Page 366 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 335

0 0.5 IO 25 2.0 25 s
delay between inifiofion of dust dispersion
and ignition

Figure 4.42 Results from explosions of aluminum/air and coal dust/air in a closed I m3 vessel. The
ignition source is a chemical ignitor at vessel center (Data from Bartknecht, 1971).

As shown by Eckhoff (1976), the data from experiments of Nagy et al. (1971) in
closed bombs of various volumes confirm the arbitrary nature of (dPldt),,, values from
closed-bomb tests. This was reemphasized by Moore (1979), who conducted further
comparative tests in vessels of different volumes and shapes.
Dahn (1991) studied the influence of the speed of a stirring propeller on the rate of
pressure rise, or the derived burning velocity, during lycopodiudair explosions in a 20
liter closed vessel. The purpose of the propeller was to induce turbulence in addition to
that generated by the dust dispersion air blast. Typically, (dPldt),, increased by a factor
of 2-25 when the propeller speed increased from 0 to 10,000rpm.
The implication of the effects illustrated by Figures 4.404.42 for predicting explo-
sion violence in practical situations in industry was neglected for some time. The strong
influence of turbulence on the rate of combustion of a dust cloud is also indeed of sig-
nificance in practical explosion situations in industry (see Chapter 6).
In the past, sufficientattention was not always paid to the influence of the ignition delay
on the violence of experimental closed-bomb dust explosions. Often continuous ignition
sources, like flowing resistance wire coils, were used, as opposed to short-duration
sources, active for only a comparatively short interval of time, allowing control of the
moment of ignition. Some consequences of using a continuous ignition source were