Duct-venting of dust explosions in a 20 L sphere at elevated static activation overpressures

Ducts are often recommended in the design of dust explosion venting in order to discharge materials to safe locations. However, the maximum reduced overpressure increases in a duct-vented vessel rather than in a simply vented vessel. This needs to be studied further for understanding the duct-venting mechanism. Numerous duct-vented dust explosion experiments were conducted, using a 20 L spherical chamber at elevated static activation overpressures, ranging from 1.8 bar to 6 bar. Duct diameters of 15 mm and 28 mm, and duct lengths of 0 m (simply venting), 1 m and 2 m, were selected. Explosion pressures both in the vessel and in the duct were recorded by pressure sensors, with a frequency of 5 kHz. Flame signals in the duct were also obtained by phototransistors. Results indicate that the secondary explosion occurring in the duct increases the maximum reduced overpressure in the vessel. The secondary explosion is greatly affected by the duct diameter and static activation overpressure, and hence influences the amplification of the maximum reduced overpressure. Larger static activation overpressure decreases the severity of the secondary explosion, and hence decreases the increment in the maximum reduced overpressure. The secondary pressure peak is more obvious as the pressure accumulation is easier in a duct with a smaller diameter. However, the increment of the maximum reduced overpressure is smaller because blockage effect, flame front distortion, and turbulent mixing due to secondary explosion are weaker in a narrow duct. The influence of duct length on the maximum reduced overpressure is small at elevated static activation overpressures, ranging from 1.8 bar to 6 bar at 15 mm and 28 mm duct diameters.     

连通容器气体密闭爆炸与泄爆过程的实验研究

利用球型容器与管道组合,开展连通容器气体爆炸与泄爆实验,分析连通条件下,火焰在管道中的传播过程及其对起爆容器和传爆容器的压力影响。实验结果表明:连通容器气体爆炸中,火焰从起爆容器到传爆容器传播经历了一段不断加速,但加速度不断减小的过程;泄爆过程中,火焰传播过程与密闭爆炸时基本一致。管道中火焰加速传播,使得传爆容器的爆炸压力和强度相较于作为起爆容器时均明显增加,危险更大,采用与起爆容器相同的泄爆面积,无法满足对连通容器中传爆容器的泄爆。同时,泄爆是一个快速的能量泄放过程应选择合理的泄爆方式,防止二次危害。     

基于1 m3泄爆装置的墨粉爆炸泄压机制研究*

为了研究墨粉在爆炸泄压过程中燃烧与流动的变化机制,通过改变泄爆片尺寸、墨粉浓度以及泄爆片的惯性力等参数对爆炸泄放过程中反应釜中压力以及外场火焰形态变化进行试验研究,同时与完全封闭空间内不同墨粉浓度的压力曲线对比。研究结果表明:相同泄爆开口尺寸下,粉尘浓度与受控爆炸压力（采用爆炸泄压保护措施后工业腔体内产生的压力）负相关;开口尺寸增加可以提升泄压效率;结合外场火焰形态的变化情况揭示声动火焰不稳定性对反应釜中压力发展的影响;通过无惯性泄爆试验的对比证明泄爆片惯性对受控爆炸压力的影响不可忽视。     

Prediction of the relief of gas explosion in a tubular vessel by venting using a mathematical model

The relief of a gas explosion in a tubular vessel by venting can be predicted by using a mathematical model. In this model, the flame acceleration is represented by an increase in the burning velocity. The movement of a vent cover can be included. The model assumes that the vent is blocked by the vent cover prior to the explosion. the venting ratio was the most influential parameter in terms of relieving the pressure. In the case of a large venting ratio, the flame acceleration made a highly significant contribution, whereas for small venting ratios, the weight of the vent cover contributed to the relief more than the flame acceleration. When the pressure is required to be reduced significantly, the venting ratio, the vent open pressure and the weight of the vent cover must all be reduced.     

泄爆口参数对氢气火焰传播过程影响的数值模拟*

为了减少管内气体爆炸造成的损失与破坏,基于大涡模拟LES模型和Zimont燃烧模型,研究泄爆尺寸(直径为40,60,80 mm)和泄爆位置(侧方距点火端1,3,5 m)等泄爆条件对受限空间中氢气燃爆特性的影响。研究结果表明:大孔径泄爆口更好的排放效果造成火焰锋面在通过泄爆口时发生严重畸变,而泄爆口与点火端距离的增加则会削弱火焰锋面畸变的程度,且不同尺寸泄爆口产生的泄压效果差异较大。因此,应考虑将合适尺寸的泄爆口设置于靠近易燃点处。通过探索不同泄爆孔径与泄爆口位置对氢气火焰传播的影响规律,可为实际应用中的安全泄爆起到指导性作用。     

Flame behaviors and pressure characteristics of vented dust explosions at elevated static activation overpressures

Dust explosion venting experiments were performed using a 20-L spherical chamber at elevated static activation overpressures larger than 1 bar. Lycopodium dust samples with mean diameter of 70 μm and electric igniters with 0.5 KJ ignition energy were used in the experiments. Explosion overpressures in the chamber and flame appearances near the vent were recorded simultaneously. The results indicated that the flame appeared as the under-expanded free jet with shock diamonds, when the overpressure in the chamber was larger than the critical pressure during the venting process. The flame appeared as the normal constant-pressure combustion when the pressure venting process finished. Three types of venting processes were concluded in the experiments: no secondary flame and no secondary explosion, secondary flame, secondary explosion. The occurrence of the secondary explosions near the vent was related to the vent diameter and the static activation overpressure. Larger diameters and lower static activation overpressures were beneficial to the occurrence of the secondary explosions. In current experiments, the secondary explosions only occurred at the following combinations of the vent diameter and the static activation overpressure: 40 mm and 1.2 bar, 60 mm and 1.2 bar, 60 mm and 1.8 bar.     

Experimental investigations on the external pressure during venting

Experiments of explosion venting in different conditions were performed in a cylindrical vessel with a vent duct; the pressure-time profiles from four transducers mounted in the line-of-sight centerline outside the vessel and the clear sequential shadowgraphs of external venting flow field taken by a high-speed shadowgraph imaging system were obtained. Based on these results, the characteristics of the external pressure field during venting were discussed systematically to explain the generation mechanism of the secondary explosion. In addition, the variations of the intensity of the secondary explosion in different venting conditions, namely the failure pressure, ignition location, area blockage ratio or equivalence ratio of the fuel, were also analyzed in detail.     

竖直分支管道泄爆开启压力对甲烷爆燃压力的影响研究

针对市政排污管网等典型受限空间内可燃气体爆燃风险,建立由水平管道和竖直分支管道构成的数值模型,研究竖直分支管道不同泄爆开启压力对甲烷爆燃压力的影响.研究结果表明:不同泄爆开启压力条件下,管道内存在爆燃压力积聚和泄放的双重效应;水平管道内各测点压力时程曲线均表现为先增大后减小而后出现亥姆霍兹振荡,随着与爆源距离的增加,初...     

Characteristics of dust explosion venting pressure and flame under high activation pressure

A 20 L spherical explosive device with a venting diameter of 110 mm was used to study the vented pressure and flame propagation characteristics of corn dust explosion with an activation pressure of 0.78–2.1 bar and a dust concentration of 400∼900 g/m³. And the formation and prevention of secondary vented flame are analyzed and discussed. The results show that the maximum reduced explosion overpressure increases with the activation pressure, and the vented flame length and propagation speed increase first and then decrease with time. The pressure and flame venting process models are established, and the region where the secondary flame occurs is predicted. Whether there is pressure accompanying or not in the venting process, the flame venting process is divided into two stages: overpressure venting and normal pressure venting. In the overpressure venting stage, the flame shape gradually changes from under-expanded jet flame to turbulent jet flame. In the normal pressure venting stage, the flame form is a turbulent combustion flame, and a secondary flame occurs under certain conditions. The bleed flames within the test range are divided into three regions and four types according to the shape of the flame and whether there is a secondary flame. The analysis found that when the activation pressure is 0.78 bar and the dust concentration is less than 500 g/m³, there will be no secondary flame. Therefore, to prevent secondary flames, it is necessary to reduce the activation pressure and dust concentration. When the dust concentration is greater than 600 g/m³, the critical dust concentration of the secondary flame gradually increases with the increase of the activation pressure. Therefore, when the dust concentration is not controllable, a higher activation pressure can be selected based on comprehensive consideration of the activation pressure and destruction pressure of the device to prevent the occurrence of the secondary flame.     

无火焰泄放装置性能检测技术研究*

为有效提高无火焰泄放装置产品质量特性和应用技术,避免或减轻爆炸事故发生造成的灾害程度,选择玉米淀粉粉尘为测试粉尘,采用1 m3爆炸罐进行扇形无火焰泄放装置爆炸泄放实验。结果表明:扇形无火焰泄放装置不适合重复使用。当扇形无火焰泄放装置重复进行爆炸泄放实验时,爆炸罐内压力会呈现升高趋势,而外场压力和温度呈现下降趋势,且阻火元件孔隙内残留大量玉米淀粉粉尘燃烧后生成的炭黑以及积聚部分高温燃烧的粉尘,致使阻火元件损坏失效。     

甲烷-空气预混气体泄爆过程的实验研究

对甲烷-空气预混气体在球形容器和球形管道连通容器内的泄爆过程进行实验研究,根据实验结果得出在较小的泄压面积时,与密闭容器爆炸实验比较,不能降低容器内的最大压力,反而会增大容器内的最大压力。通过实验结果分析,泄爆口安装在远离点火源的位置,当发生预混气体爆炸时能较好地降低容器内的最大压力,起到保护容器的作用。     

泄爆容器中粉尘爆炸的试验研究与数值模拟

为了解泄爆容器中粉尘爆炸的发展过程,采用试验和数值模拟相结合的方法对玉米淀粉在圆柱形容器内的泄爆过程进行研究。数值模型采用欧拉–拉格朗日方法模拟粉尘爆炸的两相流问题,通过求解非稳态的湍流两相反应流守恒方程对试验进行二维仿真。试验和模拟结果表明,点火位置对爆炸发展过程有明显影响,点火位置离泄爆口越远,容器中的最大泄爆压力Pred,max越高。在粉尘爆炸的安全防护设计中,应把点火位置作为重要影响因素之一加以考虑。     

Application and effect of negative pressure chambers on pipeline explosion venting

Explosion venting is a frequently-used way to lower explosion pressure and accident loss. Recently, studies of vessel explosion venting have received much attention, while little attention has been paid to pipe explosion venting. This study researched the characteristics of explosion venting for Coal Bed Methane (CBM) transfer pipe, and proposed the way of explosion venting to chamber in order to avoid the influence of explosion venting on external environment, and investigated the effects of explosion venting to atmosphere and chamber. When explosion venting to atmosphere, the average explosion impulse 4.89 kPa s; when explosion venting to 0 MPa (atmospheric pressure) chamber, average explosion impulse is 7.52 kPa s; when explosion venting to −0.01 MPa chamber, explosion flame and pressure obviously drop, and average explosion impulse decreases to 4.08 kPa s; when explosion venting to −0.09 MPa chamber, explosion flame goes out and average explosion impulse is 1.45 kPa s. Thus, the effect of explosion venting to negative chamber is far better than that to atmospheric chamber. Negative chamber can absorb more explosion gas and energy, increase stretch of explosion flame, and eliminate free radical of gas explosion. All these can promote the effect of explosion venting to negative chamber.     

泄爆面特征参数对天然气爆炸超压峰值的影响规律

为揭示泄爆面特征参数对大尺度受限空间内天然气爆炸超压峰值结构的影响机制,基于典型房间特征,借助计算流体动力学技术研究不同泄爆面开启压力、开启时间以及泄压比等参数条件下室内天然气泄爆超压峰值结构的分布规律。研究结果表明:峰值Pb随开启压力和开启时间增加均呈线性增长趋势,而泄压比对Pb影响较小;峰值Pmfa与室内最大火焰面积有关,随开启压力、开启时间的增加和泄压比的减小,气体出流速度增大,进而产生更强的湍流,导致室内火焰面积和气体燃烧率增加,最终Pmfa增大;峰值Pext随泄压比增加呈快速降低趋势,同时开启压力和开启时间对Pext影响具有协同效应,共同促进Pext快速增加。     

Effect of the vent burst pressure on explosion venting of rich methane-air mixtures in a cylindrical vessel

The effect of the vent burst pressure on explosion venting of a rich methane-air mixture was experimentally investigated in a small cylindrical vessel. The experimental results show that Helmholtz oscillation of the internal flame bubble of the methane-air mixture can occur in a vessel with a vent area much smaller than that reported by previous researchers, and the period of Helmholtz oscillation decreases slightly when the vent burst pressure increases. The maximum overpressure in the vessel increases approximately linearly with the increase in the vent burst pressure; however, the pressure peaks induced by Helmholtz oscillation always remain approximately several kilopascals. The external flame reaches its maximum length in a few milliseconds after vent failure and then oscillates in accordance with the pressure oscillation in the vessel. The maximum length of the external flame increases, but its duration time decreases with the increase in the vent burst pressure.     

The effect of vent ducts on the reduced explosion pressures of vented dust explosions

An investigation into the effects of vent ducts on reduced explosion pressures is described. Experiments were made using an 18.5m³ explosion vessel and a modified 20 1 sphere, with dusts having K_st values ranging from 144 bar ms⁻¹ to 630 bar ms⁻¹. The vent area/vessel volume ratio bursting pressure of the vent cover, and the length to diameter ratio of the vent duct have been varied. Straight vent ducts, and ducts containing sharp 45° and 90° bends have been used.A simple model to describe the effect of vent ducts on the reduced explosion pressure has been derived and compared with the experimental results. Agreement is shown to be satisfactory in nearly all cases. A comparison between the experimental results and guidance on the effect of vent ducts already available in the literature is discussed.     

Experimental study on gas explosion and venting process in interconnected vessels

A pilot scale interconnected vessels experiment system was established, and the closed and vented gas explosion characteristics in the system were studied, using 10% methane–air mixture. Regularity of pressure variation in vessels and flame propagation in linked pipes was analyzed. Furthermore, the effects of transmission style, ignition position, pipe length, and initial pressure on explosion severity were discussed. For the closed explosion: explosion in interconnected vessels presents strongly destructive power to secondary vessel, especially transmission from the big vessel to the small one; the worst ignition position is shifting from ignition in the interconnected pipe to the walls of the two vessels; as far as ignition in big vessel is concerned, the peak pressure in secondary vessel increases with the pipe length much faster than that for ignition in small vessel; the peak pressures in two vessels are approximate linear functions of initial pressure. For the vented explosion: the transmission style and interconnected pipe length have significant impacts on the effect of venting on the protection; in order to obtain the better venting effect, the use of a divergent interconnected pipe from the big vessel to the small one in industry is advised and it is necessary to reduce the interconnected pipe length as far as possible or install flame arrester in the interconnected pipe.     

加气站压缩机间气体爆炸数值模拟研究

加气站压缩机间安全设计时,需要评估内部气体爆炸危害,确定爆炸能量和影响因素。采用CFD技术,建立加气站压缩机间三维模型,模拟不同点火源位置、泄压板不同泄压压力和重量下,压缩机间气体爆炸时的爆炸压力及火焰传播行为。结果表明点火源位置以及泄压参数是影响加气站压缩机间气体爆炸的重要因素;点火源位置距离压缩机间放空位置越近,爆炸压力越小;对于泄压参数,爆炸压力与泄压板开启压力和重量之间均为正比关系。为减缓压缩机间内的气体爆炸危害,需要合理布置点火源位置,选择容重轻、泄压压力小的泄压材料,并同时需要考虑爆炸导致的物体破碎危害以及火焰次生灾害。     

粉尘爆炸泄压面积计算及影响因素分析

随着涉粉行业各类企业数量的增长,以及粉尘爆炸产生的巨大危害,企业对粉尘泄爆、防爆越来越重视。在粉尘泄爆方面,泄爆装置的泄放面积影响最大泄爆压力,国内外不同标准的计算方法也有区别,因此针对不同标准的计算方法做出了比较并举例计算说明,为泄爆装置尺寸的选择提供理论依据。     

柱形压力容器开口泄爆过程数值模拟研究

为研究柱形压力容器泄爆规律,采用经典流体力学软件FLUENT对典型的柱形压力容器泄爆过程进行数值模拟,分析从泄爆口开启到泄压结束时间段压力发展、火焰传播、气体流动及可燃气体浓度变化特性。结果表明:不同泄爆压力下容器内压力发展变化呈现不同特点,在较小泄爆压力情况下会出现压力再度上升的双峰现象。泄爆过程中产生的湍流沿泄爆口附近容器壁拉长火焰面,并加快燃烧速率。同时就容器内不同点火位置对爆炸强度影响进行研究,得出在泄爆压力为0.04 MPa时,底面点火对本柱形压力容器产生的最大升压速率约为中心点火最大升压速率的1.4倍。