Page 444 - Dust Explosions in the Process Industries

Ignition of Dust Clouds and Dust Deposits 4 1 1

velocity of the particles in quiescent air (=2 cds for lycopodium)dominate,and the par-
ticles above the spark approach the hot gas core.

30
-
E
E
u
ac NCE FROM SPARK CENTER TO
w
b- CLE WITH INITIAL DISTANCE 20 mm
z
w
u 20
x
ac
2
in
E
0
ar
LL
10
z
s
c“
[3
CONTOURS OF HOT
0
10-2 10-1 100 10’ 102 103 104 105 106
TIME AFTER INITIATION OF SPARK DISCHARGE Lpsl

Figure 5.1 6 Summary of theoretical prediction of positions of dust particles and radius of the hot
gas kernel following a ”short” discharge ofa 1.5J electric spark in a cloud of lycopodium in air (From
Ensrad, 19811.

The 1000 K and 700 K radii of the hot gas sphere as functions of time are also given
in Figure 5.16. The minimum ignition temperature of lycopodium clouds in air at
atmospheric pressure, as determined in the standard Godbert-Greenwald furnace, is
about 700 K. From Figure 5.16, it therefore follows that a dust-free, cool zone, sepa-
rating the dust cloud from the incendivepart of the hot gas core, is gradually formed from
100 ps after the spark discharge and onward, making ignition impossible. Figure 5.16
indicates that, from less than 1ps to about 100ps after the spark discharge, particles with
initial positions 2 to 5 mm from the spark center are trapped in the spark. However, this
is unlikely to cause ignition, because the induction period for “long” spark ignition of
lycopodium clouds in air, as shown by high-speed photography by Line et al. (1959), is
on the order of 1ms.
It is of interest to note that the radius of the dust-free zone 2 ms after spark discharge,
as predicted by Figure 5.16, is in close agreement with the experimentalvalue of about
10 mm found by Line et al. (1959) for the same spark energy, type of dust, and instant
after spark disch.arge.
The Schlieren flash photograph of a rising hot spark kernel in Figure 5.17 may sug-
gest that Enstad’s assumption, that the buoyancy of the spark kernel can be neglected,
may not be entirely valid.