Page 380 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 349

In general, Schuber found that MESG decreased with decreasing initial turbulence in
the dust clouds.This is in harmony with the decreaseof the minimum electric spark energy
for ignition of both gases and dust clouds with decreasing turbulence. To ensure con-
servativeresults, Schuber’sexperimentsto establishcorrelationsbetween MESG and dust
properties were conducted with comparativelylow initial turbulence in the dust clouds.
Schuber correlated his experimental MESG values with the product of minimum elec-
tric spark ignition energy and the dimensionless minimum ignition temperature (right-
most column in Table 4.14) and the result is shown in Figure 4.57, where 1 g is the length
of the gap (width of the flanges (3) in Figure 4.54).

0 lg=A mm
A lg=50 mrn
16’ 1 I I I I
1 10 102 io3 104 105
MIE x [mil
273
Figure 4.57 Correlations between MESC and ignition sensitivity of dust clouds for various gap
lengths (From Schuber, 1989).
There is an increase of MESG with increasing gap length from 0 to 50 mm by a factor
of 2 to 3. For a constant gap length, there is a fair correlation between MESG and the
ignition sensitivityparameter used. A closer examination of this parameter reveals that
(TI + 273)/273 is in Ihe range 2-3 for most of the dusts in Table 4.14, which means that
the double-logarithmic correlation in Figure 4.57 is essentially between MESG and
ME. Schuber found that MESG approached a constant value as 1g approached 50 mm.
This value was considerably smaller than the laminar quenching distance.For example,
Schuber’s value for corn starch/air at 1 g = 50 mm was 1.8 mm, whereas the laminar
quenching distance found independently by Jarosinski et al. (1987) and Proust and
Veyssiere (1988) was 6-7 mm. Schuber’s asymptotic value of 1.8 mm agrees well with
the MESG of 1.5-2.2 mm found for corn starch/air by Jarosinski et al. (1987).
An important general conclusion from Schuber’s(1988) work is that, for a fairly long
gap length of 25 mm, MESG for gases, vapors, and organic and sulhr dusts in air can
be correlated with MIE (TI + 273)/273 in one single empirical equation:
MESG (mm) = [MIE (TI +273) / 273]0.157 (4.86)
where ME is in mJand TZis in K. Equation (4.86) could, in principle, be refined by incor-
porating the gap length as a further parameter. For short gap lengths of a few m,this
would give a reduction of MESG as compared to values from equation (4.86)by a factor
of 2-3 or more.
Schuber regarded the transmission of the flame through the slot as being primarily a
process of ignition of the dust cloud downstream of the slot by the turbulent jet of hot
combustionproducts being expelled from the slot rather than flame propagation through
the slot. He attributedthe strongcorrelationbetween MESG and ignition sensitivity to this.