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Show Inertial impaction is illustrated schematically in Figure 1 for the case of a cylinder in cross flow. Two particles are illustrated as they approach the cylinder. Both respond to the gas flow field around the cylinder by beginning to move around the cylinder on approach. The inertia of both particles overwhelms the aerodynamic drag forces, and they impact on the cylinder. One is shown rebounding and the other sticking to the surface. Gas stream lines, including recirculation zones, are shown in light gray. This process is most important for large panicles (10-15 ~m or larger) and results in a coarse-grained deposit. The impaction rates are highest at the cylinder stagnation point, decreasing rather rapidly with angular position along the surface as measured from this stagnation point. At angular displacements larger than about 50° (as measured from the forward stagnation point), the rate of inertial impaction drops to essentially zero under condi tions typical of boiler operation. The impaction efficiency is indicated in Figure 2 and is defined as the ratio of number of particles that impact the tube surface to the number that are directed at the tube in the free stream. Predictions of the impaction efficiency as a function of particle, gas, and tube properties have been published, at various Parti_cle Impac.tlo n c.ptured Pattlcle c:> impacting { ... __ _ Partides .', , " , ......... -......... - -" --- " Rebounding Particle~. ~~=--....- Figure 1. Conceptual illustration of inertial impaction mechanism on a cylinder in cross flow. One rebounding and one sticking particle are also illustrated. 1.0--------------~----~--~-r~rT~~--~ >(.) c: G) 0.8 U 0.6 :: w c: o ~ 0.4 ca ~ .E 0.2 . - . Original R(Stk) 6 0 0 Calculations - Modified R(Stk) - a ........ Modified R(Stk) - b 1 Stokes Number Figure 2. Correlation of particle impaction efficiency on a tu be in cross flow as a function of Stokes number. Points and functions designated R are from the literature (ll,12). 5 |