Validation of CFD-model for hydrogen dispersion

To be able to perform proper consequence modelling as a part of a risk assessment, it is essential to be able to model the physical processes well. Simplified tools for dispersion and explosion predictions are generally not very useful. CFD tools have the potential to model the relevant physics and predict well, but without proper user guidelines based on extensive validation work, very mixed prediction capability can be expected. In this article, recent dispersion validation effort for the CFD tool FLACS–HYDROGEN is presented. A range of different experiments is simulated, including low-momentum releases in a garage, subsonic jets in a garage with stratification effects and subsequent slow diffusion, low momentum and subsonic horizontal jets influenced by buoyancy, and free jets from high-pressure vessels. LH₂ releases are also considered. Some of the simulations are performed as blind predictions.     

Morphological characteristics of flashing jet throughout superheated liquid release

Many release problems involve two-phase releases of hazardous materials of superheated liquids with high energy into the atmosphere. Such accidents are accompanied by violent phase transition and form catastrophic flashing jets. In this work, experimental and theoretical analyses were conducted to investigate dynamic characteristics of flashing jet morphology and their dependence on pressure-decay dynamics under different storage pressures, superheats, and nozzle diameters. Flashing jet morphology and angle throughout two-phase releases were captured by a high-speed camera, and the corresponding source pressure in the superheated liquid tank was measured simultaneously. Results show that three typical phases, expansion, stabilization, and decay, are characterized throughout two-phase release based on the evolution of flashing jet morphology. The jet initially expands gradually due to the enhancement of phase transition intensity, and then keeps stable when the intensity reaches its maximum, and terminally decays rapidly due to the depletion of superheated liquid. Phase transition intensity at the nozzle exit is mainly controlled by the pressure-decay dynamics. Bubbles nucleation inception sites gradually move upstream of the nozzle during the pressure decay process increasing the phase transition intensity. The increase of storage pressure, superheat and nozzle diameter promotes the mechanical and thermodynamic effects on the jet breakup. The significant increase of mechanical and thermodynamic effects effectively accelerates droplets evaporation and further affects flashing jet morphology.     

新型乙醇汽油防静电混合器数值模拟研究

设计了一种新型乙醇汽油防静电混合器,基于CFD方法对新型乙醇汽油防静电混合器及简单的T型管混合器的内部流场分别进行数值分析,通过比较混合器中各组分浓度分布、压力分布、速度分布等特征,研究乙醇与汽油在混合器中的混合效果,并分析静电产生的风险。结果表明：在新型混合器螺旋叶片的作用下,乙醇汽油在出口处几乎混合均匀,速度趋于一致,且压力损失较小,说明新型混合器可以在较小的能耗下获得较好的混合效果,从而有效减少静电积聚。     

消防脉冲水枪喷嘴结构优化研究

为了进一步提高脉冲水枪的灭火效率,采用CFD技术对维多辛斯基曲线结构、锥直结构和锥角结构3种喷嘴结构进行选型优化。CFD数值模型采用RNG k-ε方法模拟湍流,利用VOF模型追踪管道内部及外部流场的气液界面,研究了不同喷嘴结构对气液分布、能量转化、速度分布的影响,并结合水室中水的体积分数和出口速度曲线图对喷射周期进行分析。研究结果表明:维多辛斯基曲线结构射流周期T=14.8 ms、锥直结构T=15.4 ms、锥角结构T=17 ms;维多辛斯基曲线结构和锥直结构的出口速度衰减较缓慢,喷嘴前端的圆柱结构能提高射流速度的稳定性;维多辛斯基曲线结构喷嘴的出口速度更稳定、集束性更好、能量转化率更高,且产生的射流水柱呈锥式逐渐扩散,动能集中分布在轴线附近,能有效增大喷射距离,提高脉冲水枪的灭火效率。     

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。     

Behavior of buoyancy and momentum controlled hydrogen jets and flames emitted into the quiescent atmosphere

As an effort to improve the prediction of hydrogen dispersion in the atmosphere, effects of buoyancy acting on the hydrogen jets formed by releasing from high pressure vessels are investigated analytically and experimentally. For the analytical study, an integral analysis for buoyant jets and flames is carried out to yield the closed formula describing the jet and flame shapes, including their trajectory and horizontal and vertical lengths corresponding to the critical concentrations. Because the density of hydrogen after releasing from high pressure storage conditions is much close to that of helium than that of hydrogen at room temperature, helium is used as the hydrogen surrogate for the buoyant jet experiments, which were performed by visualizing the jet dispersing in the atmosphere up to the jet Reynolds number Re ～ 2400. The trajectories obtained by the integral analysis and experiments agree relatively well until the transition to turbulence occurs. A further estimate for jets and flames is made by using the integral analysis as a preliminary design to the experiments involving a much greater hydrogen release. Once the comparative investigation of the larger scale experiments with the integral analysis, we anticipate that a more universal hydrogen jet and flame data can be obtained, which perhaps leads to a better safety distance of hydrogen stations.     

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.     

Droplets behaviors of flashing jet in accidental release of superheated liquid

Structural failure of an industrial superheated liquid tank or pipe usually results in the flashing jet consisting of a mixture of massive droplets and vapor due to the violent phase transition. In this work, experiments on small-scale releases were carried out with a 20 L storage tank to investigate the droplets behaviors of flashing jets after accidental releases of superheated liquid. Distribution of droplets axial, radial and vertical velocities, as well as droplets diameter along the centreline of flashing jet, were acquired employing a Phase Doppler Anemometry (PDA). The influence of storage pressure, superheat, and nozzle diameter was also studied. Results show that the distribution of actual droplets with different velocities and diameters follows the normal distribution. Droplets mean three-dimensional velocities on the central axis of flashing jet decrease exponentially with the increase of axial distance. The droplets mean diameter first decreases exponentially and finally keeps sable at about 10 μm. Among three-dimensional velocities, the axial velocity is the highest and the vertical velocity is the lowest. Droplets mean three-dimensional velocities increase with the increase of storage pressure, superheat, and nozzle diameter. The droplets mean diameter decreases with the increase of superheat and nozzle diameter but increases with the increase of storage pressure.     

Numerical investigation of powder aerosolization in a mining rock dust dispersion chamber

We have conducted numerical simulations of dust dispersion within the NIOSH Rock Dust Dispersion Chamber. The apparatus consists of a low-speed background ventilation flow down a long box in which is placed a tray containing a rock dust powder. A nozzle upstream of the tray introduces a short pulse of a turbulent horizontal jet flow just above the powder surface. We have utilized an incompressible Reynolds-Averaged Navier-Stokes k-ω model for the turbulent flow; particles are incorporated within a one-way Euler-Lagrangian formalism. The Rock Dust Dispersion Chamber ventilation flow exhibits a recirculation zone just above the powder-containing tray. Aerosolization proceeds via the interplay of the jet pulse flow with the background recirculation flow. The air flow is not well-mixed. The aerosolized dust is convected as a concentration cloud downstream towards the detection zone. For larger particles, gravitational settling depletes the convected cloud, so the instrument behaves as a horizontal elutriator. The instrument is robust with respect to misalignment of the jet nozzle. However, reduced streamwise drift velocity allows mixing to disperse the optically detected dust cloud concentration pulse. Our large particle simulation results compare favorably with published experimental results for large, polydisperse calcium carbonate rock dust.     

离心风机噪声频谱特性分析

通过对高压离心风机叶轮常用的几种设计方法所设计的叶轮进行噪声测试，进而分析叶轮叶片以不同的组合方式和参数构成的离心风机的噪声频谱特性。     

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.     

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.     

Evaluation of multi-phase atmospheric dispersion models for application to Carbon Capture and Storage

A dispersion model validation study is presented for atmospheric releases of dense-phase carbon dioxide (CO₂). Predictions from an integral model and two different Computational Fluid Dynamics (CFD) models are compared to data from field-scale experiments conducted by INERIS, as part of the EU-funded CO₂PipeHaz project.The experiments studied consist of a 2 m³ vessel fitted with a short pipe, from which CO₂ was discharged into the atmosphere through either a 6 mm or 25 mm diameter orifice. Comparisons are made to measured temperatures and concentrations in the multi-phase CO₂ jets.The integral dispersion model tested is DNV Phast and the two CFD models are ANSYS-CFX and a research and development version of FLACS, both of which adopt a Lagrangian particle-tracking approach to simulate the sublimating solid CO₂ particles in the jet. Source conditions for the CFD models are taken from a sophisticated near-field CFD model developed by the University of Leeds that simulates the multi-phase, compressible flow in the expansion region of the CO₂ jet, close to the orifice.Overall, the predicted concentrations from the various models are found to be in reasonable agreement with the measurements, but generally in poorer agreement than has been reported previously for similar dispersion models in other dense-phase CO₂ release experiments. The ANSYS-CFX model is shown to be sensitive to the way in which the source conditions are prescribed, while FLACS shows some sensitivity to the solid CO₂ particle size. Difficulties in interpreting the results from one of the tests, which featured some time-varying phenomena, are also discussed.The study provides useful insight into the coupling of near- and far-field dispersion models, and the strengths and weaknesses of different modelling approaches. These findings contribute to the assessment of potential hazards presented by Carbon Capture and Storage (CCS) infrastructure.     

水射流技术在牙轮钻头中的应用与研究进展

介绍了喷射钻井井底流场特性、喷嘴射流破岩机理、脉冲射流喷嘴、新型水力流道钻头的研究与应用情况，以及水射流技术在石油钻井用钻头上的应用前景。     

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.     

自激振荡磨料射流的基本理论与初步试验

自激振荡磨料射流是在自激振荡脉冲射流和前混合磨料射流基础上,提出的新型射流。为此,重点讨论了自激振荡磨料射流喷嘴的结构和工作原理,分析了脉冲频率和幅值等参数对磨粒加速效果的影响。根据初步研究表明,磨粒速度受脉冲频率和幅度影响很大,磨粒速度可超过前混合磨料射流中磨粒的速度,最大钻孔速度比前混合磨料射流提高４１％ 。这一成果有望用于坚硬物料的切割和特种加工、石油钻井、大洋采矿、难采矿体的开采、坚硬岩石巷道（遂道）掘进、表面清洗等领域。     

Dust particle sedimentation in the 20 L standard vessel for dust explosion tests

According to the current international standards, to perform the correct evaluation of the explosion and flammability parameters, a uniform distribution of the dust particles should be achieved inside the 20 L and/or 1 m³ standard vessels.CFD simulations have shown that in both standard test vessels (20 L and 1 m³), the dust particles are not uniformly dispersed, being mostly concentrated at the edge of the macro-vortices generated by the injection of the fluid and particle through the nozzle. In addition, only a partial fed of the particles is obtained, and dust particles sedimentation phenomena can occur.As a result, the dust participating to the reactive process may be much lower than the expected nominal concentration in the vessel due to sedimentation and incomplete feeding. Consequently, misleading values of the flammability/explosion parameters could be measured.Particle sedimentation and incomplete feeding depends both on the Stokes number and on the Reynolds number, whereas the concentration distribution depends on the turbulence level, the fluid flow maps, and the number of particles which enter into the vessel through the nozzle.The aim of this work is to evaluate the key parameters (particle size, particle density, and fluid velocity) affecting sedimentation and incomplete feeding in 20 L vessel. To this end, CFD simulations of dust dispersion are performed at varying the particle density and size. Operating maps, in terms of the key parameters and/or their dimensionless combinations, are developed and a correlation for correction of the data is proposed.     

SK标准静态混合器应用于压缩空气泡沫系统的可行性研究

引入工业上应用较成熟的SK标准静态混合器作为压缩空气泡沫灭火系统的气液混合结构,基于FLUENT数值模拟,研究了不同混合单元个数和长径比混合器的混合效果,从压损和混合均匀性两方面进行评价,选定5个混合单元,长径比为1:1的SK混合器为较优结构,并进行了试验验证,研究结果可以为混合室的设计提供参考。     

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 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.