- Email: [email protected]
Collection efficiency of round-nozzle impactors with horizontal annular inlet
Journal of Aerosol Science 74 (2014) 63–69
Contents lists available at ScienceDirect
Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci
Collection efficiency of round-nozzle impactors with horizontal annular inlet Moon-Kyung Kim, Won-Geun Kim, Kwan-Soo Lee, Se-Jin Yook n School of Mechanical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
abstract
Article history: Received 14 October 2013 Received in revised form 26 February 2014 Accepted 18 April 2014 Available online 24 April 2014
In this study, a horizontal annular inlet was applied to a round-nozzle impactor in an effort to reduce the cut-off diameter. The collection efficiencies of round-nozzle impactors with either a vertical inlet, which has conventionally been used, or a horizontal annular inlet were investigated both numerically and experimentally. For the comparison, the nozzle width, nozzle-to-plate distance, impaction plate diameter, and flow rate of the aerosol drawn into the impactor were set the same for both the vertical inlet impactor and horizontal annular inlet impactor. A parametric study was performed to analyze the effects of the width of the horizontal annular inlet and nozzle throat length on the collection efficiency of round-nozzle impactors with nozzle Reynolds numbers ranging from 500 to 2600. By replacing the vertical inlet with the horizontal annular inlet, the square root of the Stokes number corresponding to the cut-off diameter could be reduced by approximately 30%. In other words, the value of (Stk50)1/2 was decreased from 0.49 to 0.32 or 0.33 when considering the effect of gravity. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Inertial impactor Collection efficiency Stokes number Round nozzle Horizontal annular inlet
1. Introduction Inertial impactors are widely used to collect atmospheric aerosol particles because of their transportability and easy operation. In an inertial impactor, aerosol is accelerated through a nozzle and impinges on an impaction plate, which cause the aerosol flow to abruptly change direction above the impaction plate. Then, the particles with inertia exceeding a certain value deviate from the streamline and deposit on the impaction plate, whereas the particles with less inertia remain airborne and exit the impactor stage. In this way, impactors can selectively collect aerosol particles according to their inertia. Many studies have been performed to enhance the collection efficiency of inertial impactors. The effects of various parameters such as the nozzle width, nozzle throat length, nozzle-to-plate distance, impaction plate size, and aerosol flow rate on the impactor collection efficiency have been investigated (Rader and Marple, 1985; Huang and Tsai, 2002; Grinshpun et al., 2005). The surface of an impaction plate was coated with grease or oil to reduce particle bounce (Rao and Whitby, 1978; Lee et al., 2005). The shape of the impaction plate was modified to increase the impactor collection efficiency under a heavy loading condition (Tsai and Cheng, 1995). Porous metal substrates were applied to the impaction plates to increase the collection efficiency for particles smaller than the cut-off size (Huang et al., 2001). Thermophoresis was applied by cooling the impaction plate to increase the impactor collection efficiency (Lee and Kim, 2002). Electrophoresis was employed to enhance the impactor collection efficiency for charged particles (Vinchurkar et al., 2009). The impactor stages were operated
n
Corresponding author. Tel.: þ82 2 2220 0422; fax: þ 82 2 2220 2299. E-mail address: [email protected] (S.-J. Yook).
http://dx.doi.org/10.1016/j.jaerosci.2014.04.007 0021-8502/& 2014 Elsevier Ltd. All rights reserved.
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M.-K. Kim et al. / Journal of Aerosol Science 74 (2014) 63–69
Fig. 1. Schematic of round-nozzle impactor with vertical inlet.
Fig. 2. Schematic of round-nozzle impactor with horizontal annular inlet.
at low pressures to decrease the cut-off size (Arffman et al., 2011). Elliptical concave impaction plates were used to increase the impactor collection efficiency and reduce the particle bounce (Kim and Yook, 2011; Kim et al., 2013). Even though much effort has gone into enhancing the impactor collection efficiency by modifying the geometries of the inertial impactors, changing the shape or surface condition of the impaction plate, or altering the operating environment for inertial impactors, few studies have considered modifying the impactor inlet shape to enhance the impactor collection efficiency. With round-nozzle impactors, when a particle is positioned closer to the centerline of the impactor nozzle, it has a higher probability of being collected on the impaction plate, as shown in Fig. 1. In this study, an impactor nozzle with a horizontal annular inlet was considered to allow aerosol particles to concentrate close to the centerline of the nozzle.
2. Numerical Figure 1 shows a schematic of a round-nozzle impactor with a vertical inlet, which has been commonly used to sample aerosol particles. The nozzle width is denoted as W, and the nozzle throat length is C. The nozzle-to-plate distance is S, and the impaction plate diameter is D. Figure 2 shows a schematic of a round-nozzle impactor with a horizontal annular inlet. The horizontal annular inlet was designed to allow aerosol particles to concentrate close to the centerline as a result of centrifugal force before the particles are accelerated through the nozzle. The height of the horizontal annular inlet is denoted as A, and the width of the horizontal annular inlet is B.
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The flow field in a round-nozzle impactor was simulated using the commercial code ANSYS FLUENT v13.0. The Reynolds number at the nozzle was varied in the range of 500–2600. Because the impactor collection efficiency is known to be influenced by gravity for Reynolds numbers smaller than 1500 (Huang and Tsai, 2001), the gravitational force was considered in all the simulation cases of this study. The flow was assumed to be two-dimensional axisymmetric, steady, incompressible, and laminar for nozzle Reynolds numbers smaller than 3000 (Huang and Tsai, 2002). The operating temperature and pressure were set as 20 1C and 101.3 kPa, respectively. Boundary conditions were imposed as shown in Figs. 1 and 2, and included the velocity inlet at the inlet of the impactor, pressure outlet at the outlet of the impactor, axisymmetric condition at the center of the impactor, and no-slip condition on all the walls of the impactor. A grid test was performed to select the optimum mesh, and the quad-cell numbers selected ranged from approximately 25,000 to 100,000 depending on the nozzle width. The continuity, momentum, and energy equations were iteratively solved using a convergence criterion of 10 6. The SIMPLE algorithm was used to couple the velocity and pressure. After solving the flow field, the trajectories of particles were simulated using the discrete phase models (DPM) incorporated in the FLUENT software. In order to calculate the particle trajectories, the Brownian force, gravitational force, and Stokes' drag force with slip correction were considered (FLUENT 6.3 User's Guide, 2006, chap. 22). The collection efficiency (η) of the round-nozzle impactor was predicted based on the statistical Lagrangian particle-tracking approach (Yook et al., 2007; Kim et al., 2011, 2013; Lee et al., 2011, 2012a,b; Kim and Yook, 2011; Woo et al., 2012). The particles were positioned on the velocity-inlet plane at constant intervals and injected with the same velocity as the inlet flow velocity. By considering polystyrene latex (PSL) spheres, the particle density was set at 1050 kg/m3. The influence of the number of injected particles (Nin) on the collection efficiency of round-nozzle impactors was checked, and Nin ranged from approximately 500 to 5000 depending on the impactor size. The particles were assumed to be permanently collected when they touched any wall of the inertial impactor. Two different formulae were used to predict the collection efficiency. In the case of the round-nozzle impactor with the vertical inlet (see the schematic in Fig. 1), the volumetric aerosol flow rate represented by each particle positioned on the velocity-inlet plane (where the particles were positioned at constant intervals) varied depending on the particle's radial position. Thus, the collection efficiency was calculated as follows (Kim and Yook, 2011): η¼
N in
2Δr
∑ ri f i ; R2in i ¼ 1
ð1Þ
where Δr [¼ Rin/(Nin 1)] is the constant radial interval between the particles positioned at the velocity inlet; Rin is the radius of the velocity inlet; ri is the radial position of each particle at the velocity inlet; and fi is a number used to describe the fate of each particle, where fi ¼1 if the ith particle is collected on the impaction plate and fi ¼0 if it is not collected. In the case of the round-nozzle impactor with the horizontal annular inlet (see the schematic in Fig. 2), the same volumetric flow rates were found for the particles positioned on the velocity-inlet plane at constant intervals, with no differences in regard to the particle position at the entrance of the horizontal annular inlet. Therefore, the collection efficiency was estimated by simply counting the number of particles deposited on the impaction plate (Nde), that is η¼
Nde : Nin
ð2Þ
3. Experimental Round-nozzle impactors with either a vertical inlet or horizontal annular inlet were manufactured in order to validate the numerical model. Table 1 lists the dimensions of the round-nozzle impactors used for the experiments. Here, the nozzle width, W, was determined from the following equation: 2
Stk ¼
ρp dp C c U 9μW
;
ð3Þ
Table 1 Dimensions of round-nozzle impactors with either vertical inlet or horizontal annular inlet used for experiments. Parameter
W [mm] S [mm] C [mm] D [mm] A [mm] B [mm]
PM1 nozzle
PM2.5 nozzle
Vertical inlet
Horizontal annular inlet
Vertical inlet
Horizontal annular inlet
0.9 3.8 1.0 6.0 – –
0.9 3.8 5.4 6.0 0.9 4.0
1.6 5.4 1.6 6.0 – –
1.6 5.4 9.6 6.0 1.6 3.7
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Fig. 3. Schematic of experimental setup.
where Stk is the Stokes number, ρp is the particle density, dp is the particle diameter, Cc is the slip correction factor, U is the average jet velocity through the nozzle, and μ is the dynamic viscosity of air. For round-nozzle impactors, the square root of the Stokes number corresponding to the cut-off diameter is known to be 0.49, that is, (Stk50)1/2 ¼0.49 (Hinds, 1999), where the subscript “50” implies a 50% collection efficiency. Therefore, the PM1 nozzle width of 0.9 mm was determined from Eq. (3) by considering the cut-off diameter, dp50, of 1 μm, and the PM2.5 nozzle width of 1.6 mm was determined by taking into account the cut-off diameter of 2.5 μm, at a temperature of 20 1C and pressure of 101.3 kPa. The flow rate of the aerosol drawn into each impactor, Qin, was 1.1 L/min. Therefore, the average jet velocity, U, was estimated to be 28.82 m/s through the PM1 nozzle and 9.12 m/s through the PM2.5 nozzle, resulting in nozzle Reynolds numbers of 1718 and 966, respectively. According to Huang and Tsai (2002), the effect of the impaction plate diameter on the Stokes number for the cut-off size could be negligible, if D 43W. Therefore, an impaction plate diameter of 6 mm was used for all the impactors manufactured for the experiments. Figure 3 shows a schematic of the experimental setup for measuring the collection efficiencies of the round-nozzle impactors with either a vertical inlet or horizontal annular inlet. The impaction plate was coated with vacuum grease to reduce particle bounce. PSL spheres (Thermo Scientific) were aerosolized using an atomizer, and then dried using a diffusion-dryer. The monodisperse particle diameters ranged from 0.4 to 4.0 μm. The flow rate of the aerosol introduced into the round-nozzle impactor was 1.1 L/min. A mesh plate was used in a particle disperser to evenly distribute the particles upstream of the round-nozzle impactor with either the vertical inlet or horizontal annular inlet. A three-way valve was used to allow an optical particle counter (OPC, HCT model PS-3030) to measure the particle number concentration upstream or downstream of the impactor. The impactor collection efficiency was experimentally evaluated as η¼
C up C down ; C up
ð4Þ
where Cup is the upstream particle number concentration and Cdown is the downstream particle number concentration. When the impaction plate was not installed, the collection efficiencies were evaluated to be smaller than 1% for the vertical inlet impactor and smaller than 2% for the horizontal annular inlet impactor. Therefore, the particle loss on surfaces other than the impaction plate was assumed to be negligible for the particle sizes considered in the experiments.
4. Results and discussion Figure 4 shows a comparison of the numerical results and experimental data for the collection efficiencies of roundnozzle impactors with either a vertical inlet or horizontal annular inlet, when W, S, C, D, A, and B were set as listed in Table 1. The aerosol flow rate was 1.1 L/min. The numerically simulated collection efficiencies agreed well with the experimental data, which validated the correctness of the numerical method employed in this study. When the vertical inlet was used, the cut-off diameters were evaluated to be 1.0 μm for the impactor with a W value of 0.9 mm (PM1 nozzle) and 2.5 μm for the impactor with a W value of 1.6 mm (PM2.5 nozzle). When the horizontal annular inlet was employed, the cut-off diameters were estimated to decrease to 0.8 μm for the impactor with the PM1 nozzle and 2.0 μm for the impactor with the PM2.5 nozzle, which meant that the cut-off diameter of the round-nozzle impactor could be remarkably reduced by replacing the vertical inlet with the horizontal annular inlet.
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Fig. 4. Comparison of collection efficiencies between numerical results and experimental data for round-nozzle impactors with either vertical inlet or horizontal annular inlet.
Fig. 5. Comparison of collection efficiencies between vertical inlet impactor and horizontal annular inlet impactor.
Figure 5 shows the comparison of the numerically estimated collection efficiencies of the vertical inlet impactor and horizontal annular inlet impactor. The nozzle width and aerosol flow rate were determined from Eq. (3) with (Stk50)1/2 ¼0.49, and the Reynolds number was fixed at 540. Thus, W¼0.5 mm and Qin ¼0.192 L/min for the impactor with the PM1 nozzle, W¼1.2 mm and Qin ¼0.463 L/min for the impactor with the PM2.5 nozzle, W¼2.4 mm and Qin ¼0.949 L/min for the impactor with the PM5 nozzle, and W¼4.5 mm and Qin ¼1.736 L/min for the impactor with the PM10 nozzle. The ratios of S, D, A, and C to W were set as follows: S/W¼2, D/W¼8, A/W¼0.25, and C/W¼10. In contrast, the ratio of the horizontal annular inlet width to the nozzle width, B/W, was varied from 0.5 to 4. The solid lines with no symbols represent the collection efficiencies of the vertical inlet impactors, and the symbols signify the collection efficiencies of the horizontal annular inlet impactors. It should be noted that the force of gravity was considered in the simulations for both the vertical inlet impactor and the horizontal annular inlet impactor because gravity affects the impactor collection efficiency at low Reynolds numbers (Huang and Tsai, 2001). When the vertical inlet was used, the cut-off diameters were predicted to be 1.0, 2.5, 5.0, and 10.0 μm for the PM1, PM2.5, PM5, and PM10 nozzle impactors, respectively. However, when the horizontal annular inlet was assumed instead of the vertical inlet, the cut-off diameters when considering the force of gravity were estimated to be reduced by approximately 30%, even though the nozzle width and aerosol flow rate were unchanged. The collection efficiencies of the horizontal annular inlet impactors with the PM1, PM2.5, and PM5 nozzles were unaffected by the change in the B/W ratio. In the case of the horizontal annular inlet impactor with the PM10 nozzle, the collection efficiencies for particles larger than the cut-off diameter greatly decreased with increasing B/W ratio because the particle loss at the horizontal annular inlet was enhanced by gravitational settling. However, for the B/W ratios of 0.5, 1, and 2, the particle losses at the horizontal annular inlet were negligible. Figure 6 shows the variation in the square root of the Stokes number corresponding to the cut-off diameter according to the C/W ratio, for the round-nozzle impactors with a horizontal annular inlet. The nozzle width and aerosol flow rate were the same as those used for Fig. 5. The fixed ratios were S/W¼2, D/W¼8, A/W¼0.25, and B/W¼2. The C/W ratio was varied from 2 to 18. The solid line represents the reference value of (Stk50)1/2 for the round-nozzle impactor with a vertical inlet, that is, 0.49 (Hinds, 1999). When the horizontal annular inlet was assumed, the values of (Stk50)1/2 for the horizontal annular inlet impactors with the PM1, PM2.5, PM5, and PM10 nozzles were much smaller than the reference value of 0.49, which
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Fig. 6. Square root of Stokes number for 50% collection efficiency of round-nozzle impactor with horizontal annular inlet as function of C/W ratio.
Fig. 7. Collection efficiencies of round-nozzle impactors with horizontal annular inlet for PM1, PM2.5, PM5, and PM10 nozzles, for which widths and aerosol flow rates were determined from Eq. (3) with (Stk50)1/2 ¼ 0.49.
Table 2 Nozzle widths, aerosol flow rates, and nozzle Reynolds numbers for horizontal annular inlet impactors considered in parametric study. Horizontal annular inlet impactor
W [mm]
Qin [L/min]
Nozzle Reynolds number
PM1 nozzle
0.5 0.84 1.04
0.192 0.945 1.917
540 1575 2580
PM2.5 nozzle
1.2 2.0 2.55
0.463 2.250 4.699
540 1575 2580
PM5 nozzle
2.4 4.0 5.2
0.949 4.395 9.656
540 1575 2580
PM10 nozzle
4.5 7.8 10.0
1.736 8.775 18.429
540 1575 2580
meant that the horizontal annular inlet was very useful for reducing the cut-off diameter. As the C/W ratio increased, the value of (Stk50)1/2 decreased. This was because a flow developed at the nozzle throat. Consequently, with an increase in the nozzle throat length, there was an increase in the flow velocity near the centerline, toward which the aerosol particles with sufficient inertia were concentrated as a result of using the horizontal annular inlet. However, when the C/W ratio was higher than 10, (Stk50)1/2 converged to a certain value within an approximate range of 0.32–0.33, for the round-nozzle impactors with the PM1, PM2.5, PM5, and PM10 nozzles. Figure 7 shows the collection efficiencies of the round-nozzle impactors with a horizontal annular inlet, where the nozzle width and aerosol flow rate were determined from Eq. (3) with (Stk50)1/2 ¼0.49. Table 2 lists the values of W and Qin, considered
M.-K. Kim et al. / Journal of Aerosol Science 74 (2014) 63–69
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for the parametric study, corresponding to the nozzle Reynolds numbers in the range of 500–2600. The ratios were fixed at S/ W¼2, D/W¼8, A/W¼ 0.25, B/W¼2, and C/W¼10. The collection efficiencies fell on one curve for each PM1, PM2.5, PM5, or PM10 nozzle case, in the tested range of nozzle Reynolds numbers, which implied that the effects of the horizontal annular inlet dimensions on the collection efficiency of a round-nozzle impactor could be characterized using the square root of the Stokes number. When the horizontal annular inlet was assumed to be used instead of the vertical inlet, the cut-off diameters when considering the force of gravity were estimated to decrease from 1 to 0.67 μm for the impactor with the PM1 nozzle, from 2.5 to 1.72 μm for the impactor with the PM2.5 nozzle, from 5 to 3.4 μm for the impactor with the PM5 nozzle, and from 10 to 6.54 μm for the impactor with the PM10 nozzle. A drop in the collection efficiency was anticipated for particles larger than approximately 8 μm, mainly as a result of the gravitational settling of particles in the horizontal annular inlet region. 5. Conclusions In this study, the collection efficiencies of round-nozzle impactors with a horizontal annular inlet were investigated both numerically and experimentally. A comparison between the numerical results and experimental data proved that the numerical method employed in this study could correctly predict the collection efficiency of a round-nozzle impactor with either a vertical inlet or horizontal annular inlet. For the comparison of the collection efficiencies of the vertical inlet impactor and horizontal annular inlet impactor, the same nozzle widths and aerosol flow rates were used for nozzle Reynolds numbers in the range of 500–2600. Based on a parametric study with constant ratios of S/W¼2 and D/W¼8, the geometric dimensions of the horizontal annular inlet were determined to be A/W¼0.25, B/W ¼2, and C/W¼10. With these ratios, the horizontal annular inlet reduced the square root of the Stokes number for a 50% collection efficiency by approximately 30% when considering the force of gravity, that is, from 0.49, which was valid for the round-nozzle impactor with the vertical inlet, to approximately 0.32 or 0.33. As a result, the horizontal annular inlet was found to be useful for reducing the cut-off size of a round-nozzle impactor. Acknowledgment This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (NRF-2012M3A6A7054863). References Arffman, A., Marjamaki, M., & Keskinen, J. (2011). Simulation of low pressure impactor collection efficiency curves. Journal of Aerosol Science, 42, 329–340. FLUENT 6.3 User's Guide (2006). Modeling Discrete Phase. FLUENT, Inc.: Lebanon, NH. Grinshpun, S.A., Mainelis, G., Trunov, M., Gorny, R.L., Sivasubramani, S.K., Adhikari, A., & Reponen, T. (2005). Collection of airborne spores by circular singlestage impactors with small jet-to-plate distance. Journal of Aerosol Science, 36, 575–591. Hinds, W.C. (1999). Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles 2nd ed.). John Wiley: New York(p. 126). Huang, C.H., & Tsai, C.J. (2001). Effect of gravity on particle collection efficiency of inertial impactors. Journal of Aerosol Science, 32, 375–387. Huang, C.H., & Tsai, C.J. (2002). Influence of impaction plate diameter and particle density on the collection efficiency of round-nozzle inertial impactors. Aerosol Science and Technology, 36, 714–720. Huang, C.H., Tsai, C.J., & Shih, T.S. (2001). Particle collection efficiency of an inertial impactor with porous metal substrates. Journal of Aerosol Science, 32, 1035–1044. Kim, W.K., Lee, S.C., & Yook, S.J. (2011). Numerical investigation of thermophoretic effect on particulate contamination of an inverted flat surface in a parallel airflow. Journal of the Electrochemical Society, 158, H1010–H1016. Kim, Y.J., & Yook, S.J. (2011). Enhancement of collection efficiency of inertial impactors using elliptical concave impaction plates. Journal of Aerosol Science, 42, 898–908. Kim, W.G., Yook, S.J., & Ahn, K.H. (2013). Collection efficiency of rectangular slit-nozzle inertial impactors with impaction plates of elliptical concave curvature. Aerosol Science and Technology, 47, 99–105. Lee, S.J., Demokritou, P., & Koutrakis, P. (2005). Performance evaluation of commonly used impaction substrates under various loading conditions. Journal of Aerosol Science, 36, 881–895. Lee, B.U., & Kim, S.S. (2002). The effect of varying impaction plate temperature on impactor performance: experimental studies. Journal of Aerosol Science, 33, 451–457. Lee, S.C., Kim, W.G., & Yook, S.J. (2011). Effect of plate thickness on particle deposition velocity onto a face-up flat plate situated parallel to an airflow. Journal of Applied Physics, 110, 063518. Lee, H., Yook, S.J., & Han, S.Y. (2012a). Particle deposition on face-up flat plates in parallel airflow under the combined influences of thermophoresis and electrophoresis. Journal of the Korean Physical Society, 61, 1028–1036. Lee, H., Yook, S.J., & Han, S.Y. (2012b). The effects of simultaneous electrophoresis and thermophoresis on particulate contamination of an inverted EUVL photomask surface in parallel airflow. European Physical Journal Plus, 127, 122. Rader, J.J., & Marple, V.A. (1985). Effect of ultra-Stokesian drag and particle interception on impaction characteristics. Aerosol Science and Technology, 4, 141–156. Rao, A.K., & Whitby, K.T. (1978). Non-ideal collection characteristics of inertial impactors-I. Single-stage impactors and solid particles. Journal of Aerosol Science, 9, 77–86. Tsai, C.J., & Cheng, Y.H. (1995). Solid particle collection characteristics on impaction surfaces of different designs. Aerosol Science and Technology, 23, 96–106. Vinchurkar, S., Worth Longest, P., & Peart, J. (2009). CFD simulations of the Andersen cascade impactor: model development and effects of aerosol charge. Journal of Aerosol Science, 40, 807–822. Woo, S.H., Lee, S.C., & Yook, S.J. (2012). Statistical Lagrangian particle tracking approach to investigate the effect of thermophoresis on particle deposition onto a face-up flat surface in a parallel airflow. Journal of Aerosol Science, 44, 1–10. Yook, S.J., Fissan, H., Asbach, C., Kim, J.H., Yan, P.Y., & Pui, D.Y.H. (2007). Evaluation of protection schemes for ultraviolet lithography (EUVL) masks against top-down aerosol flow. Journal of Aerosol Science, 38, 211–227.
Contents lists available at ScienceDirect
Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci
Collection efficiency of round-nozzle impactors with horizontal annular inlet Moon-Kyung Kim, Won-Geun Kim, Kwan-Soo Lee, Se-Jin Yook n School of Mechanical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
abstract
Article history: Received 14 October 2013 Received in revised form 26 February 2014 Accepted 18 April 2014 Available online 24 April 2014
In this study, a horizontal annular inlet was applied to a round-nozzle impactor in an effort to reduce the cut-off diameter. The collection efficiencies of round-nozzle impactors with either a vertical inlet, which has conventionally been used, or a horizontal annular inlet were investigated both numerically and experimentally. For the comparison, the nozzle width, nozzle-to-plate distance, impaction plate diameter, and flow rate of the aerosol drawn into the impactor were set the same for both the vertical inlet impactor and horizontal annular inlet impactor. A parametric study was performed to analyze the effects of the width of the horizontal annular inlet and nozzle throat length on the collection efficiency of round-nozzle impactors with nozzle Reynolds numbers ranging from 500 to 2600. By replacing the vertical inlet with the horizontal annular inlet, the square root of the Stokes number corresponding to the cut-off diameter could be reduced by approximately 30%. In other words, the value of (Stk50)1/2 was decreased from 0.49 to 0.32 or 0.33 when considering the effect of gravity. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Inertial impactor Collection efficiency Stokes number Round nozzle Horizontal annular inlet
1. Introduction Inertial impactors are widely used to collect atmospheric aerosol particles because of their transportability and easy operation. In an inertial impactor, aerosol is accelerated through a nozzle and impinges on an impaction plate, which cause the aerosol flow to abruptly change direction above the impaction plate. Then, the particles with inertia exceeding a certain value deviate from the streamline and deposit on the impaction plate, whereas the particles with less inertia remain airborne and exit the impactor stage. In this way, impactors can selectively collect aerosol particles according to their inertia. Many studies have been performed to enhance the collection efficiency of inertial impactors. The effects of various parameters such as the nozzle width, nozzle throat length, nozzle-to-plate distance, impaction plate size, and aerosol flow rate on the impactor collection efficiency have been investigated (Rader and Marple, 1985; Huang and Tsai, 2002; Grinshpun et al., 2005). The surface of an impaction plate was coated with grease or oil to reduce particle bounce (Rao and Whitby, 1978; Lee et al., 2005). The shape of the impaction plate was modified to increase the impactor collection efficiency under a heavy loading condition (Tsai and Cheng, 1995). Porous metal substrates were applied to the impaction plates to increase the collection efficiency for particles smaller than the cut-off size (Huang et al., 2001). Thermophoresis was applied by cooling the impaction plate to increase the impactor collection efficiency (Lee and Kim, 2002). Electrophoresis was employed to enhance the impactor collection efficiency for charged particles (Vinchurkar et al., 2009). The impactor stages were operated
n
Corresponding author. Tel.: þ82 2 2220 0422; fax: þ 82 2 2220 2299. E-mail address: [email protected] (S.-J. Yook).
http://dx.doi.org/10.1016/j.jaerosci.2014.04.007 0021-8502/& 2014 Elsevier Ltd. All rights reserved.
64
M.-K. Kim et al. / Journal of Aerosol Science 74 (2014) 63–69
Fig. 1. Schematic of round-nozzle impactor with vertical inlet.
Fig. 2. Schematic of round-nozzle impactor with horizontal annular inlet.
at low pressures to decrease the cut-off size (Arffman et al., 2011). Elliptical concave impaction plates were used to increase the impactor collection efficiency and reduce the particle bounce (Kim and Yook, 2011; Kim et al., 2013). Even though much effort has gone into enhancing the impactor collection efficiency by modifying the geometries of the inertial impactors, changing the shape or surface condition of the impaction plate, or altering the operating environment for inertial impactors, few studies have considered modifying the impactor inlet shape to enhance the impactor collection efficiency. With round-nozzle impactors, when a particle is positioned closer to the centerline of the impactor nozzle, it has a higher probability of being collected on the impaction plate, as shown in Fig. 1. In this study, an impactor nozzle with a horizontal annular inlet was considered to allow aerosol particles to concentrate close to the centerline of the nozzle.
2. Numerical Figure 1 shows a schematic of a round-nozzle impactor with a vertical inlet, which has been commonly used to sample aerosol particles. The nozzle width is denoted as W, and the nozzle throat length is C. The nozzle-to-plate distance is S, and the impaction plate diameter is D. Figure 2 shows a schematic of a round-nozzle impactor with a horizontal annular inlet. The horizontal annular inlet was designed to allow aerosol particles to concentrate close to the centerline as a result of centrifugal force before the particles are accelerated through the nozzle. The height of the horizontal annular inlet is denoted as A, and the width of the horizontal annular inlet is B.
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The flow field in a round-nozzle impactor was simulated using the commercial code ANSYS FLUENT v13.0. The Reynolds number at the nozzle was varied in the range of 500–2600. Because the impactor collection efficiency is known to be influenced by gravity for Reynolds numbers smaller than 1500 (Huang and Tsai, 2001), the gravitational force was considered in all the simulation cases of this study. The flow was assumed to be two-dimensional axisymmetric, steady, incompressible, and laminar for nozzle Reynolds numbers smaller than 3000 (Huang and Tsai, 2002). The operating temperature and pressure were set as 20 1C and 101.3 kPa, respectively. Boundary conditions were imposed as shown in Figs. 1 and 2, and included the velocity inlet at the inlet of the impactor, pressure outlet at the outlet of the impactor, axisymmetric condition at the center of the impactor, and no-slip condition on all the walls of the impactor. A grid test was performed to select the optimum mesh, and the quad-cell numbers selected ranged from approximately 25,000 to 100,000 depending on the nozzle width. The continuity, momentum, and energy equations were iteratively solved using a convergence criterion of 10 6. The SIMPLE algorithm was used to couple the velocity and pressure. After solving the flow field, the trajectories of particles were simulated using the discrete phase models (DPM) incorporated in the FLUENT software. In order to calculate the particle trajectories, the Brownian force, gravitational force, and Stokes' drag force with slip correction were considered (FLUENT 6.3 User's Guide, 2006, chap. 22). The collection efficiency (η) of the round-nozzle impactor was predicted based on the statistical Lagrangian particle-tracking approach (Yook et al., 2007; Kim et al., 2011, 2013; Lee et al., 2011, 2012a,b; Kim and Yook, 2011; Woo et al., 2012). The particles were positioned on the velocity-inlet plane at constant intervals and injected with the same velocity as the inlet flow velocity. By considering polystyrene latex (PSL) spheres, the particle density was set at 1050 kg/m3. The influence of the number of injected particles (Nin) on the collection efficiency of round-nozzle impactors was checked, and Nin ranged from approximately 500 to 5000 depending on the impactor size. The particles were assumed to be permanently collected when they touched any wall of the inertial impactor. Two different formulae were used to predict the collection efficiency. In the case of the round-nozzle impactor with the vertical inlet (see the schematic in Fig. 1), the volumetric aerosol flow rate represented by each particle positioned on the velocity-inlet plane (where the particles were positioned at constant intervals) varied depending on the particle's radial position. Thus, the collection efficiency was calculated as follows (Kim and Yook, 2011): η¼
N in
2Δr
∑ ri f i ; R2in i ¼ 1
ð1Þ
where Δr [¼ Rin/(Nin 1)] is the constant radial interval between the particles positioned at the velocity inlet; Rin is the radius of the velocity inlet; ri is the radial position of each particle at the velocity inlet; and fi is a number used to describe the fate of each particle, where fi ¼1 if the ith particle is collected on the impaction plate and fi ¼0 if it is not collected. In the case of the round-nozzle impactor with the horizontal annular inlet (see the schematic in Fig. 2), the same volumetric flow rates were found for the particles positioned on the velocity-inlet plane at constant intervals, with no differences in regard to the particle position at the entrance of the horizontal annular inlet. Therefore, the collection efficiency was estimated by simply counting the number of particles deposited on the impaction plate (Nde), that is η¼
Nde : Nin
ð2Þ
3. Experimental Round-nozzle impactors with either a vertical inlet or horizontal annular inlet were manufactured in order to validate the numerical model. Table 1 lists the dimensions of the round-nozzle impactors used for the experiments. Here, the nozzle width, W, was determined from the following equation: 2
Stk ¼
ρp dp C c U 9μW
;
ð3Þ
Table 1 Dimensions of round-nozzle impactors with either vertical inlet or horizontal annular inlet used for experiments. Parameter
W [mm] S [mm] C [mm] D [mm] A [mm] B [mm]
PM1 nozzle
PM2.5 nozzle
Vertical inlet
Horizontal annular inlet
Vertical inlet
Horizontal annular inlet
0.9 3.8 1.0 6.0 – –
0.9 3.8 5.4 6.0 0.9 4.0
1.6 5.4 1.6 6.0 – –
1.6 5.4 9.6 6.0 1.6 3.7
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Fig. 3. Schematic of experimental setup.
where Stk is the Stokes number, ρp is the particle density, dp is the particle diameter, Cc is the slip correction factor, U is the average jet velocity through the nozzle, and μ is the dynamic viscosity of air. For round-nozzle impactors, the square root of the Stokes number corresponding to the cut-off diameter is known to be 0.49, that is, (Stk50)1/2 ¼0.49 (Hinds, 1999), where the subscript “50” implies a 50% collection efficiency. Therefore, the PM1 nozzle width of 0.9 mm was determined from Eq. (3) by considering the cut-off diameter, dp50, of 1 μm, and the PM2.5 nozzle width of 1.6 mm was determined by taking into account the cut-off diameter of 2.5 μm, at a temperature of 20 1C and pressure of 101.3 kPa. The flow rate of the aerosol drawn into each impactor, Qin, was 1.1 L/min. Therefore, the average jet velocity, U, was estimated to be 28.82 m/s through the PM1 nozzle and 9.12 m/s through the PM2.5 nozzle, resulting in nozzle Reynolds numbers of 1718 and 966, respectively. According to Huang and Tsai (2002), the effect of the impaction plate diameter on the Stokes number for the cut-off size could be negligible, if D 43W. Therefore, an impaction plate diameter of 6 mm was used for all the impactors manufactured for the experiments. Figure 3 shows a schematic of the experimental setup for measuring the collection efficiencies of the round-nozzle impactors with either a vertical inlet or horizontal annular inlet. The impaction plate was coated with vacuum grease to reduce particle bounce. PSL spheres (Thermo Scientific) were aerosolized using an atomizer, and then dried using a diffusion-dryer. The monodisperse particle diameters ranged from 0.4 to 4.0 μm. The flow rate of the aerosol introduced into the round-nozzle impactor was 1.1 L/min. A mesh plate was used in a particle disperser to evenly distribute the particles upstream of the round-nozzle impactor with either the vertical inlet or horizontal annular inlet. A three-way valve was used to allow an optical particle counter (OPC, HCT model PS-3030) to measure the particle number concentration upstream or downstream of the impactor. The impactor collection efficiency was experimentally evaluated as η¼
C up C down ; C up
ð4Þ
where Cup is the upstream particle number concentration and Cdown is the downstream particle number concentration. When the impaction plate was not installed, the collection efficiencies were evaluated to be smaller than 1% for the vertical inlet impactor and smaller than 2% for the horizontal annular inlet impactor. Therefore, the particle loss on surfaces other than the impaction plate was assumed to be negligible for the particle sizes considered in the experiments.
4. Results and discussion Figure 4 shows a comparison of the numerical results and experimental data for the collection efficiencies of roundnozzle impactors with either a vertical inlet or horizontal annular inlet, when W, S, C, D, A, and B were set as listed in Table 1. The aerosol flow rate was 1.1 L/min. The numerically simulated collection efficiencies agreed well with the experimental data, which validated the correctness of the numerical method employed in this study. When the vertical inlet was used, the cut-off diameters were evaluated to be 1.0 μm for the impactor with a W value of 0.9 mm (PM1 nozzle) and 2.5 μm for the impactor with a W value of 1.6 mm (PM2.5 nozzle). When the horizontal annular inlet was employed, the cut-off diameters were estimated to decrease to 0.8 μm for the impactor with the PM1 nozzle and 2.0 μm for the impactor with the PM2.5 nozzle, which meant that the cut-off diameter of the round-nozzle impactor could be remarkably reduced by replacing the vertical inlet with the horizontal annular inlet.
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Fig. 4. Comparison of collection efficiencies between numerical results and experimental data for round-nozzle impactors with either vertical inlet or horizontal annular inlet.
Fig. 5. Comparison of collection efficiencies between vertical inlet impactor and horizontal annular inlet impactor.
Figure 5 shows the comparison of the numerically estimated collection efficiencies of the vertical inlet impactor and horizontal annular inlet impactor. The nozzle width and aerosol flow rate were determined from Eq. (3) with (Stk50)1/2 ¼0.49, and the Reynolds number was fixed at 540. Thus, W¼0.5 mm and Qin ¼0.192 L/min for the impactor with the PM1 nozzle, W¼1.2 mm and Qin ¼0.463 L/min for the impactor with the PM2.5 nozzle, W¼2.4 mm and Qin ¼0.949 L/min for the impactor with the PM5 nozzle, and W¼4.5 mm and Qin ¼1.736 L/min for the impactor with the PM10 nozzle. The ratios of S, D, A, and C to W were set as follows: S/W¼2, D/W¼8, A/W¼0.25, and C/W¼10. In contrast, the ratio of the horizontal annular inlet width to the nozzle width, B/W, was varied from 0.5 to 4. The solid lines with no symbols represent the collection efficiencies of the vertical inlet impactors, and the symbols signify the collection efficiencies of the horizontal annular inlet impactors. It should be noted that the force of gravity was considered in the simulations for both the vertical inlet impactor and the horizontal annular inlet impactor because gravity affects the impactor collection efficiency at low Reynolds numbers (Huang and Tsai, 2001). When the vertical inlet was used, the cut-off diameters were predicted to be 1.0, 2.5, 5.0, and 10.0 μm for the PM1, PM2.5, PM5, and PM10 nozzle impactors, respectively. However, when the horizontal annular inlet was assumed instead of the vertical inlet, the cut-off diameters when considering the force of gravity were estimated to be reduced by approximately 30%, even though the nozzle width and aerosol flow rate were unchanged. The collection efficiencies of the horizontal annular inlet impactors with the PM1, PM2.5, and PM5 nozzles were unaffected by the change in the B/W ratio. In the case of the horizontal annular inlet impactor with the PM10 nozzle, the collection efficiencies for particles larger than the cut-off diameter greatly decreased with increasing B/W ratio because the particle loss at the horizontal annular inlet was enhanced by gravitational settling. However, for the B/W ratios of 0.5, 1, and 2, the particle losses at the horizontal annular inlet were negligible. Figure 6 shows the variation in the square root of the Stokes number corresponding to the cut-off diameter according to the C/W ratio, for the round-nozzle impactors with a horizontal annular inlet. The nozzle width and aerosol flow rate were the same as those used for Fig. 5. The fixed ratios were S/W¼2, D/W¼8, A/W¼0.25, and B/W¼2. The C/W ratio was varied from 2 to 18. The solid line represents the reference value of (Stk50)1/2 for the round-nozzle impactor with a vertical inlet, that is, 0.49 (Hinds, 1999). When the horizontal annular inlet was assumed, the values of (Stk50)1/2 for the horizontal annular inlet impactors with the PM1, PM2.5, PM5, and PM10 nozzles were much smaller than the reference value of 0.49, which
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Fig. 6. Square root of Stokes number for 50% collection efficiency of round-nozzle impactor with horizontal annular inlet as function of C/W ratio.
Fig. 7. Collection efficiencies of round-nozzle impactors with horizontal annular inlet for PM1, PM2.5, PM5, and PM10 nozzles, for which widths and aerosol flow rates were determined from Eq. (3) with (Stk50)1/2 ¼ 0.49.
Table 2 Nozzle widths, aerosol flow rates, and nozzle Reynolds numbers for horizontal annular inlet impactors considered in parametric study. Horizontal annular inlet impactor
W [mm]
Qin [L/min]
Nozzle Reynolds number
PM1 nozzle
0.5 0.84 1.04
0.192 0.945 1.917
540 1575 2580
PM2.5 nozzle
1.2 2.0 2.55
0.463 2.250 4.699
540 1575 2580
PM5 nozzle
2.4 4.0 5.2
0.949 4.395 9.656
540 1575 2580
PM10 nozzle
4.5 7.8 10.0
1.736 8.775 18.429
540 1575 2580
meant that the horizontal annular inlet was very useful for reducing the cut-off diameter. As the C/W ratio increased, the value of (Stk50)1/2 decreased. This was because a flow developed at the nozzle throat. Consequently, with an increase in the nozzle throat length, there was an increase in the flow velocity near the centerline, toward which the aerosol particles with sufficient inertia were concentrated as a result of using the horizontal annular inlet. However, when the C/W ratio was higher than 10, (Stk50)1/2 converged to a certain value within an approximate range of 0.32–0.33, for the round-nozzle impactors with the PM1, PM2.5, PM5, and PM10 nozzles. Figure 7 shows the collection efficiencies of the round-nozzle impactors with a horizontal annular inlet, where the nozzle width and aerosol flow rate were determined from Eq. (3) with (Stk50)1/2 ¼0.49. Table 2 lists the values of W and Qin, considered
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for the parametric study, corresponding to the nozzle Reynolds numbers in the range of 500–2600. The ratios were fixed at S/ W¼2, D/W¼8, A/W¼ 0.25, B/W¼2, and C/W¼10. The collection efficiencies fell on one curve for each PM1, PM2.5, PM5, or PM10 nozzle case, in the tested range of nozzle Reynolds numbers, which implied that the effects of the horizontal annular inlet dimensions on the collection efficiency of a round-nozzle impactor could be characterized using the square root of the Stokes number. When the horizontal annular inlet was assumed to be used instead of the vertical inlet, the cut-off diameters when considering the force of gravity were estimated to decrease from 1 to 0.67 μm for the impactor with the PM1 nozzle, from 2.5 to 1.72 μm for the impactor with the PM2.5 nozzle, from 5 to 3.4 μm for the impactor with the PM5 nozzle, and from 10 to 6.54 μm for the impactor with the PM10 nozzle. A drop in the collection efficiency was anticipated for particles larger than approximately 8 μm, mainly as a result of the gravitational settling of particles in the horizontal annular inlet region. 5. Conclusions In this study, the collection efficiencies of round-nozzle impactors with a horizontal annular inlet were investigated both numerically and experimentally. A comparison between the numerical results and experimental data proved that the numerical method employed in this study could correctly predict the collection efficiency of a round-nozzle impactor with either a vertical inlet or horizontal annular inlet. For the comparison of the collection efficiencies of the vertical inlet impactor and horizontal annular inlet impactor, the same nozzle widths and aerosol flow rates were used for nozzle Reynolds numbers in the range of 500–2600. Based on a parametric study with constant ratios of S/W¼2 and D/W¼8, the geometric dimensions of the horizontal annular inlet were determined to be A/W¼0.25, B/W ¼2, and C/W¼10. With these ratios, the horizontal annular inlet reduced the square root of the Stokes number for a 50% collection efficiency by approximately 30% when considering the force of gravity, that is, from 0.49, which was valid for the round-nozzle impactor with the vertical inlet, to approximately 0.32 or 0.33. As a result, the horizontal annular inlet was found to be useful for reducing the cut-off size of a round-nozzle impactor. Acknowledgment This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (NRF-2012M3A6A7054863). References Arffman, A., Marjamaki, M., & Keskinen, J. (2011). Simulation of low pressure impactor collection efficiency curves. 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