Page 619 - Dust Explosions in the Process Industries

586 Dust Explosions in the Process Industries

9.2.2.4
Generation of Secondary Dust Clouds by Entrainment
of Dust from Layers and Deposits

As soon as a significant blast wave has been generated by the primary dust flame, this
blast may generate secondary explosive clouds ahead of the flame by entraining dust
deposits and layers (see Section 3.6 in Chapter 3). Lebecki et al. (1990) investigated such
processes in a 100m long gallery of cross section 3 m2.To establish an improved under-
standing of these processes, further experimental and theoreticd studies of the interac-
tion of blast waves with dust clouds and dust layers and deposits need to be conducted.
Work on this problem has also been performed by Ural(1992), Gelfand and Tsyganov
(personalcommunication at the Semenov Institute of Chemical Physics, Moscow, 1992),
and others. Gelfand and Tsyganov (personal communication, 1992) showed that the
presence of dust layers on solid surfaces exposed to blast waves changed the blast wave
characteristics as compared with the characteristics in the case of dust-free surfaces.
Kauffman, Sichel, and Wolanski (1992)andAustin et al. (1993) summarized their exten-
sive research on the entrainment of dust layers in long tubes by the blast wave heading
a dust explosion propagating along the tube. Tamanini and Ural (1992) summarized
their work on characterizing the dispersibility of dust layers when exposed to air blasts.
Geng,Tang, and Gronig (1993) studied the influence of clouds of maize starch in oxygen
on the peak pressures of incident shock waves of Mach 5.4 and 6.0.
Increased emphasis should be put on investigatingthe connectionbetween the param-
eters of dust cloud generation processes and the structures of the resulting dust clouds.
The structures of the clouds produced must be defined in terms of distribution of dust
concentration,quality of dust dispersion (deagglomeration),turbulencelevel, and global
velocities. Kosinski et al. (2001) presented a numerical simulationstudy of the structure
of the flow of dust clouds in branched channels.
Various test methods have been proposed for evaluating the ease with which dust
clouds can be produced from deposits and layers of powders and dusts (see Section
7.4.2 in Chapter 7). Breum (1999) investigated systematicallythe parameters influenc-
ing the “dustiness” results produced by a rotary drum tester. A general definition of the
dustiness of a material is “the tendency of a sample of the material to liberate dispersed
dust particles into the air.” In the rotary drum test, the material to be tested is placed inside
the drum and air is passed through the drum. The mass of dispersed dust particles col-
lected on a filter at the drum exit during the test period is taken as a measure of the dusti-
ness of the material tested. The materials studied by Beum were bentonite, barium
sulphate,talc, Aloxite, carbon black, and coal. Except for coal, dustiness was in general
positively correlated to the mass of the powder sample tested. A three-parametermulti-
plicative model for the dustinesspotential was developed for two of the materials tested.
Fundamental theoretical studies have been performed on the generation of dust clouds
behind shock waves sweeping across a dust layer at 90” to the layer surface. Frolov,
Mack, and Roth (1993) developed a mathematical diffusion model describing such a
process. A similar model was developed by Lu et al. (1993). This kind of work is indeed
relevant in the context of the propagationof secondary dust explosions.Nikolova (1993)
developed a source code for numerical models for the simulationof dust cloud flow. The
code can be used for cold flows as well as flows with combustion. Medvedev et al.
(1993) conducted experimental studies of the entrainment of dust from dust layers by