Chapter 10 -Comprehensive dust explosion modeling

粉尘爆炸综合建模

Dust Explosion Dynamics, 2017, Pages 567-617

Russell A. Ogle

Abstract:This chapter summarizes some of the more recent efforts to model dust explosion phenomena as multiphase, turbulent, chemically reacting flows. Dust flames are governed by physical and chemical phenomena that present difficult challenges to overcome. In Chapter 3, Thermodynamics of Dust Combustion, through Chapter 9, Flame Acceleration Effects, we examined simple, useful models that approximate some of the key features of dust combustion. If we wish to explore how dust flames are governed by the complex interplay among chemical kinetics, multiphase flow, turbulence, and thermal radiation, we must resort to the methods of computational fluid dynamics (CFD).

We take a case study approach and consider the application of CFD to four specific combustible dust problems: unconfined dust flames, confined propagating dust flames, accelerating dust flames, and flame propagation in industrial accident scenarios. For each of these problems, we will consider a first case study as a reference case. We will then compare the reference case with subsequent studies in which the investigators have introduced more realism into their models.

Methane-coal dust hybrid fuel explosion properties in a large scale cylindrical explosion chamber

大型圆筒爆室中甲烷-煤尘混合燃料爆炸特性

Journal of Loss Prevention in the Process Industries, Volume 40, March 2016, Pages 317-328

Mohammed Jabbar Ajrash, Jafar Zanganeh, Behdad Moghtaderi

Abstract:The fires and explosions caused by flammable hydrocarbon air mixtures are a major safety concern in the chemical and processing industries. The thermo-physical and chemical properties of the flammable fuels in a hybrid form appear to have a significant impact on the combustion process. This usually occurs due to substantial changes in the flammability concentration regimes. The aim of this study is to investigate the fire and explosive properties of hybrid fuels in the chemical and process industries. In addition, it examines the impact of the ignition energy and vessel geometry on the magnitude of the pressure rise and flame propagation velocity. The experimental work was conducted on a cylindrically shaped explosion chamber constructed as part of this study at The University of Newcastle, Australia. The chamber was made of mild steel and was 30 m in length and 0.5 in diameter. It included a series of high resolution pressure transducers, a pyrometer, as well as a high speed video camera. Methane and coal dust were used as fuels and chemical igniters with a known energy were used to ignite the fuels.

The results obtained from this study showed that both the ignition energy and the diluted combustible fuel dust have significant impacts on the Over Pressure Rise (OPR) in an explosion chamber. The significant findings included that the OPR doubled when 30 g m−3 of coal dust was added to a 6% methane/air mixture, and it increased by 60% when 10 kJ was used instead of a 1 kJ ignition source. The initial ignition energy was observed to considerably enhance the speed of both the pressure wave and the flame front, where the pressure wave speed doubled when using a 5 kJ instead of a 1 kJ ignition source. However, the pressure wave speed increased by five times when a 10 kJ was used instead of a 1 kJ ignition source. Additionally, the maximum flame front velocity observed for the ignition source with 5 kJ energy was twice the flame front velocity for the 1 kJ ignition source. Finally, it was observed that the time needed for the initial methane ignition was reduced by about 50% when using a 10 kJ instead of a 1 kJ ignition source.

Dust explosions in an experimental test silo: Influence of length/diameter ratio on vent area sizes

实验筒仓内的粉尘爆炸：长/直径比对排气孔面积大小的影响

Biosystems Engineering, Volume 148, August 2016, Pages 18-33

Alberto Tascón, Álvaro Ramírez-Gómez, Pedro J. Aguado

Abstract:Vented dust explosion tests have been conducted in an experimental test silo in order to analyse the effect of the length/diameter ratio (L/D). The modular design of the silo permitted the assembling of four different vessels with different geometries. The tests were carried out with wheat flour and maize starch, using three different vent area sizes. The silo was equipped with instrumentation which recorded the pressure generated by the explosion at various points in the silo, as well as the instant when the vent panel opened. The length/diameter ratio has been included in the empirical correlations currently employed in standards EN 14491 and NFPA 68 for calculating the size of vents. However, there are marked differences between the two standards when applied to certain situations, in part due to a different vent area correction for slenderness. The results obtained in this experimental test programme were compared with the standards, and indicated the advisability of applying an increase in vent area in elongated vessels when L/D > 1, as stated in EN 14491. However, this same standard appears to apply an excessively severe correction in some situations.

Scaling of dust explosion violence from laboratory scale to full industrial scale – A challenging case history from the past

粉尘爆炸强度从实验室规模到全工业规模的攀升 –令人深思的实例回顾

Journal of Loss Prevention in the Process Industries, Volume 36, July 2015, Pages 271-280

Rolf K. Eckhoff

Abstract:The standardized KSt parameter still seems to be widely used as a universal criterion for ranking explosion violence to be expected from various dusts in given industrial situations. However, this may not be a generally valid approach. In the case of dust explosion venting, the maximum pressure Pmax generated in a given vented industrial enclosure is not only influenced by inherent dust parameters (dust chemistry including moisture, and sizes and shapes of individual dust particles). Process-related parameters (degree of dust dispersion, cloud turbulence, and dust concentration) also play key roles. This view seems to be confirmed by some results from a series of large scale vented dust explosion experiments in a 500 m3 silo conducted in Norway by CMI, (now GexCon AS) during 1980–1982. Therefore, these results have been brought forward again in the present paper. The original purpose of the 500 m3 silo experiments was to obtain correlations between Pmax in the vented silo and the vent area in the silo top surface, for two different dusts, viz. a wheat grain dust collected in a Norwegian grain import silo facility, and a soya meal used for production of fish farming food. Both dusts were tested in the standard 20-L-sphere in two independent laboratories, and also in the Hartmann bomb in two independent laboratories. Pmax and (dP/dt)max were significantly lower for the soya meal than for the wheat grain dust in all laboratory tests. Because the available amount of wheat grain dust was much larger than the quite limited amount of available soya meal, a complete series of 16 vented silo experiments was first performed with the wheat grain dust, starting with the largest vent area and ending with the smallest one. Then, to avoid unnecessary laborious changes of vent areas, the first experiment with soya dust was performed with the smallest area. The dust cloud in the silo was produced in exactly the same way as with the wheat grain dust. However, contrary to expectations based on the laboratory-scale tests, the soya meal exploded more violently in the large silo than the wheat grain dust, and the silo was blown apart in the very first experiment with this material. The probable reason is that the two dusts responded differently to the dust cloud formation process in the silo on the one hand and in the laboratory-scale apparatuses on the other. This re-confirms that a differentiated philosophy for design of dust explosion vents is indeed needed. Appropriate attention must be paid to the influence of the actual dust cloud generation process on the required vent area. The location and type of the ignition source also play important roles. It may seem that tailored design has to become the future solution for tackling this complex reality, not least for large storage silos. It is the view of the present author that the ongoing development of CFD-based computer codes offers the most promising line of attack. This also applies to design of systems for dust explosion isolation and suppression.

Hybrid H2/Al dust explosions in Siwek sphere

希维克球内氢气/铝粉混合物爆炸

Journal of Loss Prevention in the Process Industries, Volume 36, July 2015, Pages 509-521

A. Denkevits, B. Hoess

Abstract:Explosion indices and explosion behaviour of Al dust/H2/air mixtures were studied using standard 20 l sphere. The study was motivated by an explosion hazard occurring at some accidental scenarios considered now in ITER design (International Thermonuclear Experimental Reactor). During Loss-of-Vacuum or Loss-of-Coolant Accidents (LOCA/LOVA) it is possible to form inside the ITER vacuum vessel an explosible atmosphere containing fine Be or W dusts and hydrogen. To approach the Be/H2 explosion problem, Be dust is substituted in this study by aluminium, because of high toxicity of Be dusts. The tested dust concentrations were 100, 200, 400, 800, and 1200 g/m3; hydrogen concentrations varied from 8 to 20 vol. % with 2% step. The mixtures were ignited by a weak electric spark. Pressure evolutions were recorded during the mixture explosions. In addition, the gaseous compositions of the combustion products were measured by a quadruple mass-spectrometer. The dust was involved in the explosion process at all hydrogen and dust concentrations even at the combination ‘8%/100 g/m3’. In all the other tests the explosion overpressures and the pressure rise rates were noticeably higher than those relevant to pure H2/air mixtures and pure Al dust/air mixtures. At lower hybrid fuel concentrations the mixture exploded in two steps: first hydrogen explosion followed by a clearly separated Al dust explosion. With rising concentrations, the two-phase explosion regime transits to a single-phase regime where the two fuel components exploded together as a single fuel. In this regime both the hybrid explosion pressures and pressure rise rates are higher than either H2 or Al ones. The two fuels compete for the oxygen; the higher the dust concentration, the more part of O2 it consumes (and the more H2 remains in the combustion products). The test results are used to support DUST3D CFD code developed at KIT to model LOCA or LOVA scenarios in ITER.