Page 301 - Dust Explosions in the Process Industries

270 Dust Explosions in the Process Industries

With respect to the role of radiative heat transfer in dust flames, Cassel (1964) rea-
soned that losses from the heat generated in the combustion zone necessarily make the
maximum temperatures actually attained considerably lower than the temperatures pre-
dicted thermodynamically for adiabatic conditions.However, in the interior of sufficiently
large dust clouds, temperatures undoubtedly approach theoretical values. Therefore, as
heat losses by radiation decrease with decreasing surface-to-volumeratio of the burn-
ing cloud, dust flames should show a positive correlation between flame size and burn-
ing velocity not encountered in combustible gas mixtures. Therefore, in the absence of
other scale effects, larger high-temperaturedust flames may be expected to burn faster
than smaller ones.
Another difference between flame propagation in a premixed gas and dust clouds has
been elucidated by Goral, Klemens, and Wolanski (1988). They studied upward propa-
gation of flames in a lean methane/air mixture to which had been added inert particles
(sand). It was found that the upward flame velocity increased with increasing sand grain
size, from 0.33 m/s for the 5.1 vol% methane/air with no sand particles, via 0.4 m/s for
40 pm particles, 0.65 m/s for 180pm particles to 0.75 m/s for 360 pm particles. The effect
was attributedmainly to the enhanced combustion due to the microturbulence generated
in the wake of the falling particles. However, thermal radiation effects were also assumed
to play a role.

4.2.3
EXPERIMENTAL BURNING VELOCITIES, FLAMETHICKNESSES,
QUENCHING DISTANCES, AND TEMPERATURES OF LAMINAR
DUST FLAMES

In the case of premixed gases, the properties of laminar flames can be investigated in
detail in special stationary burners. The same technique has been adopted in the study
of laminar dust flames. However, as Lee (1987, 1988)pointed out, laminar dust flames
are difficult to stabilizewithout causing significantcooling of the flame. Therefore,such
stabilized flames are nonadiabatic, and average burning velocities are lower than for an
adiabatic flame. In addition,the flame is not uniform over its cross section, and burning
velocities and flame thicknesses are not always easy to define. Nevertheless, much valu-
able information on the nature of laminar dust flames has been obtained from stationary
burner flame studies. Section 9.2.4.2 in Chapter 9 gives references to further works on
laminar flame propagation in dust clouds.

4.2.3.1
Metal Dusts

Cassel (1964) developed a special burner for studying stationary propagation of flat
“laminar” graphite and metal dust flames. Circular Mache-Hebra nozzles were used to
ensure a reasonably uniform distribution of the upward velocity of the dust cloud into
the flame region. Once ignited, the flat dust flame floated approximately 20-30 rnm
above the burner port. The flame was stabilized by an enveloping divergent gas stream
without using a pilot flame. Burning velocities were determined photographically both