Numerical simulation of deflagration-to-detonation transition in large confined volumes

A methodology for the computationally efficient CFD simulation of hydrogen-air explosions (including transition to detonation) in large volumes is presented. The model is validated by means of the largest ever conducted indoor DDT experiments in the RUT facility. A combination of models is proposed with a particular focus on the influence of flame-instabilities, especially of thermal-diffusive nature, which are crucial for very lean mixtures. Excellent agreement is achieved in terms of flame acceleration. The quality of DDT predictions itself depends on the underlying mechanism. Whereas DDT by shock-focusing is successfully simulated on under-resolved meshes, DDT by local explosions in the vicinity of the turbulent flame brush remains a challenge. Adaptive mesh refinement therefore emerges as a key technique to resolve more of the essential phenomena at reasonable computational costs affordable by industry. Finally, a generic case demonstrates the influence of mixture inhomogeneity, which can promote flame acceleration and ultimately DDT.     

Agricultural waste pulverised biomass: MEC and flame speeds

A modified Hartmann dust explosion tube was employed to determine the Minimum Explosible Concentration (MEC) and the flame speed for three Pakistani agricultural wastes: bagasse, rice husk and wheat straw. Agricultural biomass had a higher ash content than for woody biomass and this influenced the MEC. The dispersion, ignition and MEC were influenced by the particle size distribution, as also demonstrated by high speed video. There was a strong linear correlation between the MEC and the sum of the ash and moisture content of these and other biomasses, indicating that this inert mass in the particles acted to reduce the flame temperature and reduce the lean flammability limit or MEC. Comparison of the results was made with non-agricultural waste pulverized biomass. Peak flame speeds were approximately 2.5 m/s. The lean limits for these pulverised agricultural waste biomasses were comparable to that of pulverised wood but were much leaner than those for coal and hydrocarbon fuels, which indicate that these biomasses are highly reactive.     

Un-ignited and ignited high pressure hydrogen releases: Concentration – turbulence mapping and overpressure effects

Safety studies for production and use of hydrogen reveal the importance of accurate prediction of the overpressure effects generated by delayed explosions of accidental high pressure hydrogen releases. Analysis of previous experimental work demonstrates the lack of measurements of turbulent intensities and lengthscales in the flammable envelope as well as the scarceness of accurate experimental data for explosion overpressures and flame speeds. AIR LIQUIDE, AREVA STOCKAGE ENERGIE and INERIS join in a collaborative project to study un-ignited and ignited high pressure releases of hydrogen.The purpose of this work is to map hydrogen flammable envelopes in terms of concentration, velocity and turbulence, and to characterize the flame behaviour and the associated overpressure. These experimental results (dispersion and explosion) are also compared with blind FLACS modelling.     

Effect of scale on freely propagating flames in aluminum dust clouds

The majority of experimental tests done on combustible dusts are performed in constant volume vessels that have limited or no optical access. Over the years, McGill University has been developing alternative experimental techniques based on direct observation of dust flames, yielding reliable fundamental parameters such as flame burning velocity, temperature and structure. The present work describes two new experimental set-ups allowing direct observation of isobaric and freely propagating dust flames at two sufficiently different scales to test the influence of scale on dust flame phenomena. In the laboratory-scale experiments, a few grams of aluminum powder are dispersed in transparent, 30 cm diameter latex balloons that allow for full visualization of the spherical flame propagation. In the field experiments, about 1 kg of aluminum powder is dispersed by a short pulse of air, forming a conical dust cloud with a total volume of about 5 m³. High-speed digital imaging is used to record the particle dispersal and flame propagation in both configurations. In the small-scale laboratory tests, the measured flame speed is found to be about 2.0 ± 0.2 m/s in fuel-rich aluminium clouds. The burning velocity, calculated by dividing the measured flame speed by the expansion factor deduced from thermodynamic equilibrium calculations, correlates well with the previously measured burning velocity of about 22–24 cm/s from Bunsen dust flames. Flame speeds observed in field experiments with large-scale clouds, however, are found to be much higher, in the range of 12 ± 2 m/s. Estimations are presented that show that the presumably greater role of radiative heat transfer in larger-scale aluminium flames is insufficient to explain the six-fold increase in flame speed. The role of residual large-eddy turbulence, as well as the frozen-turbulence effect leading to large-scale dust concentration fluctuations that cause flame folding, are discussed as two possible sources for the greater flame speed.     

The characterization of heat transfer fluid P-NF aerosol combustion: Ignitable region and flame development

Heat transfer fluids tend to form aerosols due to the operating conditions at high pressure when accidental leaking occurs in pipelines or storage vessels, which may cause serious fires and explosions. Due to the physical property complexity of aerosols, it is difficult to define a standard term of “flammability limits” as is possible for gases. The study discussed in this paper primarily focuses on the characterization of ignition conditions and flame development of heat transfer fluid aerosols. The flammable region of a widely-used commercial heat transfer fluid, Paratherm NF (P-NF), was analyzed by electro-spray generation with a laser diffraction particle analysis method. The aerosol ignition behavior depends on the droplet size and concentration of the aerosol. From the adjustment of differently applied electro-spray voltages (7–10 kV) and various liquid feeding rates, a flammable condition distribution was obtained by comparison of droplet size and concentration. An appropriate amount (0.3–1.2 ppm) of smaller droplets (80–110 μm) existing in a given space could result in successful flame formation, while larger droplets (up to 190 μm) have a relatively narrowed range of flammable conditions (0.7–0.9 ppm). It is possible to generate a more useful reference for industry and lab scale consideration when handling liquids. This paper provides initial flammability criteria for analyzing P-NF aerosol fire hazards in terms of droplet size and volumetric concentration, discusses the observation of aerosol combustion processes, and summarizes an ignition delay phenomenon. All of the fundamental study results are to be applied to practical cases with fire hazards analysis, pressurized liquid handling, and mitigation system design once there is a better understanding of aerosols formed by high-flash point materials.     

阻燃纯棉织物的研究与发展

对阻燃纯棉织物的国内外研究现状、生产和应用中存在的问题，进行了论述，并提出了今后的发展方向     

Effect of Al2O3 particle size reduction on aluminium dust explosion

To investigate the effect of Al₂O₃ particle size on an aluminum explosion, the overpressure and flame velocity in a vertical duct were evaluated. The results show that the inhibitory effect of submicron Al₂O₃ is best, while the inhibitory effect increases with increasing inerting ratio. However, the inhibitory effect of micron Al₂O₃ does not increase significantly after the inerting ratio exceeds 40%. For high-concentration aluminum powder, 0.8 μm Al₂O₃ with an inerting ratio less than 20% promotes aluminum explosion. As the inerting ratio increases beyond 20%, however, the overpressure decreases. Furthermore, Al₂O₃ inhibits the formation of the intermediate product AlO and decreases the flame brightness. As the inerting ratio of 0.8 μm Al₂O₃ reaches 50%, the white patches in the flame image disappear. The results of scanning electron microscopy showed that the explosion products agglomerate and some dot-like protrusions appear on the surface of the unburned aluminum particles. The inhibition mechanism was qualitatively investigated. Physical heat absorption is proven to play a limited role. Thermal radiation and chemical inhibition play a key role. The chemical effect mainly influences the surface reaction energy source.     

Thermal effects of a sonic jet fire impingement on a pipe

Although the effects of jet fires are often limited to rather short distances, if their flames impinge on a pipe or a vessel collapse can occur in very short times. In such cases, the heat flux on the affected equipment is very high and wall temperature can increase very rapidly. This can happen in parallel pipelines, if a release occurs and impinges on another one. Nevertheless, jet fire impingement has been scarcely studied. In this communication the results obtained from an experimental set-up are presented. Sonic jet fires impinged on a pipe containing stagnant air or water. The temperatures of the flames impinging on it were measured for the worst case (flame front-bright zone), as well as the evolution with time of the pipe wall temperature at different locations. Initial temperature increases up to around twenty °C/s were registered for the air inside, with maximum values of up to 600 °C reached in 2.5 min, and 800 °C in approximately 9 min. In the case of pipe containing water, in the zone of the wall in contact with the liquid the heating rates were much lower, the maximum temperature reached being up to approximately 150 °C. From the temperatures of the jet flames and of the pipe, the heat fluxes reaching the pipe and the corresponding heat transfer coefficients were obtained. The results obtained emphasized that safe distances are essential in pipelines, together with fire proofing and other safety measures.     

A review on hybrid mixture explosions: Safety parameters,explosion regimes and criteria,flame characteristics

The hybrid mixture of combustible dusts and flammable gases/vapours widely exist in various industries, including mining, petrochemical, metallurgical, textile and pharmaceutical. It may pose a higher explosion risk than gas/vapor or dust/mist explosions since the hybrid explosions can still be initiated even though both the gas and the dust concentration are lower than their lower explosion limit (LEL) values. Understanding the explosion threat of hybrid mixtures not only contributes to the inherent safety and sustainability of industrial process design, but promotes the efficiency of loss prevention and mitigation. To date, however, there is no test standard with reliable explosion criteria available to determine the safety parameters of all types of hybrid mixture explosions, nor the flame propagation and quenching mechanism or theoretical explanation behind these parameters. This review presents a state-of-the-art overview of the comprehensive understanding of hybrid mixture explosions mainly in an experimental study level; thereby, the main limitations and challenges to be faced are explored. The discussed main contents include the experimental measurement for the safety parameters of hybrid mixtures (i.e., explosion sensitivity and severity parameters) via typical test apparatuses, explosion regime and criterion of hybrid mixtures, the detailed flame propagation/quenching characteristics behind the explosion severities/sensitivities of hybrid mixtures. This work aims to summarize the essential basics of experimental studies, and to provide the perspectives based on the current research gaps to understand the explosion hazards of hybrid mixtures in-depth.     

Experimental study on water sealing of fire barriers and explosion venting of large-scale pipeline with low-concentration gas

Low-concentration gas transported in pipelines may lead to explosion accidents because gas with a concentration of less than 30% is prone to explode. To reduce the incidence of gas explosions, water sealing of fire barriers is implemented, and explosion venting devices are installed along the pipeline. To investigate their suppression effect on low-concentration gas explosion, experiments using methane–air premixed gas under different conditions were implemented on a DN500 pipeline test system. The effects of three types of explosion venting forms (rupture disc, asbestos board, and plastic film) on explosion overpressure and flame were compared and analysed. Results show that the rupture disc, asbestos board, and plastic film can achieve adequate explosion venting, causing the peak decay rates of explosion overpressure to reach 82.37%, 81.72%, and 90.79%, respectively. The foregoing indicates that the greater the static activation pressure of the explosion venting form, the higher the peak explosion overpressure at each measurement point. Moreover, the shorter the explosion flame duration, the greater the flame propagation velocity. The research results provide an essential theoretical foundation for the effective suppression of gas explosion accidents in the process of low-concentration gas transportation.