Page 352 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 32 1

Gmurczyk and Klemens (1988) conducted an experimental and theoretical study of
the influence of the nonuniformity of the particle size distribution on the aerodynamics
of the combustion of clouds of coal dust in air. It was suggested that the nonhomoge-
neous particle size, amplifiedby imperfectdust dispersion,produces a nonhomogeneous
heat release process and leads to the formation of vortices.
Xufan et al. (1987) and Dehong (1986) studied upward flame propagation in airborne
clouds of Ca-Si dust and coal dust in a vertical cylindrical tube of internal diameter
150mm and length 2 m. The tube was open at the bottom end and closed at the top. The
Ca-Si dust contained 58% Si, 28% Ca, and 14% Fe, Al, C, and the like and had a mean
particle diameter of about 10 pm. The Chinese coal dust from Funsun contained 39%
volatiles and 14%ash and had a median particle diameterby mass of 13pm. The dust clouds
were generated by vibrating a 300 pm aperture sieve,mounted at the top of the combustion
tube and charged with the required amount of dust, in such a way that a stationary falling
dust cloud of constant concentration existed in the tube for the required period of time.
The dust concentration was measured by trapping a given volume of the dust cloud in
the tube between two parallel horizontal plates, inserted simultaneously, and weighing
the trapped dust. Ignition was accomplishedby means of a glowing resistance wire coil
at the tube bottom, after 10-20 s of vibration of the sieve. Upward flame velocities and
flame thicknesseswere determined by two photodetectorspositioned along the tube. For
the Ca-Si dust, the flame velocities were in the range 1.3-1.8 ds, and the total thick-
ness of the luminous flame extended over almost the total 2 m length of the tube. The
net thickness of the reaction zone was not determined.Figure 4.33 shows a photograph
of a Ca-Si dust flame propagating upwards in the 150mm diameter vertical tube. Figure
4.34 gives the average upwards flame velocities in clouds of various concentrations of
the Chinese coal dust in air.
On average, these flame velocities for coal/air are about half those found for the Ca-
Si under similar conditions. The data in Figure 4.34 indicate a maximum flame veloc-
ity at about 500 g/m3. If conversion of these flame velocities to burning velocities is
made by assuming some smooth convex flame front shape, the resulting estimates are
considerably higher than the expected laminar values. This agrees with the conclu-
sion of Klemens and Wolanski (1986) that this kind of dust flames in vertical tubes
easily becomes nonlaminar due to nonhomogeneous dust distribution over the tube
volume.
In the initial phase of the experiments of Proust and Veyssiere (1988) in the vertical
tube of 0.2 m x 0.2 m squarecross section, nonlaminar cellular flames, as shown in Figure
4.35, were observed. In these experiments,the height of the explosion tube was limited
to 2 m. Over the propagation distance explored, the mean flame front velocity was about
0.5 ds, as for the proper laminar flame, but careful analysis revealed a pulsating flame
movement of about 60 Hz. A corresponding60 Hz pressure oscillation,equal to the fun-
damental standing wave frequency for the one-end-open2 m long duct, was also recorded
inside the tube. Further, a characteristic sound could be heard during the propagation of
the cellular flames. Proust and Veyssiere, referring to Markstein’sdiscussion of cellular
gas flames, suggested that the observedcellular flame structureis closely linked with the
60 Hz acoustic oscillation.However, there seemed to be no straightforwardrelationship
between the cell size and the frequency of oscillation.
It is of interest to relate Proust and Veyssiere’s discussionof the role of acousticwaves
to the corn starchexplosionexperimentsof Eckhoff,Fuhre, and Pedersen (1987) in a 22 m