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Homework IV – Coagulation, Nucleation & Condensation (due April 02, 200)
1.
In an experiment using cadmium oxide smoke, the particle concentration was
recorded as follows:
Time from Start (min)
Number conc. (#/cm
3
X 10
6
)
8
0.92
24
0.47
43
0.33
62
0.24
84
0.21
It is argued that the major mechanism in this system was Brownian
coagulation only.
Determine the coagulation constant from these data and
compare with theory for monodisperse aerosols.
(10%)
Sol>
The graph shows a strong linear correlation (K
0
=8.2×10
10
cm
3
/s) and a good
assumption of the aerosol as nearly monodisperse.
The theoretical value is K
0
=3.0×10
10
cm
3
/s.
If the particle follows the theory
of monodisperse, the Cc might be 2.7 and the particle size is around 0.1μm.
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A lognormal aerosol model (BIMODALM.EXE) has been developed to help
you understand the dynamic behavior of aerosols undergoing coagulation
and/or condensation.
Instruction of how to run the program is provided in
“README.TXT” and “INPUT HELP.DOC”.
Use it to study the following
scenarios: (a) coagulation only – run for a total time of 10 s (which is equal to
the product of time step (dt) and total number of steps (IT)); (b) condensation
only – run for a total time of 1×10
2
s.
Use the default values specified in
INPUT and choose the unimodal option.
However, for each scenario,
choose two
g
’s (1 and 2).
Plot the number concentration (N), particle
diameter (d
g
), geometric standard deviation (
g
) and saturation ratio (S) as a
function of time for each scenario.
In addition, plot particle size distributions
at the beginning and at the end of each scenario.
Report your observation
and explain why it is so.
(20%)
Sol>
Change of number concentration: condensation doesn't change the number of
particles while coagulation quickly reduces number concentration of
particles.
Change of MMD: for coagulation, when s
g
is greater, dg increases faster
because large particles serve as sites and small particles can quickly attack
the sites due to diffusion. For condensation, difference is not significant.
Change of s
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This note was uploaded on 09/11/2011 for the course ENV 6130 taught by Professor Wu during the Spring '08 term at University of Florida.
 Spring '08
 Wu

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