Page 298 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 267

speed may be calculated from the following expressions:

S, = SUE= Supu/p, (4.18)

(4.19)

where M is the molecular weight, Tis the temperature (K), p is the pressure (absolute),p
is the gas density,and the u and b subscriptsrefer to the unburned and burned states,respec-
tively. In the case of spherical flame propagation, the radial flame speed is given by equa-
tions (4.18) and (4.19) if the flame thickness is negligible compared with the radius of the
spherical flame surface. For finite flame thicknesses, methods for correcting for flame
stretch have been developed,as shown by Kawakami, Okajima, and Tinuma (1988).
The burning velocity in air generally increases consistently with increasing initial
temperature, whereas for many fuels, it decreases somewhat with increasing pressure.
When the ratio of 02/Nzin the oxidizing gas is either smaller or larger than in air, the
burning velocity decreases or increases correspondingly.In pure oxygen,burning veloc-
ities are considerablyhigher than in air because of increased reaction rates and heats, par-
ticularly at stoichiometricfuel concentrations,which are much higher in oxygen than in
air at the same total pressure. Table 4.1 summarizesmaximum S, values for some gases
mixed homogeneously with air, at atmospheric pressure and normal room temperature.
Table 4.1 Maximum fundamental burning velocities S,for homogeneous mixtures of air and
various combustible gases, at atmospheric pressure and normal room temperature

Fuel gas S,[mkl
Hydrogen 3.25
Acetylene 1.60
Ethylene 0.80
Methane to n-heptane 0.42-0.47