Page 65 - Dust Explosions in the Process Industries

38 Dust Explosions in the Process Industries

't

01 I I I I
600 I

20 50 100 200 500 1000
DELAY BETWEEN DUST DISPERSION AND IGNITION [msl

Figure 1.39 Influenceof initial turbulenceon explosion rate ofa dust cloud. Experiments with 420 g/m3
oflycopodium in air in a 1.2 liter Hartmann bomb; five experimentsper delay. Bars indicate * 1 stan-
dard deviation (From Eckhoft 1977).

pressure remained fairly constant up to about 200 ms. This reflects the fact that the max-
imum pressure is essentially a thermodynamic property, as opposed to the rate of pres-
sure rise, which contains a strong kinetic component.
Christill et al. (1989), having developed a comprehensivemodel for predicting flame
propagation and pressure developmentin gas explosions,implying the k-E model of tur-
bulence (see Section 4.4.1 in Chapter 4), suggested that similar models might be devel-
oped even for turbulent dust explosions. Other work along similar lines is discussed in
Section 4.4.8 in Chapter 4.
Figure 1.40 shows the strong influence of initial turbulence on the minimum electric
spark ignition energies of dust clouds. In this case, turbulence acts in the direction of
safety, making it much more difficult to ignite the dust cloud compared with the quies-
cent state. The effect is quite dramatic, the minimum ignition energy increasing by sev-
eral orders of magnitude. This is fortunate in the context of the possible generation of
electrostatic discharges in the presence of explosible dust clouds, because such dis-
charges are normally generated when the cloud is in turbulent motion. Section 5.3.4 in