M E 433
Professor John M. Cimbala
Lecture 32
Today, we will:
•
Continue discussing about cascade impactors
•
Discuss some issues related to sampling with a probe (isokinetic, isoaxial, etc.)
•
Begin a discussion about how water drops (rain) cleans the air
Recall our cascade impactor, with several stages (j = 0, 1, 2, 3, ... J-1). This one is 8-stage,
with 8 trays.
Stage 0
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
Stage 6
Stage 7
100
90
stage 7
6
5
4
3
2 1
0
80
70
60
impaction
probability 50
(%)
40
preimpactor
30
20
10
0
0.1
1
10
aerodynamic particle diameter, Dp,aero (mm)
100
U = U0
c = c0
U = U0
c = c0
Q
Isokinetic,
isoaxial
sampling
(a)
Q
U = U0
c = c0
U < U0
c > c0
Subisokinetic,
isoaxial
sampling
(b)
U > U0
c < c0
U = U0
Q
c = c0
Superisokinetic,
isoaxial
sampling
(c)
U = U0
c = c0
Q
Isokinetic,
non-isoaxial
sampling
U = U0
c ≠ c0
(d)
Particle trajectories (dashed) and streamlines (solid) for (a) isokinetic, isoaxial sampling, (b)
subisokinetic, isoaxial sampling, (c) superisokinetic, isoaxial sampling, (d) isokinetic, but nonisoaxial sampling (misaligned sampling probe); dividing streamlines are bold and blue.