Page 628 - Dust Explosions in the Process Industries

Research and Development 595

50 mbar, were found. From this they concluded that the significant secondary explosion
pressures that can occur in the external cloud outside a vented enclosure, as a result of
primary explosions inside the enclosure, is due to very powerful flame jet ignition of the
external cloud.
Initiation of dust explosions by shock waves has been studied by several workers, and
valuable insight has been gained. One practical future application of this knowledge could
be to use induction times for shock wave ignition in models of flame propagation in tur-
bulent dust clouds (see Section 9.2.4). An informative analysis of shock wave ignition
of dust clouds was given by Wolanski (1990) and research at University of Michigan,
United States, was reported by Kauffman et al. (1992). Geng, Tang, and Griinig (199%)
measured the ignition delay behind an incident shock wave of Mach 4-6 passing through
a cloud of maize starch in oxygen. Ural (personal communication from E. A. Urd,
Factory Mutual Research, Nonvood, MA, 1992)emphasized that different induction times
are observed with incident and reflected shock waves, due to different ignition mecha-
nisms. Boiko, Papyrin, and Poplavski (1993) measured ignition delays for coal dust
clouds exposed to reflected and incident shock waves. Boiko and apyrin (1994) esti-
mated ignition delays of various dusts in incident and reflected shock waves. Geng, Lao,
and Tang (1994a) performed a numerical study of the fluid-dynamic effects of an inci-
dent shock wave passing through a dust cloud on the delay for igniting the dust behind
the shock. Geng et al. (1994b) used a vertical shock tube to measure ignition delays of
dust clouds behind an incident shock.
Lu and Fan (1994) developed a comprehensive analytical model allowing prediction
of ignition delay times of dust clouds exposed to shock waves. Good agreement between
predictions and experimental data was obtained. Hu and Sun (1994) studied the devel-
opment of aluminum dust explosions initiated by shock waves from gas explosions,
whereas Hu et al. (1994) investigated the fast ignition and combustion of wheat flour
behind a shock wave in a shock tube. Similar experiments were conducted by Elkotb
et al. (1996) with wheat flour and provender dust. Klemens et al. (1998b) presented a
mathematical model for simulating the ignition and burning of a cloud of coal particles
in air behind a shock wave passing through the cloud. The moving cloud was treated as
a two-phase, two-velocity, and two-temperature continuum, with both mechanical and
thermal interaction between the phases.

9.2.4
FLAME PROPAGATION PROCESSES IN DUST CLOUDS

9.2.4.1
General

Work up to 1990 is discussed in Chapter 4. Some central topics are the same as for flame
propagation in premixed gases:
@ Laminar flames.
@ Flame acceleration mechanisms.
@ Turbulent flames.
!a Detonation.