ABSTRACT
If a solid or liquid particle starts to fl ow with one of the impinging gas streams at the feed plane, then it accelerates from zero velocity to a certain velocity resulting from the hydrodynamics of the gas-solid fl ow (Figure 5.1). After crossing the impingement plane, the particle penetrates into the opposite stream due to its inertia and decelerates to a full stop at some distance of penetration within the domain of the opposing jet fl ow (the stagnation point). Thereafter, the particle accelerates in the opposite direction and, after crossing the impingement plane again, penetrates the original gas stream up to the opposite stagnation point. Thus, the deceleration and acceleration processes take place repeatedly. After several damped oscillations, the particle velocity in the impingement zone drops to the terminal velocity so that it is carried away with the outgoing gas stream. (For simplicity, Figure 5.1 depicts only the particle trajectory fl owing with the upward gas stream, whereas downward or both fl ows are also possible, depending on the confi guration of the impingement chamber.)
Because of such an oscillatory motion with progressively decreasing amplitude, the residence time of a single particle (of large inertia) in the impingement zone is longer than that of the gas stream. This residence time may be reduced for a multiplicity of particles because of interparticle collisions that lead to enhanced energy dissipation. Although it is possible to feed solids to both gas streams, the rate of particle collision and the resulting loss of momentum are much higher than that for a single stream feed, because particles that do experience nonelastic collisions greatly lose their momentum and are driven out of the system.
