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Propagation of Flames in Dust Clouds 255

expands rapidly from the particle surface, and vapor-phase ignition may occur near the
end of the radiant pulse. In accordance with the model proposed by Cassel (Figure 4.1),
ignition is assumed to occur at some distance from the particle surface, where conditions
(magnesium and oxygen concentrations and temperature) are optimal. The onset of igni-
tion was characterized by the rapid appearance of a large luminous zone. Radiant inten-
sities required to ignite the particles were found to increase with particle size and the
thermal conductivity of the ambient gas environment. In accordance with the results from
hot gas ignition, little change in the radiant intensities were required for ignition when
replacing air by pure oxygen.
Florko et al. (1982) investigated the structure of the combustion zone of individual mag-
nesium particles using various techniques of spectral analysis. They claimed that their
results confirmed the assumption that the oxide, after having been generated in the gas
phase in the reaction zone, condenses between this zone and the surface of the burning
particle. This observation is an interesting supplement to the observation made and the
physical model proposed by Cassel(l964).
Florko et al. (1986) estimated the temperature in the reaction zone of burning mag-
nesium particles as a function of the pressure of the ambient gas, by analyzing the spec-
trum of the unresolved electron-vibration bands of the MgO molecules in the reaction
zone. For large particles of 1.5-3 mm diameter, the reaction zone temperature was prac-
tically independent of the gas pressure and equal to 2700-2800 K in the range 0.3 to 1
bar (abs). When the pressure was reduced to 0.05 bar (abs) the reaction zone tempera-
ture dropped only slightly, to about 2600 K. The burning time of 1.5-3 mm diameter par-
ticles was proportional to the square of the particle diameter. For a 2 mm diameter
particle at atmosphere pressure, the burning time was about 6 s. Extrapolation to 60 pm
particle diameter yields a burning time of 5.4 ms, which is quite close to the times of a
few ms found by Cassel (1964) for Mg particles of this size. When the pressure was
reduced to 0.2 bar (abs), Florko et al. (1986) found a slight reduction, by about IO%, of
the burning time. See Section 9.2.3.2 in Chapter 9 for further works on Mg particles.

4.1.3
ZI RCQN I UM

Nelson and Richardson (1964) and Nelson (1965) introduced the flash light heating
technique for melting small square pieces of freely falling metal flakes to spherical
droplets. They applied this method for generating droplets of zirconium, which were sub-
sequently studied during free fall in mixtures of oxygednitrogen and oxygenhrgon.
The duration of the light flash was only on the order of a few ms. A characteristic fea-
ture was the sparking or explosive fragmentation of the drop after some time of free fall.
This was supposed to be due to forcing out the solution of nitrogen, hydrogen, and
carbon monoxide that had been chemically combined with the metal earlier in the com-
bustion process. The experimental results for air at atmospheric pressure showed, as a
first-order approximation, that the time from droplet formation to explosive fragmenta-
tion was proportional to the initial particle diameter. The relative humidity of the air had
only a marginal influence on this time. The heat initially received by a given particle by
the flash was not specified.