Tey
travel chrocsh this deviated air to be collected on the wanes. ‘Yo insure
collection, tbe wanes are coated with a grease.
For molecules, the impactor
collects by diffusion to the wanes.
Physical Laws.
Sefore consizering details of iapactor performance, the
physical laws cowering the forces on particles, diffusion phenmomens, and flow
behavior mist be exenined.
at 250,000 feer, instead of hawing a viscous flow
slowing particles as at sea lewel, slowing is a bombardcent process, since the
particle Giameter is saall compared with che mean free path of the gas wolecules.?
The relation is
Fe- 1.352 7 x way,
(5.4)
where F is force, w is average thermal velocity of molecuies, p is air density, a is particle radius, and V is the relative welocitry of the particle.
The diffusion process is well enough known that detailed discussion is
unnecessary.
Computing the flow through the filter requires two steps.
First, stagnation conditions in front of the sampler are calculated froa
sampler speed and air parameters.
use of NACA-TN-1428.9
This may be easily accooplished by rhe
Once conditions in front of tha sampler are inn,
the mass-flow rate through the impactor may be computed, since the side ports
are at about side-on pressure.
The Flow through the impactor my be considered
the flow between places.”
The expression for the mass flow through the iapactor per unic length
(q,) ts
w
(5.5)
a z (RY}qL (p +6 S)(p, ~ Pa)
where R is unit mass gas constant, T is the absolute temperature, Wis the
plate separation, 3 is viscosity, iL is plate length, P is average pressure,
€ is che slip coefficient, and (Pp, - P,) is the pressure drop across the
systea.
,
The shows Toletice 12 veliss ive viscous riow woeo tlow welocity does
not exceed acoustic.
1}
The flow through the inpactor is considered isotherzal,