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1.
The aim of this work is to determine the influence of operating parameters such as the dispersion pressure, the ignition delay and height on the dust flammability. A Computational Fluid Dynamics (CFD) simulation, based on an Euler–Lagrange approach, was developed with Ansys Fluent™ and validated experimentally. Such analysis will facilitate the choice of the most conservative conditions for a flammability test. This paper is focused on a case study performed on wheat starch with the modified Hartmann tube. The dispersion process of the powder was studied with granulometric analyses performed in situ and high speed videos. Tests were performed with injections at gas pressure ranging from 3 to 6 bars and the evolution of the particle size distribution (PSD) was recorded at different ignition heights (5, 10 and 15 cm over the dispersion nozzle). The observations highlighted the presence of agglomeration/deagglomeration processes and dust segregation. Besides, a CFD simulation analysis was aimed at evaluating the impact of a set of parameters on the PSD and the local turbulence, which are closely linked to some flammability parameters. For this computational analysis, the CFD simulation was coupled with a collision treatment based on a Discrete Element Method (DEM) in order to consider the cohesive behavior of the combustible dust. Thus the results suggest performing the injection of the gases at approximately 5 bars for the flammability tests of wheat starch in order to obtain the finest PSD at a given ignition height. It is also shown that the finest PSD are obtained at 5 cm over the dispersion nozzle. However, the local instabilities and turbulence levels are so high during the first stages of the dispersion that the flame growth can be disturbed for short ignition delays. Moreover, the stabilization of the bulk of the dust cloud requires longer periods of time when the ignition sources are located at 15 cm. As a result, the recommended height to perform a flammability test is 10 cm in this case. Finally, this study proposes some tools that might improve the procedure of dust flammability testing. 相似文献
2.
In this work, the effect of initial temperature on the explosion pressure, Pex, of various liquid fuels (isooctane, toluene and methanol) and their blends (isooctane-toluene and methanol-toluene, with three different fuel-fuel ratios) was investigated by performing experiments in a 20-l sphere at different concentrations of vaporized fuel in air. The initial temperature was varied from 333 K to 413 K.Results show that, as the fuel-air equivalence ratio, Φ, is increased, a transition occurs from a “thermodynamics-driven” explosion regime to a “radiant heat losses-driven” explosion regime. The maximum pressure, Pmax, is found in the former regime (Φ < 3), which is characterized by a trend of decreasing Pex with increasing initial temperature. This trend has been explained by thermodynamics. In the latter regime (Φ > 3), Pex increases with increasing initial temperature. This trend has been addressed to the decrease in emissivity (and, thus, radiant heat losses) with the increase in temperature. 相似文献
3.
Temperature measurement on propagating flame and minimum explosible concentration are investigated. The dust explosion experiments of nano-particle dust clouds exhibit higher temperature gradient in preheat zone and lower MEC than those of micron particle dust clouds. A heterogeneous model is proposed to describe the oxidation process under two extreme conditions: whether the alumina film is involved in the reaction or not. The new methodology allows the estimation of oxidation kinetics of growing alumina. For micron particle, the model clarifies that the activation energy which has been wrongly considered to be for aluminum oxidation should be for lattice diffusion, and the initial reaction rate is proved to be dominated by the diffusion rate of oxygen through alumina shell as diffusion controlled reaction. For nano-particle, the model explained that why the reported activation energy shows significantly lower than that for micron particle, due to initially ignorable alumina film or considered as kinetically controlled reaction. However, as reaction occurs and alumina builds up on the surface, the interference of alumina somewhat increases the activation energy. 相似文献
4.
The 20L sphere is one of the standard devices used for dust explosivity characterization. One concern about the effectiveness and reliability of this test is related to the particle size variation due to particles' agglomeration and de-agglomeration. These phenomena are related to the turbulent regime of the dust cloud during the dispersion. This variable must be considered since it determines the uncertainty level of the ignitability and severity parameters of dust combustion. In this context, this study describes the influence of the cloud turbulence on the dust segregation and fragmentation through a study combining both, experimental and computational approaches. The behavior of the gas-solid mixture evidenced with the standard rebound nozzle was compared with that observed with six new nozzle geometries. Thereafter, the time-variation of the Particle Size Distribution (PSD) within the 20L sphere was analyzed for two different powders: carbon-black and wheat starch. On the one hand, the turbulence levels and PSD variations were characterized by Particle Image Velocimetry (PIV) tests and granulometric analyses, respectively. On the other hand, a computational approach described the dispersion process with CFD-DEM simulations developed in STAR-CCM + v11.04.010. The simulation results established that the homogeneity assumption is not satisfied with the nozzles studied. Nonetheless, the particles segregation levels can be reduced using nozzles that generate a better dust distribution in the gas-solid injections. Subsequently, an additional first-approach CFD model was established to study the behavior of the combustion step for a starch/air mixture. This model considers the gas-phase reactions of the combustible gases that are produced from the devolatilization of wheat starch ( and ) and allowed to establish the approximate fraction of the particle mass that devolatilizes, as well as to confirm that the modeling of the pyrolysis stage is essential for the correct prediction of the maximum rate of pressure rise. 相似文献
5.
为了研究不同粒径的铝粉在20 L爆炸测试装置中的分散规律,基于计算模型的非结构网格划分,耦合欧拉和拉格朗日方法,实现了描述可压缩气体演化的时间平均Navier-Stokes方程组和粒子运动的DPM动量平衡方程的求解,获得了不同粒径（25,50和100 μm）的铝粉在20 L爆炸仓内分散的三维时空演化规律。研究结果表明:铝粉粒径的差异对爆炸仓点火中心的湍动能和速度的演化过程影响不显著,但对粉尘浓度的变化率和峰值均具有重要影响;随着粒径的增大,峰值浓度越小,但均高于形式浓度0.25 kg/m3,达到峰值浓度的时间越滞后。 相似文献
6.
为研究不同形状系数(SF)的铝粉在 20 L球型容器中的分散特征,通过建立描述气流携带粉尘形成两相分散系的非稳态数值模型,包括气体流动和粉尘颗粒运动轨迹方程,开展非稳态数值模拟,分析粉尘SF分别为0.2、0.4、0.6、0.8和1.0条件下的粉尘空间分布以及球室中心的湍流动能、速度随时间变化。结果表明:球室内气流携带粉尘分散可以分为进粉、扩散、稳定和沉降4个阶段,其分散均匀性和最大速度值随SF的增加而增强;SF越大,即颗粒形状越接近球形,铝粉尘的分散性越好;SF越小,粉尘容易堆积在壁面附近;标称浓度一定时,SF≤0.4,浓度峰值随SF的增加而增加;SF>0.4,浓度峰值随SF的增加而减小;铝粉-空气混合物的湍流动能最大值随SF值的增加而降低;铝粉颗粒的速度峰值随SF值的增加而升高。 相似文献
7.
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。 相似文献
8.
Measurements of flammability and explosion parameters for dust/air mixtures require uniform dispersion of the dust cloud inside the test vessel. In a previous work, we showed that, in the standard 20 L sphere, the dust injection system does not allow generation of a uniform cloud, but rather high gradients of dust concentration are established. In this work, we used a previously validated three-dimensional CFD model to simulate the dust dispersion inside the 20 L sphere at different dust nominal concentrations (and fixed dust diameter). Results of numerical simulations have shown that, as the dust nominal concentration is increased, sedimentation prevails and, thus, when ignition is provided, the dust is mainly concentrated at the vessel walls. 相似文献
9.
In the work presented in this paper, the effect of initial pressure on the lower explosion limit (LEL) of the hybrid nicotinic acid/acetone mixture was investigated through standard explosion tests carried out in the 20 L sphere. From experimental results, the flammability diagram was built in the plane (concentration/minimum explosive concentration) of nicotinic acid versus (concentration/LEL) of acetone. Interestingly, it has been found that, in going from low pressures (P < 1 atm) to high pressures (P > 1 atm), the extension of the flammability region increases. This behavior has been attributed to the fact that the turbulence kinetic energy (and thus the energy dissipation) decreases with increasing initial pressure. Bartknecht's correlation for LEL of hybrid mixtures was modified to take into account the effect of pressure, and two correlations were obtained able to give satisfactory predictions of experimental data at both low pressures and high pressures. 相似文献
10.
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。 相似文献