A numerical modelling of an influence of CH4, N2, CO2 and steam on a laminar burning velocity of hydrogen in air

The influence of additives of various chemical natures (CH₄, N₂, CO₂, and steam) at a laminar burning velocity S_u of hydrogen in air has been studied by numerical modelling of a flat flame propagation in a gaseous mixture. It was found that the additives of methane to hydrogen–air mixtures cause as a rule monotonic reduction in the S_u value with the exception of very lean mixtures (fuel equivalence ratio ? = 0.4), for which a dependence of the laminar burning velocity on the additive's concentration has a maximum. In the case of the chemically inert additives (N₂, CO₂, H₂O) the laminar burning velocity of rich near-limit hydrogen–air flames drops monotonically with an increase in the additive's content, but no more than 1.5 times, and the adiabatic flame temperature changes slowly in this case. In the case of methane as the additive, the laminar burning velocity is diminished approximately 5 times with an increase in the adiabatic flame temperature from 1200 to 2100 K. Deviations from the known empirical rule of the approximate constancy of the laminar burning velocity for near-limit flames are shown.     

Simple method for predicting pressure behavior during gas explosions in confined spaces considering flame instabilities

It is indispensable to predict the pressure behavior caused by gas explosions for the safety management against accidental gas explosions. In this study, a simple method for predicting the pressure behavior during gas deflagrations in confined spaces was examined. Previously the pressure behavior was calculated analytically assuming laminar flame propagation. However, the results of this method often provide underestimation compared with experimental data. It was known the underestimation intensifies as the scale of explosion spaces becomes larger. On the large scale gas deflagration, flame instability (especially hydrodynamic instability) might be more effective and wrinkles appeared on the flame front. Then, the flame surface area was increased and the propagating flame was gradually accelerated. The ordinary prediction methods led to the underestimation because the propagating flame was assumed to be laminar. In this study, we considered the effect of flame wrinkles caused by flame instabilities. By regarding the flame front as a fractal structure, the flame surface area could be modified. Because a flame surface starts to be wrinkled on a certain flame radius, proper determination of the critical flame radius provided accurate prediction of pressure behavior on a large scale deflagration. In addition, correction of the K_G value in a large vessel was discussed.     

Modeling of maize starch explosions in a 12 m3 silo

This paper presents a 2-dimensional numerical model of Eulerian–Lagrangian multi-phase combustion flow to predict maize starch explosions in a 12 m³ silo. The flow field after ignition, flame propagation velocity and pressure development histories etc. during the explosion, are calculated. The data of non-uniform initial conditions including dust concentration, flow velocity and turbulent RMS velocity in the silo for this model are adopted from Hauert, Vogl and Radandt (1994) [Hauert, F., Vogl, A., Radandt, S. (1994). Measurement of turbulence and dust concentration in silos and vessels. 6th international colloquium on dust explosions (pp. 71–80), Shenyang, China, August 28–September 2, 1994.]. A simple concept of dust granule taking into consideration dust dispersion efficiency is proposed and introduced. The Lagrangian method is used to trace trajectories and granules, so it is easier to consider particle size distribution. The k−ε model is used to simulate the turbulence of the gas phase, and the particle's pulsation is modeled by random vector wind generated by the surrounding gas. In the combustion model, vaporization of water, volatilization of volatile, gas phase reaction and the particle's surface reaction are taken into account.     

Modeling the initial flame acceleration in an obstructed channel using large eddy simulation

The propagation and acceleration of a flame surface past obstructions in a closed square channel was investigated using large eddy simulation. The dynamic Smagorinsky–Lilly subgrid model and the Boger flame surface density combustion model were used. The geometry is essentially two-dimensional with fence-type obstacles distributed on the top and bottom surfaces, equally spaced along the channel length at the channel height. Flame propagation, however, is three dimensional as ignition occurs at a point at the center of the channel cross-section. The effect of obstacle blockage ratio on the development of the flame structure was investigated by varying the obstacle height. Three-dimensional cases were simulated from the initiation of a combustion kernel through spark ignition to the acceleration of the flame front at speeds up to 80 m/s. The transition from laminar flame propagation to turbulent flame propagation within the “thin reaction zone” regime was observed in the simulations. By analyzing the development of the three dimensional flame surface and unburned gas flow field, the formation of several flame structures observed experimentally are explained. Global quantities such as the total flame area and centerline flame velocity were ascertained and compared to the experimental data. High amplitude oscillations in the centerline flame velocity were found to occur from a combination of the unburned gas flow field and fluctuations in the volumetric burning rate.     

Experimental investigation on explosion flame propagation of wood dust in a semi-closed tube

In order to explore flame propagation characteristics during wood dust explosions in a semi-closed tube, a high-speed camera, a thermal infrared imaging device and a pressure sensor were used in the study. Poplar dusts with different particle size distributions (0–50, 50–96 and 96–180 μm) were respectively placed in a Hartmann tube to mimic dust cloud explosions, and flame propagation behaviors such as flame propagation velocity, flame temperature and explosion pressure were detected and analyzed. According to the changes of flame shapes, flame propagations in wood dust explosions were divided into three stages including ignition, vertical propagation and free diffusion. Flame propagations for the two smaller particles were dominated by homogeneous combustion, while flame propagation for the largest particles was controlled by heterogeneous combustion, which had been confirmed by individual Damköhler number. All flame propagation velocities for different groups of wood particles in dust explosions were increased at first and then decreased with the augmentation of mass concentration. Flame temperatures and explosion pressures were almost similarly changed. Dust explosions in 50–96 μm wood particles were more intense than in the other two particles, of which the most severe explosion appeared at a mass concentration of 750 g/m³. Meanwhile, flame propagation velocity, flame propagation temperature and explosion pressure reached to the maximum values of 10.45 m/s, 1373 °C and 0.41 MPa. In addition, sensitive concentrations corresponding to the three groups of particles from small to large were 500, 750 and 1000 g/m³, separately, indicating that sensitive concentration in dust explosions of wood particles was elevated with the increase of particle size. Taken together, the finding demonstrated that particle size and mass concentration of wood dusts affected the occurrence and severity of dust explosions, which could provide guidance and reference for the identification, assessment and industrial safety management of wood dust explosions.     

Determination of turbulent burning velocities of dust air mixtures with the open tube method

Correlating turbulent burning velocity to turbulence intensity and basic flame parameters-like laminar burning velocity for dust air mixtures is not only a scientific challenge but also of practical importance for the modelling of dust flame propagation in industrial facilities and choice of adequate safety strategy. The open tube method has been implemented to measure laminar and turbulent burning velocities at laboratory scale for turbulence intensities in the range of a few m/s. Special care has been given to the experimental technique so that a direct access to the desired parameters was possible minimising interpretation difficulties. In particular, the flame is propagating freely, the flame velocity is directly accessible by visualisation and the turbulence intensity is measured at the flame front during flame propagation with special aerodynamic probes. In the present paper, those achievements are briefly recalled. In addition, a complete set of experiments for diametrically opposed dusts, starch and aluminium, has been performed and is presented hereafter. The experimental data, measured for potato dust air mixtures seem to be in accordance with the Bray Gülder model in the range of 1.5 m/s<u′<3.5 m/s. For a further confirmation, the measurement range has been extended to lower levels of turbulence of u′<1.5 m/s. This could be achieved by changing the mode of preparation of the dust air mixture. In former tests, the particles have been injected into the tube from a pressurised dust reservoir; for the lower turbulence range, the particles have been inserted into the tube from above by means of a sieve–riddler system, and the turbulence generated from the pressurised gas reservoir as before. For higher levels of turbulence, aluminium air mixtures have been investigated using the particle injection mode with pressurised dust reservoir. Due to high burning rates much higher flame speeds than for potato dusts of up to 23 m/s have been obtained.     

Numerical simulation of flame propagation and localized preflame autoignition in enclosures

A novel computational approach based on the coupled 3D Flame-Tracking–Particle (FTP) method is used for numerical simulation of confined explosions caused by preflame autoignition. The Flame-Tracking (FT) technique implies continuous tracing of the mean flame surface and application of the laminar/turbulent flame velocity concepts. The Particle method is based on the joint velocity–scalar probability density function approach for simulating reactive mixture autoignition in the preflame zone. The coupled algorithm is supplemented with the database of tabulated laminar flame velocities as well as with reaction rates of hydrocarbon fuel oxidation in wide ranges of initial temperature, pressure, and equivalence ratio. The main advantage of the FTP method is that it covers both possible modes of premixed combustion, namely, frontal and volumetric. As examples, combustion of premixed hydrogen–air, propane–air, and n-heptane–air mixtures in enclosures of different geometry is considered. At certain conditions, volumetric hot spots ahead of the propagating flame are identified. These hot spots transform to localized exothermic centers giving birth to spontaneous ignition waves traversing the preflame zone at very high apparent velocities, i.e., nearly homogeneous preflame explosion occurs. The abrupt pressure rise results in the formation of shock waves producing high overpressure peaks after reflections from enclosure walls.     

Scale effect of flame instability mechanisms on propagation velocity

Blast pressure is a major source of the damage caused by an accidental gas explosion. Its magnitude depends on the thermal expansion rate and hence on the flame propagation velocity. This paper discusses the influence of two flame instability mechanisms, i.e., diffusive-thermal and hydrodynamic instabilities, on flame propagation velocity with an emphasis on their scale effects as the flame continuously increases its size during an explosion. The Sivashinsky equation is numerically solved to simulate flame propagation behaviors. It is found that flame propagation velocity, V_f, is independent of flame size under the influence of diffusive-thermal instability, whereas V_f increases with flame size under the influence of hydrodynamic instability. The latter result is understood as a result of flame’s fractal structure. Fractal dimension is determined from the dependency of V_f on flame size, and the obtained fractal dimension is close to the known experimental value.     

Effect of the concentration distribution on the gaseous deflagration propagation in the case of H2/O2 mixture

The investigation of flame propagation accompanying the explosions of unconfined gaseous reactive clouds which are diluted in atmosphere ambient is a fundamental interest in the analysis of industrial risk assessment.Following the previous work [Sochet, I., Guelon, F., Gillard, P. (2002). Deflagrations of non-uniform mixtures: A first experimental approach, Journal of physics, 12, 7–273, 7–280], an experimental study is conducted on a deflagration of a hydrogen/oxygen gaseous cloud which is released in air. The burning velocity is directly or indirectly measured. The flammability limits of the non homogeneous cloud has been as well investigated.     

Flame propagation through aluminum particle cloud in a combustion tube

The propagation of a flame is investigated experimentally and theoretically for a large, horizontal combustion tube containing a mixture of air and aluminum powder with pre-existing turbulence. One end of the tube is closed and the other is connected to a large dump-tank. Twenty dispersion systems are used on the tube to produce a uniform suspension of aluminum dust in the tube with a mean diameter of 6 μm. The characteristics of a flame front from the ignitors at the closed end are measured using photodiodes and the development of pressure is monitored by transducers. Experimental results revealed the entire process of an accelerating flame and the development of shock waves. A set of conservation equations for two-phase turbulent combustion flow is derived, using the two-fluid model, k–ε model, Hinze–Tchen model and EBU-Arrhenius model for turbulent combustion. The SIMPLE scheme usually applied to the homogeneous turbulent combustion is extended to fit this two-phase, reactive behavior. The results of calculations show the positive feedback coupling among combustion, expansion and turbulence during flame propagation. Computed and measured results are generally in good agreement.     

Efficient simulation of flame acceleration and deflagration-to-detonation transition in smooth pipes

Evaluation of accident scenarios including flame acceleration and deflagration-to-detonation transition (DDT) in chemical plant piping systems increases the need for an efficient numerical simulation tool capable of dealing with this phenomenon. In this work, a hybrid pressure-density-based solver including deflagrative flame propagation as well as detonation propagation is presented. The initial incompressible acceleration stage is covered by the pressure-based solver until the flame velocity reaches the fast flame regime and transition to the density-based solver is done. The deflagration source term is formulated in terms of a turbulent flame speed closure model incorporating various physical effects crucial for flame acceleration at low turbulence conditions (Katzy and Sattelmayer, 2018). Modelling of the detonation source term is based on a quadratic heat release function (Hasslberger, 2017). The presented numerical approach is validated in terms of DDT locations and pressure data from Schildberg (2015) as well as recently completed flame tip position measurements. For this purpose, H₂/O₂/N₂ mixtures ranging from 25.6 vol-% H₂ to 29.56 vol-% H₂ in two different pipe geometries are considered. The focus of the current work is on predicting the DDT location correctly and good agreement is observed for the investigated cases.     

A new phenomenological model of gas explosion based on characteristics of flame surface

This paper presents a new model of explosion propagation in a closed vessel. The foundation of the formulation is a sub-model of turbulent burning velocity based on the assumption that the burning velocity of a turbulent wrinkled flame can be determined from the flame surface. In addition, model development includes simple sub-models of heat transfer and free convection. In order to verify the physics, the model was utilized to simulate the explosion of a methane–air mixture in two different test vessels. The results obtained by use of this new model were compared with results obtained by use of the classical model. While the simulations showed that both are accurate, the new model presented in this paper (called “flame surface model” for simplicity) is more flexible and can easily accommodate sub-models of different phenomena that can play an important role in fuel–air explosions.     

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.     

Propagation of ethylene–air flames in closed cylindrical vessels with asymmetrical ignition

A study of explosions in several elongated cylindrical vessels with length to diameter L/D = 2.4–20.7 and ignition at vessel's bottom is reported. Ethylene–air mixtures with variable concentration between 3.0 and 10.0 vol% and pressures between 0.30 and 1.80 bara were experimentally investigated at ambient initial temperature. For the whole range of ethylene concentration, several characteristic stages of flame propagation were observed. The height and rate of pressure rise in these stages were found to depend on ethylene concentration, on volume and asymmetry ratio L/D of each vessel. High rates of pressure rise were found in the early stage; in later stages lower rates of pressure rise were observed due to the increase of heat losses. The peak explosion pressures and the maximum rates of pressure rise differ strongly from those measured in centrally ignited explosions, in all examined vessels. In elongated vessels, smooth p(t) records have been obtained for the explosions of lean C₂H₄–air mixtures. In stoichiometric and rich mixtures, pressure oscillations appear even at initial pressures below ambient, resulting in significant overpressures as compared to compact vessels. In the stoichiometric mixture, the frequency of the oscillations was close to the fundamental characteristic frequency of the tube.     

Study of the influence of melamine polyphosphate and aluminum hydroxide on the flame propagation and explosion overpressure of aluminum magnesium alloy dust

When aluminum magnesium alloy dust floats in the air, a certain ignition energy can easily cause an accidental explosion. To prevent and control the occurrence of accidental explosions and reduce the severity of accidents, it is necessary to carry out research on the explosion suppression of aluminum magnesium alloy dust. This paper uses a vertical glass tube experimental device and a 20 L spherical explosive experimental device to carry out experimental studies on the suppression of the flame propagation and explosion overpressure of aluminum magnesium alloy dust with melamine polyphosphate (MPP) and Al(OH)₃. With increasing MPP and Al(OH)₃ concentrations, the flame brightness darkened, the flame velocity and propagation distance gradually decreased, and P_max and (dp/dt)_max decreased significantly. When the amount of MPP added reached 60%, the flame propagation distance decreased to 188 mm, which is a decrease of 68%, and the explosion overpressure decreased to 0.014 MPa, effectively suppressing the explosion of aluminum magnesium alloy dust. The experimental results showed that MPP was more effective than Al(OH)₃ in inhibiting the flame propagation and explosion overpressure of the aluminum magnesium alloy dust. Finally, the inhibitory mechanisms of the MPP and Al(OH)₃ were further investigated. The MPP and Al(OH)₃ endothermic decomposition produced an inert gas, diluted the oxygen concentration and trapped active radicals to terminate the combustion chain reaction.     

On the determination of the laminar burning velocity from closed vessel gas explosions

A methodology to determine the laminar burning velocity from closed vessel gas explosions is explored. Unlike other methods which have been used to measure burning velocities from closed vessel explosions, this approach belongs to the category which does not involve observation of a rapidly moving flame front. Only the pressure–time curve is required as experimental input. To verify the methodology, initially quiescent methane–air mixtures were ignited in a 20-l explosion sphere and the equivalence ratio was varied from 0.67 to 1.36. The behavior of the pressure in the vessel was measured as a function of time and two integral balance models, namely, the thin-flame and the three-zone model, were fitted to determine the laminar burning velocity. Data on the laminar burning velocity as a function of equivalence ratio, pressure and temperature, measured by a variety of other methods have been collected from the literature to enable a comparison. Empirical correlations for the effect of pressure and temperature on the laminar burning velocity have been reviewed and two were selected to be used in conjunction with the thin-flame model. For the three-zone model, a set of coupled correlations has been derived to describe the effect of pressure and temperature on the laminar burning velocity and the laminar flame thickness. Our laminar burning velocities are seen to fall within the band of data from the period 1953–2003. A comparison with recent data from the period 1994–2003 shows that our results are 5–10% higher than the laminar burning velocities which are currently believed to be the correct ones for methane–air mixtures. Based on this observation it is concluded that the methodology described in this work should only be used under circumstances where more accurate methods can not be applied.     

Simulation of dust explosions in complex geometries with experimental input from standardized tests

The present paper describes the development of a new CFD-code (DESC) for the assessment of accidental hazards arising from dust explosions in complex geometries. The approach followed entails the estimation of the laminar burning velocity of dust clouds from standardized laboratory-scale tests, and its subsequent use as input to the combustion model incorporated in DESC. The methodology used to obtain the laminar burning velocities is demonstrated by igniting turbulent propane-air mixtures to deflagration in a standard 20-litre USBM-vessel, and extracting the laminar burning velocity from the pressure–time curves; the results are compared with literature data. Laminar burning velocities for clouds of maize starch dust in air were estimated following the same procedure, and the resulting empirical model was used to simulate dust explosions in a 236-m³ silo.     

Prediction of distance to maximum intensity of turbulence generated by grid plate obstacles in explosion-induced flows

The interaction of unburnt gas flow induced in an explosion with an obstacle results in the production of turbulence downstream of the obstacle and the acceleration of the flame when it reaches this turbulence. Currently, there are inadequate experimental measurements of these turbulent flows in gas explosions due to transient nature of explosion flows and the connected harsh conditions. Hence, majority of measurements of turbulent properties downstream of obstacles are done using steady-state flows rather than transient flows. Consequently, an empirical based correlation to predict distance to maximum intensity of turbulence downstream of an obstacle in an explosion-induced flow using the available steady state experiments was developed in this study. The correlation would serve as a prerequisite for determining an optimum spacing between obstacles thereby determining worst case gas explosions overpressure and flame speeds. Using a limited experimental work on systematic study of obstacle spacing, the correlation was validated against 13 different test conditions. A ratio of the optimum spacing from the experiment, x_exp to the predicted optimum spacing, x_pred for all the tests was between 2-4. This shows that a factor of three higher than the x_pred would be required to produce optimum obstacle spacing that will lead to maximum explosion severity. In planning the layout of new installations, it is appropriate to identify the relevant worst case obstacle separation in order to avoid it. In assessing the risk to existing installations and taking appropriate mitigation measures it is important to evaluate such risk on the basis of a clear understanding of the effects of separation distance and congestion. It is therefore suggested that the various new correlations obtained from this work be subjected to further rigorous validation from relevant experimental data prior to been applied as design tools.     

Flame propagation behaviors of nano- and micro-scale PMMA dust explosions

Flame propagation behaviors of nano- and micro-polymethyl methacrylate (PMMA) dust explosions were experimentally studied in the open-space dust explosion apparatus. High-speed photography with normal and microscopic lenses were used to record the particle combustion behaviors and flame microstructures. Simple physical models were developed to explore the flame propagation mechanisms. High-speed photographs showed two distinct flame propagation behaviors of nano- and micro-PMMA dust explosions. For nano-particles, flame was characterized by a regular spherical shape and spatially continuous combustion structure combined with a number of luminous spot flames. The flame propagation mechanism was similar to that of a premixed gas flame coupled with solid surface combustion of the agglomerates. In comparison, for micro-particles, flame was characterized by clusters of flames and the irregular flame front, which was inferred to be composed of the diffusion flame accompanying the local premixed flame. It was indicated that smaller particles maintained the leading part of the propagating flame and governed the combustion process of PMMA dust clouds. Increasing the mass densities from 105 g/m³ to 217 g/m³ for 100 nm PMMA particles, and from 72 g/m³ to 170 g/m³ for 30 μm PMMA particles, the flame luminous intensity, scale and the average propagation velocity were enhanced. Besides, the flame front became more irregular for 30 μm PMMA dust clouds.