大断面公路隧道施工风流分布规律研究

为了采取有效的通风措施降低钻爆法施工的粉尘浓度,以辛庄隧道为例,通过对其掌子面附近风流的现场实测,系统的开展了风流流场分布规律的研究,并采用Fluent软件进行了数值模拟,模拟结果与实测数据基本吻合。结果表明:隧道内风流速度以风筒出口为界,分为射流扩张区、射流收缩区、涡流区、回流区4个部分;靠近掌子面的同一断面上:风速呈“中间小,两边大”的形式,且回流区面积稍大于射流区面积,其中射流区最大断面积约占整个断面积的40%。研究结果对隧道施工通风除尘具有一定的指导意义和使用价值。     

CFD modeling and simulation of turbulent fluid flow and dust dispersion in the 20-L explosion vessel equipped with the perforated annular nozzle

A three-dimensional CFD model was developed to simulate the turbulent flow field induced by dust feeding and the associated dust dispersion within the 20-L explosion vessel equipped with the perforated annular nozzle. The model was validated against experimental data for pressure and root mean square velocity.Simulation results have shown that the turbulent kinetic energy is rather uniformly distributed and its values are significantly lower than those attained with the rebound nozzle. Furthermore, the perforated annular nozzle is able to generate a uniform dust/air cloud. However, a consistent fraction of the dust remains trapped inside the nozzle and, thus, it does not contribute to the explosion process.     

脉冲清灰喷吹气流偏斜的优化研究

为研究改善除尘滤筒清灰过程中喷吹气流偏斜的问题,通过在多孔喷吹管上安装混流喷嘴,在滤筒迎流面和背流面6个测点分别安装压力传感器,测试不同喷吹压力和喷吹距离下的侧壁压力峰值。结果表明:安装混流喷嘴后,在最优喷吹压力0.4 MPa实验条件下,滤筒迎流面一侧与背流面一侧的压力峰值差距减小,压力动态波形图相似,清灰更加均匀,喷吹气流偏斜问题得到明显改善;在喷吹距离分别为210,240,270 mm时,随喷吹距离增加,6个测点的压力峰值整体均增加,矫正后的脉冲喷吹气流对滤筒的清灰能力得到有效提高,可以改善除尘滤筒清灰均匀性问题。     

Visualization and analysis of dispersion process of combustible dust in a transparent Siwek 20-L chamber

Dust explosion severities are closely associated with dust dispersion behaviors. To characterize the dispersion process of dust cloud, visualization experiments were conducted by using a transparent Siwek 20-L chamber. Dispersion processes of typical carbonaceous dust were recorded by a high-speed camera and, with the image processing technique, the qualitative analysis based on the transmission of dust cloud was carried out. Results have evidenced the three consecutive stages of dust dispersion process: the fast injection stage of dust particles, the stabilization stage and the sedimentation stage of dust cloud. The motion of dust particles and the variations of dust cloud in space and time can be clearly distinguished. In the stabilization stage, the good uniformity of dust dispersion is achieved when the deviation of transmission data at different locations reaches to the minimum value. Under different nominal dust concentrations, the time periods for dust dispersion stabilization are found to be significantly different, suggesting that different dust concentrations should correspond to different ignition delay in order to accurately measure the explosion characteristics in the Siwek 20-L chamber. Moreover, it is found that the decrease trend of transmission with increasing nominal dust concentration will become gradually leveling off, different from the inversely proportional relationship according to the Bouguer's law, and this indicates that the actual dust concentration will be lower than the nominal concentration or the dust cannot be fully dispersed at the case of high dust concentration. According to the experiment, when the nominal dust concentration exceeds to 1000 g/m³, the transmission will no longer vary visibly.     

Numerical study on dust movement and dust distribution for hybrid ventilation system in a laneway of coal mine

Coal dust disaster is the most serious problem in a laneway of coal mine. Dust movement regularity for comprehensive mechanized heading face is the key scientific issues for the principle and technology of dust prevention. The special topic on systematic study of the variation regularity of dust movement and dust distribution is presented with hybrid ventilation for the comprehensive mechanized heading face: Euler–Euler method was firstly established on the numerical platform for gas–solid two phase flow in a laneway. And the forces and the dynamic model of dust particles were performed in three-dimensional flow field. Then based on the visible simulations, the movement characteristics of diffusion, sedimentation and accumulation of dust particles were investigated under the action of the complex air flow, and the spatiotemporal variation of dust distribution was studied with hybrid ventilation system. Meanwhile, the obtained dust distribution regularities were compared with the obtainable experimental results. Finally, selected method on different ventilation patterns for dust control was brought out for the heading face according to the gained regularity. The research results is helpful for further understanding of the essence of dust movement with air flow, which could provide more suitable guidance for the principle of dust control and technology of ventilation.     

煤岩混合型粉尘云最低着火温度试验研究

为研究半煤岩巷道中岩粉质量分数和煤的挥发分与煤岩混合型粉尘云最低着火温度的关系,选取挥发分差异较大的5种煤样以相同比例配制煤岩混合型粉尘,利用粉尘云最小点火温度测定仪进行煤岩混合型粉尘试验。结果表明,当煤岩混合型粉尘中岩粉质量分数低于40%时,岩粉的混合会导致混合型粉尘云最低着火温度发生小幅度波动;当岩粉质量分数高于40%时,煤岩混合型粉尘最低着火温度会随岩粉质量分数的增加而大幅度升高;挥发分质量分数越小的煤粉,其混合型粉尘云最低着火温度越容易受岩粉质量分数的影响。     

Study of dust cloud behaviour in the modified Hartmann tube using the image subtraction method (ISM)

A dispersion of fine particles in the air is needed for a dust explosion to occur since an explosion is the fast combustion of particles in the air. When particles are poorly dispersed, agglomerated, or their concentration is low, the combustion velocity decreases, and deflagration would not occur. The combustion rate is strictly related to dust concentration. Therefore, the maximum explosion pressure rise occurs at dust concentration close to stoichiometric. Conversely, Minimum Explosion Concentration (MEC) is the lower limit at which self-sustained combustion and a pressure rise are possible. Dust explosion tests are designed to reproduce the dispersion and generation of dust clouds in industrial ambiences by using dispersion devices activated by pressurised air pulses. The resulting dust cloud, which has a marked transient character, is considered representative of real clouds by current standards. Over time, several studies have been carried out to optimise these devices (e.g. to reduce the inhomogeneity of the cloud in the 20 L sphere). The Minimum Ignition Energy (MIE) of dust is measured using the Mike3 modified Hartmann tube, where the ignition attempt is made 60–180 ms after dust dispersion regardless of dust characteristics.This work investigates the dust clouds’ actual behaviour inside the modified Hartmann tube before ignition using high-velocity video movies and a new image post-treatment method called Image Subtraction Method (ISM). Movies are recorded with high-speed cameras at a framerate of 2000 fps and elaborated with an on-purpose developed LabVIEW® code. Concentration (mass per volume) and dispersion pressure are varied to evaluate their effect on dust clouds. Maise starch, iron powder and silica powder are chosen to investigate the effect of particle density and size on the cloud structure and turbulence. This approach will help to investigate the structure of the dust cloud, the shape and size of the particle lumps and the change in dust concentration over time. In addition, information on the actual concentration and cloud turbulence at the ignition location and delay time were obtained, which may help identify the local turbulence scale and widen the characterisation of the cloud generated in the Hartmann tube.     

Propagation and extinction of dust flames in narrow channels

This article has investigated the propagation and extinction of aluminum dust cloud flame in a narrow channel. The burned and burning dust particles act as heat sources and the channel walls act as heat sinks. In this method, discrete heat source has been used to analyze dust combustion in a narrow channel. Using the superposition of sources and sinks, the preheat zone temperature is predicted as an indicator of flame propagation or extinction. Dust concentration and channel width are two major parameters which affect the quenching distance and flame propagating speed. Wall temperature affects the heat loss; and by preheating the walls, quenching distance is reduced and flame propagation speed is increased.     

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.     

基于多孔环形喷嘴的可燃爆粉尘分散特征的密度效应研究*

为了研究不同密度的可燃爆粉尘在内置多孔环形喷嘴的20 L爆炸特性测试装置中的分散特征,基于负载粒子流方法、耦合DPM动量平衡方程和时间平均 Navier Stokes控制方程组,实现3种不同密度的煤粉、铝粉和锆粉在20 L爆炸测试装置中粉尘分散全过程的数值模拟。研究结果表明:多孔环形喷嘴的分散较为均匀,但是约束管道末端存在局部粉尘残留区,致使爆炸仓内真实粉尘浓度远低于形式浓度;爆炸仓中心位置的最大湍动能随着粉尘密度的增加而减小,只有显著地变化粉尘密度才能展示区分度较高的浓度峰值和抵达浓度峰值的时间。     

Suppression effects of powder suppressants on the explosions of oxyhydrogen gas

Suppression tests of oxyhydrogen gas explosions were performed in an explosion tube with five types of dry powder used as the suppressants. The experimental results showed that the powder with large dust cloud density and small radius has better suppression effect, which agrees well with previous correlative results. Moreover, our results also showed that particles with chemical activity and light material density, their suppression effect are more prominent than that of the inert particles with heavy density. To discover the detailed suppression process of dust powder, governing equations were developed based on the homogeneous reactive two-phase flow. The TVD scheme and the Lax–Wendroff–Rubin scheme were adopted to solve the reactive gas phase and particle phase, respectively. The time splitting technique was employed to handle the stiffness of the coupled equations. Our calculated results showed that the dust cloud has the suppression effect on the explosion of oxyhydrogen gas, and with the increase of dust cloud density or the decrease of particle diameter, its suppression effect become more evident, which is in good agreement with our experimental results, in addition, the numerical results showed that with the same particle diameter, the suppression performance is enhanced with the reduction in particle material density.     

对喷流除尘技术在收集硫酸铵和硝酸铵粉尘中的应用研究

对喷流除尘技术利用粉尘颗粒在撞击区内来回振荡、相互碰撞并团聚的机理进行除尘.实验采用水平式对喷流除尘系统收集硫酸铵和硝酸铵的混合物粉尘,主要考察喷嘴气流速度、含尘浓度和喷雾化水润湿含尘气流对除尘效率的影响,并进行机理分析.实验表明,除尘效率随喷嘴风速的增大而升高,但喷嘴风速超过25～27 m/s后,反而下降;除尘效率随含尘浓度的增加而升高,但含尘浓度超过0 45～0.55 kg/m3后反而有所降低;喷雾化润湿含尘气流能显著提高除尘效率,最优耗水量为0.18～0.22 kg/kg粉尘,超过该值后无显著变化.实验确定的最优除尘条件为:喷嘴速度25～27 m/s、含尘浓度0.45～0.55 kg/m3、耗水量0.18～0 22 kg/kg粉尘,除尘效率最高可达96.8%.     

自进式旋转钻头钻孔修复理论及关键参数研究

瓦斯预抽钻孔在抽放过程中经常出现塌孔、堵孔等现象。针对这一问题,提出采用自进式旋转钻头修复失效钻孔的新方法。其利用自进式旋转钻头后置喷嘴的喷射反冲力作为动力,自行至钻孔堵塞段处,前置喷嘴形成的高压水射流对堵孔煤渣进行破碎,将其与水混合返出孔外,实现清孔排渣、钻孔修复的目的。对自进式旋转钻头钻孔破煤清渣的临界射流压力及流量进行分析,基于摩擦动力学计算钻头自驱的临界射流压力,在此基础上设计钻头结构及优化喷嘴布置,数值分析不同喷嘴组合情况下射流速度分布,得出最优喷嘴布置方式。并应用于鹤煤八矿-655轨道石门揭煤瓦斯预抽,实验结果表明:该方法能够实现失效钻孔的修复,并有效的提高了瓦斯抽采效果,缩短了瓦斯抽放时间。     

圆锥形喷嘴出口直径对水射流冲击动力特性的影响研究

为探讨喷嘴结构对水射流冲击动力特性的影响,以圆锥形喷嘴为研究对象,基于COMSOL数值模拟软件,建立不同出口直径的圆锥形喷嘴模型,研究出口直径对水射流冲击动力特性的影响。研究结果表明：圆锥形喷嘴水射流冲击煤岩体过程中,不同喷嘴出口直径下水射流流场分布特征相似,整个流场可分为集中区、发散区、回流区和卷吸区4个区域,随喷嘴出口直径增大,卷吸区逐渐消失,其余3个区域分布也明显减弱;煤岩体应力分布可分为中心应力集中区和两侧应力集中区,随喷嘴出口直径不断增大,中心应力集中区与两侧应力集中区的范围逐渐减小,当喷嘴出口直径为6 mm时,两侧应力集中区基本消失;主体段入口速度恒定条件下,圆锥形喷嘴优选以2～3 mm出口直径为宜,此时水射流冲击煤岩体效果较佳,且不会对喷嘴产生结构破坏。     

CFD Simulation of the Dispersion of Binary Dust Mixtures in the 20 L Vessel

There are at least two main requirements for repeatable and reliable measurements of flammability and explosibility parameters of dusts: a uniform dispersion of solid particles inside the test vessel, and a homogeneous degree of turbulence. In several literature works, it has been shown that, in the standard 20 L sphere, the dust injection system generates a non-uniform dust cloud, while high gradients characterize the turbulent flow field. In this work, the dust dispersion inside the 20 L sphere was simulated for nicotinic acid/anthraquinone mixtures (with different pure dust ratios, while keeping the total dust concentration constant) with a validated three-dimensional CFD model. Numerical results show that the fields of dust concentration, flow velocity and turbulence are strongly affected by both diameter and density of the pure dusts. These different dust properties lead to segregation phenomena with the formation of zones richer in one component and leaner in the other one and vice versa, and also result in preferential paths for the solid particles inside the sphere. Overall, the obtained results highlight the need for developing a dust injection system able to overcome the shortcomings of the actual one even when testing dust mixtures.     

Recent development of standardization of testing methods for dust explosion in Japan

The Dust Explosion Committee of the Association of Powder Process Industry and Engineering (APPIE) of Japan has acted to establish the standard for the member of the Association. Two testing methods have been authorized by the Association. The methods are established to evaluate sensitivity and severity for dust explosion.The ignition sensitivity is evaluated by the minimum explosive dust cloud concentration. Tapping sieve equipment and a strictly-specified Hartmann-type explosion tube have been selected as the testing apparatus. The tapping sieve equipment is applied to measure the minimum explosive concentration by forming a dust cloud supplied from a sieve on the top of the explosion chamber. The Hartmann-type tube is used to investigate roughly whether the test powder is explosive or not. A lot of explosion tests had been carried out to standardize the equipment and evaluate the sensitivity. This APPIE standard is being revised.Along with the ISO standard for explosion severity, Research Institute of Industrial Safety, the Ministry of Labour published a Guide to Test Method for Explosion Pressure and Rate of Pressure Rise for Combustible Dusts in 1994. Based on this Guide, The APPIE Committee has made the draft for the member.These standards and drafts are aimed to be the Japanese Industrial Standard (JIS). No other similar standard exists in Japan. In the present paper, technical data and explosion statistics which are the background of the APPIE standard for the ignition sensitivity are to be described.     

协同通风影响全断面隧道掘进机工作面岩尘分布规律研究

针对全断面掘进机施工过程中,产生大量岩尘影响施工人员工作环境的问题,采用理论分析、数值模拟与现场测定相结合的方法,以青岛地铁2号线全断面隧道掘进机工作面为对象,对协同通风影响全断面隧道掘进机工作面风流运移与岩尘污染分布规律进行研究。结果表明:在复合式通风模式和一定条件下,在隧道中段会形成由主通风与回风主导的耦合风场;当协同通风量(Qcv)从6 m3/s增大至12 m3/s时,产生的岩尘可控制在距掌子面77.2 m的范围内;当Qcv=8 m3/s时,岩尘最小扩散距离为44.7 m,工作区周围岩尘浓度约为30.1 mg/m3。     

A numerical approach to investigate the maximum permissible nozzle diameter in explosion by hot turbulent jets

The ignition of a combustible environment by hot jets is a safety concern in many industries. In explosion protection concepts, for a protection of the type “flameproof enclosures” a maximum permissible gap is of major importance. In this work a numerical framework is described to investigate the ignition processes by a hot turbulent jet which flows out from such gaps. A Probability Density Function (PDF) method in conjunction with a reaction-diffusion manifold (REDIM) technique is used to model the turbulent reactive flow. In this paper the ignition of a stoichiometric mixture of hydrogen/air gas by a hot exhaust turbulent jet is examined. The impact of the nozzle diameter on the ignition delay time is investigated, too. The method is used to explore the maximum nozzle diameter for specific boundary conditions for which there is no ignition.     

基于20 L球罐的多相混合物扩散模拟

为更好地探索多相混合物的爆炸特性，以铝粉、乙醚、空气为研究对象，基于20 L球型爆炸罐建立三维计算模型，对气固两相和气液固三相混合物的分散过程进行数值模拟，以分析不同多相混合物分散过程的差异，并为测量多相混合物爆炸下限时的点火延迟时间设定提供参考。监测分析铝粉浓度粒子分布、流场内部湍流动能以及液相体积百分数等的演化过程，讨论混合物分散效果的差异，并确定测量爆炸下限的点火延迟时间。研究结果表明:实验工况下，液相的存在会降低粉尘云团的湍流动能、降低其扩散速度，并使粉尘云内部浓度更均匀。测量多相混合物爆炸下限时，三相混合物的最佳点火延迟时间早于气固两相混合物10~20 ms。