Page 632 - Dust Explosions in the Process Industries

Research and Development 599

An excellent contribution to improved understanding of the nature of laminar dust
James was given by Dahoe, Hanjalic, and Scarlett (2002). They used a burner appara-
tus to produce stable cornstarch flames in air, and the laminar burning velocity was
measured by laser-doppler anemometry. It was found that the laminar burning velocity
varied with flame shape, and this was accounted for by introducing the “Markstein
length” of a dusthir flame. This parameter is specific for any given dust cloud. It has a
magnitude on the order of the laminar flame thickness of that specific dust cloud and
serves as a measure of the sensitivity of the laminar burning velocity to changes in the
flame shape. Dahoe et al. emphasized that neither the theoretical derivation nor the
experimental determination of the Markstein length is trivial and much remains to be
learned about its precise dependence on the chemical and physical properties of the spe-
cific combustible mixture being investigated. In the light of this work, time seems ripe
for reconsideringsome conclusionsdrawn from earlier work to determinelaminarbum-
ing velocities of dust clouds in vertical tube experiments (see Section 4.2.3 in Chapter 4).
As pointed out by Dahoe et al., buoyancy may have contributed significantly to the
upward movement of the flame front in these tubes. Also, the observation of a constant
shape (often about hemispherical) of the upward propagating flame seems to contradict
the assumptionof a constantburning velocity perpendicular to the flame surfaceimplied
in the mathematicalcorrection formula frequently used to transform the observed flame
velocity into the corresponding velocity of a plane laminar flame.

9.2.4.3
Limiting Dust Cloud Compositions for Flame Propagation
This is an important fundamental research topic for at least three different practical
applications. The first is assessment of explosive or nonexplosive; the second, assess-
ment of minimum explosive dust concentration; and the third, assessment of maximum
permissible oxygen concentration for inerting. Work up to 1990is discussed in Section
4.2.6 in Chapter 4 and Section 7.13 in Chapter 7. Recent work on some aspects of
experimental determination of limiting cloud compositions for flame propagation is
reviewed in Section 9.4.4.
Mintz (1993) found evidence for the existence of a maximum explosive dust con-
centration for dust clouds under certain circumstances. For a narrow size fraction
(106-125 pm) of maize starch, a reasonably well-defined limit of 800-1000 g/m3was
found. The results were interpreted in terms of a simple “oxygen depletion” model.
The influence of particle size distribution on the minimum explosive dust concentra-
tion was investigated by Poletaev and Korolchenko (1993),using data from experiments
with polysized polyethylene dusts. Promising agreementbetween theory and experiments
was obtained. Hanai et al. (1996) measured the minimum explosive dust concentration
for PMMA particles in air under microgravity conditions. In the absence of buoyancy,
using a point ignition source, spherical flame ball development was obtained. In the
range of particle diameters studied,the minimum explosivedust concentration increased
systematically with the particle diameter. Results from comparative experiments at
normal gravity differed only modestly from the correspondingmicrogravity results.
Hertzberg, Zlochower, and Cashdollar (19924 measured minimum explosive con-
centrations, maximum explosion pressures and maximum rates of pressure rise at con-
stant volume, and maximum flame temperatures for clouds in air of dusts of 14 metals.