AEPA
United States
Environmental Protection
Agency
Environmental Sciences Resean
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-81-107 Dec. 1981
Project Summary
A Two Stage Particle
Fractionator Using Large
Pore Nuclepore Surfaces
Gale H. Buzzard and Richard D. Parker
A fundamental study of collection
efficiency resulting from inertia!
deposition of particles onto a large
pore Nuclepore filter has been con-
ducted. The principal objectives of the
study have been to develop a compu-
tational procedure for predicting the
filter collection efficiency, to verify
these predictions experimentally, and
to apply the procedure to investigate
the effects of the pore Reynolds
number, particle density, filter thick-
ness, and filter porosity upon the
efficiency. The study has been restricted
by the assumption of continuum flow
and negligible diffusion forces.
This Project Summary was developed
by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
The work described within the report
is applicable to large pore filters, as
defined by the assumption of continuum
flow through the pore, and to a minimum
particle diameter, as defined by the
assumption of slip-flow around the
particle. The experimental work has
been conducted with a 9.5 /jm pore
diameter Nuclepore filter with a porosity
of approximately ]0 percent. For this
case, the partial efficiency of diffusion is
less than 0.1 percent. Therefore,
diffusion is negligible, and inertia!
impaction and interception are the
critical mechanisms for deposition.
A solution to the Navier-Stokes
equations for flow through a circular
pore was obtained numerically. The
flow through the pore was modelled as a
sudden contraction within a cylindrical
streamtube. The equations of motion for
a particle within the flow field were
solved numerically to obtain the particle
trajectory. The critical trajectory for
impaction was found by a trial and error
iteration. Assuming a uniform concen-
tration of particles upstream of the filter,
the impaction efficiency could be
obtained from the critical trajectory. By
considering a particle to impact when-
ever its trajectory came within a particle
radius of the filter surface, it was
possible to combine the effects of
impaction and interception and thus
avoid the need for an empirical weight-
ing factor.
Flow through the pore has been
assumed to be continuum flow, and
flow around the particle has been
assumed to be Stokes' flow modified by
the Cunningham correction for slip.
Continuum flow is generally character-
ized by a Knudsen number much less
than unity. For a 9.5 Aim pore diameter,
and a mean free path for air molecules,
A = 0.065 fjim, the Knudsen number is
given by
Kn =
= 0.0137 «1
-------
Similarly, the Knudsen number for a 1
/um diameter particle is
Kn= _*_ =0.13<1
Rpart
Since the Knudsen number for a 1 /jrr\
diameter particle is not greatly less than
unit, the inertial parameter in the
equations of motion for the particle has
been multiplied by the Cunningham slip
correction factor.
The flow field through a plexiglas
model of the filter was studied experi-
mentally using a glycerin-water mixture,
and a flow visualization technique
which used dye injection to trace the
experimental streamlines. The stream-
lines were photographed and found to
compare favorably with the streamlines
obtained from the theoretical flow field
.solutions.
Experimental collection efficiencies
have been obtained for a 9.5 fjm pore
diameter Nuclepore filter and particle
diameters ranging from 2 to 9 /um. The
particles were generated with a Berglund-
Liu monodisperse aerosol generator,
and the filter was analyzed using an
optical fluorescence technique. The
experimental efficiencies compared
quite favorably with the model in the
case of a liquid aerosol but fell well
below the predictions of the model in
the case of a solid aerosol.
Experimental Procedures
The experiments employed a 9.5 pm
pore diameter Nuclepore filter as a
primary filter and a 0.4 /um pore
diameter Nuclepore filter as a secondary
or back-up filter. Collection efficiencies
were measured for both liquid and solid
aerosols with particle diameters rang-
ing from 2 to 9 ^m. All particles not
captured by the primary filter were
assumed captured by the secondary
filter. Particles were collected by a
sampler employing a stacked primary
and secondary filter and operating at a
face velocity of 10 cm/s and 20 cm/s.
Face velocity is defined as the volu-
metric flow rate divided by the total
frontal area of the filter surface.
Collection efficiency experiments
were conducted with a wet, sticky
aerosol (oleic acid) with the view that
such an aerosol would indeed be
captured upon impact. The results of
these experiments are in agreement
with the theory and are accepted as
validation of the impaction model.
Additional experiments were conducted
with a solid aerosol (methylene blue
dye) and directed at the question of
particle capture. The results of these
experiments gave a clear indication that
the solid aerosol was not necessarily
being captured upon impact, and that
upon impact, the particles had a tendency
to bounce, become reentrained in the
flow field, and pass on through the filter.
Analysis of the filter for the quantity of
aerosol collected was done by means of
optical fluorescence. Both solutions
used in the generation of the aerosol
particles contained a known quantity of
sodium fluorescein (uranine) dye. After
collecting the sample, the filters were
washed in a known volume of aqueous
ethyl alcohol which was subsequently
analyzed with a calibrated optical
fluorimeter. The aqueous ethyl alcohol,
four parts 95 percent ethyl alcohol and
one part distilled water by volume, was
buffered five percent by volume with
1/10 normal aqueous sodium hydroxide
to counter the acidity of the aerosol
material and the effect of pH upon the
fluorescent properties of the uranine
dye. The fluorimeter output was calibrated
against known dilutions of the solution
run through the aerosol generator, and
was periodically spot-checked for drift
against a single dilution during the data
run. In the case of oleic acid, the
sensitivity of the fluorimeter was
sufficient to allow the use of one part
uranine dye to twenty parts oleic acid by
mass. The intensity of the methylene
blue dye so masked the fluorescence of
the uranine dye as to require one part
uranine dye to four parts methylene
blue dye by mass.
Results
The experimental results along with
the theoretical predictions of the model
are summarized in Figures 1 and 2.
Figure 1 presents data gathered at a
face velocity of 10 cm/s. Circles
indicate data for oleic acid (density =
700 r-
80
60
QJ
40
20
O - Oleic Acid
O - Methylene Blue
(g/cm3)
0.5
1.0
2.0
I
I
_L
0.2 0.4 0.6 0.8
Particle Diameter/Pore Diameter
1.0
Figure 1. Environmental collection efficiencies compared with the model. Face
velocity = 20 cm/s. A
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Pb.92 g
).92 gm/cc) and diamonds indicate
data for methylene blue dye (density =
1.44 gm/cc). The solid curves show the
collection efficiencies predicted by the
theory for a range of particle densities
that bracket the experimental data.
Similar data are contained in Figure 2,
but represent a face velocity of 20 cm/s.
The oleic acid data presented in both
figures agree with the theoretical
curves and support the model. The
methylene blue dye data presented in
Figure 1 fall significantly below the
theoretical curves and, in general,
below rather than above the experimen-
tal data for the less dense oleic acid
particles. The methylene blue dye data
presented in Figure 2, taken at a greater
face velocity than that presented in
Figure 1, show the same general trends
seen in Figure 1, but at a greater face
velocity are more dramatic. The fact that
the deviation from theory seen in the
solid aerosol data becomes greater with
increased face velocity or particle size
indicates an inertial effect and points
toward the probability of particle
bounce. There was no evidence in this
study that the collection efficiency was
time-dependent as-would be the case if
a particle were captured and then
Jislodged upon impact from a subse-
Jquent particle.
Conclusions and
Recommendations
The intent of this study has been to
develop and validate a theoretical
procedure for predicting the collection
efficiency of a large pore Nuclepore
filter. A theoretical procedure which
predicts the impaction efficiency of the
filter has been developed and validated.
The experimental results which validate
the impaction theory showthat particles
which impact upon the filter surface are
not necessarily captured by the surface.
For this reason, the theory is not yet one
of collection efficiency. The impaction
theory will, however, produce a collec-
tion theory if complemented with the
development of an adequate theory or
mechanism for particle capture. It
seems unlikely that the answer to this
problem depends upon being able to
predict the probability of capture for a
given particle so much as being able to
develop a reliable mechanism for
particle capture. Other researchers are
reporting qualitative success with
enhancing the collection efficiency of
large pore Nuclepore filters by means of
a grease base sticking agent applied to
he filter surface. If such techniques can
WO
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60
40
20
O- Oleic Acid
O- Methylene Blue
A
B
C
Pp (g/cm3)
0.5
1.0
2.0
1
0.2 0.4 0.6
Particle Diameter/Pore Diameter
0.8
1.0
Figure 2.
Experimental collection efficiencies compared with the model. Face
velocity =10 cm/s.
be developed to the point vyhere, within
the accuracy demanded of the sampling
results, all particles impacting upon the
filter surface are captured, then the
theory presented here is capable of
predicting collection efficiency.
Gale H. Buzzard is with Duke University, Durham, NC 27706; and Richard D.
Parker was a former graduate student at Duke University (present address
unknown)
John P. Bell is the EPA Project Officer (see below).
The complete report, entitled "A Two Stage Particle Fractionator Using Large
Pore Nuclepore Surfaces," (Order No. PB 82-110057; Cost: $9.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
U S GOVERNMENT PRINTING OFFICE, 1981 559-017/7412
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use S300
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