Finishing Stop Distance

1. L Finishing Stop Distance The vector position vs. time Broken down into x and y components UmB = a particle’s stop distance for this problem If UmB > y0 then the particle will impact For this ideal flow, y0 = x0 If the flow is coming from some finite orifice of radius and height L , then particles will be distributed in x fairly uniformly So we can expect particles to impact over a range of stop distances UmB ~ L

2. L Finishing Stop Distance For Stokes flow, we can relate the stop distance to the particle’s properties Stop distance increases with particle diameter due to the greater sensitivity of mass to diameter than mobility Particles with a stop distance > L are likely to impact. Thus particles with diameters > than a “cutoff diameter” Dcut will impact, and the rest will be transmitted. Dcutis found by setting xs = L

3. Using Stop Distance to size aerosolsMicro Orifice Uniform Depoist Impactor For each “stage”, there is a distribution of cutoff diameters based on the geometry of the orifice-impactor. The MOUDI sends air through a series of impactors, each with faster flow and shorter impaction distances

4. Effective Diameters Particles are not spheres, and we often don’t know their density So we define “effective” diameters based on what we can measure We have already defined a particle’s effective volumetric diameter if we have a known volume and number, but no distribution information. For an impactor, we are forced to define a particle’s diameter as if it were a sphere with 1 g/cc density. The “classical” aerodynamic diameter is based on the particle’s terminal velocity assuming it is a sphere of water. This is the same as the impaction diameter, but where L/U is replaced with vT/g Electrical mobility diameter is defined in the same way, but for a DMA. Since there is no inertial dependence, you don’t assume a density