Page 398 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 367

Clark and Smsot’s work confirmed that the increasinglevel of turbulencein the accel-
erating fluid is a major driving force behind the flame acceleration in coal dust flames
in coal mine galleries. Further works on modeling of turbulent dust flames are reviewed
in Section 9.2.4.7 in Chapter 9.

4.5
DETONATIONS IN DUST CLOUDS IN AIR

4.5.1
QUALUTATIVE DESCRIPTION OF DETONATION

Detonation is a singular, extreme mode of propagation of a flame through a premixed
gas or dust cloud. The transfer of heat from the burning to the unburned cloud by molec-
ular or turbulent diffusion, which is characteristicof the deflagration mode of explosion
propagation discussed so far, is replaced by direct ignition by extreme compression of
unburned cloud in a shock wave driven through the cloud at supersonic speed by the
explosionitself.As will be mentioned in Section 4.5.3.2, the detailed mechanism of igni-
tion and combustion inside the shocked detonation front is still a subject of research.
The necessary condition for self-sustained detonation propagation is that the shock
wave is sufficiently strong for the volume inside it to ignite and react chemically before
the shock wave has traveled a significant distance away. In this way, the shock wave and
the chemical reaction zone remain closely coupled, and the shock wave speed and
strength is maintained. Typical maximum detonation velocities in premixed hydrocar-
bon gadair and dudair mixtures at normal pressure and temperature and optimum fuel
concentrationsare in the range 1500-2000 m/s. This is on the order of 5 times the veloc-
ity of sound in the unburned and uncompressed premixed gadair or dust cloud in air;
hence, the unburned mixture obtains no gas dynamic signal from the approaching det-
onation front until being caught by the front itself. Therefore, reducing the maximum
explosion pressure of a detonation by venting is impossible.
It follows from what has been said that a detonation in a premixed gas or a dust cloud
can be initiated only by a sufficiently strong shock wave. This can be supplied by either
an explosive charge or similar external means of generatingthe initial shock or by grad-
ual buildup of a strong shock by turbulent acceleration of the explosion itself, as illus-
trated in Figure 4.63.
Wolanski (1987) gave a comprehensivereview of experimental evidence and theory
of dust cloud detonations up to that time.

4.5.2
EXPERIMENTAL EVIDENCE OF DETONATIONS IN DUST
CLOUDS IN AIR

4.5.2 .I
Experiments in Ducts and Large-Scale Galleries

Figure 4.63 illustrates how detonation may develop in ducts of large LID via enhanced
combustion due to flow-generatedturbulence. As already mentioned in Section 4.4.7,