Page 256 - Dust Explosions in the Process Industries

228 Dust Explosions in the Process Industries

air velocity for dispersion of the ridge. It was further suggested that the tensile strength,
o,,of the powder deposit (Section 3.4.2) was a significant factor.
Based on resolution of velocity vectors, Singer et al. (1967) proposed a simple empir-
ical model for estimation of lift and drag coefficients on particles in deposits exposed to
airflow.The model neglected the pressure differencebetween both the windward and the
leeward sides of the dust ridge and the surface roughness. It took the following form:

C, = k(Re)" cosp
(3.27)
CL= k(Re)" sinp

where C, and CLare the drag and lift coefficients;Re is a special Reynolds number based
on the upstream air velocity at midheight of the dust ridge, the ridge height, and the den-
sity and viscosity of air; p is the angle between the base and the windward side of the
ridge; and m and k are empirical constants.
Singer et al. (1967) also found that large-amplitudeairstreampulsations,of up to 33 Hz,
superimposedon the main airflow by a rotating vane in a vent duct,broke dust ridges into
lumps. The lumps were lifted almost vertically into the turbulent pulsating airstream,
where they were eventually dispersed as individual particles into the turbulent core.
Iversen (1985) determined reentrainment rates of fine powders of Al, Cu, Mo, and W
of average particle size 5 ,um in a wind tunnel of width 0.50 m and height 0.71 m. The
length of the powder layer was 1.8 m and its width 0.14 m. The bed was prepared by
dispersing dust via air guns and allowing the dispersed dust to settle under gravity and
form the bed.
The data for particle mass collected as a function of height above the surface,wind speed,
and particle density were analyzed using the following solution of the equations for dif-
fusion from a two-dimensional source oriented laterally to the mean wind direction:

(3.28)

Here Cis the dust concentration at height z above the powder bed surface,and n is a veloc-
ity profile exponent defined by

v = VI (z/z,)'/" (3.29)

and

where A is a diffusion coefficient and y is the coordinate in the wind direction.
Equation (3.28) was used for calculating the averagevertical flux, qv,of particles from
the bed surface (equal to horizontal flux divided by the area of the powder bed) for
molybdenum particles. The following empirical equation was found to fit the experimental
data for all four powders: