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Aerosol Science and Technology
ISSN: 0278-6826 (Print) 1521-7388 (Online) Journal homepage: http://www.tandfonline.com/loi/uast20
Design and Performance Test of a Lab-Made
Single-Stage Low-Pressure Impactor for
Morphology Analysis of Diesel Exhaust Particles
Junho Hyun, Ali Mohamadi Nasr, Nak-Kyoung Choi, Dongho Park & Jungho
Hwang
To cite this article: Junho Hyun, Ali Mohamadi Nasr, Nak-Kyoung Choi, Dongho Park &
Jungho Hwang (2015) Design and Performance Test of a Lab-Made Single-Stage Low-Pressure
Impactor for Morphology Analysis of Diesel Exhaust Particles, Aerosol Science and Technology,
49:10, 895-901, DOI: 10.1080/02786826.2015.1081669
To link to this article: http://dx.doi.org/10.1080/02786826.2015.1081669
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Design and Performance Test of a Lab-Made Single-Stage
Low-Pressure Impactor for Morphology Analysis of Diesel
Exhaust Particles
Junho Hyun,
1
Ali Mohamadi Nasr,
2
Nak-Kyoung Choi,
3
Dongho Park,
4
and Jungho Hwang
1,2
1
Graduate Program in Clean Technology, Yonsei University, Seodaemoon-gu, Seoul, Korea
2
Department of Mechanical Engineering, Yonsei University, Seodaemoon-gu, Seoul, Korea
3
RAC Product Development, LG Electronics, Seongsan-gu, Changwon, Korea
4
Korea Institute of Industrial Technology, Ipjang-myeon, Seobuk-gu, Cheonan, Korea
A serial method is described for estimating the particle
effective density and dynamic shape factor of particles, i.e., diesel
exhaust particles (DEPs). For this purpose, we designed a single
stage low-pressure impactor with a cutoff diameter of 130 nm.
The collection efficiency curve of the impactor was obtained using
mobility-classified sodium chloride (NaCl) particles as a function
of the mobility diameter. Then by converting the mobility
diameter of the NaCl particle into the aerodynamic equivalent
diameter, the efficiency curve can be expressed as a function of
the aerodynamic diameter. We also obtained the efficiency curve
numerically by using a commercial computational fluid dynamics
software package. After confirming the design and performance
of the impactor (experimentally 135 nm and numerically 137 nm
of cutoff diameter), we measured the currents carried by
mobility-classified DEPs downstream and upstream of the
impactor so that the collection efficiency value for DEP could
be obtained at each mobility diameter of DEPs. By making this
value equal to that of the efficiency curve, the relationship
between the mobility diameter of DEPs and the aerodynamic
diameter was obtained; this enabled us to determine the effective
density and dynamic shape factor of DEPs. The effective density
decreased from 1.06 to 0.51 g/cm
3
and the dynamic shape factor
increased from 1.28 to 1.64 as the particle size increased from 60
to 105 nm, regardless of the engine type or operating conditions.
1. INTRODUCTION
Particulate emissions from diesel vehicles are a major
source of ultrafine particles in the atmosphere. The size distri-
bution of diesel exhaust particles (DEPs) differs for different
engines and operation conditions, but the typical mean mobil-
ity diameter of DEPs is 50–100 nm (Burtscher 2005; Kittelson
et al. 2006; Asbach et al. 2009). DEPs are chain-shaped
agglomerates, similar to other combustion-generated particles.
The structure of particles is directly related to their transport
properties and plays an important role in determining their
deposition pattern in the human respiratory system. Particles
can be characterized by many different properties such as
their number concentration, mass concentration, particle size,
density, dynamic shape factor, etc. The effect of an irregular
shape on the drag force of a particle is expressed as the
dynamic shape factor. It is defined as the ratio of the actual
resistance force of the nonspherical particle to the resistance
force of a sphere having the same volume and velocity as the
nonspherical particle (Hinds 1999). Particle density is also an
important property for aerosol particles. For example, know-
ing the particle density is required to determine the relation-
ship between the Stokes and aerodynamic diameters and to
convert the measured number concentration into the mass
concentration, which is used in governmental regulation. The
particle effective density is a parameter that describes the
combined effects of the particle density and shape upon aero-
sol motion.
To measure the particle effective density, it is typical to use
two instruments connected in series. Kelly and McMurry
(1992) introduced the basic approach used in this methods and
utilized this technique to analyze lab-generated particles. They
used a differential mobility analyzer (DMA) and a microori-
fice uniform deposit impactor (MOUDI) with known cutoff
diameters. Additionally, McMurry et al. (2002) and Park et al.
(2003) measured the effective density of atmospheric aerosols
and DEPs, respectively; they classified particles with a DMA
and then measured the particle mass with an aerosol particle
mass analyzer (APM). For each APM voltage, the particle
number concentration was measured with a condensation
nucleus counter, and the peak APM voltage, which corre-
sponds to the peak mass for the mobility-selected particles,
was determined. The particle effective density was obtained
by carrying out sequential measurements on atmospheric aero-
sols (or diesel particles) and polystyrene spheres of the same
Received 9 June 2015; accepted 30 July 2015.
Address correspondence to Jungho Hwang, Department of Mechani-
cal Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-
gu, Seoul 120-749, Korea. E-mail: hwangjh@yonsei.ac.kr
895
Aerosol Science and Technology, 49:895–901, 2015
Copyright ÓAmerican Association for Aerosol Research
ISSN: 0278-6826 print / 1521-7388 online
DOI: 10.1080/02786826.2015.1081669
Downloaded by [Yonsei University] at 06:14 16 September 2015
mobility size. Olfert et al. (2007) used a Couette centrifugal
particle mass analyzer (CPMA), which is similar to the APM
but with an improved transfer function, to measure the effec-
tive density of particles emitted from a light-duty diesel vehi-
cle. Malloy et al. (2009) used an APM and a scanning
mobility particle sizer (SMPS) for the density analysis of sec-
ondary organic aerosol density evolution.
In the studies mentioned above, the effective density was
determined by the particle mass that was measured. However,
the effective density can also be calculated once the particle
mobility diameter and the aerodynamic diameter have been
determined. An electrical low-pressure impactor (ELPI),
which measures the aerodynamic particle diameter, can also
be used (instead of APM or CPMA). Thus, Virtanen et al.
(2002) used a DMA and an ELPI in experiments using three
different engine speeds and two types of diesel fuel with dif-
ferent sulfur contents. Maricq and Xu (2004) studied the effec-
tive density of diesel particles by using both a DMA and an
ELPI. They classified particles with a DMA and then mea-
sured the aerodynamic size of the classified particles with an
ELPI. Van Gulijk et al. (2004) also used a DMA and an ELPI
and verified that the estimated mobility diameter based on the
aerodynamic diameter gave a better indication of the apparent
particle size compared to the results of aerodynamic particle
size analysis based on scanning electron microscopy (SEM).
In addition, Tavakoli and Olfert (2014) classified the aerody-
namic diameter using an aerodynamic aerosol classifier and
then measured the mobility diameter of the classified particles
with a DMA. The effective density was obtained with the
mobility diameter and the corresponding aerodynamic
diameter.
In the present work, a single-stage impactor was designed
and fabricated based on classical impactor design theory. For
performance evaluation, the collection efficiency curve of the
impactor was obtained using mobility-classified sodium chlo-
ride (NaCl) particles as a function of their mobility diameter.
Then by converting the mobility diameter of NaCl particles
into the aerodynamic equivalent diameter with the known
dynamic shape factor and particle density of NaCl particles of
1.08 and 2.165 g/cm
3
, respectively, the efficiency curve was
transformed as a function of the aerodynamic diameter. After-
ward, the currents carried by the mobility-classified DEPs
downstream and upstream of the impactor were measured, and
the collection efficiency value for DEPs was calculated at each
mobility diameter. Hence, the relationship between the mobil-
ity diameter of DEPs and the aerodynamic diameter could be
obtained, enabling the determination of the effective density
and dynamic shape factor of DEPs.
In addition to evaluating the performance of impactors
experimentally, numerical methods have also been used; these
allow parameters to be changed more easily. Thus, Huang and
Tsai (2001) investigated the effect of gravity upon the particle
collection efficiency of a single round-nozzle inertial impactor.
They also investigated the influence of the impaction plate
diameter and particle density upon the particle collection effi-
ciency (Huang and Tsai 2002). Gulak et al. (2009) used a sin-
gle jet model to analyze the performance of an Andersen
cascade impactor by using ANSYS FLUENT, which is a com-
mercial computational fluid dynamics software package, and
compared the results to experimental data obtained by
Vaughan (1989), Nichols et al. (1998), Srichana et al. (1998)
and Zhou et al. (2007). Regarding low-pressure impactor per-
formance, Leduc et al. (2006) studied an ELPI with laminar
and different kinds of turbulent models by using ANSYS FLU-
ENT; however, their simulations were not accurate for nano-
particles. They insisted that simulations are not accurate for
flow velocities above 50 m/s. Arffman et al. (2011) also stud-
ied low-pressure ELPI stages based on time-averaged flow
field simulation with ANSYS FLUENT and used Matlab for
particle trajectories. They concluded that equations based on
the time-averaged Navier–Stokes equations could not be used,
and that the effect of turbulent dispersion must be taken into
account if the local Reynolds number exceeds 1800.
In this article, we designed a single stage low-pressure
impactor and carried out performance tests with NaCl par-
ticles. Numerical calculations were also conducted to predict
the performance of our impactor. Then, we applied a serial
method to estimate the effective density and dynamic shape
factor of DEP emitted from two different engines.
2. EFFECTIVE DENSITY AND SHAPE FACTOR
To calculate the effective density, the following equation is
needed (Maricq et al. 2000; Park et al. 2003):
reff DrH2O
d1
aCa
d2
mcm [1]
where d
a
and d
m
are the aerodynamic diameter and mobility
diameter, respectively; C
a
and C
m
are the slip correction fac-
tors for the aerodynamic diameter and mobility diameter,
respectively; rH2ois the density of water (1.0 g/cm
3
); and reff
is the effective density. reff is expressed as follows (Kelly and
McMurry 1992; Schumid et al. 2007):
reff Drp
dve
dm
3
;[2]
where r
p
is the bulk density of a particle and d
ve
is the volume
equivalent diameter.
In this study, the mobility diameter and the aerodynamic
diameter were determined via DMA and impactor measure-
ments, respectively. Then, the effective density was calculated
with Equation (1). Using this calculated effective density, we
estimated the volume equivalent diameter with Equation (2)
and calculated the shape factor by using Equation (3) (Kelly
896 J. HYUN ET AL.
Downloaded by [Yonsei University] at 06:14 16 September 2015
and McMurry 1992; Schmid et al. 2007):
xDCve
cm
rp
reff
1=2
[3]
where xis the dynamic shape factor and C
ve
is the slip correc-
tion factor for the volume equivalent diameter of the particle.
3. DESIGN OF THE IMPACTOR
Our impactor was designed based on the procedure
described by Hillamo and Kauppinen (1991). Additional
details are provided in the online supplementary information
(SI). In the present study, the mass flow rate of air and nozzle
diameter were assumed and then the flow velocity was calcu-
lated with an assumed value of pressure pby using Equation
(S2). An additional flow velocity, based on the assumed value
of p, was calculated with Equations (S3) and (S4). The value
of pressure pwas iterated until the error between these two cal-
culated flow velocities was minimized. Once the proper flow
velocity was determined, the cutoff diameter was calculated
based on the assumed nozzle diameter by using Equation (S6).
The calculation process was repeated until the cutoff diameter
was 130 nm (aerodynamic diameter), which is the design
value of this study. This value of 130 nm was determined by
using Equation (1) and was based on the selected mobility
diameter of 180 nm and the effective density of 0.6 (Olfert
et al. 2007). In this study, the mass flow rate of air was 5.0 £
10
¡5
kg/s, the number of nozzles was 1, the nozzle diameter
was 0.56 mm, and the jet-to-plate distance was 0.86 mm.
Therefore, for the selected cutoff diameter (aerodynamic
diameter of 130 nm), the calculated flow velocity, pressure,
and temperature at the exit of the nozzle were 348 m/s,
44.2 kPa, and 232 K, respectively.
4. METHODS
4.1. Performance Test of the Impactor
An impactor made of stainless steel was fabricated with the
design parameters presented in Section 3. The impactor was
cylindrically shaped, with an outer diameter of 29 mm and a
total height of 55 mm. The radius of the impaction plate was
3.8 mm. A reservoir was located before the nozzle to form the
stagnation state (Figure S1). The pressure difference between
the inlet and the outlet of the impactor was measured with a
pressure transmitter (DPLH0100R, Sensys, Korea).
The experimental setup for generating the test aerosol particles
is shown in Figure 1. Compressed air (0.6 MPa) was used as a car-
rier gas, once oil droplets, moisture, and contamination particles
were removed by a clean air supply system (which consisted of an
oil trap, a diffusion dryer, and a HEPA filter). For the test particles,
we used NaCl particles that were generated from an electrically
heated tube furnace (GTF12/25/364, Lenton Furnaces, UK). The
heating length and inner diameter of the ceramic tube were
364 mm and 25.4 mm, respectively. The furnace temperature was
controlled to be between 700 and 900C. The NaCl particles were
passed through a charge neutralizer (Soft X-ray charger 4530,
HCT, Korea) to obtain a Boltzmann charge distribution. The test
particles exiting the neutralizer were classified according to the
required mobility sizes with a DMA (3081, TSI, MN, USA). Test
particles exiting the DMA were diluted with clean air (2.3 L/min)
before entering the impactor. Another branch of test particles,
classified with the DMA, were delivered to a condensation particle
counter (CPC, 3022A, TSI, MN, USA) for size distribution mea-
surement. An aerosol electrometer (3068A, TSI, MN, USA) was
used to measure the current carried by the particles that were clas-
sified with the DMA. The collection efficiency of the impactor
(h(d
m
)) was calculated as
h.dm/D1¡Idown.dm/
Iup.dm/;[4]
FIG. 1. Schematics of experimental apparatus for (a) performance test and (b) diesel engine test.
LOW-PRESSURE IMPACTOR FOR MORPHOLOGY ANALYSIS OF DEPS 897
Downloaded by [Yonsei University] at 06:14 16 September 2015
where I
down
and I
up
are the currents measured downstream and
upstream of the impactor, respectively. Because the impactor
was designed based on the aerodynamic diameter size scale, the
following equations were used to convert the mobility diameter
into its corresponding aerodynamic diameter:
Cve
dvex
DCm
dm
;[5]
rpdm2Cm
dve
dm
3
DrH2Oda2Ca:[6]
Equation (5) was derived from Equations (2) and (3), while
Equation (6) was derived from Equations (1) and (2). The
NaCl particles were cubic (xD1.08) and had a density of
2.163 g/cm
3
(Perry and Green 1999). A theoretical assessment
of the impactor performance was also carried out by a numeri-
cal procedure (see the SI).
4.2. Morphology Analysis of Diesel Exhaust Particles
Two diesel engines were tested in this study. Engine 1 was
a four-cylinder single overhead camshaft diesel engine running
at an idling mode of 1500 revolutions per minute (RPM) with
no load. Engine 2 was a single-cylinder common rail direct
injection diesel engine running at an idling mode of 1500
RPM, with a load of 75%. Table 1 lists the specifications of
the test engines.
The experimental setup used to measure DEPs is illustrated
in Figure 1. A dilution system (MD19-2E, Matter Eng., Swit-
zerland) was used to reduce the emitted DEPs number concen-
tration, because it is typically very high (>10
12
particles/cm
3
)
(Hueglin et al. 1997). A SMPS consisting of a DMA and a
CPC was used to measure the size distribution of the diluted
DEPs aerosols. For a specific mobility diameter, the particles
classified by the DMA passed through the impactor before
approaching the aerosol electrometer. Using Equation (4), the
collection efficiency value of the impactor for DEPs was
obtained at each mobility diameter of DEPs. By setting this
value equal to that of the efficiency curve determined with the
NaCl particles, the relationship between the mobility diameter
of DEPs and the aerodynamic diameter was obtained. Finally,
the effective density and the dynamic shape factor of DEPs
were calculated using Equations (1)–(3).
5. RESULTS AND DISCUSSION
5.1. Performance Tests
To evaluate the collection efficiency of the impactor experi-
mentally, NaCl particles with two different size distributions
were used. One size distribution was chosen such that its aero-
dynamic mean diameter was smaller than the design cutoff
diameter (130 nm) and the other was chosen to have an aero-
dynamic mean diameter larger than the design value. The geo-
metric mean diameters (mobility diameters) of these two
particle distributions were 85.1 and 113.4 nm, respectively,
corresponding to aerodynamic diameters of 120.86 and
199.3 nm. The experimentally obtained cutoff aerodynamic
diameter was 135 nm (Figure 2), which matched the design
value of 130 nm well. The collection efficiency data were fit-
ted using the Boltzmann sigmoidal function defined by
Demokritou et al. (2004):
h.da/Da1
1Cexp[¡da¡d50
b]
Ca2[7]
where bis the width of fitting and a
1
and a
2
are the coefficients
calculated by regression. These variables were determined by
Excel solver to minimize the sum of the squares of the errors.
In this study, the values for a
1
,a
2
, and bwere 0.923, 0.088,
and 19.2, respectively. Then by putting aerodynamic diameters
into the fitting equation, the correlation coefficient (r
2
)
between the collection efficiency data points and calculated
ones obtained from the fitting equation was determined as 0.99
from Excel regression analysis. Figure 2 also shows the col-
lection efficiency, which was determined by means of numeri-
cal calculation. The numerically calculated cutoff diameter
was 137 nm, which also matched the design value well.
TABLE 1
Specifications of test engines 1 and 2
Engine 1 Engine 2
Engine model 2.2 MAGMA diesel engine RSI 090 research engine
Engine type Four-cylinder Single-cylinder
Injection system Mechanical fuel injection Common rail diesel injection
Bore £stroke (mm) 86 £94 83 £92
Displacement volume (L) 2.184 0.498
Stroke £bore (mm) 94 £86 92 £83
Compression ratio 22.9:1 19.5:1
898 J. HYUN ET AL.
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5.2. Effective Density and Dynamic Shape Factor of
Diesel Exhaust Particles
The size distributions of the DEPs are shown in Table 2.
The correlation between the mobility diameter and the aerody-
namic diameter is shown in a double-logarithmic plot (Fig-
ure 3). By comparing our results with those of Van Gulijk
et al. (2004), we determined that the correlation was similar
even though a lab-made impactor was used in this study. The
effective density distributions shown in Figure 4 were calcu-
lated for Engines 1 and 2 by using Equation (1). We found
that the effective density of Engine 1 decreased from 0.82 to
0.53 g/cm
3
as the aerodynamic diameter increased from 83 to
105 nm, whereas the effective density of Engine 2 decreased
from 1.06 to 0.51 g/cm
3
as the aerodynamic diameter
increased from 60 to 101 nm. These results agreed well with
those obtained in previous studies (Park et al. 2003; Maricq
and Xu 2004; Olfert et al. 2007).
After the effective density was determined by using Equa-
tion (1), the dynamic shape factor was determined with Equa-
tions (2) and (3). In these calculations, the bulk density of
DEPs was assumed to be 1.77 g/cm
3
, which is very close to
that of amorphous elemental carbon, 2.0 g/cm
3
(Park et al.
2004; Burtscher 2005). As the aerodynamic diameter
increased from 60 to 105 nm (Figure 5), the dynamic shape
factor increased from 1.40 to 1.62 for Engine 1 and from 1.28
to 1.64 for Engine 2. Park et al. (2004) used transmission elec-
tron microscopy to study the structural properties of diesel par-
ticles that ranged in size from 59 to 133 nm; they reported
dynamic shape factors ranging from 1.11 to 2.21.
The mass-mobility exponent was estimated as
reff /dDf¡3
m;[8]
where D
f
is the mass-mobility exponent (Tavakoli and
Olfert 2014). For Engine 1, the fractal dimension was 2.28,
and that of Engine 2 was 2.34. These values agreed well with
previous reports that the fractal dimensions of DEPs were
between 2 and 3 (Virtanen et al. 2002; Park et al. 2003).
So far, a serial method for estimating the effective density
and dynamic shape factor of DEPs has been presented in the
following way: (1) obtain the collection efficiency curve of the
impactor as a function of the aerodynamic diameter by using
TABLE 2
Size distributions of particles from test engines (number of repetition D3)
Engine 1 Engine 2
Geometric mean diameter (nm) 92 221.1
Geometric standard deviation 1.47 1.52
Total number concentration (particles/cm
3
) 2.18 £10
6
1.78£10
6
FIG. 2. Experimental and simulated collection efficiencies of the impactor for
NaCl particles.
FIG. 3. Log-log plot of mobility diameter versus aerodynamic diameter.
LOW-PRESSURE IMPACTOR FOR MORPHOLOGY ANALYSIS OF DEPS 899
Downloaded by [Yonsei University] at 06:14 16 September 2015
NaCl particles, DMA, and an aerosol electrometer; (2) find the
relationship between the mobility diameter of DEPs and the
aerodynamic diameter; and (3) apply the equations introduced
in this article to calculate the effective density and dynamic
shape factor of DEPs.
However, using this method, potential sources of uncer-
tainty can be encountered. These include electrical noise in
the aerosol electrometer, diffusional and electrostatic wall
loss of particles in the impactor, and multiply charged par-
ticles exiting from the DMA. Details of uncertainties were
stated in the SI.
6. CONCLUSIONS
In this study, a single-stage low-pressure impactor with a
cutoff diameter of 130 nm was designed. The performance of
this impactor was tested both numerically and experimentally
with NaCl particles. The collection efficiency curve of the
impactor provided the aerodynamic diameter of particles with
a certain mobility diameter. From this relationship, the correla-
tion between the mobility diameter and the aerodynamic diam-
eter was obtained. The impactor was used in series with a
DMA to estimate the effective density and dynamic shape fac-
tor of DEPs emitted from two diesel engines. The effective
density decreased from 1.06 to 0.51 g/cm
3
as the particle size
increased from 60 to 105 nm, regardless of the engine type or
operating conditions.
FUNDING
This research is supported by the Korean Ministry of
Environment as “The Converging Technology Program.”
SUPPLEMENTAL MATERIAL
Supplemental data for this article can be accessed on the
publisher’s website.
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LOW-PRESSURE IMPACTOR FOR MORPHOLOGY ANALYSIS OF DEPS 901
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