Transition from the Diffusion to Inertial Ranges

The often expressed intuitive belief that small particles are more difficult to remove From a ga.s than laige ones is usually not correct. The particles most difficult to collect are those in the size range corresponding to the transition from diffusional to inertial deposition, usually between 0.1 and 1 iim. The transition may be strongly influenced by direct interception (finite particle diameter effect) depending on the dimensions of the system. 

Calculating the collection efficiency in the transition region when both inertia and diffusion are important is very difficult The usual practice is to calculate the efficiencies separately for each effect and then add them to produce a composite curve of efficiency as a function of particle size. More complete analy.ses are passible such as computer 

1 A panicle is injected vertically upward with a velocity wo into a stationary gas. Derive an expression for the maximum distance traveled by the particle against the gravitational fieid. Assume the motion can be described by Stokes law. 

2 The drag coefficient for non-Stokesian particles can be represented by the expression 

3 A duct 4 ft in diameter with a 90 bend has been designed to carry particles In the range I dp 20 which adhere when they strike the wall. Before construction, it is proposed to carry out bench scale experiments to determine the particle deposition rate in the bend. The model is to be built to 1/10 scale, and the same aerosol will be used as in the full-scale system, Show that it is not pos.siblc to ntainlain both Stokes and Reynolds number similarity in the full-scale and model systems. If Stokes similarity is to be preserved, calculate the Reynolds number ratio for the model to full-scale systems. Why is it more important to preserve Stokes than Reynolds similarity in such experiments  

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