Current Approaches to APSD Measurements of OIPs Based on Inertial Impaction - Good Cascade Impactor Practices, AIM and EDA for Orally Inhaled Products - page 22

Fig. 2.5 Idealized n -stage
cascade impactor
As the incoming flow passes through the nozzle plate, the streamlines diverge on
approach to the collection surface, whereas the finite inertia of the particles causes
them to cross the streamlines, if sufficiently large. The dimensionless Stokes
number ( St ), which is the ratio of the stopping distance of a particle to a character-
istic dimension, in this case the nozzle diameter, W (or average diameter, for a
multiorifice stage), describes the process. It will be seen later that the square root of
St defines a critical particle size in terms of aerodynamic diameter that will succeed
in leaving the airstream and reaching the collection surface for a particular stage
geometry (Fig. 2.6 ).
The modern theory underlying impactor function has been developed over the
past 35 years based firstly upon solving the Navier-Stokes equations for steady,
incompressible, and isothermal conditions that define the gas flow field having
viscosity, h , and density, r g , modeling the simplified geometry of a single-stage
“ideal” impactor. This operation is undertaken in the absence of particles.