Features of Particle Adhesion in an Air Stream

Adhesion and Thermal Processes. When particles touch the walls of a duct, we may find not only the forces already considered (see Sections 5,6,15-17), but also additional forces due to melting of the particles in the contact zone and a triboeffect, both favoring strong retention of the dust particles on the surface. 

With regard to thermal stability or heat resistance, all particles can be divided arbitrarily into two groups according to whether they are sensitive or insensitive to the rise of temperature caused by friction. 

The second group of substances does not have any such heat-induced tackiness. 

The adhesion characteristics of these two groups of substances were considered by Rumpf [262]. In the case of particles that are not temperature sensitive, the force of adhesion will predominate over the elastic detaching force when the velocity is low (Fig. IX.S.b). As the flow velocity increases, particle rebound is possible. The rebound of spherical glass particles with diameters up to 30 /im from a steel surface mounted perpendicular to the dusty stream is observed at a flow velocity of about 13 m/sec. 

In the case of particles that are temperature sensitive (Fig. IX.8a), the initial sticking is due to the same processes as in the case of elastic particles, except that this initial section (up to the point Ic) is restricted to narrower limits. As the flow velocity is increased, the impact energy becomes greater, and the particles will melt in the contact zone giving an increase in adhesion (point 2c). As the flow velocity is further increased, the quantity of evolved heat remains unchanged, thus leading to the appearance of a third critical point (3c). 

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