Page 317 - Dust Explosions in the Process Industries

286 Dust Explosions in the Process Industries

0.10

7 0.05
2
L
2
I I I
0.6 0.8 1.0
b
0.4
m
a
1
0.3

0.2
Figure 4.1 9 Burning velocities of clouds of two
coal dusts in air at zero gravity as functions of
equivalence ratio (=I for stoichiometric mixtures).
0.1 COAL The data points are experimental values. The solid
-
032-12 pm line is a comprehensive theory. The dotted lines are
39% VOLATILES a simplified theory neglecting either radiative losses
only, radiative losses and chemical reaction time,
0
0 0.4 0.6 0.8 1.0 or particle swelling. Percentages indicate the roles
@ of the respective factors (From Ballal, 7 983).

heat loss term. Once 6, has been calculated, the correspondingS, can be obtained from
equation (4.31).
Figure 4.19 shows that the theoretical prediction of S, (solid lines) agrees well with
the experimentaldata. Figure 4.19 also shows the predicted relative influence of the fac-
tors t,, Q, (radiative loss from particles), and$
Figure 4.20 gives the theoretically predicted dimensionless flame thickness (the real
flame thickness divided by average surfaceholume particle diameter D32)as functions
of the equivalenceratio (dimensionless dust concentration).
The 37% volatiles coal in Figure 4.19(a) has a burning velocity of about 0.11 m/s at
stoichiometricconcentration.According to Figure 4.20, the corresponding 6,/D,, value
is about 25, which for 032= 0.047 mm gives 6, = 1.18 111111.This is somewhat smaller
than the experimental values in Section 4.2.3.2 and illustrates the limitations of the
theory. Ballal(l983) pointed out that his theory is not applicable if
1. The equivalenceratio q >> 1;in which case, radiation contributespositively to flame
propagation.
2. Radiative heat transfer from shielding walls or pilot flames is significant.
3. The combustion is or becomes turbulent.
4. Gravitationaleffects play a significant role (particle diameter >5 pm).