted with arrestor elements made of crimped metal ribbon. Depending on the reactivity of the fuel gas/air mixture and the tube geometry the running up to detonation and hence the stressing of a flame arrestor by a detonation is possible in longer tubes with relatively small diameters. Only with reactive gas phases of explosion group I this stressing case for a flame arrestor can be excluded. With detonative gas phases the stressing of the flame arrestor decisively depends on the place of installation with respect to the point of transition from deflagration to detonation in the system considered. Five different stressing cases with a probably very different limiting pressure of safety against flame transmission must be distinguished. The results of the investigations will help to evaluate the results from testing of flame arrestors carried out according to the requirements in national and international standards or regulations.     

瓦斯爆炸阻隔爆装置失效原因的实验研究

通过对水平管道内瓦斯爆炸的火焰结构及压力结构的实验研究 ,分析了瓦斯爆炸阻隔爆装置的失效原因。结果表明 ,瓦斯爆炸火焰是沿着管道的底部向前传播的 ,火焰长度较长 ,并具有较高的内聚力。阻隔爆装置的失效原因是由于其动作延迟时间与火焰到达装置位置所需的时间不一致 ,使释放出的抑制剂不能有效地覆盖整个火焰区 ,造成具有较高内聚力的火焰 ,在其后部巨大爆炸产物膨胀压力的推动下继续向前传播     

Development of a mitigation system against hydrogen-air deflagrations in nuclear power plants

A novel mitigation system against hydrogen-air deflagrations in nuclear power plant buildings is proposed and developed through a series of field experiments using explosion vessels of different volume sizes. The mitigation system is installed on the outer surface of the vessels, and it comprises flame arrester and explosion air bag. The flame arrester is made by stacking 10–20 sheets of fine-mesh wire screens, and the air bag is connected for holding explosion gas. The successful mitigation mechanism is the sequence of pressure-rise reduction by the air bag expansion, flame quenching by the flame arrester, and the slow burning of the gas mixture sucked from the air bag back into the vessel due to the negative pressure caused by the rapid condensation of water vapor inside the vessel. Necessary conditions for the successful mitigation system are discussed, and the practical unit size of flame arrester sheet is recommended.     

Experimental investigation on explosion flame propagation of wood dust in a semi-closed tube

In order to explore flame propagation characteristics during wood dust explosions in a semi-closed tube, a high-speed camera, a thermal infrared imaging device and a pressure sensor were used in the study. Poplar dusts with different particle size distributions (0–50, 50–96 and 96–180 μm) were respectively placed in a Hartmann tube to mimic dust cloud explosions, and flame propagation behaviors such as flame propagation velocity, flame temperature and explosion pressure were detected and analyzed. According to the changes of flame shapes, flame propagations in wood dust explosions were divided into three stages including ignition, vertical propagation and free diffusion. Flame propagations for the two smaller particles were dominated by homogeneous combustion, while flame propagation for the largest particles was controlled by heterogeneous combustion, which had been confirmed by individual Damköhler number. All flame propagation velocities for different groups of wood particles in dust explosions were increased at first and then decreased with the augmentation of mass concentration. Flame temperatures and explosion pressures were almost similarly changed. Dust explosions in 50–96 μm wood particles were more intense than in the other two particles, of which the most severe explosion appeared at a mass concentration of 750 g/m³. Meanwhile, flame propagation velocity, flame propagation temperature and explosion pressure reached to the maximum values of 10.45 m/s, 1373 °C and 0.41 MPa. In addition, sensitive concentrations corresponding to the three groups of particles from small to large were 500, 750 and 1000 g/m³, separately, indicating that sensitive concentration in dust explosions of wood particles was elevated with the increase of particle size. Taken together, the finding demonstrated that particle size and mass concentration of wood dusts affected the occurrence and severity of dust explosions, which could provide guidance and reference for the identification, assessment and industrial safety management of wood dust explosions.     

Z型管道中气体火焰传播规律的实验研究

本文实验研究了丙烷／氧气／空气的当量比气体燃烧火焰通过两个90°弯管形成的“z”型管道的传播规律,通过改变第一个弯管前加速段的长度和改变气体浓度,实验研究了稳定爆轰波,非稳定爆轰波以及爆燃火焰通过“z”型管道的传播规律。利用光电传感器记录弯管前后的火焰传播速度,运用燃烧理论和爆轰波的理论对实验结果做了分析。结果表明:稳定爆轰波通过“z”型管道时传播速度有明显的下降;但“z”型管道对非稳定爆轰波的传播作用受到非稳定爆轰波自身速度的影响;爆燃火焰通过“z”型管道时火焰传播速度的变化呈现不确定性。