Page 370 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 339

Tezok et al. (1985) extended the work of Kauffman et al. (1984a) to measure turbu-
lent burning velocities in the 0.95 m3 spherical explosion bomb. Radial turbulent burn-
ing velocities of 0.45-1.0 m/s were measured for mixed grain dust/air and 0.70-3.3 ds
for corn starchhir in the range of turbulence intensities of 1.54.2 m/s and dust con-
centrations between 50 and 1300 g/m3.The ratio of turbulent to laminar burning veloc-
ity was found to correlate well with the ratio of the rms turbulence velocity to laminar
burning velocity as well as with the Reynolds number. Some data from experiments with
<74pm maize starch of 4% moisture content are shown in Figure 4.47. The laminar burn-
ing velocities SL were the same as those derived by Kauffman et al. (1984a) by extra-
polating measured burning velocities in the 0.95 m3bomb to zero turbulence intensity.
The S, value of 0.7 m/s for 700 g/m3is, however, considerably higher than the highest
value of 0.27 ds arrived at for corn starch/air at constantpressureby Proust arad Veyssiere

01 I I
0 5 10
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Figure 4.47 Variation of normalized turbulent burning velocity for corn starch/air clouds, with nor-
malized turbulence intensity of the air. The experiments were done in a 0.95 m3spherical closed bomb
(From Tezok et al., 1985).

Tezok et al. also conducted some indicative measurements of the total thickness of the
turbulent flame, using an optical probe. They found it to be in the range of 0.15 to 0.70 m
and increasing with increasing turbulence intensity and dust concentration.This would
mean that the total flame thickness was on the same order as the dimensions of the
experimental vessel.
It should be mentioned that Lee, Yi Kang Pu, and Knystautus (1987) studied some fur-
ther aspects of the influence of turbulence on (dPldt),,, and P,, in closed-bomb dust
explosions.
In an investigationfollowing up the work of Tezok et al. (1985),Tai et al. (1988) used
laser doppler anemometry for studying turbulent dust explosions in the 0.95 m3explo-
sion vessel. They found that the dust had little effect on the turbulenceintensity,as com-
pared to that in pure gas under the same conditions of turbulence generation. Turbulent
burning velocities were determined for a range of dusts at turbulence intensities up to
3.3 m/s. Laminar burning velocities were estimated by extrapolatingto zero turbulence