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Measurement of emission factor of road dust in a wind tunnel
Powder Technology 118 Ž2001. 10–15 www.elsevier.comrlocaterpowtec
Measurement of emission factor of road dust in a wind tunnel Shin-Fu Chiou, Chuen-Jinn Tsai ) Institute of EnÕironmental Engineering, National Chiao Tung UniÕersity, 75 Poai Street, Hsin-Chu 300, Taiwan
Abstract One of the critical problems in making an effective strategy to control road dust emission is to estimate the emission factor accurately. A wind tunnel was built in the laboratory to measure the emission factor of road dust under different wind acceleration conditions. Test road dusts were collected from two sites, one from the construction site where a DRAM factory was being constructed in the Science Industrial Park of Hsin-Chu Ždust sample a1. and the other from an unpaved road at Nan Laio area of Hsin-Chu Ždust sample a2.. The dust samples were first sieved using a standard No. 325 mesh to remove particles greater than 44 mm. Thereafter, the dust samples were dried and then contained within an aluminum cell of 5 Žwidth. = 5 Žlength. = 0.2 cm Ždepth., which was embedded in a working platform in the wind tunnel. The surface of dust samples was kept flush with the plate before testing. Test results show that the air acceleration rate affects the emission factor significantly. As the air acceleration rate is increased from 0.1 to 1.5 mrs 2 , the emission factor is increased linearly from 1.0 to 7.0 = 10y4 kgrm2-s for sample a1, and from 0.1 to 1.0 = 10y4 kgrm2-s for sample a2. Test results show that the air acceleration rate has no significant effect on the threshold wind speed for reentrainment, which was found to be 10–12 mrs for sample a1, and 9–10 mrs for sample a2. While the edge effect at the surface of the dust sample has no significant effect on the threshold wind speed for reentrainment, it increases both reentrained dust concentration and emission factor significantly. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Road dust; Reentrainment; Wind tunnel; Emission factor
1. Introduction PM 10 Žsuspended particulate matter with aerodynamic diameters less than 10 mm. has been one of the most serious air pollutants for many years in Taiwan w1,2x. According to field studies, the daily average PM 10 mass concentrations often exceed the PM 10 air quality standard, which is 150 mgrm3. To develop a better control strategy, one of the major uncertainties in estimating PM 10 contribution from road dust is the emission factor. Many factors will affect the emission factor of the road dust in the field, including water content of the road dust, wind speed, mechanical disturbance, silt loading and so on w3,4x. In this study, we investigated the influence of different air acceleration rates and different surface conditions of the dust sample on the emission factor by conducting the experiments in a wind tunnel. It is expected that the results from this study will be helpful for estimating the emission factor
)
Corresponding author. Tel.: q886-3573-1880; fax: q886-3572-7835. E-mail address: [email protected] ŽC.-J. Tsai..
of the road dust in the field and for better understanding the behavior of the particles reentrained from the surface. The mechanism of the particle reentrainment from the surface has been widely studied both theoretically and experimentally by many researchers w5–10x. Three different models have been proposed to explain the phenomenon of the particle reentrainment from the surface by the airflow, which are based on the force balance, moment balance and energy balance, respectively. In the force balance model, the critical condition for the particle reentrainment is when the lift force exerted on the particles by the airflow exceeds the adhesion force between the particles and the attached surface. In the moment balance model, the airflow is assumed to impose a bending moment on the particles. When it overcomes the moment from the adhesion force on the particles, the reentrainment of the particles will occur and particles will roll on the surface first and subsequently detach from the surface w11–14x. In the energy balance model, the attached particles are assumed to accumulate the energy obtained from the flow field through vibration. Once the energy is greater than the proposed adhesive potential well of the particles, the particles will escape from the surface w15,16x.
0032-5910r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 1 . 0 0 2 8 9 - 3
S.-F. Chiou, C.-J. Tsai r Powder Technology 118 (2001) 10–15
However, these models can’t be applied directly to this study because most of them are related to the reentrainment of the monolayer or monodispersed particles from the surface. Because in actual field conditions, most particles are polydispersed and multilayers of particle reentrainment must be considered.
2. Experimental 2.1. Wind tunnel system Fig. 1Ža. shows a wind tunnel system used in this study to simulate the reentrainment of road dust from a surface. The working section of the wind tunnel is 30 cm in diameter. At the entrance of the wind tunnel, HEPA filters were installed to prevent the suspended particles in the air from entering the wind tunnel. The air was sucked into the wind tunnel by a 5-hp fan and its rotational speed, hence the wind speed was controlled by an inverter. The maximum wind speed was controlled to under 15 mrs by a damper in this study. Air temperature was from 178C to 248C, and relative humidity was between 55% and 75%. A honeycomb was used to smooth the airflow before a hollow working platform, on which dust samples were tested. To avoid the edge effect, the sample surface was kept flush with the top surface of the working platform, as shown in Fig. 1Žb.. An aluminum cell with inner dimen-
11
sions of 5 Žlength. = 5 Žwidth. = 0.2 cm Ždepth. was used to contain the dust samples. The cell containing the dust sample was then embedded into an opening on the working platform. Front and rear edges of the working platform were smoothed to prevent flow separation. Ten sampling tubes Ži.d.: 0.75 cm. were installed in the downstream of the working platform to collect the reentrained particles by filter holders, as shown in Fig. 2. Filter holders containing glass fiber filter Ž47 mm in diameter. were operated at a flow rate of about 10 lrmin. The inlets were located at the same cross-section of the wind tunnel, which was 70 cm away from the downstream of the dust sample. The relative positions of the 10 sampling tubes were adjusted vertically and horizontally, until no particles were detected between the sampling area and the wall of the wind tunnel. The inlet edge of each sampling tube was sharpened to reduce the particle bounce. After sampling, the tubes were purged with clean air for a short time to remove particles lost inside the tubes. To determine the threshold reentrainment velocity, the filter holder at S5 position was replaced by a DustTrak PM 10 sampler ŽModel 8520, TSI, St. Paul, MN, USA. for real-time measurement. In most cases, the surface of the dust sample was smooth and flush with the platform. In some cases, the rear edge of the dust sample was cut into two cavities to determine the edge effect on the threshold reentrainment velocity and emission factor. The total projected area of the two cavities were about 3% of the total surface area of the sample, which is 25 cm2 .
Fig. 1. Ža. Schematic drawing of the wind tunnel, Žb. the enlarged diagram of the working platform with the dust sample. ŽUnits in centimeters..
12
S.-F. Chiou, C.-J. Tsai r Powder Technology 118 (2001) 10–15
2.3. Calculation of emission factor In this study, 10 filter holders connected to 10 sampling tubes were used to measure the total mass of the reentrained particles for further calculation of the emission factor of the dust sample. When measuring the particle concentration, aspiration efficiency and transmission loss of the sampling tube have to be considered. Sampling efficiency Es , is defined as w17x Es s
Fig. 2. Relative positions of 10 sampling inlets. ŽUnits in centimeters..
measured mass concentration actual mass concentration
s Ea Et
Ž 1.
where Ea s aspiration efficiency and Et s transmission efficiency. In this study, we used the clean air to purge the particles lost in the sampling tube into the filter for a short time after the reentrainment experiment. Therefore, the transmission efficiency can be considered as 1.0. For the aspiration efficiency, Durham and Lundgren w18x proposed the following model Ea s
Ci Cw
ž
s1y 1y
1
Ž 2 q 0.62 k . Stk , Stk - 6.0 k 1 q Ž 2 q 0.62 k . Stk Ž 2.
/
Wind speed distribution at the location of the sampling cross-section was measured using a hot-wire anemometer ŽModel 8352-3, TSI. and was used to calculate the total mass of the reentrained particles. The wind speed was also measured at 0.5 cm above the sample surface. Water content of the dust sample during each experiment was under 0.6%. Two MOUDIs Žmicroorifice uniform deposit impactor, Model 100, MSP, Minneapolis, MN, USA. were connected to the S2 and S9 sampling tubes for determining the initial size distribution of the dust samples, which were dispersed by a dust feeder ŽWright Dust Feeder, WDF-II, BGI, Waltham, MA, USA. installed at the H1 opening, as shown in Fig. 1Ža.. The reentrained dust distribution was measured by a MOUDI connected to the S5 sampling tube. The temperature and relative humidity in the wind tunnel were measured by a Qtrak ŽModel 8551, TSI..
where Ci is the particle mass concentration at the inlet of the sampling tube; C w is the particle mass concentration in the free stream; k s ŽUi .rŽUw ., Ui s the sampling speed; Uw s the wind speed, Stk s Ž rp d p2 Uw .rŽ18 m D i . Ž rp s particle density; d p s particle diameter; m s air viscosity; D i s inlet diameter.. A MOUDI connected to the S5 sampling tube was used to measure the reentrained size distribution, and the size distribution was used to calculate the aspiration efficiency, hence the actual mass concentration in the free stream. Total reentrained mass of the particles, M, can be calculated from the mass concentration, sampling flow rate and total sampling time. Therefore, emission factor of the dust sample can be derived as
2.2. Dust samples
Es
Test road dusts were collected from two sites, one from the construction site where a DRAM factory was being constructed at the Science Industrial Park of Hsin-Chu Ždust sample a1. and the other from an unpaved road at Nan Laio area of Hsin-Chu Ždust sample a2.. A vacuum cleaner was used to collect the dust samples from the ground. The dust samples were dried at 1058C for 24 h in an oven. The dried dust samples were sieved using a standard No. 325 mesh to remove particles greater than 44 mm. The sieved dust sample was put into the cell Ž5 cm long = 5 cm wide = 0.2 cm deep. and the surface was made flush using a sharp knife edge. The dust-loaded cell was further dried for 30 min, and weighed after it was cooled down in a sealed container.
where A 0 is the surface area of the dust sample, and T is the total sampling time.
M A0T
Ž 3.
3. Results and discussion Table 1 shows the size distributions of the initial dust samples and the reentrained dust at an air acceleration rate of 1.5 mrs 2 . Most particles in the initial test samples are seen to be greater than 10 mm in aerodynamic diameter, so are the reentrained dusts. In terms of percentage of total mass, particles greater than 10 mm were 99.2% and 91.7% for initial dust samples a1 and a2, respectively, and 90.6% and 92.6% for reentrained samples a1 and a2,
S.-F. Chiou, C.-J. Tsai r Powder Technology 118 (2001) 10–15 Table 1 The size distribution of the initial dust samples and reentrained dusts Žair acceleration rate of 1.5 mrs 2 .
13
respectively. According to the initial size distribution shown in Table 1, dust sample a2 has more small particles than dust sample a1 for particles less than 3.2 mm, which are referred to as clay. These small particles play an important role in particle reentrainment. If more small particles occupy the vacancy between the large particles, the increasing contact area may result in the increase of attraction force between the particles. Therefore, it is expected that it is more difficult to reentrain large particles of sample a2 than sample a1. That is, the emission factor for sample a2 is smaller than that for sample a1 at the same air acceleration rate. A DustTrak connected to the S5 sampling tube was used to determine the real-time reentrained dust concentration and threshold wind speed of reentrainment. The measured PM 10 concentration of a dichotomous PM 10 sampler ŽModel SA241, Graseby Andersen, 500 Technology Court, Smyrna, GA, USA. was compared to that measured by the DustTrak at field conditions and a coefficient of correlation Ž1.16. was found. Fig. 3 shows that time-averaged
dichot-equivalent PM 10 concentration measured by the DustTrak with the maximum wind speed at two different air acceleration rates for sample a1. The time-averaged dichot-equivalent PM 10 concentration was calculated based on the real-time dust concentrations sampled from zero wind speed and continued for 180 s until after the maximum wind speed was reached. Particle reentrainment does not seem to occur until the maximum wind speed exceeds a critical value, which is referred to as the threshold wind speed for reentrainment. In this experiment, the threshold wind speed was found to be 10–12 mrs, irrespective of air acceleration rate. When two cavities Žabout 3% of the surface area of the test sample. appeared on the surface of test sample a2, Fig. 4 shows that the threshold wind speed, 9–10 mrs, does not change too much. However, the edge effect due to the cavities increases the reentrained dust concentration by about four times compared to that without edge effect at the maximum wind speed of 15 mrs. In order to know the difference in the reentrained dust concentrations between two consecutive dust reentrainments, a second reentrainment experiment was conducted immediately after the first experiment was over for sample a1 at air acceleration rate of 1.5 mrs 2 . Fig. 5 shows that when there is no edge effect Ži.e. no cavities on the sample surface., the reentrained dust concentration of the second reentrainment test is much lower than that of the first reentrainment test, and approaches zero concentration. That is, most of the loose particles in the uppermost layers were reentrained at the first reentrainment test, leaving particles that were tightly connected and difficult to remove. Lazaridis and Drossinos w16x also found that particles in the lower layer were harder to reentrain than those in the upper layer. As seen in Fig. 5, the reentrained dust concentration decays quickly after a maximum concentration is reached.
Fig. 3. Relationship of time-averaged dichot-equivalent PM 10 concentration measured by the DustTrak with maximum wind speed. Two different air acceleration rates, flat surface Žno edge effect., sample a1.
Fig. 4. Relationship of time-averaged dichot-equivalent PM 10 concentration measured by the DustTrak with maximum wind speed. With and without edge effect, air acceleration rate: 1.5 mrs 2 , sample a2.
Particle size Žmm.
Sample a1 Žmass %. Initial Reentrained
Sample a2 Žmass %. Initial Reentrained
18–29.1 10–18 5.6–10 3.2–5.6 1.8–3.2 1.0–1.8 0.56–1.0 0.32–0.56 0.18–0.32 - 0.053
94.0 5.2 0.3 0.2 0.1 0.1 0.1 0.1 0.0 0.0
76.9 14.8 2.5 1.9 1.8 1.1 0.5 0.4 0.3 0.0
83.0 7.6 2.3 1.7 1.5 1.3 1.3 1.3 0.0 0.0
79.7 12.9 2.0 1.9 0.9 0.9 1.8 0.0 0.0 0.0
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S.-F. Chiou, C.-J. Tsai r Powder Technology 118 (2001) 10–15
Fig. 5. Time-averaged dichot-equivalent PM 10 concentration measured by the DustTrak vs. sampling time. Two consecutive reentrainments, air acceleration rate: 1.5 mrs 2 , maximum wind speed: 15 mrs, flat surface Žno edge effect., sample a1.
Fig. 7. Emission factors for the test samples with cavities Žwith edge effect. and without cavities Žno edge effect. at different air acceleration rates. Maximum wind speed: 15 mrs, sample a1.
This phenomenon was also observed by Matsusaka and Masuda w14x. They stated that the decay rate of the reentrained dust concentration is higher for larger air acceleration rates, which was observed in this study as well. Fig. 6 shows the emission factor calculated from dust concentrations measured by 10 filters vs. the different air acceleration rates for two dust samples with flat surface Žno edge effect.. As the air acceleration rate is increased from 0.1 to 1.5 mrs 2 , the emission factor is increased linearly from 1.0 to 7.0 = 10y4 kgrm2 s for sample a1, and from 0.1 to 1.0 = 10y4 kgrm2 s for sample a2. It is seen that the emission factor of sample a1 is much greater than that of sample a2 at the same air acceleration rate. The former is about seven times as much as the latter. This is caused by the existence of more small particles in
sample a2, thus enhancing the interaction between the particles, as explained earlier. The existence of cavities on the surface of the dust sample increase the emission factor greatly, as shown in Fig. 7 for sample a1. The two cavities occupying 3% of the sample surface are seen to result in an edge effect, which increases the emission factor by as much as 20 times, although the threshold wind speed is not affected, as shown in Fig. 4.
Fig. 6. Emission factor vs. air acceleration rate. Maximum wind speed: 15 mrs, flat surface Žno edge effect..
4. Conclusions One of the critical problems in making an effective strategy to control road dust emission is to estimate the emission factor accurately. A wind tunnel was built in the laboratory to measure the emission factor of road dust under different wind acceleration conditions. Test road dusts were collected from two sites, one from the construction site where a DRAM factory was being constructed in the Science Industrial Park of Hsin-Chu Ždust sample a1., and the other from an unpaved road at the Nan Laio area of Hsin-Chu Ždust sample a2.. The dust samples were first sieved using a standard No. 325 mesh to remove particles greater than 44 mm. Thereafter, the dust samples were dried and then contained within an aluminium cell of 5 cm Žwidth. = 5 cm Žlength. = 0.2 cm Ždepth., which was embedded in a working platform in the wind tunnel. The surface of dust samples was kept flush with the plate before testing. Test results show that the air acceleration rate affects the emission factor very much. As the air acceleration rate is increased from 0.1 to 1.5 mrs 2 , the emission factor is increased linearly from 1.0 to 7.0 = 10y4 kgrm2 s for sample a1, and from 0.1 to 1.0 = 10y4 kgrm2 s for sample a2.
S.-F. Chiou, C.-J. Tsai r Powder Technology 118 (2001) 10–15
Some conclusions can be drawn from this study. 1. The air acceleration rate has no significant effect on the threshold wind speed for reentrainment, which was found to be 9–12 mrs. 2. The edge effect at the surface of the dust sample has no significant effect on the threshold wind speed for reentrainment either. 3. When the edge effect appears, both reentrained dust concentration and emission factor will become much higher than those without the edge effect. For sample a1, the edge effect increases the emission factor by as much as 20 times. 4. As the air acceleration rate is increased from 0.1 to 1.5 mrs 2 , the emission factor is increased linearly from 1.0 to 7.0 = 10y4 kgrm2 s for sample a1, and from 0.1 to 1.0 = 10y4 kgrm2 s for sample a2. Much more clay in sample a2 than that in sample a1 results in a stronger adhesion between particles, which leads to a smaller emission factor in the former.
Acknowledgements The authors gratefully acknowledge the financial support of the Taiwan National Science Council of the Republic of China under grant NSC88-EPA-Z009-001.
References w1x C.H. Hung, C.S. Yuan, C.H. Shih, E. Wu, Characteristics of air quality in Kaohsiung harbor tunnel,The 1998 International Conference on Aerosol Science and Technology, PingTong, Taiwan, 1998, pp. 385–397. w2x W.L. Lan, C.T. Chang, Y.P. Wu, J.H. Lin, U.M. Chen, Study on the fugitive dust emission characteristics in gravel processing sites,The 1998 International Conference on Aerosol Science and Technology, PingTong, Taiwan, 1998, pp. 445–449.
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w3x C.J. Tsai, D.Y. Miaw, S.F. Chiou, T.Y. Lin, J.B. Chong, C.H. Huang, S.H. Wang, The investigation of domestic control technologies for fugitive particle pollutants,The 1998 International Conference on Aerosol Science and Technology, PingTong, Taiwan, 1998, pp. 417–426. w4x M.T. Cheng, Y.C. Lin, S.J. Jeang, S.L. Yen, Characteristics and emission factor of traffic road dust,The 1998 International Conference on Aerosol Science and Technology, PingTong, Taiwan, 1998, pp. 427–435. w5x B.V. Deryagin, V.M. Muller, Y.P. Toporov, Effect of contact deformation on the adhesion of particles, J. Colloid Interface Sci. 53 Ž1975. 314–326. w6x K.L. Johnson, K. Kendall, A.D. Roberts, Surface energy and the contact of elastic solids, Proc. R. Soc., Ser. A 324 Ž1971. 301–313. w7x J.W. Cleaver, B. Yates, Mechanism of detachment of colloidal particles from a flat substrate in a turbulent flow, J. Colloid Interface Sci. 44 Ž1973. 464–473. w8x H.Y. Wen, G. Kasper, On the kinetics of particle reentrainment from surfaces, J. Aerosol Sci. 20 Ž1989. 483–498. w9x D.A. Braaten, R.H. Shaw, U.K.T. Paw, Particle resuspension in a turbulent boundary layer, J. Aerosol Sci. 21 Ž1990. 613–628. w10x S. Matsusaka, T. Aoyagi, H. Masuda, Unsteady particle-reentrainment model based on the internal adhesive strength distribution of a fine powder layer, Kagaku Kogaku Ronbunshu 17 Ž1991. 1194– 1200. w11x Y. Kousaka, K. Okuyama, Y. Endo, Reentrainment of small aggregate particles from a plane surface by air stream, J. Chem. Eng. Jpn. 13 Ž1980. 143–147. w12x H.C. Wang, Effects of inceptive motion on particle detachment from surfaces, J. Aerosol Sci. 13 Ž1990. 386–393. w13x C.J. Tsai, D.Y.H. Pui, B.Y.H. Liu, Elastic flattening and particle adhesion, Aerosol Sci. Technol. 15 Ž1991. 239–255. w14x S. Matsusaka, H. Masuda, Particle reentrainment from a fine powder layer in a turbulent air flow, Aerosol Sci. Technol. 24 Ž1996. 69–84. w15x M.W. Reeks, J. Reed, D. Hall, On the resuspension of small particles by a turbulent flow, J. Phys. D: Appl. Phys. 21 Ž1988. 574–589. w16x M. Lazaridis, Y. Drossinos, Multilayer resuspension of small identical particles by turbulent flow, Aerosol Sci. Technol. 28 Ž1998. 548–560. w17x S. Hangal, K. Willeke, Aspiration efficiency: unified model for all forward sampling angles, Environ. Sci. Technol. 24 Ž1990. 688–691. w18x M.D. Durham, D.H. Lundgren, Evaluation of aerosol aspiration efficiency as a function of Stokes number, velocity ratio and nozzle angle, J. Aerosol Sci. 11 Ž1980. 179–188.
Measurement of emission factor of road dust in a wind tunnel Shin-Fu Chiou, Chuen-Jinn Tsai ) Institute of EnÕironmental Engineering, National Chiao Tung UniÕersity, 75 Poai Street, Hsin-Chu 300, Taiwan
Abstract One of the critical problems in making an effective strategy to control road dust emission is to estimate the emission factor accurately. A wind tunnel was built in the laboratory to measure the emission factor of road dust under different wind acceleration conditions. Test road dusts were collected from two sites, one from the construction site where a DRAM factory was being constructed in the Science Industrial Park of Hsin-Chu Ždust sample a1. and the other from an unpaved road at Nan Laio area of Hsin-Chu Ždust sample a2.. The dust samples were first sieved using a standard No. 325 mesh to remove particles greater than 44 mm. Thereafter, the dust samples were dried and then contained within an aluminum cell of 5 Žwidth. = 5 Žlength. = 0.2 cm Ždepth., which was embedded in a working platform in the wind tunnel. The surface of dust samples was kept flush with the plate before testing. Test results show that the air acceleration rate affects the emission factor significantly. As the air acceleration rate is increased from 0.1 to 1.5 mrs 2 , the emission factor is increased linearly from 1.0 to 7.0 = 10y4 kgrm2-s for sample a1, and from 0.1 to 1.0 = 10y4 kgrm2-s for sample a2. Test results show that the air acceleration rate has no significant effect on the threshold wind speed for reentrainment, which was found to be 10–12 mrs for sample a1, and 9–10 mrs for sample a2. While the edge effect at the surface of the dust sample has no significant effect on the threshold wind speed for reentrainment, it increases both reentrained dust concentration and emission factor significantly. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Road dust; Reentrainment; Wind tunnel; Emission factor
1. Introduction PM 10 Žsuspended particulate matter with aerodynamic diameters less than 10 mm. has been one of the most serious air pollutants for many years in Taiwan w1,2x. According to field studies, the daily average PM 10 mass concentrations often exceed the PM 10 air quality standard, which is 150 mgrm3. To develop a better control strategy, one of the major uncertainties in estimating PM 10 contribution from road dust is the emission factor. Many factors will affect the emission factor of the road dust in the field, including water content of the road dust, wind speed, mechanical disturbance, silt loading and so on w3,4x. In this study, we investigated the influence of different air acceleration rates and different surface conditions of the dust sample on the emission factor by conducting the experiments in a wind tunnel. It is expected that the results from this study will be helpful for estimating the emission factor
)
Corresponding author. Tel.: q886-3573-1880; fax: q886-3572-7835. E-mail address: [email protected] ŽC.-J. Tsai..
of the road dust in the field and for better understanding the behavior of the particles reentrained from the surface. The mechanism of the particle reentrainment from the surface has been widely studied both theoretically and experimentally by many researchers w5–10x. Three different models have been proposed to explain the phenomenon of the particle reentrainment from the surface by the airflow, which are based on the force balance, moment balance and energy balance, respectively. In the force balance model, the critical condition for the particle reentrainment is when the lift force exerted on the particles by the airflow exceeds the adhesion force between the particles and the attached surface. In the moment balance model, the airflow is assumed to impose a bending moment on the particles. When it overcomes the moment from the adhesion force on the particles, the reentrainment of the particles will occur and particles will roll on the surface first and subsequently detach from the surface w11–14x. In the energy balance model, the attached particles are assumed to accumulate the energy obtained from the flow field through vibration. Once the energy is greater than the proposed adhesive potential well of the particles, the particles will escape from the surface w15,16x.
0032-5910r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 1 . 0 0 2 8 9 - 3
S.-F. Chiou, C.-J. Tsai r Powder Technology 118 (2001) 10–15
However, these models can’t be applied directly to this study because most of them are related to the reentrainment of the monolayer or monodispersed particles from the surface. Because in actual field conditions, most particles are polydispersed and multilayers of particle reentrainment must be considered.
2. Experimental 2.1. Wind tunnel system Fig. 1Ža. shows a wind tunnel system used in this study to simulate the reentrainment of road dust from a surface. The working section of the wind tunnel is 30 cm in diameter. At the entrance of the wind tunnel, HEPA filters were installed to prevent the suspended particles in the air from entering the wind tunnel. The air was sucked into the wind tunnel by a 5-hp fan and its rotational speed, hence the wind speed was controlled by an inverter. The maximum wind speed was controlled to under 15 mrs by a damper in this study. Air temperature was from 178C to 248C, and relative humidity was between 55% and 75%. A honeycomb was used to smooth the airflow before a hollow working platform, on which dust samples were tested. To avoid the edge effect, the sample surface was kept flush with the top surface of the working platform, as shown in Fig. 1Žb.. An aluminum cell with inner dimen-
11
sions of 5 Žlength. = 5 Žwidth. = 0.2 cm Ždepth. was used to contain the dust samples. The cell containing the dust sample was then embedded into an opening on the working platform. Front and rear edges of the working platform were smoothed to prevent flow separation. Ten sampling tubes Ži.d.: 0.75 cm. were installed in the downstream of the working platform to collect the reentrained particles by filter holders, as shown in Fig. 2. Filter holders containing glass fiber filter Ž47 mm in diameter. were operated at a flow rate of about 10 lrmin. The inlets were located at the same cross-section of the wind tunnel, which was 70 cm away from the downstream of the dust sample. The relative positions of the 10 sampling tubes were adjusted vertically and horizontally, until no particles were detected between the sampling area and the wall of the wind tunnel. The inlet edge of each sampling tube was sharpened to reduce the particle bounce. After sampling, the tubes were purged with clean air for a short time to remove particles lost inside the tubes. To determine the threshold reentrainment velocity, the filter holder at S5 position was replaced by a DustTrak PM 10 sampler ŽModel 8520, TSI, St. Paul, MN, USA. for real-time measurement. In most cases, the surface of the dust sample was smooth and flush with the platform. In some cases, the rear edge of the dust sample was cut into two cavities to determine the edge effect on the threshold reentrainment velocity and emission factor. The total projected area of the two cavities were about 3% of the total surface area of the sample, which is 25 cm2 .
Fig. 1. Ža. Schematic drawing of the wind tunnel, Žb. the enlarged diagram of the working platform with the dust sample. ŽUnits in centimeters..
12
S.-F. Chiou, C.-J. Tsai r Powder Technology 118 (2001) 10–15
2.3. Calculation of emission factor In this study, 10 filter holders connected to 10 sampling tubes were used to measure the total mass of the reentrained particles for further calculation of the emission factor of the dust sample. When measuring the particle concentration, aspiration efficiency and transmission loss of the sampling tube have to be considered. Sampling efficiency Es , is defined as w17x Es s
Fig. 2. Relative positions of 10 sampling inlets. ŽUnits in centimeters..
measured mass concentration actual mass concentration
s Ea Et
Ž 1.
where Ea s aspiration efficiency and Et s transmission efficiency. In this study, we used the clean air to purge the particles lost in the sampling tube into the filter for a short time after the reentrainment experiment. Therefore, the transmission efficiency can be considered as 1.0. For the aspiration efficiency, Durham and Lundgren w18x proposed the following model Ea s
Ci Cw
ž
s1y 1y
1
Ž 2 q 0.62 k . Stk , Stk - 6.0 k 1 q Ž 2 q 0.62 k . Stk Ž 2.
/
Wind speed distribution at the location of the sampling cross-section was measured using a hot-wire anemometer ŽModel 8352-3, TSI. and was used to calculate the total mass of the reentrained particles. The wind speed was also measured at 0.5 cm above the sample surface. Water content of the dust sample during each experiment was under 0.6%. Two MOUDIs Žmicroorifice uniform deposit impactor, Model 100, MSP, Minneapolis, MN, USA. were connected to the S2 and S9 sampling tubes for determining the initial size distribution of the dust samples, which were dispersed by a dust feeder ŽWright Dust Feeder, WDF-II, BGI, Waltham, MA, USA. installed at the H1 opening, as shown in Fig. 1Ža.. The reentrained dust distribution was measured by a MOUDI connected to the S5 sampling tube. The temperature and relative humidity in the wind tunnel were measured by a Qtrak ŽModel 8551, TSI..
where Ci is the particle mass concentration at the inlet of the sampling tube; C w is the particle mass concentration in the free stream; k s ŽUi .rŽUw ., Ui s the sampling speed; Uw s the wind speed, Stk s Ž rp d p2 Uw .rŽ18 m D i . Ž rp s particle density; d p s particle diameter; m s air viscosity; D i s inlet diameter.. A MOUDI connected to the S5 sampling tube was used to measure the reentrained size distribution, and the size distribution was used to calculate the aspiration efficiency, hence the actual mass concentration in the free stream. Total reentrained mass of the particles, M, can be calculated from the mass concentration, sampling flow rate and total sampling time. Therefore, emission factor of the dust sample can be derived as
2.2. Dust samples
Es
Test road dusts were collected from two sites, one from the construction site where a DRAM factory was being constructed at the Science Industrial Park of Hsin-Chu Ždust sample a1. and the other from an unpaved road at Nan Laio area of Hsin-Chu Ždust sample a2.. A vacuum cleaner was used to collect the dust samples from the ground. The dust samples were dried at 1058C for 24 h in an oven. The dried dust samples were sieved using a standard No. 325 mesh to remove particles greater than 44 mm. The sieved dust sample was put into the cell Ž5 cm long = 5 cm wide = 0.2 cm deep. and the surface was made flush using a sharp knife edge. The dust-loaded cell was further dried for 30 min, and weighed after it was cooled down in a sealed container.
where A 0 is the surface area of the dust sample, and T is the total sampling time.
M A0T
Ž 3.
3. Results and discussion Table 1 shows the size distributions of the initial dust samples and the reentrained dust at an air acceleration rate of 1.5 mrs 2 . Most particles in the initial test samples are seen to be greater than 10 mm in aerodynamic diameter, so are the reentrained dusts. In terms of percentage of total mass, particles greater than 10 mm were 99.2% and 91.7% for initial dust samples a1 and a2, respectively, and 90.6% and 92.6% for reentrained samples a1 and a2,
S.-F. Chiou, C.-J. Tsai r Powder Technology 118 (2001) 10–15 Table 1 The size distribution of the initial dust samples and reentrained dusts Žair acceleration rate of 1.5 mrs 2 .
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respectively. According to the initial size distribution shown in Table 1, dust sample a2 has more small particles than dust sample a1 for particles less than 3.2 mm, which are referred to as clay. These small particles play an important role in particle reentrainment. If more small particles occupy the vacancy between the large particles, the increasing contact area may result in the increase of attraction force between the particles. Therefore, it is expected that it is more difficult to reentrain large particles of sample a2 than sample a1. That is, the emission factor for sample a2 is smaller than that for sample a1 at the same air acceleration rate. A DustTrak connected to the S5 sampling tube was used to determine the real-time reentrained dust concentration and threshold wind speed of reentrainment. The measured PM 10 concentration of a dichotomous PM 10 sampler ŽModel SA241, Graseby Andersen, 500 Technology Court, Smyrna, GA, USA. was compared to that measured by the DustTrak at field conditions and a coefficient of correlation Ž1.16. was found. Fig. 3 shows that time-averaged
dichot-equivalent PM 10 concentration measured by the DustTrak with the maximum wind speed at two different air acceleration rates for sample a1. The time-averaged dichot-equivalent PM 10 concentration was calculated based on the real-time dust concentrations sampled from zero wind speed and continued for 180 s until after the maximum wind speed was reached. Particle reentrainment does not seem to occur until the maximum wind speed exceeds a critical value, which is referred to as the threshold wind speed for reentrainment. In this experiment, the threshold wind speed was found to be 10–12 mrs, irrespective of air acceleration rate. When two cavities Žabout 3% of the surface area of the test sample. appeared on the surface of test sample a2, Fig. 4 shows that the threshold wind speed, 9–10 mrs, does not change too much. However, the edge effect due to the cavities increases the reentrained dust concentration by about four times compared to that without edge effect at the maximum wind speed of 15 mrs. In order to know the difference in the reentrained dust concentrations between two consecutive dust reentrainments, a second reentrainment experiment was conducted immediately after the first experiment was over for sample a1 at air acceleration rate of 1.5 mrs 2 . Fig. 5 shows that when there is no edge effect Ži.e. no cavities on the sample surface., the reentrained dust concentration of the second reentrainment test is much lower than that of the first reentrainment test, and approaches zero concentration. That is, most of the loose particles in the uppermost layers were reentrained at the first reentrainment test, leaving particles that were tightly connected and difficult to remove. Lazaridis and Drossinos w16x also found that particles in the lower layer were harder to reentrain than those in the upper layer. As seen in Fig. 5, the reentrained dust concentration decays quickly after a maximum concentration is reached.
Fig. 3. Relationship of time-averaged dichot-equivalent PM 10 concentration measured by the DustTrak with maximum wind speed. Two different air acceleration rates, flat surface Žno edge effect., sample a1.
Fig. 4. Relationship of time-averaged dichot-equivalent PM 10 concentration measured by the DustTrak with maximum wind speed. With and without edge effect, air acceleration rate: 1.5 mrs 2 , sample a2.
Particle size Žmm.
Sample a1 Žmass %. Initial Reentrained
Sample a2 Žmass %. Initial Reentrained
18–29.1 10–18 5.6–10 3.2–5.6 1.8–3.2 1.0–1.8 0.56–1.0 0.32–0.56 0.18–0.32 - 0.053
94.0 5.2 0.3 0.2 0.1 0.1 0.1 0.1 0.0 0.0
76.9 14.8 2.5 1.9 1.8 1.1 0.5 0.4 0.3 0.0
83.0 7.6 2.3 1.7 1.5 1.3 1.3 1.3 0.0 0.0
79.7 12.9 2.0 1.9 0.9 0.9 1.8 0.0 0.0 0.0
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S.-F. Chiou, C.-J. Tsai r Powder Technology 118 (2001) 10–15
Fig. 5. Time-averaged dichot-equivalent PM 10 concentration measured by the DustTrak vs. sampling time. Two consecutive reentrainments, air acceleration rate: 1.5 mrs 2 , maximum wind speed: 15 mrs, flat surface Žno edge effect., sample a1.
Fig. 7. Emission factors for the test samples with cavities Žwith edge effect. and without cavities Žno edge effect. at different air acceleration rates. Maximum wind speed: 15 mrs, sample a1.
This phenomenon was also observed by Matsusaka and Masuda w14x. They stated that the decay rate of the reentrained dust concentration is higher for larger air acceleration rates, which was observed in this study as well. Fig. 6 shows the emission factor calculated from dust concentrations measured by 10 filters vs. the different air acceleration rates for two dust samples with flat surface Žno edge effect.. As the air acceleration rate is increased from 0.1 to 1.5 mrs 2 , the emission factor is increased linearly from 1.0 to 7.0 = 10y4 kgrm2 s for sample a1, and from 0.1 to 1.0 = 10y4 kgrm2 s for sample a2. It is seen that the emission factor of sample a1 is much greater than that of sample a2 at the same air acceleration rate. The former is about seven times as much as the latter. This is caused by the existence of more small particles in
sample a2, thus enhancing the interaction between the particles, as explained earlier. The existence of cavities on the surface of the dust sample increase the emission factor greatly, as shown in Fig. 7 for sample a1. The two cavities occupying 3% of the sample surface are seen to result in an edge effect, which increases the emission factor by as much as 20 times, although the threshold wind speed is not affected, as shown in Fig. 4.
Fig. 6. Emission factor vs. air acceleration rate. Maximum wind speed: 15 mrs, flat surface Žno edge effect..
4. Conclusions One of the critical problems in making an effective strategy to control road dust emission is to estimate the emission factor accurately. A wind tunnel was built in the laboratory to measure the emission factor of road dust under different wind acceleration conditions. Test road dusts were collected from two sites, one from the construction site where a DRAM factory was being constructed in the Science Industrial Park of Hsin-Chu Ždust sample a1., and the other from an unpaved road at the Nan Laio area of Hsin-Chu Ždust sample a2.. The dust samples were first sieved using a standard No. 325 mesh to remove particles greater than 44 mm. Thereafter, the dust samples were dried and then contained within an aluminium cell of 5 cm Žwidth. = 5 cm Žlength. = 0.2 cm Ždepth., which was embedded in a working platform in the wind tunnel. The surface of dust samples was kept flush with the plate before testing. Test results show that the air acceleration rate affects the emission factor very much. As the air acceleration rate is increased from 0.1 to 1.5 mrs 2 , the emission factor is increased linearly from 1.0 to 7.0 = 10y4 kgrm2 s for sample a1, and from 0.1 to 1.0 = 10y4 kgrm2 s for sample a2.
S.-F. Chiou, C.-J. Tsai r Powder Technology 118 (2001) 10–15
Some conclusions can be drawn from this study. 1. The air acceleration rate has no significant effect on the threshold wind speed for reentrainment, which was found to be 9–12 mrs. 2. The edge effect at the surface of the dust sample has no significant effect on the threshold wind speed for reentrainment either. 3. When the edge effect appears, both reentrained dust concentration and emission factor will become much higher than those without the edge effect. For sample a1, the edge effect increases the emission factor by as much as 20 times. 4. As the air acceleration rate is increased from 0.1 to 1.5 mrs 2 , the emission factor is increased linearly from 1.0 to 7.0 = 10y4 kgrm2 s for sample a1, and from 0.1 to 1.0 = 10y4 kgrm2 s for sample a2. Much more clay in sample a2 than that in sample a1 results in a stronger adhesion between particles, which leads to a smaller emission factor in the former.
Acknowledgements The authors gratefully acknowledge the financial support of the Taiwan National Science Council of the Republic of China under grant NSC88-EPA-Z009-001.
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