Effect of shock strength on dust entrainment behind a moving shock wave

Secondary dust explosion is a serious industrial issue because it occurs under conditions corresponding to an increased quantity and concentration of dispersed, combustible dust when compared with the primary explosion. The problems of lifting and dispersion of a dust layer behind a propagating shock wave must therefore be understood to ensure safety regarding secondary dust explosion hazards. Using a new shock-tube facility for studying shock propagation over dust layers, limestone dust was subjected to Mach numbers ranging from 1.10 to 1.60. A shadowgraph technique was applied by using a high-speed camera (15,000 fps) for visualization of the dust-layer height change behind the moving shock wave. Also, the effect of dust-layer thickness on the entrainment process was observed by performing tests with two different layer depths, namely 3.2- and 12.7-mm thicknesses. New correlations were developed between the shock strength and the dust entrainment height as a function of time for each layer depth. In general, the results herein are in agreement with trends found in previous work, where there is a linear relationship between dust growth rate and shock Mach number at early times after shock passage. Also, new data were collected for image analyses over longer periods, where the longer observation time and higher camera framing rates led to the discovery of trends not previously observed by earlier studies, namely a clear transition time between the early, linear growth regime and a much-slower average growth regime. This second regime is however accompanied by surface instabilities that can lead to a much larger variation in the edge of the dust layer than seen in the early growth regime. In addition, for the linear growth regime, there was no significant difference in the dust-layer height growth between the two layer thicknesses; however, the larger thickness led to higher growth rates and much larger surface instabilities at later times.     

Numerical modelling of dust-layered detonation structure in a narrow tube

The problems of lifting and dispersing of a dust layer behind the propagating shock wave as well as ignition, combustion of coal particles and dust-layered detonation formation in a tube are numerically investigated. The layered detonation is formed at large distance from the place of the primary shock wave initiation (～100 diameters of the tube). The strong oblique transverse shocks caused by combustion zone were discovered. The acceleration of leading shock wave and dust-layered detonation formation are connected with increasing and intensification of combustion zone which strongly depends on arising system of the oblique waves due to the development of the dust layer instabilities and vice versa. In the applied model, the moving medium is treated as a two-phase, two-velocity and two-temperature continuum with mechanical and thermal interphase interaction. The numerical procedure is based on the finite-volume approach and is implemented for parallel computing. The results obtained are of interest for applications in predictive modelling of accidents in industrial systems with reactive dust.     

Explosibility of cork dust in methane/air mixtures

Explosibility studies of hybrid methane/air/cork dust mixtures were carried out in a near-spherical 22.7 L explosibility test chamber, using 2500 J pyrotechnic ignitors. The suspension dust burned as methane/air/dust clouds and the uniformity of the cork dust dispersion inside the chamber was evaluated through optical dust probes and during the explosion the pressure and the temperature evolution inside the reactor were measured. Tested dust particles had mass median diameter of 71.3 μm and the covered dust cloud concentration was up to 550 g/m³. Measured explosions parameters included minimum explosion concentration, maximum explosion pressures and maximum rate of pressure rise. The cork dust explosion behavior in hybrid methane/air mixtures was studied for atmospheres with 1.98 and 3.5% (v/v) of methane. The effect of methane content on the explosions characteristic parameters was evaluated. The conclusion is that the risk and explosion danger rises with the increase of methane concentration characterized by the reduction of the minimum dust explosion concentration, as methane content increases in the atmosphere. The maximum explosion pressure is not very much sensitive to the methane content and only for the system with 3.5% (v/v) of methane it was observed an increase of maximum rate of pressure rise, when compared with the value obtained for the air/dust system.     

Numerical study of dust lifting in a channel with vertical obstacles

In the paper, several results of numerical computation of multiphase flows in a channel with complex geometry are considered. The objective of the research was to study the dust lifting process from a layer behind a shock wave in a rectangular channel with vertical obstacles in the upper part of the tube. It is to be shown that that kind and also any sort of geometry may crucially change the whole phenomena of dust enhancement and of combustion. This is very important for safety in, for example, coal mines where channels are usually of more sophisticated structure than is usually assumed by most researchers.     

Experimental investigation of the influence of inert solids on ignition sensitivity of organic powders

This work presents the results of the experimental characterization of the ignition sensitivity of solid inertant/combustible powders mixtures. Three inert solids (alumina, Kieselguhr, aerosil) and eleven organic powders have been considered and the following parameters have been determined: (1) the minimum ignition energy, (2) the minimum ignition temperature in cloud and (3) the minimum ignition temperature in 5 mm layer. The effects of the addition of inert solids are described and a simple model is proposed to represent the experimental results.Generally, increasing inert solid content in a powder leads to a higher minimum ignition energy as well as a higher minimum ignition temperatures in cloud and in layer. In some cases, the flammability is influenced above a threshold concentration value, which can be quite high (up to 85 wt.%). Indeed, the proposed model shows a zone below the minimum ignition concentration (MIC), which does not enable an efficient or safe inerting: either the admixed inert solid does not provide a sufficient effect, or it can even facilitate the ignition of the dust by notably improving its dispersability.The influence of key parameters such as the thermal conductivity or optical properties on the efficiency of the inerting by admixed solid need to be further assessed in a future work in view to better understand the mechanisms involved and to extend the scope to other types of oxidizable materials.     

Ignitability assessment of shredder dusts of refrigerator and the prevention of the dust explosion

Explosibility of polyurethane dusts produced in the recycling process of refrigerator and the ways to prevent the dust explosion were studied. In recent years, cyclopentane is often used as the foaming agent and this produces explosive atmosphere in the shredding process. The minimum explosive concentration of polyurethane dust, influence of coexisting cyclopentane gas on the explosibility, effect of relative humidity on the minimum explosive concentration of polyurethane dusts, the minimum ignition energy, influence of cyclopentane mixture on the explosion severity, etc. were investigated.The minimum explosive dust concentration decreased with the increase of cyclopentane concentration and increased with the increase of relative humidity. The minimum ignition energy was about 11 mJ. The ignition energy decreased with the increase of the cyclopentane gas concentration. The cyclopentane gas concentration up to about 5300 ppm did not influence too much on the explosion index (K_st) and maximum explosion pressure. From these, it would be a good way to increase the relative humidity and to regulate the cyclopentane concentration in the shredding process to prevent the dust explosion hazard.     

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.     

Velocity and number density profiles of particles across upward and downward flame propagating through iron particle clouds

Behaviors of particles across upward and downward flame propagating through iron particle clouds have been recorded on photomicrographs by using a high-speed video camera with a microscopic optical system. The velocity profiles of iron particles across flames were measured by using the high-speed photomicrographs, and the number density profiles of iron particles near the flames were calculated by using the velocity profiles. It is shown that the number density of iron particles changes in the range of x smaller than 11.0 mm, where x is the distance from the leading edge of the combustion zone. The number density increases with the decrease of x in the range 0<x<11.0 mm, reaches a maximum at leading edge of the combustion zone, and then decreases. For upward propagating flame, the maximum value of the number density is about 3.5 times larger than that at the region far ahead of the flame (x>10.0 mm), however, for downward propagating flame, it is only 2.3 times larger than that at the region far ahead of the flame.     

Explosibility of fine graphite and tungsten dusts and their mixtures

To evaluate the explosion hazard of ITER-relevant dusts, a standard method of 20-l-sphere was used to measure the explosion indices of fine graphite and tungsten dusts and their mixtures. The effect of dust particle size was studied on the maximum overpressures, maximum rates of pressure rise, and lower explosive concentrations of graphite dusts in the range 4 μm to 45 μm. The explosion indices of 1 μm tungsten dust and its mixtures with 4 μm graphite dust were measured. The explosibility of these dusts and mixtures were evaluated. The dusts tested were ranked as St1 class. Dust particle size was shown to be very important for explosion properties. The finest graphite dust appeared to have the lowest minimum explosion concentration and be able to explode with 2 kJ ignition energy.     

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.     

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.     

Investigations of flame front propagation between interconnected process vessels. Development of a new flame front propagation time prediction model

For the case where a dust or gas explosion can occur in a connected process vessel, it would be useful, for the purpose of designing protection measures and also for assessing the existing protection measures such as the correct placement, to have a tool to estimate the time for flame front propagation along the connecting pipe. Measurements of data from large-scale explosion tests in industrially relevant process vessels are reported. To determine the flame front propagation time, either a 1 m³ or a 4.25 m³ primary process vessel was connected via a pipe to a mechanically or pneumatically fed 9.4 m³ secondary silo. The explosion propagation started after ignition of a maize starch/air mixture in the primary vessel. No additional dust was present along the connecting pipe. Systematic investigations of the explosion data have shown a relationship between the flame front propagating time and the reduced explosion over-pressure of the primary explosion vessel for both vessel volumes. Furthermore, it was possible to validate this theory by using explosion data from previous investigations. Using the data, a flame front propagation time prediction model was developed which is applicable for:

• •gas and dust explosions up to a K value of 100 and 200 bar m s⁻¹, respectively, and a maximum reduced explosion over-pressure of up to 7 bar;
• •explosion vessel volumes of 0.5, 1, 4.25 and 9.4 m³, independent of whether they are closed or vented;
• •connecting pipes of pneumatic systems with diameters of 100–200 mm and an air velocity up to 30 m s⁻¹;
• •open ended pipes and pipes of interconnected vessels with a diameter equal to or greater than 100 mm;
• •lengths of connecting pipe of at least 2.5–7 m.

     

Influence of the ignition source location on the determination of the explosion pressure at elevated initial pressures

Explosion pressures are determined for rich methane–air mixtures at initial pressures up to 30 bar and at ambient temperature. The experiments are performed in a closed spherical vessel with an internal diameter of 20 cm. Four different igniter positions were used along the vertical axis of the spherical vessel, namely at 1, 6, 11 and 18 cm from the bottom of the vessel. At high initial pressures and central ignition a sharp decrease in explosion pressures is found upon enriching the mixture, leading to a concentration range with seemingly low explosion pressures. It is found that lowering the ignition source substantially increases the explosion pressure for mixtures inside this concentration range, thereby implying that central ignition is unsuitable to determine the explosion pressure for mixtures approaching the flammability limits.     

Behavior of flames propagating through lycopodium dust clouds in a vertical duct

The structure of flame propagating through lycopodium dust clouds has been investigated experimentally. Upward propagating laminar flames in a vertical duct of 1800 mm height and 150×150 mm square cross-section are observed, and the leading flame front is also visualized using by a high-speed video camera. Although the dust concentration decreases slightly along the height of duct, the leading flame edge propagates upwards at a constant velocity. The maximum upward propagating velocity is 0.50 m/s at a dust concentration of 170 g/m³. Behind the upward propagating flame, some downward propagating flames are also observed. Despite the employment of nearly equal sized particles and its good dispersability and flowability, the reaction zone in lycopodium particles cloud shows the double flame structure in which isolated individual burning particles (0.5–1.0 mm in diameter) and the ball-shaped flames (2–4 mm in diameter; the combustion time of 4–6 ms) surrounding several particles are included. The ball-shaped flame appears as a faint flame in which several luminous spots are distributed, and then it turns into a luminous flame before disappearance. In order to distinguish these ball-shaped flames from others with some exceptions for merged flames, they are defined as independent flames in this study. The flame thickness in a lycopodium dust flame is observed to be 20 mm, about several orders of magnitude higher than that of a premixed gaseous flame. From the microscopic visualization, it was found that the flame front propagating through lycopodium particles is discontinuous and not smooth.     

Evaluation of Abelmoschus esculentus (lady's finger) seed as a novel biosorbent for the removal of Acid Blue 113 dye from aqueous solutions

The use of a new biosorbent derived from Abelmoschus esculentus (A. esculentus) seed for the removal of Acid Blue 113 (AB113) in aqueous solutions was investigated in batch mode. Biosorption studies were carried out under varying operational parameters including initial pH, biosorbent dosage, contact time, initial dye concentration and temperature. The results indicated that the biosorption properties were strongly dependent on initial pH. Fourier transform infrared spectroscopy analysis revealed that hydroxyl, carboxylic and amide functional groups present on the biosorbent surface were involved in the dye removal process. Equilibrium data were best fitted by the Langmuir model. The maximum biosorption capacity was 169.9 ± 3.1 mg g⁻¹ at 25 °C and initial pH 5.5. The kinetic data were in good agreement with the pseudo-second-order kinetic model. The process was controlled by diffusion through boundary layer at the initial stage followed by intra-particle diffusion at the later stage. Thermodynamic evaluation showed that the process was endothermic and spontaneous. The present study suggests that A. esculentus seed with maximum biosorption capacity which compared well with values reported in the literature can be a potential biosorbent for AB113 dye removal.     

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.     

Electrowinning studies for metallic zinc production from double leached Waelz oxide

Waelz oxide is a zinc and lead concentrate pyrometallurgically derived from electric arc furnace dust. There are clear incentives to leaching and purify this oxide to produce liquors that can be electrowinned to obtain recycled metallic zinc. This study is focused on this electrolytic zinc production from previously obtained sulphuric liquor. This liquor was obtained from double leached Waelz oxide (DLWO) throughout a hydrometallurgical process. The electrolytic liquor, after the corresponding leaching and purification stages, contains around 49 kg/m³. This liquor was fed to an electrowinning process where platinum or lead anodes and aluminium cathodes were used. After optimizing the different electrowinning critical parameters, and working at these optimal conditions, almost 88 secondary zinc kg were precipitated per each initial DLWO tonne in one-through process. The zinc purity was higher than 99.5%. After some mass balance calculations, it can be concluded that, optimizing the process configuration through internal recycling, around 210 Zn kg/DLWO t with purity near to 100% could be achieved.     

Experimental simulation of airbag deployment for pipeline closing

Small scale tests were carried out at ISL's shock tube facility STA (100 mm inner diameter) to study the problem of closing a pipeline by means of an airbag in case of explosions or gas leakages. Experiments were carried out to simulate the flow in a pipeline at velocities and gas pressures as present in pipeline flows. In this study the gas used was nitrogen at static pressures of 0.2 up to 5 MPa and at flow velocities of 25 m/s up to 170 m/s. A special Nylon airbag, deployed from the tube wall into the pipe, was used to simulate the airbag inflation in a real pipeline. For this purpose a special gas filling system consisting of a gas generator with a reservoir volume of up to 500 cm³ which permits air pressures up to 17 MPa to be generated inside the airbag was developed at ISL. With a fast pyrotechnically opened valve the reservoir gas was released for airbag filling. The airbag inflation was triggered in such a way that it opened in nearly 3 ms into the pipe flow generated by the shock tube and continued for about 10 ms. For this application a special measuring chamber was designed and constructed with 20 measuring ports. Through two window ports, located one in front of the other, the airbag inflation could be visualized with up to 50 successive flash sparks illuminating a fast rotating film inside a drum camera. Pressure measurements using commercially available PCB pressure gauges at 9 measuring ports placed along the inner tube surface gave some hints on the behaviour of the wall pressure during airbag deployment. As a result from the experiments performed it is to conclude, that, with the Nylon airbag samples available, the pipe flow cannot be blocked by the inflating airbag. The flow forces acting on the airbag during deployment are in the shock tube experiments of the order of about 1000 N, which are not balanced by the airbags' neck, fixing it to the shock tube wall. This outcome suggests that a mechanical support is required to fix the airbag in its place during inflation.     

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.