Shielded Blast Effects From Detonations and Large Volume, Confined Deflagrations cont'd

DOE-HDBK-3010-94

4.0 Solids; Powders

(6 to 80 m diameters) and free-flowing (100 to 150 m diameters). Different entrainment

mechanisms govern the types of materials. A variety of observations on the sequences or

phenomena governing entrainment are reported.

For velocities following weak or marginal explosions, Singer, Cook and Grumer (1972)

reported entrainment resulting from the erratic rupture and removal of large clumps from the

surface of cohesive dust ridges and their dispersal in the midstream. They attributed the

entrainment to a five step process: 1) detachment of single particles from the loose material

on the surface; 2) detachment of small clumps and particles; 3) partial fracturing and

subsequent entrainment of large clumps; 4) ridge breakup and complete breakup of large

clumps; and, 5) continued breakup and dispersal of clumps in midstream. Steps 1 through 4

required from 0.1 to 0.5 seconds to entrain 2 grams of material from cohesive dust ridges.

Fresh deposits were more readily dispersed and deposits became cohesive when slightly

compacted (such as after spill of material).

Singer, Harris and Grumer (1976) observed that explosions generated oscillatory flow

(increased flow followed by flow reversal) in wind tunnel experiments. Entrainment

appeared to be a weak function of the instantaneous air speed over the bed. Wetted or

wetted-dried layers of coal and rock dust dispersed faster due to the selective lifting of

relatively large briquetted fragments. Entrainment proceeded simultaneously by longitudinal

regression of the leading edge of a cohesive bed and the lifting of material from the surface

layer. The threshold instantaneous air velocity to entrain bulk quantities from cohesive beds

ranged from 5 to 30 m/s (11 to 67 mph). If the threshold velocity is exceeded for one

component of a mixed layer but not the other, the components can be entrained individually.

Chepil (1945) observed that the entrainment process begins with a rolling or sliding particle

motion as drag forces exceed friction forces. Punjrath and Heldman (1937) attributed

entrainment to the sudden increase in aerodynamic shear stress from the transition from

laminar to turbulent flow. Einstein (1942) observed that entrainment depended on the

fluctuation of the air velocity at the surface rather than of critical fluid properties (e.g., mean

velocity of bulk flow). Kalinske (1947) and Graf and Acaroglu (1968) reported entrainment

was due to fluctuating pressure and velocity components. Parthenaides and Passwell (1970)

presented the entrainment rate equations for the erosion of cohesive beds based on the

fluctuating flow components inducing instantaneous tensile stresses within the bed that

exceed the weakest bond holding particles in the bed.

No theory for entrainment due to the pressure waves generated by explosion appears to be

generally accepted. Various equations were uncovered that estimated the entrainment rate

under these conditions but all required either experimentally derived/empirical factors or

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