Two apparatuses have been developed to characterize the entrainability of dust deposits. The minimum aerodynamic wall shear stress required for dust removal (Critical Shear Stress), measured by the first apparatus, is an indicator of the ease with which dust particles can be removed from a layer. The second apparatus measures the settling velocity distribution of dust clouds, which is an important factor controlling the extent of dispersion as well as the time period during which the entrained dust cloud can remain suspended.
Testing has shown that the critical shear stress depends not only on the nature of the dust, but on the way the dust deposit is formed. Therefore, a portable version of this apparatus was developed to allow field testing of dust deposits. This methodology has also been used to evaluate the effect of oil treatment sometimes used in the grain handling industry to reduce dust emission.
Simulated explosion tests have demonstrated the importance of critical shear stress on dust entrainability. A simple model calculating unsteady dust concentration in elongated structures has been developed. The model shows that flammable dust concentrations induced in a gallery by a primary explosion exist over a finite length and time period. Therefore, the probability of a secondary explosion depends not only on the dust and layer properties, but on the details of the primary explosion and of the enclosure geometry. A simple criterion for transmission of a primary explosion into an adjoining elongated structure is discussed in the paper.