Page 363 - Dust Explosions in the Process Industries

332 Dust Explosions in the Process Industries

For this reason, dust flame propagation should be studied in large-scale apparatus. It
should be emphasized,however, that from a practical standpoint, large orfill scale is not
an unambiguous term. For example, a dust extraction duct of diameter 150 mm is full
industrialscale and, at the same time, the scale of laboratoryequipment.On the other hand,
the importantfeatures of an explosionin a large grain silo cell of diameter 9 m and height
70 m are unlikely to be reproduced in a laboratory silo model of 150 mm diameter.
It should be mentioned here that Abdel-Gayed, Bradley, and Lawes (1987) identified
generally applicable correlations in terms of dimensionless groups, enabling prediction
of accelerationof flames in turbulentpremixed gases. A similarapproachmight, in some
cases, offer a means of scaling even dust explosions. The role of radiative heat transfer
in dust flames then needs to be discussed, as done by Lee (1987). His conclusion was
that conductive and convectiveheat transfer are probably more important than radiative
transfer. This may be valid for coal and organic dusts but probably not for metal dusts
like silicon and aluminum.
Amyotte, Chippett, and Pegg (1989) reviewed more than a hundred publications on
various effects of turbulence on ignition and propagation of dust explosions. They con-
sidered the influence of both initial and explosion-induced turbulence on flame propa-
gation in both vented and fully confined explosions. They suggested two possible
approaches toward an improved understanding: concurrent investigations of dust and gas
explosions and direct measurement of turbulent scales and intensities in real experi-
ments as well as in industrial plants. See also Sections 9.2.4.4 and 9.2.4.7 in Chapter 9.

4.4.3
EXPERIMENTALSTUDIES OF TURBULENT DUST FLAMES
IN CLOSED VESSELS

4.4.3.1
Common Features of Experiments

The majority of the published experimentalstudiesof turbulent dust explosions in closed
vessels have been conducted in apparatus of the type illustrated in Figure 4.39.
The closed explosion vessel of volume VI and initialpressure P, is equipped with a dust
dispersion system, a pressure sensor, and an ignition source.In most equipment, the dust

I
IGNITION
IGNITION
PRESSURE
t SENSOR
DUST I
SAMPLE

Figure 4.39 The type of apparatus commonly used in closed-vessel turbulent dust explosion
experiments.