Flame propagation and flow field measurements in a Hartmann dust explosion tube (original) (raw)

Flame Speed Measurements in Dust Explosions

The main objective of this work was the determination of the near-laminar and turbulent free flame speeds, and explosion enhancement factors of dust-air explosions in fire balls of approximately up to 1 m in diameter. Such information could find immediate application in the more effective design of dust-explosion protection systems in industry. In many respects, this work was a feasibility study and an evaluation of the capability of the proposed measurement techniques in delivering reliable data. Dust-air mixtures were exploded in a 1 m 3 industry standard vessel for the determination of explosion indices of dusts. Maize starch, phenolic resin, pulverised coal, biscuit flour and dried skimmed milk powders were tested. The tests were carried out under turbulent conditions of variable rms turbulent velocity, which was controlled by the ignition delay time from the onset of dust injection. The flame speeds and the K st values were measured at increasing ignition time delays and hence decreasing levels of turbulence, and were extrapolated to their laminar values (limit values at infinite time delay). A methodology was developed for dealing with problem of dust settlement, and other areas where further work is needed (such as the ignition mechanism) were identified. The laminar flame speeds and K st values for 5 dusts tested are reported. Maximum enhancements factors of up to 4 with regard to the laminar K st value were recorded in the ISO vessel, and up to about 6 with regard to the flame speed.

Turbulent flame propagation in large dust clouds

Journal of Loss Prevention in the Process Industries

In order to be able to model correctly dust explosion propagation, data are needed to couple the flame velocity to the characteristics of the turbulence : intensity and scale. So far most of the available data were obtained with laboratory equipments. In this paper, large scale experiments (up to 100 m 3) were performed during which both the flame velocity and turbulence characteristics were measured. Results are presented exhibiting a good correlation with the smaller scale data

A parametric study of lycopodium dust flame

Journal of Engineering Mathematics

Dust flames are associated with two-phase combustion phenomena where flame characteristics depend on interactions between solid and gas phases. Since organic dust particles can be effectively utilized in energy production systems, investigation of this phenomenon is essential. In this study, an analytical model is presented to simulate the combustion process of moist organic dust. The flame structure is divided into three zones: preheat zone, reaction zone, and postflame zone. To determine the effects of moisture content and volatile evaporation, the preheat zone is also divided into four subzones: first heating subzone and drying subzone, second heating subzone, and volatile evaporation subzone. The results obtained from the presented model are in reasonable agreement with experimental data for lycopodium particles. An increase in moisture content causes a reduction in burning velocity owing to moisture evaporation resistance. Consequently, the effects of some important parameters,...

Determination of the burning velocity of gas/dust hybrid mixtures

Process Safety and Environmental Protection, 2017

The laminar flame speed is an essential input for Computational Fluid Dynamics simulation programs aiming to predict the effects of explosions. In this study, an approach to assess fundamental flame propagation properties from the analysis of the flame velocity as a function of its stretching and hydrodynamic instabilities was developed. A numerical tool was developed to analyse videos of propagating flames in order to estimate their unstretched burning velocities. Markstein's theory, developed for gases and assuming a linear relation between the flame stretch and its speed, was then extended to dust clouds and hybrid mixtures of starch and methane. At first, the approach was validated with pure methane and was extended to pure starch and hybrid mixtures of both compounds. Finally, it appears that hybrid mixtures, especially when the gas concentration is greater than the lower explosive limit, can present a synergetic effect enabling faster flame propagation with regard to pure gas flames. Indeed, the stretching of a gas flame is strongly influenced by the addition of dusts. Nevertheless, for lower gas concentrations and larger dust concentrations called 'dust-driven regime', the presence of powders tends to limit the flame velocity to that of the less reactive compound, i.e. the dust.

Flame Propagation of Dust and Gas-Air Mixtures in a Tube

The flame propagation of hybrid (nicotinic acid/methane-air) mixtures has been studied using the open-tube method. During the flame propagation four phases can be distinguished and two phases, the first and the third, have been selected as useful for the evaluation of the burning velocity. The first phase has a spherical shape propagation mode while during the third phase the flame propagation can be considered as pseudo stationary and planar. In both phases, the role of the pre-ignition turbulence and of the turbulence induced by the flame itself has been addressed. The main issues which arise when estimating the burning velocity of hybrid mixtures have been evaluated and suggestions for future work are addressed. In the first phase the flame propagation is laminar but mixing between methane and nicotinic acid is prevented. In this case, in order to guarantee a homogeneous dispersion of the dust as well as a good mixing with the flammable gas, the injection of the dust/gas mixture should be realized from the reservoir. In the third phase, the turbulence induced by the flame propagation plays a major role and the determination of the turbulence level is required in order to evaluate the burning velocity.

Thermal radiation in dust flame propagation

Journal of Loss Prevention in the Process Industries, 2017

The role of thermal radiation in premixed flame propagation has been a matter of debate for decades. And it is not only a challenging scientific point, it has significant practical implications. For instance, a route to explain the Buncefield explosion (HSL, 2009) was the implication of tiny particles raised by the blast and promoting flame acceleration through enhanced heat exchanges by thermal radiation in the flame front. In dust explosion protection, the flame is implicitly supposed to propagate like a in a gaseous mixtures but if thermal radiation is dominant for some dusts, many aspects concerning the way to mitigate the explosions for those particular dusts would need to be revised (Proust and al., 2013). In this paper, new experimental measurements of thermal radiation in dust flames (methane air, methane air seeded with inert particles, aluminum dust air flames) are presented together with a physical interpretation.

Understanding the role of thermal radiation in dust flame propagation

2020

International audienceThe role of thermal radiation in premixed flame propagation has been a matter of debate for decades. And it is not only a challenging scientific point, it has significant practical implications. For instance, a proposed explanation of the Buncefield explosion (HSL, 2009) was tiny particles were raised by the blast and promoted flame acceleration through enhanced heat exchanges by thermal radiation through the flame front. In dust explosion protection, the flame is implicitly supposed to propagate like a in a gaseous mixtures but if it happens that thermal radiation is dominant for some dusts, many aspects concerning the way to mitigate the explosions for those particular dusts would need to be revised. The present research team (Ben Moussa et al., 2013, 2017; Proust et al., 2017a, 2017b) and another one (Julien et al., 2015) have been working on this subject for some time. The scientific problem was settled and a significant experimental effort was done. It was...

Determination of the laminar burning velocity and the Markstein length of powder–air flames

Powder Technology, 2002

This work deals with the determination of the laminar burning velocity and introduces the Markstein length of powder-air mixtures. A powder burner was used to stabilize laminar cornstarch-air dust flames and the laminar burning velocity was determined by means of laser Doppler anemometry. The dust concentration was varied from 0.26 to 0.38 kg m y3. The measured laminar burning velocities were found to be sensitive to the shape of the flame. With the same dust concentration, parabolic flames were found to have a laminar burning Ž y1 y1. velocity, which was almost twice that of a planar flame ca. 30 cm s for the latter as compared with ca. 54 cm s for the former. From this discrepancy and the flame curvature, the Markstein length could be determined. It was found to have a value of 11.0 mm. This Markstein length was subsequently used to correct the measured laminar burning velocities at various dust concentrations in order to obtain the unstretched laminar burning velocity. The unstretched laminar burning velocity lies between 15 and 30 cm s y1 and is thought to be a property of the dust and of the concentration.

Modes of particle combustion in iron dust flames

Proceedings of the Combustion Institute, 2011

The so-called argon/helium test is proposed to identify the combustion mode of particles in iron dust flames. Iron powders of different particle sizes varying from 3 to 34 lm were dispersed in simulated air compositions where nitrogen was replaced by argon and helium. Due to the independence of the particle burning rate on the oxygen diffusivity in the kinetic mode, the ratio between the flame speeds in helium and argon mixtures is expected to be smaller if the particle burning rate is controlled by reaction kinetics rather than oxygen diffusion. Experiments were performed in a reduced-gravity environment on a parabolic flight aircraft to prevent particle settling and buoyancy-driven disruption of the flame. Uniform suspensions of the iron powders were produced inside glass tubes and a flame was initiated at the open end of the tube. Quenching plate assemblies of various channel widths were installed inside the tube and pass or quench events were used to measure the quenching distance. Flame propagation was recorded by a high-speed digital camera and spectral measurements were used to determine the temperature of the condensed emitters in the flame. The measured flame speeds and quenching distances were in good agreement with previously developed one-dimensional, dust flame model where the particles are assumed to burn in a diffusive mode and heat losses are described on a volumetric basis. However, a significant drop of the ratio of flame speeds in helium and argon mixtures was observed for finer 3 lm particles and was attributed to a transition from the combustion controlled by diffusion for larger particles to kinetically controlled burning of micron-size particles. In helium mixtures, the lower flame temperatures measured in suspensions of fine particles in comparison to larger particles reinforces this assumption.