A numerical study of the evolution of the blast wave shape in rectangular tunnels

When the explosion of condensed materials occurs in square or circular cross-section tunnel, the subsequent blast wave reveals two patterns: three-dimensional close to the explosive charge and one-dimensional far from the explosion. Pressure decays for these two patterns have been thoroughly studied. However, when the explosion occurs in rectangular cross-section tunnel, which is the most regular geometry for underground networks, the blast wave exhibits a third, two-dimensional, patterns. In order to assess the range of these three patterns, several numerical simulation of blast waves were carried out varying the width and the height of the rectangular cross-section as well as the mass of the charge. Laws are presented to localize the transition zones between the 3D and the 2D patterns, and between the 2D and the 1D patterns, as functions of non-dimensional width and height. The numerical results of the overpressure are compared to existing 3D and 1D laws. An overpressure decay law is proposed to represent the 2D pattern. Knowing the two transition zones and the overpressure decays within these zones, an algorithm is presented to efficiently predict an overpressure map. This algorithm is validated by comparison with experimental data.     

Vented confined explosions in Stramberk experimental mine and AutoReaGas simulation

Research Mining Institute, Inc., Ostrava-Radvanice, in cooperation with Dept. of Theory And Technology of Explosives of University of Pardubice and Klokner Institute of CTU in Prague, has performed three series of experiments examining methane–air mixture explosions and their impact on 14 and 29 cm thick wall. The project was named ‘Modeling Pressure Fields Effects on Engineering Structures During Accidental Explosions of Gases in Buildings’ and was sponsored by Grant Agency of Czech Republic (project No. 103/01/0039). The project is aimed at deeper understanding of pressure field effect upon the structures. Methane-air mixture explosion was used to generate the blast wave. The geometrical configuration of the environment resembled a room of an average size, such as larger kitchen. Preliminary simulations were made by AutoReaGas code (Century Dynamics and TNO). The design phase was followed by tests in an experimental mine in Stramberk. Two masonry dams were build in the mine, with cross-section areas of 10.2 m² and longitudinal distance of 5.7 m, creating an explosion chamber with a volume of 58 m³. Two vent openings with an adjustable free cross-section were used to control the maximum overpressure inside the chamber. The concentration of methane-air mixture was approximately 9.5% (vol.) and the volumes of the clouds were 5.25, 10.2 and 15.3 m³ respectively. The generated blast wave overpressures inside the chamber ranged between 1 and 150 kPa. According to experimental results a calibration of the code was performed. After the calibration it is possible to make relatively accurate simulations in similar geometry and to calculate the pressure loading of the structure at any spot in the simulated space. This paper describes the experiments performed and compares experimental and computational results.     

A study on the effect of trees on gas explosions

The downstream as well as the upstream oil and gas industry has for a number of years been aware of the potential for flame acceleration and overpressure generation due to obstacles in gas clouds caused by leaks of flammable substances. To a large extent the obstacles were mainly considered to be equipment, piping, structure etc. typically found in many installations. For landbased installations there may however also be a potential for flame acceleration in regions of vegetation, like trees and bushes. This is likely to have been the case for the Buncefield explosion that occurred in 2005 (Buncefield Major Incident Investigation Board, 2008), which led to the work described in the present paper. The study contains both a numerical and an experimental part and was performed in the period 2006–2008 (Bakke and Brewerton, 2008, Van Wingerden and Wilkins, 2008).The numerical analysis consisted of modelling the Buncefield tank farm and the surrounding area with FLACS. The site itself was not significantly congested and it was not expected to give rise to high overpressures in case of a hydrocarbon leak. However, alongside the roads surrounding the site (Buncefield Lane and Cherry Tree Lane), dense vegetation in the form of trees and bushes was included in the model. This was based on a site survey (which was documented by video) performed in the summer of 2006.A large, shallow, heavier-than-air gas cloud was defined to cover part of the site and surroundings. Upon ignition a flame was established in the gas cloud. This flame accelerated through the trees along the surrounding roads, and resulted in high overpressures of several barg being generated by FLACS. This is to the authors’ knowledge the first time a possible effect of vegetation on explosions has been demonstrated by 3D analyses.As a consequence of these results, and since the software had been validated against typical industrial congestion rather than dense vegetation, a set of experiments to try to demonstrate if these effects were physical was carried out as well. The test volume consisted of a plastic tunnel, 20 m long with a semi-circular cross-section 3.2 m in diameter allowing for representing lanes of vegetation. The total volume of the tent was approximately 80.4 m³. The experimental programme involved different degrees of vegetation size, vegetation density (blocking ratio) and number of vegetation lanes (over the full length of the tunnel). The experiments were performed with stoichiometric propane–air mixtures resulting in continuously accelerating flames over the full length of the tunnel for some of the scenarios investigated.The main conclusions of the study are that trees can have an influence on flame acceleration in gas–air clouds, and that advanced models such as FLACS can be used to study such influence. More research is needed, however, because even if FLACS predicts flame acceleration in dense vegetation, no evidence exists that applying the code to trees rather than rigid obstacles provides results of acceptable accuracy.     

爆炸冲击波在多级穿廊结构坑道内传播规律的数值分析

数值模型尺寸参照总参工程兵科研三所所建的穿廊结构坑道实体模型,采用ANSYS/LS-DYNA建立三维穿廊端部不同开闭的数值模型,得出冲击波在多级穿廊结构坑道内的传播规律,并与长直坑道内爆炸冲击波传播规律进行对比分析。结果表明多级穿廊结构端部开放坑道对爆炸冲击波的削弱作用非常显著,一级穿廊结构削弱冲击波强度62％,级数越高,削弱效果越明显;端部封堵时冲击波反射效果明显。     

爆破冲击波在台阶法隧道内的传播特性研究

为了解决目前隧道施工过程中掌子面开挖爆破产生的冲击波容易造成工程灾害的问题,基于宁攀高速胜利隧道台阶法开挖的实际情况,采用动力分析软件ANSYS/LS-DYNA及现场测试的方法研究冲击波在隧道上下台阶区段的传播规律。研究结果表明：初始冲击波传播至下台阶断面时先扩散至中上区域,与拱顶及侧壁碰撞反射后再传播至下部区域;冲击波在下台阶段0～10 m范围内,经多次反射叠加形成提升幅度约30%的第2次超压峰值;冲击波由上台阶较小断面进入下台阶较大断面传播时,超压衰减速度相对加快,过渡界面前后10 m范围内衰减系数由1.01%最高提升至1.05%。现场实测数据与数值模拟结果在上下台阶段得到的波形传播规律基本一致,验证数值模拟的可靠性。研究结果可为实际工程中爆破冲击波在变截面隧道中的控制措施提供参考。     

Artificial neural network modelling of As(III) removal from water by novel hybrid material

The present study reported a method for removal of As(III) from water solution by a novel hybrid material (Ce-HAHCl). The hybrid material was synthesized by sol–gel method and was characterized by XRD, FTIR, SEM–EDS and TGA–DTA. Batch adsorption experiments were conducted as a function of different variables like adsorbent dose, pH, contact time, agitation speed, initial concentration and temperature. The experimental studies revealed that maximum removal percentage is 98.85 at optimum condition: pH = 5.0, agitation speed = 180 rpm, temperature = 60 °C and contact time = 80 min using 9 g L⁻¹ of adsorbent dose for initial As(III) concentration of 10 mg L⁻¹. Using adsorbent dose of 10 g L⁻¹, the maximum removal percentage remains same with initial As(III) concentration of 25 mg L⁻¹ (or 50 mg L⁻¹). The maximum adsorption capacity of the material is found to be 182.6 mg g⁻¹. Subsequently, the experimental results are used for developing a valid model based on back propagation (BP) learning algorithm with artificial neural networking (BP-ANN) for prediction of removal efficiency. The adequacy of the model (BP-ANN) is checked by value of the absolute relative percentage error (0.293) and correlation coefficient (R² = 0.975). Comparison of experimental and predictive model results show that the model can predict the adsorption efficiency with acceptable accuracy.     

Determination of the applicable exhaust airflow rate through a ventilation shaft in the case of road tunnel fires

In this study, a model experiment in a long road tunnel employing the longitudinal ventilation system with a ventilation shaft is carried out during a fire accident to determine the optimum exhaust airflow rate through the ventilation shaft. The appropriate operation of the shaft fan according to the position of fire is investigated, and the optimum exhaust airflow rate for prevention of smoke spreading through the tunnel is determined based on the entire exhaust of both smoke and airflow generated by fire and jet fan operation, respectively. As a result of using the amount of smoke, the critical air velocity produced by jet fans, the effective cross-sectional area of a tunnel and the correction factor, a formula for exhaust airflow rate is drived. In addition, a correction factor (α = 1.1) for the thermal expansion caused by heat of a 20 MW fire is determined theoretically and experimentally. It is expected that this study will contribute to plan the shaft operation for the emergency ventilation as well as provide the preliminary data to design the airflow rate of shaft.     

Full-scale experimental studies for backdraft using solid materials

The backdraft experiments involved three full-scale room fire tests that used solid furnishing, loveseats. From experimental data, a backdraft caused two temperature peaks. The first one was below 600 °C. Then, an abrupt opening of the front door led to a supply of a large amount of fresh air, followed by an indication of sudden temperature rise. The second peak temperature was over 600 °C. Meanwhile, the deflagration resulted in the gases heating and expanding within the fire space, thus forcing unburned gases out of the vent ahead of the flame front. Comparing both cases with natural gas and solid loveseat as the fuel in backdraft, the former can achieve pre-mixture state and readily create an instant explosion wave phenomenon; however, this wave disappeared immediately. On the other hand, the solid loveseat used as the fuel in this study produced backdraft within 30–50 s after opening of the door. After the occurrence of backdraft, fire maintained a period of fully developed stage, which was consistent with the conditions in actual fires.     

An experimental investigation of the coupling between gaseous explosion pressures and a water volume in a closed vessel

An experimental test program has been undertaken on the pressure coupling between gaseous deflagration and detonations and an underlying volume of water. The two forms of gaseous explosions were initiated in an ullage space within of a closed cylindrical metal vessel. The vessel, placed in a vertical orientation, and was 2 m high and 0.247 m diameter. The depth of water used for the experiments was 1.44 m. For the combustion tests the maximum pressure recorded in the ullage was also developed in the water volume. For detonation tests however a distinct pressure wave developed in the water filled region, significantly modifying the time resolved pressure history at the vessel wall.     

The explosion overpressure field and flame propagation of methane/air and methane/coal dust/air mixtures

The explosion of the methane/air mixture and the methane/coal dust/air mixture under 40 J center spark ignition condition was experimentally studied in a large-scale system of 10 m³ vessel. Five pressure sensors were arranged in space with different distances from the ignition point. A high-speed camera system was used to record the growth of the flame. The maximum overpressure of the methane/air mixture appeared at 0.75 m away from the ignition point; the thickness of the flame was about 10 mm and the propagation speed of the flame fluctuated around 2.5 m/s with the methane concentration of 9.5%. The maximum overpressure of the methane/coal dust/air mixture appeared at 0.5 m. The flame had a structure of three concentric zones from outside were the red zone, the yellow illuminating zone and the bright white illuminating zone respectively; the thickness and the propagation speed of the flame increased gradually, the thickness of red zone and yellow illuminating zone reached 3.5 cm and 1 cm, the speed reached 9.2 m/s at 28 ms.     

Experiments on the influence of pre-ignition turbulence on vented gas and dust explosions

Experiments were performed on the influence of pre-ignition turbulence on the course of vented gas and dust explosions. A vertical cylindrical explosion chamber of approximately 100 l volume and a length-to-diameter ratio (l/d) of 4.7 consisting of a steel bottom segment and three glass sections connected by steel flanges was used to perform the experiments. Sixteen small fans evenly distributed within the chamber produced turbulent fluctuations from 0 to 0.45 m/s. A Laser-Doppler-anemometer (LDA) was used to measure the flow and turbulence fields. During the experiments the pressure and in the case of dust explosions the dust concentration were measured. In addition, the flame propagation was observed by a high-speed video camera. A propane/nitrogen/oxygen mixture was used for the gas explosion experiments, while the dust explosions were produced by a cornstarch/air mixture.It turned out that the reduced explosion pressure increased with increasing turbulence intensity. This effect was most pronounced for small vents with low activation pressures, e.g. for bursting disks made from polyethylene foil. In this case, the overpressure at an initial turbulence of 0.45 m/s was twice that for zero initial turbulence.     

Degradation of Rhodamine B by an electrochemical ozone generating system consist of a Ti anode coated with nanocomposite of Sn–Sb–Ni oxide

An ozonation process was performed using a recycled electrochemical ozone generator system. A titanium based electrode, coated with nanocomposite of Sn–Sb–Ni was applied as anode in a laboratory-made electrochemical reactor. A constant flow rate of 192 mg/h of generated ozone was entered to an ozonation reactor to contact with a typical target pollutant, i.e., Rhodamine B (Rh.B) molecules in aqueous solution. Four operational parameters such as: initial dye concentration, pH, temperature and the contact time were evaluated for the ozonation process. Experimental findings revealed that for a solution of 8 mg/L of the dye, the degradation efficiency could reach to 99.5% after 30 min at pH 3.7 and temperature of 45 °C as the optimum conditions. Kinetic studies showed that a second order equation can describe the ozonation adequately well under different temperatures. Also, considering to the importance of process simulation, a three-layered feed forward back propagation artificial neural network model was developed. Sensitivity analysis indicated order of the operational parameter's relative importance on the model output as: time ≫ pH > Rh . B initial concentration > temperature.     

Explosions and deflagration-to-detonation transitions in epoxy propane/air mixtures

The explosion and deflagration-to-detonation transition (DDT) in epoxy propane (E.P.) vapor/air mixture clouds under weak ignition conditions has been studied in an experimental tube of diameter 199 mm and length 29.6 m. E.P. vapor clouds were formed by injecting liquid E.P. into the experimental tube and evaporating of the fine E.P. droplets. The dimension and the evaporating process of the E.P. droplet were measured and analyzed. The E.P. vapor/air mixture clouds were ignited by an electric spark with an ignition energy of 40 J. The characteristics and the stages of the DDT process in the E.P. vapor/air mixtures have been studied and analyzed. A self-sustained detonation wave formed, as was evident from the existence of a transverse wave and a cellular structure. Moreover, a retonation wave formed during the DDT process in the E.P. vapor/air mixture. The influence of the E.P. vapor concentration on the DDT process has been studied. The minimum E.P. vapor concentration for the occurrence of the DDT in the E.P. vapor/air mixture has been evaluated and the variation of DDT distance with E.P. vapor concentration has been analyzed.     

Pb(II) adsorption from aqueous solutions by raw and treated biomass of maize stover – A comparative study

The potential to remove Pb(II) ion from wastewater treatment systems using raw and treated maize stover through adsorption was investigated in batch experiments. To achieve this, batch mode experiments were conducted choosing specific parameters such as pH (2–8), dosage concentration (2–30 g L⁻¹), contact time (5–180 min), temperature (20–45 °C) and metal ion concentrations (10–50 mg L⁻¹). Adsorption was pH-dependent showing a maximum at pH value 5. The equilibrium sorption capacities of raw and treated maize stover were 19.65 and 27.10 mg g⁻¹, respectively. The adsorption data fitted well to the Langmuir isotherm model. Kinetic studies revealed that the adsorption process followed pseudo-second-order model. The calculated thermodynamic parameters showed that the adsorption of Pb(II) was spontaneous and exothermic in nature. Consequently, this study demonstrated that both raw and treated maize stover could be used as adsorbents for the treatment of Pb(II) from industrial wastewaters.     

Measurement of turbulent burning velocities by means of the open tube method

Fraunhofer-ICT is one of the participants of EU-project DESC. The final objective of DESC is the development of a numerical code to be able to support a safe design of dust handling facilities. Fh-ICT designed an apparatus based on the so called open tube method (Andrews, G. E., & Bradley, D. (1972). Determination of burning velocities: a critical review. Combustion and Flame 18, 133–153) by means of which the relation between turbulent burning velocity and turbulence intensity in the range of 1 m/s<u′<3 m/s can be determined. This knowledge is important as an input for the code. A special feature of the experiment is the measurement of turbulence intensity in situ with sensors of the Pitot-type.     

A numerical model to simulate smouldering fires in bulk materials and dust deposits

A numerical model is presented which consists of a set of partial differential equations for the transport of heat and mass fractions of eight chemical species to describe the onset of self-ignition and the propagation of smouldering fires in deposits of bulk materials or dust accumulations. The chemical reaction sub-model includes solid fuel decomposition and the combustion of char, carbon monoxide and hydrogen.The model has been validated against lab-scale self-ignition and smouldering propagation experiments and then applied to predictions of fire scenarios in a lignite coal silo. Predicted reaction temperatures of 550 K and propagation velocities of the smouldering front of about 6 mm/h are in good agreement with experimental values derived from lab-scale experiments.     

Hydrogen sulphide removal from landfill gas

Control of odours should be considered to be a fundamental issue in order to site, design and manage sanitary landfills. With regard to construction and demolition (C&;D) debris, landfilling was the mainly adopted solution in many European Countries; in particular, gypsum drywalls can produce high concentrations of hydrogen sulphide (H₂S) in landfill gas ranging from 7 ppm to 100 ppm. In some cases also dangerous concentrations until to 12,000 ppm were detected. In this paper H₂S removal efficiency in a lab-scale vertical packed scrubber was investigated. Hydrogen sulphide abatement was evaluated for inlet H₂S concentrations of 1000–100–10 ppm, adjusting scrubbing liquid pH in the range 9–12.5 by means of caustic soda (NaOH 2N solution). Moreover, best operating conditions for the system were defined as well as H₂S abatement along the tower and liquid recirculation effectiveness in case of inlet H₂S concentration of 10 ppm (typical odour concentration). Results showed that pH of 11.5 in scrubbing liquid could be considered the best value for removal of different inlet H₂S concentrations, also taking into account parasitical consumption of NaOH due to CO₂ absorption. Moreover, in case of continuous working of the system at H₂S concentration of 10 ppm, strong removal efficiency was already obtained with a packed bed height of about 70 cm. Significant performances were ensured after 1 h of constant activity, consuming about 3 ml of soda per cubic meter of polluted air. Subsequently liquid blowdown was necessary.     

Removal mechanism of trace oxytetracycline by aerobic sludge

To investigate the mechanism of removal of selected pharmaceuticals in activated sludge systems, laboratory-scale batch experiments were conducted to assess the adsorption and degradation behavior of trace oxytetracycline (OTC). The adsorption equilibrium of OTC was observed in 30 min and the adsorption process could be well described by a pseudo-second-order model with a rate of 0.362 L μg^?1 min^?1. The OTC adsorption rate decreased with increasing temperature and could be fitted by the Freundlich isotherm. The linear partition coefficients (K_d) were 1.19, 0.999, and 0.841 L g^?1 at temperatures of 15, 20, and 25 °C, respectively. Thermodynamic analysis revealed that the adsorption of OTC onto the inactivated sludge was spontaneous (ΔG = ?16.7 to ?17.0 kJ mol^?1), enthalpy-driven (ΔH = ?24.9 kJ mol^?1), entropy-retarded (ΔS = ?27.4 J (mol K)^?1), and predominantly a physical adsorption.     

Investigation on removal of p-nitrophenol using a hybridized photo-thermal activated persulfate process: Central composite design modeling

The aim of this work is the study of p-nitrophenol (PNP) removal, as a nitroaromatic compound, using a hybridized photo-thermally activated potassium persulfate (KPS) in a fully recycled batch reactor. Response surface method was used for modeling the process. Reaction temperature, KPS initial dosage and initial pH of the solution were selected as variables, besides PNP degradation efficiency was selected as the response. ANOVA analysis reveals that a second order polynomial model with F-value of 41.7, p-value of 0.0001 and regression coefficient of 0.95 is able to predict the response. Based on the model, the process optimum conditions were introduced as initial pH of 4.5, [KPS]₀ = 1452 mg/L and T = 66 °C. Also experiments showed that using thermolysis and photolysis of the persulfate simultaneously, the role of thermolysis is not considerable. A pseudo first order kinetic model was established to describe the degradation reaction. Operational cost, as a vital industrial criterion, was estimated so that the condition of initial pH of 4.5, [KPS]₀ = 1452 mg/L and T = 25 °C showed the highest cost effective case. Under the preferred mild condition, the process will reach to 84% and 89% of degradation and mineralization efficiencies, after 60 and 120 min, respectively.     

A methodology to predict shock overpressure decay in a tunnel produced by a premixed methane/air explosion

A methodology for estimating the blast wave overpressure decay in air produced by a gas explosion in a closed-ended tunnel is proposed based on numerical simulations. The influence of the tunnel wall roughness is taken into account in studying a methane/air mixture explosion and the subsequent propagation of the resulting shock wave in air. The pressure time-history is obtained at different axial locations in the tunnel outside the methane/air mixture. If the shock overpressure at two, or more locations, is known, the value at other locations can be determined according to a simple power law. The study demonstrates the accuracy of the proposed methodology to estimate the overpressure change with distance for shock waves in air produced by methane/air mixture explosions. The methodology is applied to experimental data in order to validate the approach.