Thermo-kinetic modelling of dust explosions

The guidelines for protection and mitigation against hazard coming from dust explosion require the knowledge and then the evaluation either experimentally or theoretically of the thermo-kinetic parameters (i.e. K_St, P_max). We developed a numerical tool for the evaluation of the thermo-kinetic parameters of dust explosion. This model is based on the simulations of the combustion reaction by means of a detailed reaction mechanism assuming that the pyrolysis/devolatilization step is very fast and then gas combustion is controlling dust explosion. The model allows then the determination of the most conservative values of K_St, S_l, P_max. In the present paper we calculated the deflagration index and the laminar burning velocity for dusts utilized in various process industries (i.e. cornstarch, polyethylene, cellulose) as function of dust concentration. The obtained data were successfully compared with the available experimental results.     

Explosibility of hydrogen–graphite dust hybrid mixtures

To evaluate the hazard of combined hydrogen/dust explosions under severe accident conditions in International Thermonuclear Experimental Reactor (ITER), standard method of 20-L-sphere was used to measure the explosion indices of 4-μm fine graphite dust in lean hydrogen/air mixtures. The mixtures were ignited by a weak electric spark. The tested fuel concentrations were 8–18 vol% H₂ and 25–250 g/m³ dust. If the hydrogen content is higher than 10 vol%, the dust constituent can be induced to explode by the hydrogen explosion initiated by a weak electric spark. Depending on the fuel component concentrations, the explosions proceed in either one or two stages. In two-stage explosions occurring at low hydrogen and dust concentrations, the mixture ignition initiates first a fast hydrogen explosion followed by a slower phase of the dust explosion. With increasing dust concentration, the dust explodes faster and can overlap the hydrogen-explosion stage. At higher hydrogen concentrations, the hybrid mixtures explode in one stage, with hydrogen and dust reacting at the same time scale. Maximum overpressures of hybrid explosions are higher than those observed with hydrogen alone; maximum rates of pressure rise are lower in two-phase explosions and, generally, higher in one-stage explosions, than those characteristic of the corresponding H₂/air mixtures.     

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.     

Measuring the violence of dust explosions with the “20 l sphere” and with the standard “ISO 1 m vessel”: Systematic comparison and analysis of the discrepancies

On the basis of a systematic testwork with a number of different dusts, the explosion indices as determined within the 20 l sphere and with the ISO-VDI 1 m³ vessel have been compared. The repeatability has been assessed and since some systematic deviations appear a refined physical analysis of the explosion processes is developed. It appears in particular that the cube root law supposed to link both vessels is not verified. A striking illustration of this appears when a dust with a significant explosion severity inside the 20 l sphere is not even explosible in the larger vessel. It is strongly suggested that the ignition energy is forcing very significantly the explosion in the smaller vessel inducing several tens of Celsius degrees of preheating. It is shown also that the inner level of turbulence is decreasing very fast in the 20 l sphere during the flame development so that difficult-to-ignite mixtures would tend to burn at a lower combustion rate. It is further demonstrated that the major bias between the chambers can be explained and quantified with these elements. A correlation with the standard 1 m³ vessel and a grid of interpretation of the data is proposed.     

Fast flame speeds and rates of pressure rise in the initial period of gas explosions in large L/D cylindrical enclosures

Flame speeds and rates of pressure rise for gaseous explosions in a 76 mm diameter closed cylindrical vessel of large length to diameter ratio (L/D = 21.6), were quantitatively investigated. Methane, propane, ethylene and hydrogen mixtures with air were studied across their respective flammability ranges. Ignition was affected at one end of the vessel. Very fast flame speeds corresponding to high rates of pressure rise were measured in the initial 5–10% of the total explosion time. During this period 20–35% of the maximum explosion pressure was produced, and over half of the flame propagation distance was completed. Previous work has concentrated on the later stages of this type of explosion; the development of tulip flames, pressure wave effects and transition to turbulence. The initial fast phase is very important and should dominate considerations in pressure relief vent design for vessels of large L/D.     

An influence of oxygen content in an oxidizing atmosphere on inhibitive action of fluorinated agents on a hydrogen flame

An experimental investigation of the influence of inhibitors of various chemical natures on flammability limits in mixtures H₂+oxidizer (O₂+N₂)–suppressant (C₂HF₅; CHF₃; C₄F₁₀; inhibitor AKM, which is a mixture of olefins) was carried out. Compositions of N₂ and O₂ with elevated (25 vol%) and reduced (15 vol%) oxygen concentrations and air were used as oxidizing atmospheres. Experiments were done at room temperature and atmospheric pressure. Flammability limits were determined in a closed vessel of volume of 4.2 dm³ (internal diameter 20 cm). Mixtures were prepared immediately in the preliminary evacuated reaction vessel by partial pressures. The mixtures were ignited by an electrical spark of energy near 1 J in the center of the reaction vessel. A flame propagation was detected by a pressure transducer. Twelve flammability curves were measured, which allowed to compare effectiveness of the inhibitors at various oxygen contents in the atmosphere. A qualitative analysis of the obtained results was done, which showed an important role of an inhibitor regeneration.     

Study on the early stage of runaway reaction using Dewar vessels

In this paper, compared with a UN cylindrical 500 mL Dewar (H.4 in the UN tests), a spherical 1 L Dewar vessel was used to study the early stages of runaway reactions of several liquid and solid samples, including three organic peroxides and a reactive material. The samples were filled in the vessels and the temperature profiles versus times at different positions of the samples were measured. As a result, the minimum temperatures, defined as the SADT, were averagely 10 K lower than those measured in the cylindrical Dewar vessels. At the same time, the temperature profiles of solids in the spherical Dewars tended to be homogeneous. The heat transfer coefficient of a spherical Dewar is only 0.18 W/K/m, one-eighth of a conventional cylindrical Dewar vessel. Meanwhile it has a low phi factor. These factors are essential to simulate low heat loss bulk conditions in the equilibrium process and at the early stage of a runaway reaction. To characterize the ability of the adiabaticity of a storage vessel, it can be seen that a spherical Dewar could simulate the plant process having critical storage size of a reactive-material, r₀, approximately 0.6 m. It is recommended that such a technique is used to investigate the SADT of an unstable material in larger scale packaging or a material with very weak heat release in industry.     

Large-scale dust explosions in vessel-pipe systems

This study investigates dust explosions in vessel-pipe systems to develop a better understanding of dust flame propagation between interconnected vessels and implications for the proper application of explosion isolation systems. Cornstarch dust explosions were conducted in a large-scale setup consisting of a vented 8-m³ vessel and an attached pipe with a diameter of 0.4 m and a length of 9.8 m. The ignition location and effective dust reactivity were varied between experiments. The experimental results are compared against previous experiments with initially quiescent propane-air mixtures, demonstrating a significantly higher reactivity of the dust explosions due to elevated initial turbulence, leading to higher peak pressures and faster flame propagation. In addition, a physics-based model developed previously to predict gas explosion dynamics in vessel-pipe systems was extended for dust combustion. The model successfully predicts the pressure transients and flame progress recorded in the experiments and captures the effects of ignition location and effective dust reactivity.     

Ignitability characteristics of aluminium and magnesium dusts that are generated during the shredding of post-consumer wastes

The authors investigated the ignitability of aluminium and magnesium dusts that are generated during the shredding of post-consumer waste. The relations between particle size and the minimum explosive concentration, the minimum ignition energy, the ignition temperature of the dust clouds, etc. the relation between of oxygen concentration and dust explosion, the effect of inert substances on dust explosion, etc. were studied experimentally.

The minimum explosive concentration increased exponentially with particle size. The minimum explosive concentrations of the sample dusts were about 170 g/m³ (aluminium: 0–8 μm) and 90 g/m³ (magnesium: 0–20 μm). The minimum ignition energy tended to increase with particle size. It was about 6 mJ for the aluminium samples and 4 mJ for the magnesium samples. The ignition temperature of dust clouds was about 750 °C for aluminium and about 520 °C for magnesium. The lowest concentrations of oxygen to produce a dust explosion were about 10% for aluminium and about 8% for magnesium. A large mixing ratio (more than about 50%) of calcium oxide or calcium carbonate was necessary to decrease the explosibility of magnesium dust. The experimental data obtained in the present investigation will be useful for evaluating the explosibility of aluminium and magnesium dusts generated in metal recycling operations and thus for enhancing the safety of recycling plants.     

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.     

Combined effects of initial pressure and turbulence on explosions of hydrogen-enriched methane/air mixtures

Hydrogen-enrichment has been proposed as a useful method to overcome drawbacks (local flame extinction, combustion instabilities, lower power output, etc.) associated to turbulent premixed combustion of natural gas in both stationary and mobile systems. For the safe use of hydrogen-enriched hydrocarbon fuels, explosion data are needed.In this work, a comparative experimental study of the explosion behavior of stoichiometric hydrogen-enriched methane/air (with 10% of hydrogen molar content in the fuel) and pure methane/air mixtures is presented. Tests were carried out in a 5 l closed cylindrical vessel at different initial pressures (1, 3 and 6 bar), and starting from both quiescent and turbulent conditions.Results allow quantifying the combined effects of hydrogen substitution to methane, pressure and turbulence on maximum pressure, maximum rate of pressure rise, burning velocity and Markstein lengths.     

Effect of spark duration on explosion parameters of methane/air mixtures in closed vessels

The paper outlines an experimental study on influence of the spark duration and the vessel volume on explosion parameters of premixed methane–air mixtures in the closed explosion vessels. The main findings from these experiments are: For the weaker ignition the spark durations in the range from 6.5 μs to 40.6 μs had little impact on explosion parameters for premixed methane–air mixtures in the 5 L vessel or 20 L vessel; For the same ignitions and volume fractions of methane in air the explosion pressures and the flame temperatures in both vessels of 5 L and 20 L were approximately the same, but the rates of pressure rises in both vessels of 5 L and 20 L were different; The explosion indexes obtained from the measured pressure time histories for both vessels of 5 L and 20 L were approximately equal; For the weaker ignition with the fixed spark duration 45 μs the ignition energies in the range from 54 mJ to 430 mJ had little impact on the explosion parameters; For the same ignition and the volume fractions of methane in air, the vessel volumes had a significant impact on the flame temperatures near the vessel wall; The flame temperatures near the vessel wall decreased as the vessel volumes increased.     

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.     

Explosion limits of mixtures relevant to the production of 1,2-dichloroethane (ethylene dichloride)

A simple method exists to estimate the limiting oxygen concentration (LOC) based upon the lower explosion limit (LEL) by assuming (1) that the LOC lies at the apex of the explosion area, (2) that the LEL is unaffected by nitrogen addition and (3) that the apex of the explosion area lies on the stoichiometric line. This estimation method is assessed for mixtures relevant to the production of 1,2-dichloroethane. To this end, the explosion areas of ethylene/hydrogen/nitrogen/air, ethylene/nitrogen/air and ethylene/1,2-dichloroethane/hydrogen chloride/nitrogen/air mixtures are determined at typical process conditions. The experiments are performed in a closed spherical 8 l vessel. The mixtures are ignited by fusing a coiled tungsten wire, placed at the centre of the vessel. A 5% pressure rise criterion is used to determine the explosion limits. The experimental procedure is based upon EN 14756. It is found that a safe estimate of the LOC of ethylene/hydrogen/nitrogen/air mixtures can be found based upon the LEL of these mixtures.     

Vent Sizing Criteria for Partial Volume Deflagration

The accidental spill of volatile solvents or the release of flammable gases within equipment and buildings is likely to form fuel concentration gradients unless efficient mixing is provided. As a consequence, even small amounts of fuel can form flammable clouds, and partial volume deflagrations may occur. Nevertheless, few indications are given in international guidelines for vent sizing and only over-conservative well-mixed stoichiometric assumptions are used. In this paper, we propose a predictive methodology for the evaluation of the dynamics of partial volume deflagration, aiming at defining useful correlations for the design of vent devices, starting from the fundamental equation for the rate of pressure rise and flame propagation in closed vessel. We define a ‘stratified gas deflagration index’ K_G(m), where m is the filling ratio, and use it with the most common design equations for vent sizing. The approach has been validated by means of a CFD code for the simulation of stratified laminar methane–air explosion by varying both filling ratio and volume.     

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.     

Experimental investigation of gas explosion in single vessel and connected vessels

Gas explosion in connected vessels usually leads to high pressure and high rate of pressure increase which the vessels and pipes can not tolerate. Severe human casualties and property losses may occur due to the variation characteristics of gas explosion pressure in connected vessels. To determine gas explosion strength, an experimental testing system for methane and air mixture explosion in a single vessel, in a single vessel connected a pipe and in connected vessels has been set up. The experiment apparatus consisted of two spherical vessels of 350 mm and 600 mm in diameter, three connecting pipes of 89 mm in diameter and 6 m in length. First, the results of gas explosion pressure in a single vessel and connected vessels were compared and analyzed. And then the development of gas explosion, its changing characteristics and relevant influencing factors were analyzed. When gas explosion occurs in a single vessel, the maximum explosion pressure and pressure growth rate with ignition at the center of a spherical vessel are higher than those with ignition on the inner-wall of the vessel. In conclusion, besides ignition source on the inner wall, the ignition source at the center of the vessels must be avoided to reduce the damage level. When the gas mixture is ignited in the large vessel, the maximum explosion pressure and explosion pressure rising rate in the small vessel raise. And the maximum explosion pressure and pressure rising rate in connected vessels are higher than those in the single containment vessel. So whenever possible, some isolation techniques, such as fast-acting valves, rotary valves, etc., might be applied to reduce explosion strength in the integrated system. However, when the gas mixture is ignited in the small vessel, the maximum explosion pressures in the large vessel and in the small vessel both decrease. Moreover, the explosion pressure is lower than that in the single vessel. When gas explosion happens in a single vessel connected to a pipe, the maximum explosion pressure occurs at the end of the pipe if the gas mixture is ignited in the spherical vessel. Therefore, installing a pipe into the system can reduce the maximum explosion pressure, but it also causes the explosion pressure growth rate to increase.     

Effects of vacuum levels,carbon dioxide,and structural sizes of linked vessels on dump vessel vented

The method of explosion venting is widely used in industrial explosion-proof design due to its simple operation, economical and practical features. A dump vessel vented platform was built. By changing the vacuum level and the gas in the dump vessels and the structural size of linked vessels, the pressure in the explosion vessel and the dump vessel was compared, and the influencing factors of explosion venting investigated. The main conclusions are as follows: In the explosion venting process, the higher the vacuum in the dump vessel, the smaller the pressure peak of the explosion vessel and the dump vessel, and the faster the explosion pressure is lowered. When the dump vessel is under the same vacuum level and the gas in the dump vessel is CO₂, the maximum pressure of the explosion vessel and the dump vessel is less than the maximum pressure when the containment medium is air. Under the same vacuum condition, the larger the volume ratio of the dump vessel and the explosion vessel, the smaller the pressure peak of the explosion vessel, the faster the explosion pressure drops, and the volume of the dump vessel reaches or exceeds the explosion vessel. Increasing the volume ratio of the containment vessel to the explosion vessel facilitates protection of the explosion vessel and the containment vessel. Under the same vacuum condition, when the gas explosion in 113 L vessel vents into 22 L vessel, the longer the length of the pipe, the greater the maximum pressure in the spherical vessel. When the gas explosion in 22 L vessel vents into 113 L dump vessel, as the pipeline grows, the maximum pressure in the two vessels decreases, but the reduction is not significant. In practical application, it is recommended to use a vacuum of 0.08Mpa or more for the dump vessel vented, and the containment medium is CO_2.In terms of the structural size of the container, it is recommended that the ratio of the receiving container to the explosion container be as large as possible, and the pipe length be as long.     

Influence of ignition position and obstacles on explosion development in methane–air mixture in closed vessels

The paper outlines an experimental study of influence of the ignition position and obstacles on explosion development in premixed methane–air mixtures in an elongated explosion vessel. As the explosion vessel, 1325 mm length tube with 128.5 mm diameter was used. Location of the ignition was changeable, i.e., fitted in the centre or at one of ends of the tube, when the tube was in a horizontal position. When it was in a vertical position, three locations of the ignition (bottom, centre and top) were used. In the performed study, the influence of obstacles on the course of pressure was investigated. Two identical steel grids were used as the obstacles. They were placed 405 mm from either end of the tube. Their blockage ratio (grid area to tube cross-section area) was determined as 0.33 for most of experiments. A few additional experiments (with smaller blockage ratio—0.16) were also conducted in order to compare the influence of the blockage ratio on the explosion development. Also some experiments were conducted in a semi-cylindrical vessel with volume close to 40 l.

All the experiments were performed under stabilized conditions, with the temperature and pressure inside the vessel settled to room values and controlled by means of electronic devices. The pressure–time profiles from two transducers placed in the centreline of the inner wall of the explosion vessel were obtained for stoichiometric (9.5%), lean (7%) and rich (12%) methane–air mixture. The results obtained in the study, including maximum pressures and pressure–time profiles, illustrate a quite distinct influence of the above listed factors upon the explosion characteristics. The effect of ignition position, obstacles location and their BR parameters is discussed.

The additional aim of the performed experiments was to find the data necessary to validate a new computer code, developed to calculate an explosion hazard in industrial installations.     

初始压力对连通容器甲烷-空气混合物泄爆压力的影响

建立球形容器与管道、2个球形容器与管道组成的2种形式的连通容器试验装置,研究初始压力对连通容器甲烷-空气混合物泄爆压力的影响。结果表明:连通容器内泄爆超压随初始压力增加而增大,并与初始压力近似成线性关系;对于2个球形容器与管道组成的连通容器,起爆容器的泄爆超压始终小于传爆容器;泄爆方式和点火方式对连通容器泄爆超压有较大影响,大容器点火时,2个容器的泄爆压力差随初始压力增加而增大,但小容器点火时,2个容器的泄爆压力差随初始压力的增加变化较小;初始压力对不同结构和尺寸的连通容器的泄爆压力的影响不同,当令初始压力对大容器点火时,小容器内泄爆压力受影响最大,而当对单球形容器与管道组成的连通容器的小容器点火时,小容器内泄爆压力受影响最小。