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     

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.     

Effects of multiple annular obstacles on flame propagation of local corn starch dust in a vertical pipe

With high-speed camera technology, the propagation behavior of explosion flame for the local dust cloud of corn starch in a semi-open vertical pipe under the action of the annular obstacle was studied experimentally, and the blockage rate and the annular obstacle numbers as well as impact of dust cloud concentration on the flame propagation were investigated. The researches showed that both the blockage rate and the annular obstacle numbers have significant effects on the flame speed and propagation process for the dust cloud explosion of corn starch. The increase of the blockage rate of such annular obstacles will cause that the combustion of dust cloud with high concentration is mainly concentrated in the lower part of the pipe. The increase of the annular obstacle numbers will lead to the acceleration of combustion of the dust cloud. With the increase of the blockage rate and the annular obstacle numbers, the maximum flame speed shows a trend of the first increasing and then decreasing, and the phenomenon of accelerated propagation of the flame becomes more and more obvious, however, the distance of continuous acceleration for the flame is gradually decreased and the maximum flame speed is farther from the outlet of the pipe. Under the action of such annular obstacles, the concentration of dust cloud has a significant effect on the flame speed and shape of the dust cloud of the corn starch. The increase of the concentration of the dust cloud will decrease the acceleration effect of such annular obstacles to result in maximum flame speed showing a trend of the first increasing and then decreasing. However, the acceleration distance of the flame is longer, and the maximum flame speed is closer to the outlet of the pipe. The increasing concentration will make the flame speed develop more slowly, the flame color will be darker, and the flame segmentation phenomenon will be more obvious.     

Explosions in closed pipes containing baffles and 90 degree bends

There is a general lack of information on the effects of full-bore obstacles on combustion in the literature, these obstacles are prevalent in many applications and knowledge of their effects on phenomena including burning rate, flame acceleration and DDT is important for the correct placing of explosion safety devices such as flame arresters and venting devices. In this work methane, propane, ethylene and hydrogen–air explosions were investigated in an 18 m long DN150 closed pipe with a 90 degree bend and various baffle obstacles placed at a short distance from the ignition source. After carrying out multiple experiments with the same configuration it was found that a relatively large variance existed in the measured flame speeds and overpressures, this was attributed to a stochastic element in how flames evolved and also how they caused and interacted with turbulence to produce flame acceleration. This led to several experiments being carried out for one configuration in order to obtain a meaningful average. It was shown that a 90 degree bend in a long tube had the ability to enhance flame speeds and overpressures, and shorten the run-up distance to DDT to a varying degree for a number of gases. In terms of the qualitative effects on these parameters they were comparable to baffle type obstacles with a blockage ratios of between 10 and 20%.     

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.     

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 investigation of effects on premixed hydrogen/air flame propagation in pipes with different contraction or expansion angles

Numerical simulations of premixed hydrogen-air flame propagation in a pipe with different contraction or expansion angles are carried out in this study. The effects on the flame propagation characteristics are investigated, including flame shape, the speed of flame front and overpressure. Results show that the flame propagation at different contraction angles experiences 6 flame stages: spherical flame stage, finger-shaped flame stage, stage of flame front touching the sidewalls, classic tulip flame stage, dissipation stage of tulip flame and its re-formation stage. The formation of tulip flame and the following stages are promoted by the contraction structure. Meanwhile, the development of the flow and pressure fields near the contraction are analyzed and it is found that the paraclinical effects induced by the contraction angle enhance the tulip re-formation. In the sudden expansion pipes, a triple flame stage appears in the pipes. The flame front remains relatively static for a period of time. However, the flame would continue to propagate when the expansion angle becomes larger and the flame propagation distance in the ducts increased obviously with the larger expansion angle. Baroclinic effect can inhibit the intensity of the vortex in the flow field, and hence weaken the forward transport of fuel. This inhibit effects decrease with the expansion angle becomes larger. The results of this study have implications concerning designs for pipe geometry of hydrogen and may help get better hydrogen transportation.     

Effect of bifurcation on premixed methane-air explosion overpressure in pipes

It is well known that bifurcation structures have a significant influence on gas explosions in pipelines or roadways. In this work, three different types of bifurcation, namely, bifurcation with two right angles (BTRAs), bifurcation with two obtuse angles (BTOAs), and bifurcation with an obtuse angle and an acute angle (BOAA), were used to study the effect of bifurcation on premixed methane–air explosion overpressure in pipes. The effect of the position of bifurcation on gas explosions was also discussed. Our results suggest that the peak overpressure evolution in pipes exhibits a downtrend before the bifurcation, a sharp increase after the bifurcation until reaching the maximum, and a downward trend when propagating into the pipe end. It was also found that gas-explosion propagation was affected by the joint action of turbulence induced by obstacles and the abrupt increase of the cross-sectional area. In addition, the bifurcation’s position had only a small effect on the maximum peak overpressure in pipes.     

有障碍物半开口管道内氢气燃烧数值模拟研究

基于有障碍物氢气燃烧实验装置进行数值模拟研究,采用Fluent软件分析了半开口管道内障碍物对氢气/空气燃烧特性的影响。结果表明:障碍物会促进实验管段内氢气火焰加速,随着障碍物阻塞率和数量的增加,火焰加速更快且燃烧压力峰值更大;在相同阻塞率下,障碍物形状对氢气火焰速度和燃烧压力峰值的影响很小;燃烧压力随障碍物间距的增大先增大后减小,障碍物间距为3倍管道内径时产生的燃烧压力峰值最大。     

Flame propagation in hybrid mixture of coal dust and methane

To investigate the flame propagation through hybrid mixture of coal dust and methane in a combustion chamber, a high-speed video camera with a microscopic lens and a Schlieren optical system were used to record the flame propagation process and to obtain the direct light emission photographs. Flame temperature was detected by a fine thermocouple. The suspended coal dust in the mixture of methane and air was ignited by an electric spark. The flame propagation speeds and maximum flame temperatures of the mixture were analyzed. The results show that the co-presence of coal dust and methane improves the flame propagation speed and maximum flame temperature notably, which become much higher than that of the single-coal dust flame. The flame front temperature varies with the coal dust concentration.     

Flame acceleration in unconfined hydrogen/air deflagrations using infrared photography

Flame behavior and blast waves generated during unconfined hydrogen deflagrations were experimentally studied using infrared photography. Infrared photography enables expanding spherical flame behaviors to be measured and flame acceleration exponents to be evaluated. In the present experiments, hydrogen/air mixtures of various concentrations were filled in a plastic tent of thin vinyl sheet of 1 m³ and ignited by an electric spark. The onset of accelerative dynamics on the flame propagation was analyzed by the time histories of the flame radius and the stretched flame speed. The results demonstrated that the self-acceleration of the flame, which was caused by diffusional-thermal and hydrodynamic instabilities of the blast wave, was influenced by hydrogen deflagrations in unconfined areas. In particular, it was demonstrated that the overpressure rapidly increased with time. The burning velocity acceleration was greatly enhanced with spontaneous-turbulization.     

Effects of particle thermal characteristics on flame microstructures during dust explosions of three long-chain monobasic alcohols in a half-closed chamber

To reveal clearly the effects of particle thermal characteristics on flame microstructures during organic dust explosions, three long-chain monobasic alcohols, solid at room temperature and similar in physical-chemical properties, were chosen to conduct experiments in a half-closed chamber. In the experiments, the dust materials were dispersed into the chamber by air to form dust clouds and the hybrids were ignited by an electrical spark. A high-speed optical schlieren system was used to record the flame propagation behaviors. A fine thermocouple and an ion current probe were respectively used to measure the flame temperature profile and the reaction behaviors of the combustion zone. Based on the experimental results, combustion behaviors and flame microstructures in dust clouds with different thermal characteristics were analyzed in detail. As a result, it was found that the dust flame surfaces were completely covered by cellular structures that significantly increased the flame frontal areas. Flame propagated more quickly and the number of the cellular cells increased as increasing the volatility of the particles. On the contrary, maximum temperature and the thickness of the preheated zone decreased as increasing the volatility of the particles. According to the ion current profile, the particles in the preheat zone were pyrolyzed to intermediate radicals and the radicals' fraction in the higher volatile dust flame was higher than that in the lower volatile dust flame.     

Isometric Pull and Push Strengths of Paraplegics in the Workspace: 2. Statistical Analysis of Spatial Factors

The effect of reach levels, horizontal angles, and vertical angles on isometric pull and push strengths of male and female was determined. Highly significant increases in men’s push strength were found between extreme to maximum reaches, and from extreme to normal reaches. However, for women’s push strength, a significant increase was found only between extreme to maximum reach. Significant or highly significant increases were found in men’s and women’s pull strength between the horizontal angle (θ) sagittal through the active shoulder (90°) and other angles (0, 45, and 135°). However, for men’s push strength, highly significant increases were found between the horizontal angle 45° from the frontal plane, and other angles. For women’s push strength, significant or highly significant increases were found between the horizontal angles 0° and angles of 90 and 135°. For men’s and women’s pull strength, significant or highly significant increases were found between the vertical angle (?), 90°, and other angles (–20, 0, and 45°). Similar increases were found for women’s push strength between the 45° angle and other angles. In the design of a workstation for paraplegics that requires pull and push forces, consideration must be given to the spatial factors.     

试样放置角度与外加辐射强度对可碳化固体表面火蔓延的影响

选取了尺寸为400mm×90mm×3mm质地较为均匀的白木作为研究对象,对不同试样放置角度和不同外加辐射强度作用下的火焰高度、火蔓延速率和失重速率等火蔓延参数进行了测量,以研究试样放置角度和辐射强度对可炭化固体表面火蔓延的作用规律。研究发现,试样表面的火蔓延速率随辐射强度的增加而增加。当角度在负角度范围内时,角度的变化对火蔓延速率的影响不大,当角度在正角度范围内时,火蔓延速率随着角度的增加而迅速上升。试样失重速率随着辐射强度以及试样放置角度的增加而增加。火焰高度会随着外加辐射强度的增加而增加,并与试样失重速率的0.4次方呈正比关系。     

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.     

Effects of cross-wise obstacle position on methane–air deflagration characteristics

A vented chamber, with internal dimensions of 150 mm × 150 mm × 500 mm, is constructed in which the premixed methane–air deflagration flame, propagating away from the ignition source, interacts with obstacles along its path. Three obstacle configurations with different cross-wise positions are investigated. The cross-wise obstacle positions are found to have significant effects on deflagration characteristics, such as flame structure, flame front location, flame speed, and overpressure transients. The rate of flame acceleration, as the flame passes over the last obstacle, is the highest at the configuration with three centrally located obstacles, whereas the lowest is observed at the configuration with three obstacles mounted on one side of the chamber. Compared with the side configuration, the magnitude of overpressure generated increases by approximately 80% and 165% for the central and staggered configurations, respectively. Furthermore, flame propagation speeds and generated overpressures for both the central and staggered configurations are greater, which should to be avoided to reduce the risk associated with turbulent premixed deflagrations in practical processes.     

Large eddy simulation and experimental study of the effect of wire mesh on flame behaviours of methane/air explosions in a semi-confined pipe

Preventing the propagation of flames in a pipeline is an effective measure for avoiding gas explosion accidents and reducing losses. To evaluate the effect of wire mesh, acting as a porous media, experimental and simulation studies are conducted to determine the influence of the wire mesh on the dynamics of premixed methane/air flame propagation in a semi-closed pipe. Four different kinds of wire mesh with different numbers of layers are chosen in the experiments and simulation, and the mechanism of wire mesh quenching of the flame is investigated. The experimental and simulation results are consistent. Flames are quenched when 4 layers of 40-mesh or 3 layers of 60-mesh wire mesh are used; however, once the flame propagates through the wire mesh, the risk of methane combustion may increase. The wire mesh becomes the key factor causing flame folds and acceleration, and the greater the number of layers or the larger the mesh size is, the more obvious the folds after the flame passes through the wire mesh. Moreover, the combination of heat absorption and disruption of the continuous flame surface by the mesh causes flame quenching. Wire mesh can effectively attenuate the flame temperature during premixed flame propagation in a pipe, and the attenuated maximum rate reaches approximately 79% in the case of adding 3 layers of 60-mesh wire mesh.     

Flame propagation in lycopodium/air mixtures below atmospheric pressure

Industrial processes are often operated at conditions deviating from atmospheric conditions. Safety relevant parameters normally used for hazard evaluation and classification of combustible dusts are only valid within a very narrow range of pressure, temperature and gas composition. The development of dust explosions and flame propagation under reduced pressure conditions is poorly investigated. Standard laboratory equipment like the 20 l Siwek chamber does not allow investigations at very low pressures. Therefore an experimental device was developed for the investigations on flame propagation and ignition under reduced pressure conditions. Flame propagation was analysed by a video analysis system the actual flame speed was measured by optical sensors. Experiments were carried out with lycopodium at dust concentrations of 100 g/m³, 200 g/m³ and 300 g/m³. It was found that both flame shapes and flame speeds were quite different from those obtained at atmospheric pressure. Effects like buoyancy of hot gases during ignition and flame propagation are less strong than at atmospheric conditions. For the investigated dust concentrations the flame reaches speeds that are nearly an order of a magnitude higher than at ambient conditions.     

Modeling the initial flame acceleration in an obstructed channel using large eddy simulation

The propagation and acceleration of a flame surface past obstructions in a closed square channel was investigated using large eddy simulation. The dynamic Smagorinsky–Lilly subgrid model and the Boger flame surface density combustion model were used. The geometry is essentially two-dimensional with fence-type obstacles distributed on the top and bottom surfaces, equally spaced along the channel length at the channel height. Flame propagation, however, is three dimensional as ignition occurs at a point at the center of the channel cross-section. The effect of obstacle blockage ratio on the development of the flame structure was investigated by varying the obstacle height. Three-dimensional cases were simulated from the initiation of a combustion kernel through spark ignition to the acceleration of the flame front at speeds up to 80 m/s. The transition from laminar flame propagation to turbulent flame propagation within the “thin reaction zone” regime was observed in the simulations. By analyzing the development of the three dimensional flame surface and unburned gas flow field, the formation of several flame structures observed experimentally are explained. Global quantities such as the total flame area and centerline flame velocity were ascertained and compared to the experimental data. High amplitude oscillations in the centerline flame velocity were found to occur from a combination of the unburned gas flow field and fluctuations in the volumetric burning rate.     

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.