Page 283 - Dust Explosions in the Process Industries

252 Dust Explosions in the Process Industries

oxide accumulated on the burning aluminum droplet. Because of this, the combustion
process was terminated by fragmentationof the droplet (as shown by Nelson, 1965,for
zirconium), The very fast flash-heatingmethod generated fully developed metal droplets
with practically no oxide on the surface. This presented initial conditions for studying
the subsequent ignition and combustion processes, when the virgin droplets interacted
with the surrounding air. Detailed SEM studies of the oxide layer buildup revealed a
porous structurewith a great number of fumaroles.Over the experimentalrange, the burn-
ing time to fragmentationincreased linearly with the particle diameter from about 200
ms at 300 pm to 600 ms at 500 pm. Prentice studied the combustion of aluminum
droplets in dry air over a range of pressures up to 4.5 bar (abs). The particles were found
to fragmentin dry air at pressures up to about 2.4 bar (abs). Fragmentation became quite
weak and sporadic at this pressure and finally ceased as the pressure was raised to
approximately 4.0 bar (abs). The time to fragmentationwas found to be inversely pro-
portional to the air pressure, that is, to the oxygen concentration.
Prentice also found that the nitrogen in the air played an active role in the combustion
process, causing the oxide generated to adhere to the droplet surface and form an asym-
metrical, spin-generatingoxide layer that appeared to be a precondition for fragmenta-
tion. The driving gas causing particle fragmentationis in part aluminum vapor, but for
combustion in air, the major constituent is nitrogen from nitride.
Frolov, Pokhil, and Logachev (1972) studied ignition and combustion of single alu-
minum particles in high-temperature oxidizing gases, as a function of particle size and
state of the gas. Various theories were reviewed.
Grigorev and Grigoreva (1976) modified the theory of aluminum particle ignition
by Khaikin, Bloshenko,and Merzhanov (1970), by including a fractional oxidation law
accounting for possible changes of the structure of the oxide film during the preflame
heating period. Experiments had revealed that the minimum ignition temperature of
aluminum particles was independent of particle size, and Grigorev and Grigoreva
attributed this to the oxidation rate depending very little on the thickness of the oxide
layer.
Razdobreev, Skorik, and Frolov (1976) studied the ignition and combustion of indi-
vidual 230-680 pm diameter aluminum particles in air, following exposure to station-
ary laser light fluxes.At incident fluxes approaching 150W/cm2,the particle melted, but
ignition occurred only at fluxes higher than 250 W/cm2.Coefficients of reflection were
not measured but assumed to be in the range 96-50%, which means that less than half
the incident light flux was absorbed by the particle. The time from onset of radiant heat-
ing to ignition increased with particle diameter from 100ms for 230 pm, through 270 ms
for 400 pm, to 330 ms for 680 pm.
Ermakov et al. (1982) measured the surfacetemperatureof 400-1200 pm diameter alu-
minum particles at the moment of ignition. The heating was performed by a continuous
laser of wavelength 10.6 pn at a constant flux incident on the particle in the range
1500-4500W/cm2,that is, much higher than the experimentalrange of Razdobreev et al.
(1976). The particle temperature was measured by a tungsten-rheniumthermocouple,
whose junction of thickness 18-20 pm was located at the center of the particle.
Microscopichigh-speed film records were made synchronously with recording the par-
ticle temperature at a rate up to 4500 frames/s. The simultaneous recording permitted
detailed simultaneous comparison of the temperature of the particle with physical phe-
nomena observed on the particle surface. The appearance of a flame in the form of a