- Email: [email protected]
Thermophoretic deposition of ultrafine particles in a turbulent pipe flow: Simulation of ultrafine particle behaviour in an automobile exhaust pipe
Aerosol Science 37 (2006) 1788 – 1796 www.elsevier.com/locate/jaerosci
Thermophoretic deposition of ultrafine particles in a turbulent pipe flow: Simulation of ultrafine particle behaviour in an automobile exhaust pipe Byung Uk Leea , Du Sub Byunb , Gwi-Nam Baeb,∗ , Jin-Ha Leec a Department of Mechanical Engineering, Konkuk University, 1 Hwayang-Dong, Gwangjin-Gu, Seoul 143-701, Republic of Korea b Hazardous Substances Research Center, Korea Institute of Science and Technology (KIST), 39-1 Hawolgok-Dong, Seongbuk-Gu,
Seoul 136-791, Republic of Korea c Corporate Research and Development Division, Hyundai and Kia Motors, 104 Mabuk-Dong, Giheung-Gu, Yongin-Si,
Gyunggi-Do 449-912, Republic of Korea Received 26 April 2006; received in revised form 27 July 2006; accepted 28 July 2006
Abstract Thermophoresis of ultrafine particles in a turbulent pipe flow was studied using high-temperature and high-concentration polydisperse ultrafine particles. Experimental conditions were designed to simulate particle behaviour in an automobile exhaust pipe, with a particular focus on establishing similar particle residence time. From the experimental results, thermophoresis was found to be a dominant mechanism for ultrafine particle deposition in the turbulent pipe flow. A previous thermophoretic deposition model was found to be inadequate with respect to estimating the results of the experimental conditions. In this study, the experimental data and the computational analysis results reflect the necessity of establishing a new formula for thermophoretic deposition for high-concentration polydisperse ultrafine particles in a pipe flow. 䉷 2006 Elsevier Ltd. All rights reserved. Keywords: Ultrafine particles; Thermophoresis; Turbulence; Pipe; Automobile; Emission
1. Introduction Thermophoresis is a phenomenon in which aerosol particles migrate in the direction of decreasing temperature. Thermophoresis has both negative and positive effects in application areas. Negative effects of thermophoresis include reduction of thermal conductivity of heat exchanger pipes and reduction of production yield of specialty powders manufactured in high-temperature aerosol reactors. On the other hand, the concept of thermophoresis provides a working principle to fabricate optical fibre in a modified chemical vapour deposition (MCVD) process. It also can be employed to remove or sample atmospheric particles from the air in a thermal precipitator. Thermophoresis has been reviewed by a number of authors including Talbot, Cheng, Schefer, and Willis (1980), Bakanov (1995), Li and Davis (1995a,b), and Lee and Kim (2001). ∗ Corresponding author. Tel: +82 2 958 5676; fax: +82 2 958 5805.
E-mail address: [email protected] (G.-N. Bae). 0021-8502/$ - see front matter 䉷 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaerosci.2006.07.006
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
1789
Typical characteristics of processes of thermophoresis include a hot aerosol stream flowing through a tube or an annulus, and the presence of a non-negligible thermal gradient between the aerosol stream and the cooled walls of the tube or of an outer tube of the annulus. Accordingly, many thermophoresis studies have targeted these geometries. Thermophoretic deposition of particles in an annular flow was studied theoretically by Weinberg (1983) and Fiebig, Hilgenstock, and Riemann (1988). Weinberg (1983) suggested that complete collection was possible with thermophoresis and that a smaller separation distance between concentric cylinders resulted in higher deposition efficiency. Fiebig et al. (1988) showed that when the annulus was oriented vertically, as a result of the buoyancy effect, the deposition efficiency tended to increase for a smaller ratio of inner to outer tube radius. Chang, Ranade, and Gentry (1992, 1995) carried out experiments and numerical simulations to quantify thermophoretic deposition in an annular flow system with fixed thermal gradients between two concentric cylinders. They found good agreement between experimental results and computational results using the model of Talbot et al. (1980). Lee and Kim (2001) studied thermophoretic deposition experimentally and numerically in an annular flow system using several models suggested by Derjaguin, Ravinovich, Storozhilova, and Shcherbina (1976) and Talbot et al. (1980) in a cryogenic temperature range. They found that the thermophoretic models required modification in the cryogenic temperature range. A tube flow with a thermal gradient has been utilized in many applications including heat exchanger pipes and automobile exhaust pipes. Therefore, it is necessary to study thermophoresis in a tube flow in order to understand and innovate a number of systems that are employed in a variety of applications. Thermophoresis in a laminar tube flow has been studied thoroughly. Stratmann and Fissan (1989) experimentally studied thermophoretic deposition using 0.005–0.1 m monodisperse aerosols in a laminar tube flow. Montassier, Bouland, and Renoux (1990) carried out experiments with an experimental set-up that was similar to that employed by Stratmann and Fissan (1989) except for the direction of the flow and the measuring technique. It was found that cumulative deposition increased with decreasing flow rate, and thermophoretic deposition increased with decreasing particle size (Montassier et al., 1990). There are a number of current applications that can be modelled as a turbulent aerosol pipe flow, such as automobile exhaust pipe flow. However, few have dealt with thermophoresis in turbulent pipe flows (Romay, Takagaki, Pui, & Liu, 1998). In addition, with heightened concern over ultrafine particles, there is growing demand for research that will shed light on the behaviour of ultrafine particles in a turbulent pipe flow. However, little data is available for ultrafine particles in relation to thermophoresis in a turbulent pipe flow. In the present study, the thermophoresis effect on ultrafine particles in a turbulent pipe flow was studied with the primary goal of elucidating the behaviour of ultrafine particles emitted from automobile engines via the exhaust pipe. Exhaust particles emitted from automobiles have received special attention due to their adverse health effects (Burtscher, 2005; Lighty, Veranth, & Sarofim, 2000; Wichmann & Peters, 2000). Recent concerns associated with health effects of ultrafine particles and stricter regulations pertaining to automobile emissions require reduction of ultrafine particles emitted from automobiles. Ultrafine particles could be controlled by an electric air ion emission (Lee, Yermakov, & Grinshpun, 2004b) or filtering a flow (Lee, Yermakov, & Grinshpun, 2004a, 2005). However, at present, there is no efficient technology to control ultrafine particles in automobile emission systems. Ultrafine particles generated from automobile engines pass through the exhaust pipe and several catalysts before being emitted to the environment. Therefore, in order to reduce ultrafine particle emissions, it is necessary to first understand the behaviour of ultrafine particles in the exhaust pipe. The ultrafine particle flow in an exhaust pipe could be modelled as a turbulent particle flow with thermal gradients. As stated above, in contrast to other areas of thermophoresis studies, few studies have dealt with thermophoresis of ultrafine particles in turbulent pipe flows. Both theoretical and experimental studies of thermophoresis in a turbulent pipe flow have been conducted by Byers and Calvert (1969), Nishio, Kitani, and Takahashi (1974), and Romay et al. (1998). However, these studies focused on submicron particles, which were larger than 0.1 m. The aims of the present study are to augment currently available experimental data pertaining to thermophoresis to the area of ultrafine particles in a turbulent pipe flow and to shed light on the behaviour of ultrafine particles in the automobile exhaust system using a simulation system for the exhaust pipe. From the present experimental results, it was found that thermophoresis is a major mechanism for ultrafine particle behaviour in the exhaust pipe system. Along with the experimental studies, theoretical-based computations have been carried out.
1790
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
2. Experimental method Automobiles emit an abundance of particles at high volumes with frequent variations depending on engine loads. As such, it is difficult to study the mechanisms of particle movements using particles that are directly emitted from automobiles. In order to understand the particle movement in detail, a surrogate experimental set-up for aerosols has been employed (Lee, Bae, Kim, Lee, & Yeo, 2005). In the present study, a surrogate system was employed in order to elucidate the ultrafine particle behaviour. Fig. 1 shows a schematic diagram of the experimental set-up. Ultrafine particles having similar particle size distributions as those emitted from engines were generated artificially using a manufactured combustion aerosol generator. In this combustion aerosol generator, flow rates of butane gas, air, and nitrogen gas were controlled to allow for adjustment of combustion conditions, and to create variable particle size distributions (Lee, Bae et al., 2005). Temperature of the generated aerosol flow and the carrier gas was increased by passing the flow through an electrically heated tube furnace in order to attain the level of normal engine exhaust temperature. After passing through the tube furnace, the high-temperature aerosol particles were flowed through a horizontal simulation exhaust pipe. The simulation exhaust pipe is 1.5 m in length and 0.7 cm in diameter. It is made of pyrex glass wrapped by a fabric insulator, which functions to mitigate the decrease of temperature of the flow and to maintain temperature levels similar to those measured in automobiles. This pipe was designed with a particular focus on establishing a similar particle residence time to that of an automobile exhaust pipe (∼ 0.1 s), as the particle residence time is considered to be the most important parameter in terms of particle behaviour in the pipe. As the aerosol particles passed through the pipe, they were sampled and measured through five sampling ports. The sampling ports were located at intervals of 30 cm. Particle size distributions and concentrations of the sampled aerosol particles were measured using a Scanning Mobility Particle Sizer (SMPS, (DMA 3085, CPC 3025A), TSI, USA), which could measure particle sizes as low as 3 nm. Under the experimental conditions, particle concentrations and temperature of the aerosol flow were too high. Therefore, a rotating dilutor (rotating disk dilutor type MD 19-2E, Matter Engineering AG), which could dilute particle concentrations and allow for adjustment of temperature to the measurable range of the SMPS, was utilized. At least three measurements were conducted under the same conditions and a dilution factor of 50 was used. The same dilution factor is considered in plotting the particle size distributions. Temperatures of the tube flow and the tube wall were measured at the sampling ports using a thermocouple thermometer (Hanyoung, Korea). Quenching N2
Compressed Air
CAST
thermometer Particles
MFM
Port 1
Hood Port 2
30cm Burner
Furnace
MFC
MFC
MFC
N2
Filter C3H8
30 cm
Port 4
Port 5
30 cm
Samping
Diluter
Gas In
Filter
30 cm
Port3
Air
Fig. 1. Schematics of the experimental set-up.
SMPS
Computer
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
1791
3. Results and discussion 3.1. Experimental results In order to simulate the automobile exhaust conditions, the temperature of the aerosol flow was increased to 360 ◦ C, which is within the range of automobile engine exhaust flow temperature (Hwang, Han, Yun, Kim, & Kim, 2005), by flowing the aerosol through a tube furnace. Fig. 2 shows flow temperatures and wall temperatures measured at the sampling ports. The values in Fig. 2 are the average values of three measurements. Deviations of each temperature measurement at the same conditions were less than 2 ◦ C. The temperature difference between the flow and wall was largest at port 1, approximately 250 ◦ C. The differences decreased along the pipe: 175, 125, 91 and 77 ◦ C at ports 2–5, respectively. The flow rate was 30.5 L min−1 and the Reynolds number of the experimental system at the standard condition was 5800. From these experimental conditions, the aerosol flow could be modelled as a turbulent pipe flow with thermophoresis. Fig. 3(A) shows the particle size distributions measured at five ports along the pipe at these conditions. At port 1, the first port from the combustion aerosol generator, the total concentration of particles was 1.4 × 108 particles cm−3 and the mode diameter was 31 nm. Along the simulation exhaust pipe, a large number of ultrafine aerosol particles, ranging from 20 to 50 nm in diameter, were deposited in the pipe, particularly between ports 1 and 2, as shown in Fig. 3(A). The total particle number concentrations decreased from 1.4 × 108 to 3.5 × 107 particles cm−3 , representing 75% total particle loss, between ports 1 and 2 for the conditions of Fig. 3(A). Possible deposition mechanisms in a turbulent pipe flow include thermophoretic deposition, turbulent eddy impaction, Brownian diffusion, turbulent eddy diffusion, and electrostatic precipitation (Romay et al., 1998). In a previous study, involving similar experimental conditions as the present study (a pipe diameter, 0.49 cm; Reynolds number, 1379, 5517, 9656) (Romay et al., 1998), it was found that thermophoretic deposition and turbulent eddy impaction were the two dominant mechanisms of particle deposition. In this study, in order to elucidate the deposition mechanism of ultrafine particles in the simulation pipe, the thermophoresis effect in particular was targeted and tested by varying the entry temperature of the aerosol flow from 360 to 30 ◦ C while maintaining the other experimental conditions constant. If non-thermophoretic deposition mechanisms are dominant in this system, particle loss will be similar regardless of the temperature of the aerosol flow upon entry to the pipe. This assumption was partly supported by previous findings that turbulent eddy impaction, one of two dominant mechanisms in the previous study, was not affected by an increase of temperature (Romay et al., 1998).
400 350
Flow temperature Wall temperature
Temperature (°C)
300 250 200 150 100 50 0
Port 1 0
Port 2
Port 3
Port 4
Port 5
40 60 80 20 100 120 Sampling locations on the simulation exhaust pipe (cm)
Fig. 2. Flow temperatures and wall temperatures along the simulation exhaust pipe. Temperature of aerosol flow at port 1 was 360 ◦ C.
1792
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
7e+7 port 1 2 3 4 5
4e+8 3e+8
dN/dlogDp (particles/cm3)
dN/dlogDp (particles/cm3)
5e+8
2e+8 1e+8 0 10
4e+7 3e+7 2e+7 1e+7
100
Particle diameter (nm)
(A)
1
10
100
Particle diameter (nm)
(B) 1e+8
7e+7 port 1 2 3 4 5
6e+7 5e+7
dN/dlogDp (particles/cm3)
dN/dlogDp (particles/cm3)
5e+7
0 1
4e+7 3e+7 2e+7 1e+7
port 1 2 3 4 5
8e+7 6e+7 4e+7 2e+7 0
0 1 (C)
port 1 2 3 4 5
6e+7
10 Particle diameter (nm)
100
1 (D)
10
100
Particle diameter (nm)
Fig. 3. Effect of temperature of entering aerosol flow on particle behaviours in the simulation exhaust pipe: temperatures of aerosol flow at port 1 were (A) 360 ◦ C, (B) 210 ◦ C, (C) 108 ◦ C, and (D) 30 ◦ C.
If thermophoresis were the dominant mechanism, particle loss would become much smaller when the temperature of the aerosol flow upon entry was decreased. Figs. 3(B)–(D) show the particle behaviour along the pipe with varying entry temperature, 210, 108, and 30 ◦ C, respectively. As the figures clearly indicate, as the temperature of the entering aerosol flow is decreased, the loss of ultrafine particles becomes smaller. In the case of the conditions for Fig. 3(D), at which there was almost no thermal gradient in the tube, distinct ultrafine particle loss could not be observed in the experiments. The above findings are also supported by consideration of the total number concentrations of particles. Fig. 4 shows the normalized total number concentrations along the simulation exhaust pipe. As the temperature of port 1 decreases, the loss of total number concentrations decreases. In the case of 360 ◦ C, a maximum of 75% loss of total particle number concentrations was observed whereas in the case of 30 ◦ C, the total particle concentrations fluctuated in a range of 10% of the total particle number concentrations. The particle size distributions at 108 and 30 ◦ C showed the fluctuation with no distinct particle loss in Figs. 3(C) and (D), and this fluctuation caused the partial increase of the total particle concentrations in Fig. 4. From these results, it is clear that thermophoresis is the dominant mechanism of ultrafine particle deposition in this experimental system. In Fig. 3, particles smaller than 20 nm maintained their concentration levels while passing through the pipe. The additional condensation of combustion by-products could account for this result. When a combustion aerosol generator is used, most aerosols are produced before entering a pipe system; however, by-products in vapour form could be passed
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
1793
Total particle concentrations normalized by port 1 values
2.0 360°C 210°C 108°C 30°C
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
Port 1 0
Port 2 20
Port 4
Port 3 40
60
80
Port 5 100
120
Sampling locations on the simulation exhaust pipe (cm) Fig. 4. Effect of temperature of entering aerosol flow on total particle concentrations measured at sampling ports. Temperature values on the graph legend represent temperatures of entering aerosol flow. The total particle concentrations were normalized by the values at port 1.
to the pipe system and condensed in the pipe. Therefore, particles smaller than 20 nm could be generated from this condensation while these by-products would pass the pipe with decreasing flow temperature. Therefore, it is thought that this additional condensation could maintain the concentration levels of particles smaller than 20 nm. In order to assess the replicability of the experimental results, especially in Fig. 3(A), several iterations of the experiments were conducted with different particle size distributions, which included peak concentrations ranging from 2.0 × 108 to 3.5 × 108 particles cm−3 in Figs. 5(A) and (B). In the replicated experiments the same trends were obtained. All experimental measurements shown in Fig. 3 were conducted with at least three replications. The data points shown in Fig. 3 are average values of the three measurements. The standard deviations of particle measurements were mostly around 10% (the error bars are not shown here to clearly show the trends of the results), and sometimes ranged from 10% to 30% with the exception of very low particle number concentration cases. 3.2. Theoretical analysis Several formulas have been suggested for the description of thermophoretic deposition in a turbulent pipe flow (Byers & Calvert, 1969; Nishio et al., 1974; Romay et al., 1998). These formulas are based on the assumption that the wall temperature is constant. However, in many applications, including this study, the wall temperature at the entrance is elevated due to a high aerosol flow, and as the aerosol flow cools along the pipe, the wall temperature decreases. Therefore, the aforementioned theoretical formulas are not exactly appropriate for the experimental conditions of the present study. However, for comparison, thermophoretic deposition was calculated using the most recent formula suggested by Romay et al. (1998) with some modifications of the temperature conditions. The deposition efficiencies of ultrafine particles between the ports for the conditions described in Figs. 3(A) and (B) were estimated from the following formula: L = 1 −
Tw + (Te − Tw ) exp(−DhL/QC p ) Te
P rK th ,
(1)
where is the deposition efficiency, L the pipe length, Te the gas temperature at the entrance, Tw the temperature at the pipe wall, D the pipe diameter, h represents the convective heat transfer coefficient, the gas density, Q the flow rate, Cp the gas specific heat at constant pressure, Pr the Prandtl number, and Kth is the thermophoretic coefficient.
1794
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
dN/dlogDp (particles/cm3)
2.5e+8 port 1 2 3 4 5
2.0e+8
1.5e+8
1.0e+8
5.0e+7
0.0 1 (A)
10 Particle diameter (nm)
100
10 Particle diameter (nm)
100
3.5e+8 port 1 2 3 4 5
dN/dlogDp (particles/cm3)
3.0e+8 2.5e+8 2.0e+8 1.5e+8 1.0e+8 5.0e+7 0.0 1 (B)
Fig. 5. Particle behaviour in the simulation exhaust pipe with different particle size distributions at port 1. Other experimental conditions correspond with the conditions of Fig. 3(A), including the inlet temperature. The same trend as that of Fig. 3(A) can be observed.
In the computation, L represents the distance from port 1, Te is the value as measured in the experiments, and Tw is assumed to be a middle value of wall temperatures of port 1 and other ports. D is 7 mm, and h was calculated by the following empirical formula for convective heat transfer coefficient in a pipe flow (Incropera & DeWitt, 1990): (f/8)(ReD − 1000)P r hD , = N uD = k 1 + 12.7(f/8)1/2 (P r 2/3 − 1)
(2)
f = (0.79 ln ReD − 1.64)−2 .
(3)
Kth was calculated based on the following formula (Talbot et al., 1980) : V = −2Cs
(kg /kp + Ct /R)(1 + (/R)[A1 + A2 exp(−A3R/)]) ∇T , (1 + 3Cm /R)(1 + 2kg /kp + 2Ct /R) T0
(4)
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
1795
Table 1 Particle deposition efficiencies between ports when the temperature of the aerosol flow at port 1 was 360 ◦ C Particle diameter (nm)
30 40 50
Experimental results
Computations based on formulas (1)–(4)
Ports 1–2 (%)
Ports 1–5 (%)
Ports 1–2 (%)
Ports 1–5 (%)
95 97 94
96 99 99
8 8 8
18 18 18
Table 2 Particle deposition efficiencies between ports when the temperature of the aerosol flow at port 1 was 210 ◦ C Particle diameter (nm)
20 25 30
Experimental results
Computations based on formulas (1)–(4)
Ports 1–2 (%)
Ports 1–5 (%)
Ports 1–2 (%)
Ports 1–5 (%)
36 37 29
77 93 97
6 6 6
12 12 12
where A1, A2, and A3 are the coefficients used in the Millikan drag formula, and have values of 1.2, 0.41, and 0.88, respectively. It is known that the values of coefficients are Cs =1.17, Ct =2.18, and Cm =1.14. The values of properties at the middle temperature between port 1 and other ports were used in the calculations. Tables 1 and 2 show the experimental results and the computational results based on formulas (1)–(4). In Table 1, when the temperature of the entering aerosol flow was 360 degr C, the particle deposition efficiencies for ultrafine particles, such as particles 30, 40, and 50 nm in diameter, were approximately 95% between ports 1 and 2 in the experiments. However, the deposition efficiencies as calculated using formulas (1)–(4) were only 8%. Deposition efficiencies between ports 1 and 5 were approximately 99% in the experiments, compared with 18% in the computations. In Table 2, when the temperature of the entering aerosol flow was 210 ◦ C, the condition for Fig. 3(B), the particle deposition efficiencies between ports 1 and 2 were approximately 30% in the experiments whereas the computations yielded approximately 6% deposition efficiencies. Deposition efficiencies between ports 1 and 5 ranged from 77% to 97% in the experiments compared with 12% in the computations. From this comparison, it is evident that the previous formulas are not adequate with respect to estimating experimental results in the case of high-temperature and high-concentration polydisperse ultrafine particles in a turbulent flow in a pipe characterized by decreasing wall temperature. The previous formulas are based on several assumptions such as uniform aerosol concentration distribution at each cross-section, uniform wall temperature, a fully developed turbulent flow, and no particle sources in the flow domain. Also, in previous works, monodisperse particles of small concentration were generated to test and develop the formulas. However, as in the present study, when there are a large number of polydisperse ultrafine particles, these assumptions and formulas are not appropriate. Therefore, it is necessary to develop new formulas that can estimate ultrafine particle depositions in a high-temperature aerosol pipe flow with decreasing wall temperature and a large number of polydisperse ultrafine particles. 4. Conclusions From this study, it was found that thermophoresis is the dominant mechanism for ultrafine particle deposition in a turbulent aerosol pipe flow, where the employed experimental conditions simulate the conditions of an automobile exhaust pipe flow. A discrepancy was found between the results yielded by the previous formulas and the present experimental results when different wall temperature conditions were considered. A thermophoretic deposition formula for high-concentration, high-temperature ultrafine particles in a turbulent pipe flow, which is accompanied by decreasing wall temperature, should be developed in order to estimate experimental results and for application in future studies.
1796
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
Acknowledgements This work is part of the project ‘Development of Partial Zero Emission Technology for Future Vehicle’ funded by Ministry of Commerce, Industry and Energy and we are grateful for its financial support. References Bakanov, S. P. (1995). Future directions for experiments in thermophoresis: A commentary. Journal of Aerosol Science, 26, 1–4. Burtscher, H. (2005). Physical characterization of particulate emissions from diesel engines: A review. Journal of Aerosol Science, 36, 896–932. Byers, R. L., & Calvert, S. (1969). Particle deposition from turbulent streams by means of thermal force. Industrial and Engineering Chemistry, Fundamentals, 8(4), 646–655. Chang, Y. C., Ranade, M. B., & Gentry, J. W. (1992). Experimental measurements of thermophoretic deposition of glass spheres in an annulus. Journal of Aerosol Science, 23, S31–S34. Chang, Y. C., Ranade, M. B., & Gentry, J. W. (1995). Thermophoretic deposition in flow along an annular cross-section: Experiment and simulation. Journal of Aerosol Science, 26(3), 407–428. Derjaguin, B. V., Ravinovich, Ya. I., Storozhilova, A. I., & Shcherbina, G. I. (1976). Measurement of the coefficient of thermal slip of gases and the thermophoretic velocity of large-size aerosol particles. Journal of Colloid Interface Science, 57, 451–461. Fiebig, M., Hilgenstock, M., & Riemann, H.-A. (1988). The modified chemical vapor deposition process in a concentric annulus. Aerosol Science and Technology, 9, 237–249. Hwang, S., Han, B., Yun, S., Kim, D., & Kim, Y. (2005). A study on the characteristics of exhaust particles from diesel engines with fuel injection types and after-treatment systems. Particle and Aerosol Research, 1, 61–68 (in Korean). Incropera, F. P., & DeWitt, D. P. (1990). Fundamentals of heat and mass transfer (3rd ed., p. 497). New York: Wiley. Lee, B.U., Bae, G.N., Kim, J.K., Lee, J.H., & Yeo, G.K. (2005). The behaviour of combustion aerosols in an exhaust pipe. Proceedings of the 13th international Pacific conference on automotive engineering (pp. 1036–1041). Gyeongju, Korea, August 22–24. Lee, B. U., & Kim, S. S. (2001). Thermophoresis in the cryogenic temperature range. Journal of Aerosol Science, 32, 107–119. Lee, B. U., Yermakov, M., & Grinshpun, S. A. (2004a). Unipolar ion emission enhances respiratory protection against fine and ultrafine particles. Journal of Aerosol Science, 35, 1359–1368. Lee, B. U., Yermakov, M., & Grinshpun, S. A. (2004b). Removal of fine and ultrafine particles from indoor air environments by the unipolar ion emission. Atmospheric Environment, 38(29), 4815–4823. Lee, B. U.,Yermakov, M., & Grinshpun, S. A. (2005). Filtering efficiency of N95- and R95-type facepiece respirators, dust-mist facepiece respirators, and surgical masks operating in unipolarly ionized indoor air environments. Aerosol and Air Quality Research, 5, 25–38. Li, W., & Davis, E. J. (1995a). Measurement of the thermophoretic force by electrodynamic levitation: Microspheres in air. Journal of Aerosol Science, 26, 1063–1083. Li, W., & Davis, E. J. (1995b). The effects of gas and particle properties on thermophoresis. Journal of Aerosol Science, 26, 1085–1099. Lighty, J. S., Veranth, J. M., & Sarofim, A. F. (2000). Combustion aerosols: Factors governing their size and composition and implications to human health. Journal of the Air and Waste Management Association, 50, 1565–1618. Montassier, N., Bouland, D., & Renoux, A. (1990). Experimental study of thermophoretic deposition of particles in laminar tube flow. Proceedings of the third international aerosol conference (pp. 395–398). Kyoto, Japan. Nishio, G., Kitani, S., & Takahashi, K. (1974). Thermophoretic deposition of aerosol particles in a heat-exchanger pipe. Industrial and Engineering Chemistry Process Design Development, 13(4), 408–415. Romay, F. J., Takagaki, S. S., Pui, D. Y. H., & Liu, B. Y. H. (1998). Thermophoretic deposition of aerosol particles in turbulent pipe flow. Journal of Aerosol Science, 29, 943–959. Stratmann, F., & Fissan, H. (1989). Experimental and theoretical study of submicron particle transport in cooled laminar tube flow due to combined convection, diffusion, and thermophoresis. Journal of Aerosol Science, 20, 899–902. Talbot, L., Cheng, R. K., Schefer, R. W., & Willis, D. R. (1980). Thermophoresis of particles in a heated boundary layer. Journal of Fluid Mechanics, 101, 737–758. Weinberg, M. C. (1983). Thermophoretic deposition of particles in laminar flow in a concentric annulus. Journal of the American Ceramic Society, 66, 439–443. Wichmann, H. E., & Peters, A. (2000). Epidemiological evidence of the effects of ultrafine particle exposure. Philosophical Transactions of the Royal Society of London, Series A, 358, 2751–2768.
Thermophoretic deposition of ultrafine particles in a turbulent pipe flow: Simulation of ultrafine particle behaviour in an automobile exhaust pipe Byung Uk Leea , Du Sub Byunb , Gwi-Nam Baeb,∗ , Jin-Ha Leec a Department of Mechanical Engineering, Konkuk University, 1 Hwayang-Dong, Gwangjin-Gu, Seoul 143-701, Republic of Korea b Hazardous Substances Research Center, Korea Institute of Science and Technology (KIST), 39-1 Hawolgok-Dong, Seongbuk-Gu,
Seoul 136-791, Republic of Korea c Corporate Research and Development Division, Hyundai and Kia Motors, 104 Mabuk-Dong, Giheung-Gu, Yongin-Si,
Gyunggi-Do 449-912, Republic of Korea Received 26 April 2006; received in revised form 27 July 2006; accepted 28 July 2006
Abstract Thermophoresis of ultrafine particles in a turbulent pipe flow was studied using high-temperature and high-concentration polydisperse ultrafine particles. Experimental conditions were designed to simulate particle behaviour in an automobile exhaust pipe, with a particular focus on establishing similar particle residence time. From the experimental results, thermophoresis was found to be a dominant mechanism for ultrafine particle deposition in the turbulent pipe flow. A previous thermophoretic deposition model was found to be inadequate with respect to estimating the results of the experimental conditions. In this study, the experimental data and the computational analysis results reflect the necessity of establishing a new formula for thermophoretic deposition for high-concentration polydisperse ultrafine particles in a pipe flow. 䉷 2006 Elsevier Ltd. All rights reserved. Keywords: Ultrafine particles; Thermophoresis; Turbulence; Pipe; Automobile; Emission
1. Introduction Thermophoresis is a phenomenon in which aerosol particles migrate in the direction of decreasing temperature. Thermophoresis has both negative and positive effects in application areas. Negative effects of thermophoresis include reduction of thermal conductivity of heat exchanger pipes and reduction of production yield of specialty powders manufactured in high-temperature aerosol reactors. On the other hand, the concept of thermophoresis provides a working principle to fabricate optical fibre in a modified chemical vapour deposition (MCVD) process. It also can be employed to remove or sample atmospheric particles from the air in a thermal precipitator. Thermophoresis has been reviewed by a number of authors including Talbot, Cheng, Schefer, and Willis (1980), Bakanov (1995), Li and Davis (1995a,b), and Lee and Kim (2001). ∗ Corresponding author. Tel: +82 2 958 5676; fax: +82 2 958 5805.
E-mail address: [email protected] (G.-N. Bae). 0021-8502/$ - see front matter 䉷 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaerosci.2006.07.006
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
1789
Typical characteristics of processes of thermophoresis include a hot aerosol stream flowing through a tube or an annulus, and the presence of a non-negligible thermal gradient between the aerosol stream and the cooled walls of the tube or of an outer tube of the annulus. Accordingly, many thermophoresis studies have targeted these geometries. Thermophoretic deposition of particles in an annular flow was studied theoretically by Weinberg (1983) and Fiebig, Hilgenstock, and Riemann (1988). Weinberg (1983) suggested that complete collection was possible with thermophoresis and that a smaller separation distance between concentric cylinders resulted in higher deposition efficiency. Fiebig et al. (1988) showed that when the annulus was oriented vertically, as a result of the buoyancy effect, the deposition efficiency tended to increase for a smaller ratio of inner to outer tube radius. Chang, Ranade, and Gentry (1992, 1995) carried out experiments and numerical simulations to quantify thermophoretic deposition in an annular flow system with fixed thermal gradients between two concentric cylinders. They found good agreement between experimental results and computational results using the model of Talbot et al. (1980). Lee and Kim (2001) studied thermophoretic deposition experimentally and numerically in an annular flow system using several models suggested by Derjaguin, Ravinovich, Storozhilova, and Shcherbina (1976) and Talbot et al. (1980) in a cryogenic temperature range. They found that the thermophoretic models required modification in the cryogenic temperature range. A tube flow with a thermal gradient has been utilized in many applications including heat exchanger pipes and automobile exhaust pipes. Therefore, it is necessary to study thermophoresis in a tube flow in order to understand and innovate a number of systems that are employed in a variety of applications. Thermophoresis in a laminar tube flow has been studied thoroughly. Stratmann and Fissan (1989) experimentally studied thermophoretic deposition using 0.005–0.1 m monodisperse aerosols in a laminar tube flow. Montassier, Bouland, and Renoux (1990) carried out experiments with an experimental set-up that was similar to that employed by Stratmann and Fissan (1989) except for the direction of the flow and the measuring technique. It was found that cumulative deposition increased with decreasing flow rate, and thermophoretic deposition increased with decreasing particle size (Montassier et al., 1990). There are a number of current applications that can be modelled as a turbulent aerosol pipe flow, such as automobile exhaust pipe flow. However, few have dealt with thermophoresis in turbulent pipe flows (Romay, Takagaki, Pui, & Liu, 1998). In addition, with heightened concern over ultrafine particles, there is growing demand for research that will shed light on the behaviour of ultrafine particles in a turbulent pipe flow. However, little data is available for ultrafine particles in relation to thermophoresis in a turbulent pipe flow. In the present study, the thermophoresis effect on ultrafine particles in a turbulent pipe flow was studied with the primary goal of elucidating the behaviour of ultrafine particles emitted from automobile engines via the exhaust pipe. Exhaust particles emitted from automobiles have received special attention due to their adverse health effects (Burtscher, 2005; Lighty, Veranth, & Sarofim, 2000; Wichmann & Peters, 2000). Recent concerns associated with health effects of ultrafine particles and stricter regulations pertaining to automobile emissions require reduction of ultrafine particles emitted from automobiles. Ultrafine particles could be controlled by an electric air ion emission (Lee, Yermakov, & Grinshpun, 2004b) or filtering a flow (Lee, Yermakov, & Grinshpun, 2004a, 2005). However, at present, there is no efficient technology to control ultrafine particles in automobile emission systems. Ultrafine particles generated from automobile engines pass through the exhaust pipe and several catalysts before being emitted to the environment. Therefore, in order to reduce ultrafine particle emissions, it is necessary to first understand the behaviour of ultrafine particles in the exhaust pipe. The ultrafine particle flow in an exhaust pipe could be modelled as a turbulent particle flow with thermal gradients. As stated above, in contrast to other areas of thermophoresis studies, few studies have dealt with thermophoresis of ultrafine particles in turbulent pipe flows. Both theoretical and experimental studies of thermophoresis in a turbulent pipe flow have been conducted by Byers and Calvert (1969), Nishio, Kitani, and Takahashi (1974), and Romay et al. (1998). However, these studies focused on submicron particles, which were larger than 0.1 m. The aims of the present study are to augment currently available experimental data pertaining to thermophoresis to the area of ultrafine particles in a turbulent pipe flow and to shed light on the behaviour of ultrafine particles in the automobile exhaust system using a simulation system for the exhaust pipe. From the present experimental results, it was found that thermophoresis is a major mechanism for ultrafine particle behaviour in the exhaust pipe system. Along with the experimental studies, theoretical-based computations have been carried out.
1790
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
2. Experimental method Automobiles emit an abundance of particles at high volumes with frequent variations depending on engine loads. As such, it is difficult to study the mechanisms of particle movements using particles that are directly emitted from automobiles. In order to understand the particle movement in detail, a surrogate experimental set-up for aerosols has been employed (Lee, Bae, Kim, Lee, & Yeo, 2005). In the present study, a surrogate system was employed in order to elucidate the ultrafine particle behaviour. Fig. 1 shows a schematic diagram of the experimental set-up. Ultrafine particles having similar particle size distributions as those emitted from engines were generated artificially using a manufactured combustion aerosol generator. In this combustion aerosol generator, flow rates of butane gas, air, and nitrogen gas were controlled to allow for adjustment of combustion conditions, and to create variable particle size distributions (Lee, Bae et al., 2005). Temperature of the generated aerosol flow and the carrier gas was increased by passing the flow through an electrically heated tube furnace in order to attain the level of normal engine exhaust temperature. After passing through the tube furnace, the high-temperature aerosol particles were flowed through a horizontal simulation exhaust pipe. The simulation exhaust pipe is 1.5 m in length and 0.7 cm in diameter. It is made of pyrex glass wrapped by a fabric insulator, which functions to mitigate the decrease of temperature of the flow and to maintain temperature levels similar to those measured in automobiles. This pipe was designed with a particular focus on establishing a similar particle residence time to that of an automobile exhaust pipe (∼ 0.1 s), as the particle residence time is considered to be the most important parameter in terms of particle behaviour in the pipe. As the aerosol particles passed through the pipe, they were sampled and measured through five sampling ports. The sampling ports were located at intervals of 30 cm. Particle size distributions and concentrations of the sampled aerosol particles were measured using a Scanning Mobility Particle Sizer (SMPS, (DMA 3085, CPC 3025A), TSI, USA), which could measure particle sizes as low as 3 nm. Under the experimental conditions, particle concentrations and temperature of the aerosol flow were too high. Therefore, a rotating dilutor (rotating disk dilutor type MD 19-2E, Matter Engineering AG), which could dilute particle concentrations and allow for adjustment of temperature to the measurable range of the SMPS, was utilized. At least three measurements were conducted under the same conditions and a dilution factor of 50 was used. The same dilution factor is considered in plotting the particle size distributions. Temperatures of the tube flow and the tube wall were measured at the sampling ports using a thermocouple thermometer (Hanyoung, Korea). Quenching N2
Compressed Air
CAST
thermometer Particles
MFM
Port 1
Hood Port 2
30cm Burner
Furnace
MFC
MFC
MFC
N2
Filter C3H8
30 cm
Port 4
Port 5
30 cm
Samping
Diluter
Gas In
Filter
30 cm
Port3
Air
Fig. 1. Schematics of the experimental set-up.
SMPS
Computer
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
1791
3. Results and discussion 3.1. Experimental results In order to simulate the automobile exhaust conditions, the temperature of the aerosol flow was increased to 360 ◦ C, which is within the range of automobile engine exhaust flow temperature (Hwang, Han, Yun, Kim, & Kim, 2005), by flowing the aerosol through a tube furnace. Fig. 2 shows flow temperatures and wall temperatures measured at the sampling ports. The values in Fig. 2 are the average values of three measurements. Deviations of each temperature measurement at the same conditions were less than 2 ◦ C. The temperature difference between the flow and wall was largest at port 1, approximately 250 ◦ C. The differences decreased along the pipe: 175, 125, 91 and 77 ◦ C at ports 2–5, respectively. The flow rate was 30.5 L min−1 and the Reynolds number of the experimental system at the standard condition was 5800. From these experimental conditions, the aerosol flow could be modelled as a turbulent pipe flow with thermophoresis. Fig. 3(A) shows the particle size distributions measured at five ports along the pipe at these conditions. At port 1, the first port from the combustion aerosol generator, the total concentration of particles was 1.4 × 108 particles cm−3 and the mode diameter was 31 nm. Along the simulation exhaust pipe, a large number of ultrafine aerosol particles, ranging from 20 to 50 nm in diameter, were deposited in the pipe, particularly between ports 1 and 2, as shown in Fig. 3(A). The total particle number concentrations decreased from 1.4 × 108 to 3.5 × 107 particles cm−3 , representing 75% total particle loss, between ports 1 and 2 for the conditions of Fig. 3(A). Possible deposition mechanisms in a turbulent pipe flow include thermophoretic deposition, turbulent eddy impaction, Brownian diffusion, turbulent eddy diffusion, and electrostatic precipitation (Romay et al., 1998). In a previous study, involving similar experimental conditions as the present study (a pipe diameter, 0.49 cm; Reynolds number, 1379, 5517, 9656) (Romay et al., 1998), it was found that thermophoretic deposition and turbulent eddy impaction were the two dominant mechanisms of particle deposition. In this study, in order to elucidate the deposition mechanism of ultrafine particles in the simulation pipe, the thermophoresis effect in particular was targeted and tested by varying the entry temperature of the aerosol flow from 360 to 30 ◦ C while maintaining the other experimental conditions constant. If non-thermophoretic deposition mechanisms are dominant in this system, particle loss will be similar regardless of the temperature of the aerosol flow upon entry to the pipe. This assumption was partly supported by previous findings that turbulent eddy impaction, one of two dominant mechanisms in the previous study, was not affected by an increase of temperature (Romay et al., 1998).
400 350
Flow temperature Wall temperature
Temperature (°C)
300 250 200 150 100 50 0
Port 1 0
Port 2
Port 3
Port 4
Port 5
40 60 80 20 100 120 Sampling locations on the simulation exhaust pipe (cm)
Fig. 2. Flow temperatures and wall temperatures along the simulation exhaust pipe. Temperature of aerosol flow at port 1 was 360 ◦ C.
1792
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
7e+7 port 1 2 3 4 5
4e+8 3e+8
dN/dlogDp (particles/cm3)
dN/dlogDp (particles/cm3)
5e+8
2e+8 1e+8 0 10
4e+7 3e+7 2e+7 1e+7
100
Particle diameter (nm)
(A)
1
10
100
Particle diameter (nm)
(B) 1e+8
7e+7 port 1 2 3 4 5
6e+7 5e+7
dN/dlogDp (particles/cm3)
dN/dlogDp (particles/cm3)
5e+7
0 1
4e+7 3e+7 2e+7 1e+7
port 1 2 3 4 5
8e+7 6e+7 4e+7 2e+7 0
0 1 (C)
port 1 2 3 4 5
6e+7
10 Particle diameter (nm)
100
1 (D)
10
100
Particle diameter (nm)
Fig. 3. Effect of temperature of entering aerosol flow on particle behaviours in the simulation exhaust pipe: temperatures of aerosol flow at port 1 were (A) 360 ◦ C, (B) 210 ◦ C, (C) 108 ◦ C, and (D) 30 ◦ C.
If thermophoresis were the dominant mechanism, particle loss would become much smaller when the temperature of the aerosol flow upon entry was decreased. Figs. 3(B)–(D) show the particle behaviour along the pipe with varying entry temperature, 210, 108, and 30 ◦ C, respectively. As the figures clearly indicate, as the temperature of the entering aerosol flow is decreased, the loss of ultrafine particles becomes smaller. In the case of the conditions for Fig. 3(D), at which there was almost no thermal gradient in the tube, distinct ultrafine particle loss could not be observed in the experiments. The above findings are also supported by consideration of the total number concentrations of particles. Fig. 4 shows the normalized total number concentrations along the simulation exhaust pipe. As the temperature of port 1 decreases, the loss of total number concentrations decreases. In the case of 360 ◦ C, a maximum of 75% loss of total particle number concentrations was observed whereas in the case of 30 ◦ C, the total particle concentrations fluctuated in a range of 10% of the total particle number concentrations. The particle size distributions at 108 and 30 ◦ C showed the fluctuation with no distinct particle loss in Figs. 3(C) and (D), and this fluctuation caused the partial increase of the total particle concentrations in Fig. 4. From these results, it is clear that thermophoresis is the dominant mechanism of ultrafine particle deposition in this experimental system. In Fig. 3, particles smaller than 20 nm maintained their concentration levels while passing through the pipe. The additional condensation of combustion by-products could account for this result. When a combustion aerosol generator is used, most aerosols are produced before entering a pipe system; however, by-products in vapour form could be passed
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
1793
Total particle concentrations normalized by port 1 values
2.0 360°C 210°C 108°C 30°C
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
Port 1 0
Port 2 20
Port 4
Port 3 40
60
80
Port 5 100
120
Sampling locations on the simulation exhaust pipe (cm) Fig. 4. Effect of temperature of entering aerosol flow on total particle concentrations measured at sampling ports. Temperature values on the graph legend represent temperatures of entering aerosol flow. The total particle concentrations were normalized by the values at port 1.
to the pipe system and condensed in the pipe. Therefore, particles smaller than 20 nm could be generated from this condensation while these by-products would pass the pipe with decreasing flow temperature. Therefore, it is thought that this additional condensation could maintain the concentration levels of particles smaller than 20 nm. In order to assess the replicability of the experimental results, especially in Fig. 3(A), several iterations of the experiments were conducted with different particle size distributions, which included peak concentrations ranging from 2.0 × 108 to 3.5 × 108 particles cm−3 in Figs. 5(A) and (B). In the replicated experiments the same trends were obtained. All experimental measurements shown in Fig. 3 were conducted with at least three replications. The data points shown in Fig. 3 are average values of the three measurements. The standard deviations of particle measurements were mostly around 10% (the error bars are not shown here to clearly show the trends of the results), and sometimes ranged from 10% to 30% with the exception of very low particle number concentration cases. 3.2. Theoretical analysis Several formulas have been suggested for the description of thermophoretic deposition in a turbulent pipe flow (Byers & Calvert, 1969; Nishio et al., 1974; Romay et al., 1998). These formulas are based on the assumption that the wall temperature is constant. However, in many applications, including this study, the wall temperature at the entrance is elevated due to a high aerosol flow, and as the aerosol flow cools along the pipe, the wall temperature decreases. Therefore, the aforementioned theoretical formulas are not exactly appropriate for the experimental conditions of the present study. However, for comparison, thermophoretic deposition was calculated using the most recent formula suggested by Romay et al. (1998) with some modifications of the temperature conditions. The deposition efficiencies of ultrafine particles between the ports for the conditions described in Figs. 3(A) and (B) were estimated from the following formula: L = 1 −
Tw + (Te − Tw ) exp(−DhL/QC p ) Te
P rK th ,
(1)
where is the deposition efficiency, L the pipe length, Te the gas temperature at the entrance, Tw the temperature at the pipe wall, D the pipe diameter, h represents the convective heat transfer coefficient, the gas density, Q the flow rate, Cp the gas specific heat at constant pressure, Pr the Prandtl number, and Kth is the thermophoretic coefficient.
1794
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
dN/dlogDp (particles/cm3)
2.5e+8 port 1 2 3 4 5
2.0e+8
1.5e+8
1.0e+8
5.0e+7
0.0 1 (A)
10 Particle diameter (nm)
100
10 Particle diameter (nm)
100
3.5e+8 port 1 2 3 4 5
dN/dlogDp (particles/cm3)
3.0e+8 2.5e+8 2.0e+8 1.5e+8 1.0e+8 5.0e+7 0.0 1 (B)
Fig. 5. Particle behaviour in the simulation exhaust pipe with different particle size distributions at port 1. Other experimental conditions correspond with the conditions of Fig. 3(A), including the inlet temperature. The same trend as that of Fig. 3(A) can be observed.
In the computation, L represents the distance from port 1, Te is the value as measured in the experiments, and Tw is assumed to be a middle value of wall temperatures of port 1 and other ports. D is 7 mm, and h was calculated by the following empirical formula for convective heat transfer coefficient in a pipe flow (Incropera & DeWitt, 1990): (f/8)(ReD − 1000)P r hD , = N uD = k 1 + 12.7(f/8)1/2 (P r 2/3 − 1)
(2)
f = (0.79 ln ReD − 1.64)−2 .
(3)
Kth was calculated based on the following formula (Talbot et al., 1980) : V = −2Cs
(kg /kp + Ct /R)(1 + (/R)[A1 + A2 exp(−A3R/)]) ∇T , (1 + 3Cm /R)(1 + 2kg /kp + 2Ct /R) T0
(4)
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
1795
Table 1 Particle deposition efficiencies between ports when the temperature of the aerosol flow at port 1 was 360 ◦ C Particle diameter (nm)
30 40 50
Experimental results
Computations based on formulas (1)–(4)
Ports 1–2 (%)
Ports 1–5 (%)
Ports 1–2 (%)
Ports 1–5 (%)
95 97 94
96 99 99
8 8 8
18 18 18
Table 2 Particle deposition efficiencies between ports when the temperature of the aerosol flow at port 1 was 210 ◦ C Particle diameter (nm)
20 25 30
Experimental results
Computations based on formulas (1)–(4)
Ports 1–2 (%)
Ports 1–5 (%)
Ports 1–2 (%)
Ports 1–5 (%)
36 37 29
77 93 97
6 6 6
12 12 12
where A1, A2, and A3 are the coefficients used in the Millikan drag formula, and have values of 1.2, 0.41, and 0.88, respectively. It is known that the values of coefficients are Cs =1.17, Ct =2.18, and Cm =1.14. The values of properties at the middle temperature between port 1 and other ports were used in the calculations. Tables 1 and 2 show the experimental results and the computational results based on formulas (1)–(4). In Table 1, when the temperature of the entering aerosol flow was 360 degr C, the particle deposition efficiencies for ultrafine particles, such as particles 30, 40, and 50 nm in diameter, were approximately 95% between ports 1 and 2 in the experiments. However, the deposition efficiencies as calculated using formulas (1)–(4) were only 8%. Deposition efficiencies between ports 1 and 5 were approximately 99% in the experiments, compared with 18% in the computations. In Table 2, when the temperature of the entering aerosol flow was 210 ◦ C, the condition for Fig. 3(B), the particle deposition efficiencies between ports 1 and 2 were approximately 30% in the experiments whereas the computations yielded approximately 6% deposition efficiencies. Deposition efficiencies between ports 1 and 5 ranged from 77% to 97% in the experiments compared with 12% in the computations. From this comparison, it is evident that the previous formulas are not adequate with respect to estimating experimental results in the case of high-temperature and high-concentration polydisperse ultrafine particles in a turbulent flow in a pipe characterized by decreasing wall temperature. The previous formulas are based on several assumptions such as uniform aerosol concentration distribution at each cross-section, uniform wall temperature, a fully developed turbulent flow, and no particle sources in the flow domain. Also, in previous works, monodisperse particles of small concentration were generated to test and develop the formulas. However, as in the present study, when there are a large number of polydisperse ultrafine particles, these assumptions and formulas are not appropriate. Therefore, it is necessary to develop new formulas that can estimate ultrafine particle depositions in a high-temperature aerosol pipe flow with decreasing wall temperature and a large number of polydisperse ultrafine particles. 4. Conclusions From this study, it was found that thermophoresis is the dominant mechanism for ultrafine particle deposition in a turbulent aerosol pipe flow, where the employed experimental conditions simulate the conditions of an automobile exhaust pipe flow. A discrepancy was found between the results yielded by the previous formulas and the present experimental results when different wall temperature conditions were considered. A thermophoretic deposition formula for high-concentration, high-temperature ultrafine particles in a turbulent pipe flow, which is accompanied by decreasing wall temperature, should be developed in order to estimate experimental results and for application in future studies.
1796
B.U. Lee et al. / Aerosol Science 37 (2006) 1788 – 1796
Acknowledgements This work is part of the project ‘Development of Partial Zero Emission Technology for Future Vehicle’ funded by Ministry of Commerce, Industry and Energy and we are grateful for its financial support. References Bakanov, S. P. (1995). Future directions for experiments in thermophoresis: A commentary. Journal of Aerosol Science, 26, 1–4. Burtscher, H. (2005). Physical characterization of particulate emissions from diesel engines: A review. Journal of Aerosol Science, 36, 896–932. Byers, R. L., & Calvert, S. (1969). Particle deposition from turbulent streams by means of thermal force. Industrial and Engineering Chemistry, Fundamentals, 8(4), 646–655. Chang, Y. C., Ranade, M. B., & Gentry, J. W. (1992). Experimental measurements of thermophoretic deposition of glass spheres in an annulus. Journal of Aerosol Science, 23, S31–S34. Chang, Y. C., Ranade, M. B., & Gentry, J. W. (1995). Thermophoretic deposition in flow along an annular cross-section: Experiment and simulation. Journal of Aerosol Science, 26(3), 407–428. Derjaguin, B. V., Ravinovich, Ya. I., Storozhilova, A. I., & Shcherbina, G. I. (1976). Measurement of the coefficient of thermal slip of gases and the thermophoretic velocity of large-size aerosol particles. Journal of Colloid Interface Science, 57, 451–461. Fiebig, M., Hilgenstock, M., & Riemann, H.-A. (1988). The modified chemical vapor deposition process in a concentric annulus. Aerosol Science and Technology, 9, 237–249. Hwang, S., Han, B., Yun, S., Kim, D., & Kim, Y. (2005). A study on the characteristics of exhaust particles from diesel engines with fuel injection types and after-treatment systems. Particle and Aerosol Research, 1, 61–68 (in Korean). Incropera, F. P., & DeWitt, D. P. (1990). Fundamentals of heat and mass transfer (3rd ed., p. 497). New York: Wiley. Lee, B.U., Bae, G.N., Kim, J.K., Lee, J.H., & Yeo, G.K. (2005). The behaviour of combustion aerosols in an exhaust pipe. Proceedings of the 13th international Pacific conference on automotive engineering (pp. 1036–1041). Gyeongju, Korea, August 22–24. Lee, B. U., & Kim, S. S. (2001). Thermophoresis in the cryogenic temperature range. Journal of Aerosol Science, 32, 107–119. Lee, B. U., Yermakov, M., & Grinshpun, S. A. (2004a). Unipolar ion emission enhances respiratory protection against fine and ultrafine particles. Journal of Aerosol Science, 35, 1359–1368. Lee, B. U., Yermakov, M., & Grinshpun, S. A. (2004b). Removal of fine and ultrafine particles from indoor air environments by the unipolar ion emission. Atmospheric Environment, 38(29), 4815–4823. Lee, B. U.,Yermakov, M., & Grinshpun, S. A. (2005). Filtering efficiency of N95- and R95-type facepiece respirators, dust-mist facepiece respirators, and surgical masks operating in unipolarly ionized indoor air environments. Aerosol and Air Quality Research, 5, 25–38. Li, W., & Davis, E. J. (1995a). Measurement of the thermophoretic force by electrodynamic levitation: Microspheres in air. Journal of Aerosol Science, 26, 1063–1083. Li, W., & Davis, E. J. (1995b). The effects of gas and particle properties on thermophoresis. Journal of Aerosol Science, 26, 1085–1099. Lighty, J. S., Veranth, J. M., & Sarofim, A. F. (2000). Combustion aerosols: Factors governing their size and composition and implications to human health. Journal of the Air and Waste Management Association, 50, 1565–1618. Montassier, N., Bouland, D., & Renoux, A. (1990). Experimental study of thermophoretic deposition of particles in laminar tube flow. Proceedings of the third international aerosol conference (pp. 395–398). Kyoto, Japan. Nishio, G., Kitani, S., & Takahashi, K. (1974). Thermophoretic deposition of aerosol particles in a heat-exchanger pipe. Industrial and Engineering Chemistry Process Design Development, 13(4), 408–415. Romay, F. J., Takagaki, S. S., Pui, D. Y. H., & Liu, B. Y. H. (1998). Thermophoretic deposition of aerosol particles in turbulent pipe flow. Journal of Aerosol Science, 29, 943–959. Stratmann, F., & Fissan, H. (1989). Experimental and theoretical study of submicron particle transport in cooled laminar tube flow due to combined convection, diffusion, and thermophoresis. Journal of Aerosol Science, 20, 899–902. Talbot, L., Cheng, R. K., Schefer, R. W., & Willis, D. R. (1980). Thermophoresis of particles in a heated boundary layer. Journal of Fluid Mechanics, 101, 737–758. Weinberg, M. C. (1983). Thermophoretic deposition of particles in laminar flow in a concentric annulus. Journal of the American Ceramic Society, 66, 439–443. Wichmann, H. E., & Peters, A. (2000). Epidemiological evidence of the effects of ultrafine particle exposure. Philosophical Transactions of the Royal Society of London, Series A, 358, 2751–2768.