Experimental investigation on spontaneous ignition caused by pressurized hydrogen suddenly release into an S-shaped tube

An experimental investigation on the effects of continuous semicircular curved structure on spontaneous ignition during pressurized hydrogen suddenly release was conducted. An S-shaped tube with 700 mm in length and 10 mm in diameter was used in our experiments, and a straight tube with the same configuration was adopted for comparison. The results show that the continuously generated rarefaction waves and reflected shock waves make the pressure curves in the S-shaped tube more complicated. Meanwhile, the mean velocity and intensity of the leading shock wave undergo considerable attenuation when it propagates in the S-shaped structure. By comparing with the straight tube, the minimum critical pressure condition for spontaneous ignition in the S-shaped tube is slightly difficult to reach, but the difference is not huge. Nevertheless, the S-shaped structure can effectively promote hydrogen-air mixing and make combustion more intense. A secondary overpressure peak detected by the pressure transducer near the nozzle occurs in the spontaneous ignition cases and no such pressure increase is caught in the non-ignition cases. The transition from spontaneous combustion flame to a jet flame at the nozzle and the complete out-tube jet flame development process are captured and discussed.     

Mechanisms of high-pressure hydrogen gas self-ignition in tubes

This paper describes a numerical and experimental investigation of hydrogen self-ignition occurring as a result of the formation of a shock wave. The shock wave is formed in front of high-pressure hydrogen gas propagating in a tube. The ignition of the hydrogen–air mixture occurs at the contact surface of the hydrogen and oxidant mixture and is due to the temperature increase produced as a result of the shock wave. The required condition for self-ignition is to maintain the high temperature in the mixture for a time long enough for inflammation to take place. The experimental technique employed was based on a high-pressure chamber pressurized with hydrogen, to the point of a burst disk operating to discharge pressurized hydrogen into a tube of cylindrical or rectangular cross section containing air. A physicochemical model involving gas-dynamic transport of a viscous gas, detailed kinetics of hydrogen oxidation and heat exchange in the laminar approach was used for calculations of high-pressure hydrogen self-ignition. The reservoir pressure range, when a shock wave is formed in the air that has sufficient intensity to produce self-ignition of the hydrogen–air mixture, is found. An analysis of governing physical phenomena based on the experimental and numerical results of the initial conditions (the hydrogen pressure inside the vessel, and the shape of the tube in which the hydrogen was discharged) and physical mechanisms that lead to combustion is presented.     

Experimental investigation on shock waves generated by pressurized gas release through a tube

An experimental investigation on the flow structures and the strength of shock waves generated by high-pressure gas release through a tube into air was conducted. The results demonstrated that a leading shock wave was generated in front of the compressed gas jet and the shock wave speed increased firstly, then decreased and finally kept constant with an increase of the propagation distance in the tube. The experimentally measured Mach numbers of shock waves were close to those calculated from the theory of ideal shock tube flow. After spouting out of the tube, the normal shock quickly developed into a hemispherical shape. The Mach disk was observed in the under-expanded jet. For high-pressure combustible gas release, the concept of theoretical critical pressure of ignition was introduced and several theoretical critical pressures of common gaseous fuels were obtained.     

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.     

DDT and detonation propagation limits in an obstacle filled tube

Experiments with hydrogen–air and ethylene–air mixtures at atmospheric pressure were carried out in a 6.1 m long, 0.1 m diameter tube with different obstacle configurations and ignition types. Classical DDT experiments were performed with the first part of the tube filled with equally spaced 75 mm (44% area blockage ratio) orifice-plates. The DDT limits, defining the so-called quasi-detonation regime, where the wave propagates at a velocity above the speed of sound in the products, were found to be well correlated with d/λ = 1, where d is orifice-plate diameter and λ is the detonation cell size. The only exception was the rich ethylene limit where d/λ = 1.9 was found. In a second experiment detonation propagation limits were measured by transmitting a CJ detonation wave into an obstacle filled (same equally spaced 44% orifice plates) section of the tube. An oxy-acetylene driver promptly initiated a detonation wave at one end. In this experiment the quasi-detonation propagation limits were found to agree very well with the d/λ = 1 correlation. This indicates that the d/λ = 1 represents a propagation limit. In general, one can conclude that the classical DDT limits measured in an orifice-plate filled tube are governed by the wave propagation mechanism, independent of detonation initiation (DDT process) that can occur locally in the obstacles outside these limits. For rich mixtures, transmission of the quasi-detonation into the smooth tube resulted in CJ detonation wave. However, in a narrow range of mixtures on the lean side, the detonation failed to transmit in the smooth tube. This highlights the critical role that shock reflection plays in the propagation of quasi-detonation waves.     

Effect of ignition position on the run-up distance to DDT for hydrogen-air explosions

The method described in this paper enabled reliable and accurate positioning of an overdriven detonation by calculation of shock wave velocities (detonation and retonation) for hydrogen explosions in a closed 18 m long horizontal DN150 pipe. This enabled an empirical correlation between the ignition position and the run-up distance to DDT to be determined. It was shown that the initial ability of the flame to expand unobstructed and the piston-like effect of burnt gas expanding against the closed end of the tube contributed to initial flame acceleration and hence were able to affect the run-up distance to overdriven detonation. Flame speeds and rates of initial pressure rise were also used to explain how these two competing effects were able to produce a minimum in the run-up distance to DDT. The shortest run-up distance to DDT, relative to the ignition position, for this pipe and gas configuration was found when the ignition position was placed 5.6 pipe diameters (or 0.9 m) from the closed pipe end. The shortest run-up distance to DDT relative to the end of the pipe was recorded when the ignition source was placed 4.4 pipe diameters or 0.7 m from the pipe end.     

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.     

Shock-induced ignition of hydrogen gas during accidental or technical opening of high-pressure tanks

This paper is devoted to the numerical and experimental investigation of hydrogen self-ignition as a result of the formation of a primary shock wave in front of a cold expanding hydrogen gas jet. Temperature increase, as a result of this shock wave, leads to the ignition of the hydrogen–air mixture formed on the contact surface. The required condition for hydrogen self-ignition is to maintain the high temperature in the area for a time long enough for hydrogen and air to mix and inflammation to take place.

Calculations of the self-ignition of a hydrogen jet are based on a physicochemical model involving the gas-dynamic transport of a viscous gas, the kinetics of hydrogen oxidation, the multi-component diffusion, and the heat exchange. We found that the reservoir pressure range, when a shock wave formed in the air during depressurization, has sufficient intensity to produce self-ignition of the hydrogen–air mixture formed at the front of a jet of compressed hydrogen. We present an analysis of the initial conditions (the hydrogen pressure inside the vessel, the temperature of the compressed hydrogen and the surrounding air, and the diameter of the hole through which the jet was emitted), which leads to combustion.     

Large scale experiments to study fires following the rupture of high pressure pipelines conveying natural gas and natural gas/hydrogen mixtures

As part of the EC funded Naturalhy project, two large scale experiments were conducted to study the hazard presented by the rupture of high pressure transmission pipelines conveying natural gas or a natural gas/hydrogen mixture containing approximately 22% hydrogen by volume. The experiments involved complete rupture of a 150 mm diameter pipeline pressurised to nominally 70 bar. The released gas was ignited and formed a fireball which rose upwards and then burned out. It was followed by a jet fire which continued to increase in length, reaching a maximum of about 100 m before steadily declining as the pipeline depressurised. During the experiments, the flame length and the incident radiation field produced around the fire were measured. Measurements of the overpressure due to pipeline rupture and gas ignition were also recorded. The results showed that the addition of the hydrogen to the natural gas made little difference to radiative characteristics of the fires. However, the fraction of heat radiated by these pipeline fires was significantly higher than that observed for above ground high pressure jet fires (also conducted as part of the Naturalhy project) which achieved flame lengths up to 50 m. Due to the lower density, the natural gas/hydrogen mixture depressurised more quickly and also had a slightly reduced power. Hence, the pipeline conveying the natural gas/hydrogen mixture resulted in a slightly lower hazard in terms of thermal dose compared to the natural gas pipeline, when operating at the same pressure.     

Explosions and deflagration-to-detonation transitions in epoxy propane/air mixtures

The explosion and deflagration-to-detonation transition (DDT) in epoxy propane (E.P.) vapor/air mixture clouds under weak ignition conditions has been studied in an experimental tube of diameter 199 mm and length 29.6 m. E.P. vapor clouds were formed by injecting liquid E.P. into the experimental tube and evaporating of the fine E.P. droplets. The dimension and the evaporating process of the E.P. droplet were measured and analyzed. The E.P. vapor/air mixture clouds were ignited by an electric spark with an ignition energy of 40 J. The characteristics and the stages of the DDT process in the E.P. vapor/air mixtures have been studied and analyzed. A self-sustained detonation wave formed, as was evident from the existence of a transverse wave and a cellular structure. Moreover, a retonation wave formed during the DDT process in the E.P. vapor/air mixture. The influence of the E.P. vapor concentration on the DDT process has been studied. The minimum E.P. vapor concentration for the occurrence of the DDT in the E.P. vapor/air mixture has been evaluated and the variation of DDT distance with E.P. vapor concentration has been analyzed.     

Methane/coal dust/air explosions and their suppression by solid particle suppressing agents in a large-scale experimental tube

Methane/coal dust/air explosions under strong ignition conditions have been studied in a 199 mm inner diameter and 30.8 m long horizontal tube. A fuel gas/air manifold assembly was used to introduce methane and air into the experimental tube, and an array of 44 equally spaced dust dispersion units was used to disperse coal dust particles into the tube. The methane/coal dust/air mixture was ignited by a 7 m long epoxypropane mist cloud explosion. A deflagration-to-detonation transition (DDT) was observed, and a self-sustained detonation wave characterized by the existence of a transverse wave was propagated in the methane/coal dust/air mixtures.The suppressing effects on methane/coal dust/air mixture explosions of three solid particle suppressing agents have been studied. Coal dust and the suppressing agent were injected into the experimental tube by the dust dispersion units. The length of the suppression was 14 m. The suppression agents examined in this study comprised ABC powder, SiO₂ powder, and rock dust powder (CaCO₃). Methane/coal dust/air explosions can be efficiently suppressed by the suppression agents characterized by the rapid decrease in overpressure and propagating velocity of the explosion waves.     

燃烧管中烟煤粉粉尘云传播参数的试验研究

利用自行设计的长29.6 m,内径199 mm,配有特殊扬尘装置的大犁卧式燃烧爆炸管道试验系统,对弱点火条件下烟煤粉与空气两相悬浮流中的爆炸过程进行了试验研究,用压电传感器测量了管内各测点的压力信号,观测到快速爆燃的状态稳定,分析了爆燃波稳定传播机理.结果表明:在煤粉浓度为300g/m~3及弱点火条件下,悬浮烟煤粉粉尘云中爆燃波能够稳定传播,且稳态传播距离持续20 m以上,峰值超压和波速平均值分别约为70 kPa和430m/s.     

Self-ignition and explosion during discharge of high-pressure hydrogen

The phenomenon of self-ignition and explosion during discharge of high-pressure hydrogen was investigated. To clarify the ignition conditions of high-pressure hydrogen jets, rapid discharge of the high-pressure hydrogen was examined experimentally. A diaphragm was used to allow rapid discharge of the high-pressure hydrogen. The burst pressure was varied from 4 to 30 MPa. The downstream geometry of the diaphragm was a flange and extension pipes, with the pipe length varying from 3 to 300 mm. The diameter of the nozzle was 5 or 10 mm. When short pipes were used, the hydrogen jet did not ignite. However, the hydrogen jet showed an increasing tendency to ignite in the pipe as the length of the pipe became longer. At higher burst pressures, a diffusion jet flame was formed from the pipe. The blast wave from the fireball formed on self-ignition of the hydrogen jet resulted in an extremely rapid pressure rise.     

Small-scale physical explosions in shock tubes in comparison with condensed high explosive detonations

A series of small-scale experiments involving physical explosions in a 1.6 l pressure vessel was carried out. Explosions were initiated by spontaneous rupture of an aluminium membrane on one side of the vessel at a pressure in the range 1–1.2 MPa. The pressure waves released were measured at different distances along two separate shock tubes, one 10 m long and 200 mm in diameter (closed at one end by the high pressure vessel) and the other 15 m long and 100 mm in diameter.TNT equivalency was used for predicting the blast wave characteristics after vessel rupture. TNT equivalency was used because equations for prediction of peak pressure and impulse of the blast wave in 1-D geometry after detonations of condensed explosives are known. Some experiments with an equivalent amount of real explosive were carried out for comparison with the theoretical and experimental data obtained. The applicability of the TNT equivalency method presented for calculations of maximum pressure and shock wave impulse generated after rupture of the pressure vessel in 1-D geometry is discussed.     

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.     

Explosibility of polyamide and polyester fibers

The current research is aimed at investigating the explosion behavior of hazardous materials in relation to aspects of particulate size. The materials of study are flocculent (fibrous) polyamide 6.6 (nylon) and polyester (polyethylene terephthalate). These materials may be termed nontraditional dusts due to their cylindrical shape which necessitates consideration of both particle diameter and length. The experimental work undertaken is divided into two main parts. The first deals with the determination of deflagration parameters for polyamide 6.6 (dtex 3.3) for different lengths: 0.3 mm, 0.5 mm, 0.75 mm, 0.9 mm and 1 mm; the second involves a study of the deflagration behavior of polyester and polyamide 6.6 samples, each having a length of 0.5 mm and two different values of dtex, namely 1.7 and 3.3. (Dtex or decitex is a unit of measure for the linear density of fibers. It is equivalent to the mass in grams per 10,000 m of a single filament, and can be converted to a particle diameter.) The explosibility parameters investigated for both flocculent materials include maximum explosion pressure (P_max), size-normalized maximum rate of pressure rise (K_St), minimum explosible concentration (MEC), minimum ignition energy (MIE) and minimum ignition temperature (MIT). ASTM protocols were followed using standard dust explosibility test equipment (Siwek 20-L explosion chamber, MIKE 3 apparatus and BAM oven). Both qualitative and quantitative analyses were undertaken as indicated by the following examples. Qualitative observation of the post-explosion residue for polyamide 6.6 indicated a complex interwoven structure, whereas the polyester residue showed a shiny, melt-type appearance. Quantitatively, the highest values of P_max and K_St were obtained at the shortest length and finest dtex for a given material. For a given length, polyester displayed a greater difference in P_max and K_St at different values of dtex than polyamide 6.6. Long ignition delay times were observed in the BAM oven (MIT measurements) for polyester, and video framing of explosions in the MIKE 3 apparatus (MIE measurements) enabled observation of secondary ignitions caused by flame propagation after the initial ignition occurring at the spark electrodes.     

Ignition wave in a two-velocity two-temperature multiphase mixture

The theory of stationary adiabatic and non-adiabatic ignition waves in magnesium aerosuspension is developed on the basis of the mathematical model based on two-velocity two-temperature approach of mechanics of heterogeneous media. Specifically, the conditions such that the particle cloud ignites under the action of an initiating shock wave (SW) are defined. An agreement between the adiabatic one-velocity model and the adiabatic two-velocity model on the ignition delay time for small particles is shown. Influence of particle size on this characteristic is compared for both models. Validation of the two-velocity model is performed on the basis of comparison with the experimental data. The unified formula for calculation of the induction period of the magnesium particle mixture in oxygen which takes into account its dependence on the SW Mach number and particle radius is found.     

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.     

Experimental simulation of airbag deployment for pipeline closing

Small scale tests were carried out at ISL's shock tube facility STA (100 mm inner diameter) to study the problem of closing a pipeline by means of an airbag in case of explosions or gas leakages. Experiments were carried out to simulate the flow in a pipeline at velocities and gas pressures as present in pipeline flows. In this study the gas used was nitrogen at static pressures of 0.2 up to 5 MPa and at flow velocities of 25 m/s up to 170 m/s. A special Nylon airbag, deployed from the tube wall into the pipe, was used to simulate the airbag inflation in a real pipeline. For this purpose a special gas filling system consisting of a gas generator with a reservoir volume of up to 500 cm³ which permits air pressures up to 17 MPa to be generated inside the airbag was developed at ISL. With a fast pyrotechnically opened valve the reservoir gas was released for airbag filling. The airbag inflation was triggered in such a way that it opened in nearly 3 ms into the pipe flow generated by the shock tube and continued for about 10 ms. For this application a special measuring chamber was designed and constructed with 20 measuring ports. Through two window ports, located one in front of the other, the airbag inflation could be visualized with up to 50 successive flash sparks illuminating a fast rotating film inside a drum camera. Pressure measurements using commercially available PCB pressure gauges at 9 measuring ports placed along the inner tube surface gave some hints on the behaviour of the wall pressure during airbag deployment. As a result from the experiments performed it is to conclude, that, with the Nylon airbag samples available, the pipe flow cannot be blocked by the inflating airbag. The flow forces acting on the airbag during deployment are in the shock tube experiments of the order of about 1000 N, which are not balanced by the airbags' neck, fixing it to the shock tube wall. This outcome suggests that a mechanical support is required to fix the airbag in its place during inflation.