The shock wave ignition of dusts

Abstract

Dust explosions pose a serious hazard in many industries. The detonability and flam inability of dust/oxidizer mixtures depend on the ignition delay of the dust particles when suddenly exposed to a high temperature environment. Consequently, the ignition delay time of dust particles behind a shock wave in the Mach number range of 4.0-5.0 has been measured using a photomultiplier tube to determine the onset of ignition. The dusts investigated included Pittsburgh Seam Coal, graphite, diamond, oats, and RDX. The experimental arrangement, consisting of a shock tube and two different dust injection devices, is described in detail, and experimental results for dusts ranging in particle size from 2 to 74 /*m are presented. In the Mach number range considered, ignition delay times varied from 2 to 100 /*s. A detailed analytical model based on a solution of the heat conduction equations for the particle interior coupled with a solution of the particle equation of motion has been developed. Heterogeneous reactions occurring on the particle surface and in the pores within the particle are used to model the chemistry. The results were in reasonable agreement with most of the data. Approximate analyses based on a comparison of characteristic thermal and chemical times were also developed. A key conclusion is that the ignition delay is determined mainly by the heat-up time of the particle surface.

The interaction between shock waves and solid spheres arrays in a shock tube

“…It was because of lack of the aerodynamic knowledge, in their study of particles with diameters from a few micro to a few hundred micro, Rogue et al [6] did not show clearly how to treat the drag coefficient for the particles in a dense cloud. Sichel et al [7] also indicated that in the study of shock wave ignition of dusts, both experiments and analyses involve too many difficulties and approximations. In this paper we present an experimental study on the shock interaction with spheres, whose diameters are i0 mm and 20 mm, respectively, because they can be easily framed by using 220 smaller diameter rods.…”

Mathematical modeling of heterogeneous detonation in gas suspensions of aluminum and coal-dust particles

“…It is also taken into account that the ignition delays of coal particles in transient shock waves are comparable with the characteristic time of particle heating. Sichel et al [6] made an attempt to take into account the thermal dynamics inside the volume of an individual particle, which allowed experimental and calculated ignition delays to be more or less reconciled. In gas suspensions with a significant fraction (in terms of mass) of the discrete phase, however, the mutual effect of the phases should be taken into account.…”

“…These parameters of the mixture corresponded to those of detonation experiments [3]. Similar particles corresponding to bituminized coal of the Pittsburg stratum both in terms of size (53-74 μm) and in terms of the volatile contents were considered in [6], as applied to ignition problems. The reaction constants were borrowed from [12][13][14][15] for a similar type of coal.…”

“…Conditions behind the detonation-wave front differ both from static conditions of slow heating and from conditions behind reflected shock waves. Experimental values of the ignition delay of coal-dust particles in transient shock waves (SW) [5,6] show that the ignition process occurs in a mixture with velocity nonequilibrium. Ignition is accelerated by the increase in heat-transfer intensity (increase in the Nusselt number) and by the increase in temperature in the boundary layer of the particle owing to flow deceleration.…”

Dust Related Detonations

“…The process of dust particle ignition in shock waves was analyzed by many authors [17], [21]- [25]. Very detailed calculations of particle ignition were first made by Ural [22], and Medviediev et al [23].…”