Page 631 - Dust Explosions in the Process Industries

598 Dust Explosions in the Process Industries

more-detailed analysis of upward propagation laminar lycopodiudair flames in a ver-
tical duct. At 170 g/m3 a global approximately steady upward flame speed of about
0.50 m/s was observed, and global flame thickness of about 20 mm could be estimated.
However, the global faint parabolic flame front contained a complex microstructure of
small, more luminous flamelets,and a complex flow pattern in the unburned dust cloud
ahead of the flame was resolved. This suggests that the determination of fundamental
burning velocities of dust clouds in vertical tube experiments is far from straightforward.
In addition to flow ahead of the flame, it is also likely that the parabolic flame front is
subjected to the flame stretch phenomena.
In their experimental investigation, Krause, Kasch, and Gebauer (1996) studied the
effects of flow velocity and dust concentration on the apparent measured laminar dust
cloud burning velocity in vertical tubes. Similar experimental studies were conducted
by Glinka et al. (1996). Ju, Dobashi, and Hirano (1998a) studied the detailed structure
offlames propagating through clouds of stearic acid particles. The reaction zone was
explored by means of an electrostaticmicroprobe and Schlierenphotography. Klemens,
Szatan, and Wolanski (1998a) studied flames in clouds of stearic acid particles in air in
a vertical duct of square cross section. Schlieren photography was used to record flame
structure and flame development.
Extensive experimental research in Japan on the detailed structure of flames propa-
gating through clouds of 1-octadecanol,stearic acid, and iron particles in air is described
in a series of papers by Chen,Dobashi,and Hirano (1996); Ju, Dobashi, and Hirano (1998a,
199Xb, 1998~);J.-H. Sun, Dobashi, and Hirano (1998,2000,2001); Dobashi, Sun, and
Hirano (2000); and Dobashi (2001). Global flamepropagation velocities were measured
by direct and Schlieren high-speed photomicrography, whereas laser MIE-scattering
techniques were used to study the behavior of individual particles. The structure of the
ionized reaction zone was studied using a micro-scale electrostaticprobe, whereas tem-
perature projiles across the flame fronts were measured by means of thermocouples.
Similar detailed experimental studies of pulverized coal flames, comprising chemical
species concentrations(CO, C02,NO, and O,), flame velocities, and flame temperatures
were performedby Nazeer, Pickett, and Tree (1999) and Pickett,Jackson, and Tree (1999).
Special attention was paid to the effect of swirl on the flame structure. Klemens et al.
(2000a) studied details of the structures of flames in quasi-laminar clouds of coal, lig-
nite, and stearic acid in air in various apparatus.The diagnostics included Mach-Zehnder
interferometry and Schlieren and other photographic techniques. (Similar earlier work
by Klemens and coworkers is described in Section 4.3 in Chapter 4.)
In their experimental and theoretical investigation Goroshin, Bidabadi, and Lee (1996)
studied the combustion of well-dispersed, laminar clouds of fine atomized aluminumpar-
ticles of Sauter mean diameter about 5 pm. The quenching distance in airwas measured
to be about 5 mm, which yielded an estimatedflamethickness of about 2.8 mm. The down-
ward laminar flame front velocity in air was on the order of 80 cm/s. The experimental
data did not permit estimation of the correspondingfundamentallaminar burning veloc-
ity. The mathematical analysis of the problem at hand yielded calculated quenching dis-
tances in good agreement with those determined experimentally.Colver, Kim, and Yu
(1995), using an electrostatic method for generating homogeneous experimental dust
clouds, found that the minimum quenching distances for laminar dust clouds in air was
in excess of 5 mm for 17.5 pm diameter aluminum particles and about 14 mm for an
American coal.