Finishing Stop Distance

1
Finishing Stop Distance
The vector position vs. time
Broken down into x and y components
UmB a particles stop distance for this
problem If UmB gt 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
Finishing Stop Distance
For Stokes flow, we can relate the stop distance
to the particles properties Stop distance
increases with particle diameter due to the
greater sensitivity of mass to diameter than
mobility Particles with a stop distance gt L are
likely to impact. Thus particles with diameters
gt than a cutoff diameter Dcut will impact, and
the rest will be transmitted. Dcut is 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 dont
know their density So we define effective
diameters based on what we can measure
We have already defined a particles effective
volumetric diameter if we have a known volume and
number, but no distribution information.
For an impactor, we are forced to define a
particles diameter as if it were a sphere with 1
g/cc density.
The classical aerodynamic diameter is based on
the particles 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 dont assume a density