Page 372 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 34 1

Table 4.13 K,, values measured for clouds of maize starch dust in air in different closed vessels and
arranged according to vessel volume: Ks, = (dP/dt),,, V’l3

*Arithmetic mean values, 11% moisture in starch.
Source: Extended and modified version of table from Pu, 1988.

It should be mentioned that Bradley et al. (1988) were able to express K,, in terms of
a “massburning rate” and the initial and final pressure. The K,, concept was then defined
by equation (4.85).

4.4.4
TURBULENT FLAME PROPAGATION IN PARTLY OR FULLY
UNCONFINED GEOMETRIES

The important work of Tamanini (1989), and Tamanini and Chaffee (1989) is discussed
more extensively in Chapter 6 on venting of dust explosions. In the present context, it
should only be briefly mentioned that explosion experimentswere conducted in a 64 m3
vented vessel ah a series of different,known turbulence intensitiesat the moment of igni-
tion. The turbulenceintensitieswere measured by means of a bidirectional impact probe.
For a given dust, dust concentration, and vent characteristics,the maximum pressure in
the vented explosion increased systematically with increasing initial turbulence inten-
sity in the experimental range 2-12 ds.
Hayes et al. (1983) investigatedthe influence of the speed of four shroudedaxial fans,
mounted above the channel floor, on the dust flame speed in a horizontal channel of 1.5 m
length and 0.15 m x 0.15 m square cross section, open at both ends. A cloud of dried
wheat flour of mean particle size 100 pm was produced in the channel and ignited by a
propane/air flarme while the fans were running. Some results are shown in Figure 4.48.
It was anticipated that the flame speed would increase markedly with fan speed, and
this was also observed up to a fan speed of about 1500 rpm. However, as the fan speed
was increased further, the flame speed exhibited a marked decrease, to about 3000 rpm,
beyond which ignition of the dust cloud by the propane flame was no longer possible.