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An experimental study on short-time particle resuspension from inner surfaces of straight ventilation ducts
Building and Environment 53 (2012) 119e127
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
An experimental study on short-time particle resuspension from inner surfaces of straight ventilation ducts Shuo Wang a, Bin Zhao a, *, Bin Zhou a, Zhongchao Tan b a b
Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, PR China Department of Mechanical & Mechatronics Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2011 Received in revised form 15 January 2012 Accepted 17 January 2012
In this work, effects of particle size, air speed, and time factor on the short-term resuspension of micron and submicron particles in straight ventilation ducts were experimentally studied with a focus on the particles airborne from the inner surface of ventilation ducts. The experimental method was designed to monitor continuously the particle resuspension rate owing to lift-off. The duct systems were made of typical commercial materials. The particles tested were in eleven size groups in the range of 0.4e10 mm. The experiments were conducted at six air speeds: 3.8, 5.1, 6.3, 7.1, 8.0, and 8.8 m/s and each test lasted for 180 s. The measured particle resuspension rates increased with the increases of the air speed to a certain values, ranging from 9.2 1011 s1 to 3.7 109 s1 corresponding to the particle size of 0.4e10 mm, while the measured particle resuspension rate was smaller than previous reported results. The measurements also showed that particle resuspension rates were independent of time during the first 60 s and then decreased sharply. The experiments suggest that more particles slide and roll off rather than being lifted into the air flow and that higher air speed may reduce the amount of particles lifted into air flow as more of the particles may become airborne due to sliding and rolling off. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Indoor air quality Particle Resuspension Ventilation duct Experiment
1. Introduction Inhalation of airborne particles smaller than 10 mm in diameter may adversely affect human health [1]. These particles usually enter commercial buildings through ventilation ducts and are often circulated through ventilation ducts. Field surveys showed that there was considerable amount of dust on the inner surfaces of ventilation ducts. The amount of accumulated dust in ducts could reach as high as 5 g/m2 in new buildings [2] and those occupied for less than one year [3]. For older buildings, the amount of dust in ventilation ducts was found to exceed the local hygiene standards [4]. When air passes through the ducts, the dust particles on the inner surfaces are likely to be removed from their original positions, resulting in particle resuspension, increased concentration of airborne particles indoors, and consequent increase of occupational exposure to particulate matter. Many researchers have studied the resuspension of a single particle adhering to a wall and presented several models to clarify the resuspension mechanisms [5e7]. However, the research on the resuspension of a single particle may not reflect the actual behavior
* Corresponding author. E-mail address: [email protected] (B. Zhao). 0360-1323/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2012.01.005
of polydispersed particles, which aggregate easily after they are deposited onto a solid surface [8]. More recent theoretical studies on particle resuspension were performed by including the effect of momentum balance [9], turbulent fluctuations [10] and wall surfaces [11]. Computational fluid dynamics (CFD) was also employed to combine with the effect of particle resuspension using source term models [12]. Experimental studies include those focused on the influence of air flow characteristics [13] and indoor human activities [14,15] on particle resuspension from indoor surfaces. The measurement of particle adhesion force indoors was also reported [16]. Previous experimental studies on particle resuspension were mainly carried out by measuring the change of particle mass loading on the surfaces to determine the particle resuspension rate (e.g [17,18].). This approach requires a high resolution in measurement of particle mass load on the test surface. And, the particle resuspension rate determined by this method is the mean value over a period of testing time; it cannot determine the instant particle resuspension rate. Furthermore it does not distinguish the different mechanisms of particle resuspension: liftoff, sliding or rolling off. The initial motion of the resuspension may be caused by lift-off, sliding or rotation [5]. For the purpose of studying the effect of in-duct particle resuspension on indoor air quality, we only considered those particles lifted into air flow through the ducts excluding the sliding and rolling off ones.
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Nomenclature Cðx;tÞ (mg/m3) Particle concentration Cðx1 ;tÞ (mg/m3) Particle concentration upstream Cðx2 ;tÞ (mg/m3) Particle concentration downstream C ðtÞ (mg/m3) Average particle concentration of the test section ~ (mg/m3) Particle concentration difference between C ðtÞ upstream and downstream H (m) Height of the ventilation duct L0 (mg/m2) Initial mass load of particles on the test section Rðx;tÞ (s1) Particle resuspension rate RðtÞ Average particle resuspension rate of the test section t (s) Time T (s) Time step of particle concentration measurement U (m/s) Speed of the air in the ventilation duct Vd (m/s) Velocity of particle deposition W (m) Width of the ventilation duct x (m) Coordinate of the axes along the ventilation duct x1 (m) Particle concentration sampling position upstream (start point of the test section) x2 (m) Particle concentration sampling position downstream(end point of the test section) X (m) Length of the test section
The objective of this work was to experimentally study the resuspension behavior of polydispersed particles in straight ventilation ducts. The test particles were seeded in multiple layers. An experimental method was developed and employed to determine the particle resuspension rate attributed to lift-off into air over time.
Fig. 1. Sketch diagram of the testing method for straight ventilation duct.
Fig. 2. Sketch diagram of the testing straight ventilation duct.
Our previous study showed that the particle concentration along a straight duct is an exponential distribution [19], thus we can fit the distribution and calculate the integration of the particle concentration along the straight ventilation duct by the particle concentration upstream and downstream. Defining
Zx2
2. Experimental methodology
Cðx;tÞ dx x1
2.1. Experiment design
C ðtÞ ¼
As the resuspension rate is widely used to characterize the particle resuspension processes [17,18], the resuspension rate herein is defined by
~ ¼ C C ðtÞ ðx1 ;tÞ Cðx2 ;tÞ
ðx2 x1 Þ
Particle resuspension flux from surface to air flow mg=m2 $s R ¼ Particle mass load of the surface mg=m2
The experimental approach is based on the particle mass/ number balance principle under the combined effect of inflow, outflow of particles, particle deposition flux and resuspension flux in the test section. Consider the straight ventilation duct as Fig. 1 shows. The particle mass balance in the test section with length X gives that the particle mass increment equals to the net particle mass flow rate between upstream and downstream and the resuspended particle flux minus deposited particle flux.
1 0 x Z2 Zx2 d@ A Cðx;tÞ WHdx ¼UWH Cðx1 ;tÞ Cðx2 ;tÞ þ Rðx;tÞ L0 Wdx dt x1
x1
Zx2
(4)
(1)
Zx2 RðtÞ $X ¼
Rðx;tÞ dx
(5)
x1
One can transform Eq. (2) into
XWH
dC ðtÞ ~ þ XWL $R XWV $C ¼ UWH$C 0 ðtÞ ðtÞ ðtÞ d dt
(6)
For a short enough time interval, Eq. (6) can be further transformed as follow:
C ðtÞ C ðtTÞ ~ þ XWL $R XWV $C ¼ UWH$C XWH 0 ðtÞ ðtÞ ðtÞ d T (7)
Vd Cðx;tÞ Wdx x1
(3)
ð2Þ
The values of the air speed, U, the ventilation duct width and height, respectively, W and H, the length of the test section, X, and
S. Wang et al. / Building and Environment 53 (2012) 119e127
2.2. Experiment setup
Table 1 Experimental conditions. Air relative Turbulence Air Air Initial mass speed temperature humidity (%) intensity (%) load L0 (g/m2) (Mean value S.D.) U (m/s) (Mean value S.D.) ( C) 3.8 5.1 6.3 7.1 8.0 8.8
21.06 19.52 18.92 21.22 18.74 18.34
0.2% 0.6% 0.4% 0.3% 0.5% 0.4%
121
20.0 20.0 20.0 20.0 20.0 20.0
1 1 1 1 1 1
40 40 40 40 40 40
3 3 3 3 3 3
0.5 0.5 0.6 0.6 0.6 0.7
2% 3% 7% 4% 6% 6%
Fig. 3. Volume percentage distribution of the testing particles.
the initial mass load, L0, can be measured directly. Vd is the particle deposition velocity onto the duct surfaces, and it was obtained from the experimental results of Sippola and Nazaroff [20]. By continuous measurement of the particle concentrations both up- and downstream of the test section, the average particle resuspension rate Rt from the inner surface of the test section can be determined according to Eq. (7) as a function of time. With the proposed methodology, we can measure the particle concentration changes upstream and downstream of the test section in a ventilation duct to estimate the continuous change of particle resuspension rate over time. This approach allows us to avoid measuring the mass load change that usually requires high resolution measurement of mass load for many times, which is significantly influenced by the operation conditions.
The experimental setup is illustrated in Fig. 2. The horizontal stainless steel ventilation duct has a 0.25 0.25 m size of crosssection, which is typical in a real ventilation duct and is 4.0 m in length. A turbulent grid located upstream of the duct was employed to produce fully developed turbulent air flow. The test section was 0.2 m long and 0.25 m wide, and it was placed 2.0 m downstream of the grid. During the experiments, the wind velocities were controlled by a variable frequency device (VFD) connected to the fan upstream of the turbulent grid. The experiments were conducted at six air speeds: 3.8, 5.1, 6.3, 7.1, 8.0, and 8.8 m/s, which are typically found in straight ducts of actual ventilation systems [21]. The turbulence intensity was measured by the Hot Wire Anemometer and Turbulence Intensity LSI BSV107 (LSI-LASTEM Ltd., Milan, Italy) with an accuracy of 4%. The measured turbulence intensities (mean values) corresponding to each air speed are 0.5%, 0.5%, 0.6%, 0.6%, 0.6%, 0.7%, as shown in Table 1. During the tests, the fan was turned on after particles had been settled down. Time zero is defined by the moment when particle concentration in the air flow rised, which implies occurrence of the particle resuspension. Each test lasted for 180 s. The temperature and the relative humidity of the air during the tests were 20.0 C 1 C and 40% 3%, respectively. They were controlled by the same air handling unit during all experiments. The polydispersed test particles made of calcium salt were deposited onto the floor of the test section. The particles deposited onto the surfaces formed multiple layers. The density of the testing particles was 2540 kg/m3 with a standard deviation of 0.15. It was measured by using the AccuPyc 1330 pycnometer (Micromeritics Instrument Corporation, Norcross, GA, USA). The size distribution of the testing particles was measured by the Mastersizer 2000 particle size analyzer (Malvern Instruments Ltd, Malvern, UK) with an accuracy of 0.1%. The measured size distribution of the testing particles was stable and the volume percentage is shown in Fig. 3. They were nearly spherical and in the size range of 0.25e10 mm in mass median diameter with a lognormal distribution. The initial mass load of particles on the test section was measured by the Hangping JA1003 microbalance (ShangHai Balance Ltd., Shanghai, China) with an accuracy of 0.01 g. The mass of particles in each size group was calculated with the aforementioned size distribution and density of the testing particles. The initial particle mass load on the test section was at the order of 20 g/ m2, which was close to the threshold limit values for dust accumulation amount in ducts in China [22]. The experiments were
Fig. 4. Repeated experimental results (air speed U ¼ 8.0 m/s).
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Fig. 5. Measured particle resuspension rate against time.
conducted with six initial particle mass loads (mean values): 21.06, 19.52, 18.92, 21.22, 18.74, and 18.34 g/m2. The information of experimental conditions is summarized in Table 1. Grimm dust monitor 1.209 (GRIMM Technologies, Inc., Douglasville, GA, USA) was used for the measurements of the particle concentrations. It measures and records the concentrations of particles in fifteen different size ranges (0.3e0.4 mm, 0.4e0.5 mm, 0.5e0.65 mm, 0.65e0.8 mm, 0.8e1.0 mm, 1.0e1.6 mm, 1.6e2.0 mm, 2.0e3.0 mm, 3.0e4.0 mm, 4.0e5.0 mm, 5.0e7.5 mm, 7.5e10 mm, 10e15 mm, 15e20 mm, and 20.0 mm) with a reproducibility of 2%. The monitor has been calibrated by the manufacturer before the measurements. The particle concentrations were measured at both ends of the test section in the wind duct following the
isokinetic sampling procedure. The measuring nozzles were connected to the instrument alternatively every 6 s by an automatic switch valve so that the particle concentrations of each end (upstream and downstream) were measured alternatively by the same instrument. Therefore, the particle concentrations were determined for consecutive integrated period of every 12 s. Our experiments focus on the initial 180 s. This is shorter than what has been reported by other researchers [23]. However, such a time scale may be important when considering the period when the ventilation system is just started to run, during which the particle resuspension may be dominant [17,18]. Due to the number limitation of the dust monitor, we could only measure the particle concentration at one certain point for
S. Wang et al. / Building and Environment 53 (2012) 119e127
123
Fig. 5. (continued).
each test. To get the average particle concentration in one certain cross section (e.g., the cross section in upstream) of the ventilation duct, we virtually divided the measured cross section into 9 equivalent areas, and the particle concentrations at these 9 areas were measured respectively in 9 repeated tests. Then the average particle concentration in this cross section could be calculated with the 9 measured values. We measured the average particle concentration in upstream and downstream by the automatic switch valve as mentioned above. The turbulence intensities between the 9 measurements were controlled at a certain value by the turbulent grid, where the standard error (S.D.) of the 9
times measured turbulence intensities is less than 7% (see Table 1). 3. Results and discussion The experiment for the case corresponding to an air speed of 8.0 m/s was repeated for three times. For a clear presentation, only the repeated experimental results for three particle size groups are shown in Fig. 4. In these three experiments, the measured particle resuspension rates were close to each other. Thus the measurement method was deemed stable to give repeatable results.
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Fig. 5. (continued).
The uncertainties shown in Figs. 4e6 represent the minimum and maximum values of measured particle resuspension rates, which were determined based on the uncertainties of particle concentration, initial particle mass loading and air speed measurements. 3.1. The dependence of particle resuspension rate on time The measured particle resuspension rate vs. time is shown in Fig. 5. For each experiment, the particle resuspension rate was
observed to be stable for the first 60 s followed by a rapid drop. The time dependence of particle resuspension rate in some previous studies were described as a negative exponential [24,25], whilst others proposed an inverse relationship between resuspension rate and time [26,27]. Our experimental results, however, showed otherwise. It is likely that most theoretical analyses and experimental studies mentioned above were for monolayer dust that deposited on the surfaces, whereas the experiments herein were for multilayer dust. For the case of multilayer dust deposited on the surfaces, particles in the bottom layer are attached to the duct
S. Wang et al. / Building and Environment 53 (2012) 119e127
125
Fig. 5. (continued).
surface by strong forces (e.g., Van der Waals force between the particles and surface) and, theoretically, it is difficult for them to resuspend. However, particles in the upper layers were attached to the surrounding particles by much weaker adhesion forces. For example, the Van der Waals force exerted on the particles in the upper layers (particle between particle mode) is weaker than that exerted on the particles in the bottom layer (particles between duct surface mode) [9], and they were expected to be subjected to constant adhesion, lifting and drifting forces. Consequently, the rate of particle resuspension is expected to remain constant until it reaches the bottom layer of particles, when the resuspension rate dropped abruptly. This is also confirmed by the visual observation
during the experiments. This may experimentally verify the “loose particles” concept mentioned by Mortazavi [28]. He pointed out there might be loose particles in the dust layer when particle resuspension started and it is easier for these loose particles to resuspend than those adhered to the duct surfaces in lower layers. The measured results confirmed the “loose particles” concept indirectly. On the other hand, the measured particle resuspension rates were much smaller than some data in the literature (e.g. 107 to 103 s1 [17,29]), although they fall in the range of 1013 to 104 s1 as previously reported by others [30e36]. This can also be explained by the fact that our experiments focused only on the
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S. Wang et al. / Building and Environment 53 (2012) 119e127
Fig. 6. Measured particle resuspension rate against air speed and diameter in ventilation duct.
particles lifted into air flow while the previous studies (of larger resuspension rate) measured the particle removal rate owing to all the three mechanisms. The difference is likely attributed to the measuring errors of the experiments. We measured the particle resuspension owing to lift off in this measurement because we focused on the particle mass balance principle in the test section, where the control volume is the air within the test section. However, the previous studies measured the particle resuspension rate by weighing the mass load change of particles on the surfaces in the test section, which considered the combined effects of particle sliding off, rolling off and lifting off. The difference also implies that the amount of particles sliding and rolling off was much larger than lifting. This is consistent with previous theoretical analyses [28,34]. It is suggested that more particles slide and roll off than being lifted into air flow. 3.2. The dependence of particle resuspension rate on the particle size and the air speed The measured particle resuspension rates against air speed and particle diameter in the ventilation duct are shown in Fig. 6 (particle resuspension rates vs. particle diameters are also shown in Fig. 5). The experimental data confirmed the theoretical analysis that particle resuspension or reentrainment strongly depends on the particle size. The resuspension rate increases with the size of the particles as indicated in our experiments, which is consistent with previous experimental [30e36] and theoretical studies [9,28,35]. The measured particle resuspension rate also increased with the air speed when it was low, which is also consistent with previous studies [30e36]. However, the resuspension rate decreased when
the air speed reached a certain value (ranging from 7.1 to 8.8 m/s corresponding to the studied particle size, see Fig. 6 and Table 2). And the value depended on the particle size. Consequently, there is a peak value, ranging from 9.2 1011 s1 to 3.7 109 s1, for the particle sizes of 0.4e10 mm. We define the air speed corresponding to the maximum resuspension rate (the peak value of the resuspension rate) as the scale air speed. The peak values of the particle resuspension rates and the scale air speeds corresponding to the particle sizes are listed in Table 2. These results indirectly show that the larger the particles, the easier to be resuspended. Generally, the scale air speed decreases with the increase of the particle size. When the air speed exceeds the scale air speed, the particle resuspension rate did not increase as expected. The methodology presented in this paper is based on the measured concentration changes upstream and downstream of the test section; it cannot reflect the change of the mass of particles on the surfaces of ventilation ducts caused by particles sliding and rolling off. Therefore, the experimental results may imply that particles are more likely to slide or roll off than being lifted into the high-speed air flow. Other mechanisms may cause such a complicated phenomenon. Further theoretical analysis is needed for a better understanding the mechanism. 3.3. Limitations of current work Although the experiment methodology was novel and some of the experimental results differed from those previously reported, there are some limitations in this study. First of all, more experimental data is needed with different temperature and humidity of the air as well as the materials of the particles and duct surfaces before a more general conclusion can be made.
Table 2 The peak values of the particle resuspension rates and the scale air speeds corresponding to the particle size. Particle size (mm)
0.3e0.4 0.4e0.5 0.5e0.65 0.65e0.8 0.8e1.0
9.24 Peak values 1011 of R (s1) Scale air speed (m/s) 8.8
1.17 1010 8.8
1.51 1010 8.8
2.13 1010 8.8
1.0e1.6
1.6e2.0
2.0e3.0
3.0e4.0
4.0e5.0
5.0e7.5
7.5e10.0
2.96 1010 3.05 1010 1.19 109 8.37 1010 1.16 109 1.95 109 2.33 109 3.66 109 8.8
8.8
8.8
8.8
8.0
8.0
7.1
7.1
S. Wang et al. / Building and Environment 53 (2012) 119e127
We used one dust monitor to measure the particle concentration at two locations (up- and downstream of the test section), there was time lag for the two measurements inevitably, while it demands simultaneous measuring theoretically. The standard sampling time of the dust monitor is 6 s in its “fast mode”. The response time was not listed in the manual and it was only mentioned that the monitor measure particle concentration in a real time. Therefore, it is deemed to be reasonable to ignore the effect of the response time. During our experiments, however, there was a 6-s gap between the measurements of up and downstream concentrations. This can be improved by simultaneous measurement of up- and down-stream particle concentrations in the future. Despite the limitations, this study does suggest a simple method to measure the particle resuspension rate in ventilation ducts, which avoids measuring the mass load change that requires high resolution measurement of mass load for many times. 4. Conclusions In this study, an experimental approach was developed to study the resuspension rate of particles lifted into air flow. Without laborintensive measurement of particle mass load on the surfaces, the particle resuspension rate against time could be measured by only measuring the concentrations of the airborne particles at both ends of the test section continuously. This method concerned the case where polydispersed particles deposited in multiple layers on top of a straight ventilation duct. Based on the experiments and analyses within this work, the following conclusions may be drawn. (1) By using polydispersed particles deposited on a horizontal solid surface, one can determine the particle resuspension rate attributed to lift-off excluding sliding and rolling. The experimental data confirmed the theoretical analysis that larger particles were easier to be lifted and that a higher air speed in a certain range (7.1e8.8 m/s) increased the resuspension rate for the particles in the size range of 0.4e10 mm in diameter. (2) The measured particle resuspension rates were much smaller than data in literature because our method was for measuring the resuspension rate due to lift-off only. The experimental results suggested that more particles slided and rolled off rather than being lifted into air flow. A high air speed (higher than 7.1e8.8 m/s corresponding to the particle size of 0.4e10 mm) may prevent the particles from being lifted into air flow as more of the particles may be removed by rolling and sliding off. (3) The particle resuspension rate increased as the air speed increased until it reached a certain value (ranging from 9.2 1011 s1 to 3.7 109 s1 for the particle size of 0.4e10 mm), when it started to drop quickly. The maximum resuspension rate depended on the particle size. (4) The particle resuspension rate was independent of time during the first 60 s and then decreased suddenly under the conditions tested. Acknowledgement This work was sponsored by the National Natural Science Foundation of China (Grant No. 50908127). We are much appreciated that Prof. Wai-tin (Daniel) Chan provided the dust monitor for measurements. References [1] Pope CA. Review: epidemiological basis for particulate air pollution health standards. Aerosol Sci Tech 2000;32:4e14.
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An experimental study on short-time particle resuspension from inner surfaces of straight ventilation ducts Shuo Wang a, Bin Zhao a, *, Bin Zhou a, Zhongchao Tan b a b
Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, PR China Department of Mechanical & Mechatronics Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2011 Received in revised form 15 January 2012 Accepted 17 January 2012
In this work, effects of particle size, air speed, and time factor on the short-term resuspension of micron and submicron particles in straight ventilation ducts were experimentally studied with a focus on the particles airborne from the inner surface of ventilation ducts. The experimental method was designed to monitor continuously the particle resuspension rate owing to lift-off. The duct systems were made of typical commercial materials. The particles tested were in eleven size groups in the range of 0.4e10 mm. The experiments were conducted at six air speeds: 3.8, 5.1, 6.3, 7.1, 8.0, and 8.8 m/s and each test lasted for 180 s. The measured particle resuspension rates increased with the increases of the air speed to a certain values, ranging from 9.2 1011 s1 to 3.7 109 s1 corresponding to the particle size of 0.4e10 mm, while the measured particle resuspension rate was smaller than previous reported results. The measurements also showed that particle resuspension rates were independent of time during the first 60 s and then decreased sharply. The experiments suggest that more particles slide and roll off rather than being lifted into the air flow and that higher air speed may reduce the amount of particles lifted into air flow as more of the particles may become airborne due to sliding and rolling off. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Indoor air quality Particle Resuspension Ventilation duct Experiment
1. Introduction Inhalation of airborne particles smaller than 10 mm in diameter may adversely affect human health [1]. These particles usually enter commercial buildings through ventilation ducts and are often circulated through ventilation ducts. Field surveys showed that there was considerable amount of dust on the inner surfaces of ventilation ducts. The amount of accumulated dust in ducts could reach as high as 5 g/m2 in new buildings [2] and those occupied for less than one year [3]. For older buildings, the amount of dust in ventilation ducts was found to exceed the local hygiene standards [4]. When air passes through the ducts, the dust particles on the inner surfaces are likely to be removed from their original positions, resulting in particle resuspension, increased concentration of airborne particles indoors, and consequent increase of occupational exposure to particulate matter. Many researchers have studied the resuspension of a single particle adhering to a wall and presented several models to clarify the resuspension mechanisms [5e7]. However, the research on the resuspension of a single particle may not reflect the actual behavior
* Corresponding author. E-mail address: [email protected] (B. Zhao). 0360-1323/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2012.01.005
of polydispersed particles, which aggregate easily after they are deposited onto a solid surface [8]. More recent theoretical studies on particle resuspension were performed by including the effect of momentum balance [9], turbulent fluctuations [10] and wall surfaces [11]. Computational fluid dynamics (CFD) was also employed to combine with the effect of particle resuspension using source term models [12]. Experimental studies include those focused on the influence of air flow characteristics [13] and indoor human activities [14,15] on particle resuspension from indoor surfaces. The measurement of particle adhesion force indoors was also reported [16]. Previous experimental studies on particle resuspension were mainly carried out by measuring the change of particle mass loading on the surfaces to determine the particle resuspension rate (e.g [17,18].). This approach requires a high resolution in measurement of particle mass load on the test surface. And, the particle resuspension rate determined by this method is the mean value over a period of testing time; it cannot determine the instant particle resuspension rate. Furthermore it does not distinguish the different mechanisms of particle resuspension: liftoff, sliding or rolling off. The initial motion of the resuspension may be caused by lift-off, sliding or rotation [5]. For the purpose of studying the effect of in-duct particle resuspension on indoor air quality, we only considered those particles lifted into air flow through the ducts excluding the sliding and rolling off ones.
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Nomenclature Cðx;tÞ (mg/m3) Particle concentration Cðx1 ;tÞ (mg/m3) Particle concentration upstream Cðx2 ;tÞ (mg/m3) Particle concentration downstream C ðtÞ (mg/m3) Average particle concentration of the test section ~ (mg/m3) Particle concentration difference between C ðtÞ upstream and downstream H (m) Height of the ventilation duct L0 (mg/m2) Initial mass load of particles on the test section Rðx;tÞ (s1) Particle resuspension rate RðtÞ Average particle resuspension rate of the test section t (s) Time T (s) Time step of particle concentration measurement U (m/s) Speed of the air in the ventilation duct Vd (m/s) Velocity of particle deposition W (m) Width of the ventilation duct x (m) Coordinate of the axes along the ventilation duct x1 (m) Particle concentration sampling position upstream (start point of the test section) x2 (m) Particle concentration sampling position downstream(end point of the test section) X (m) Length of the test section
The objective of this work was to experimentally study the resuspension behavior of polydispersed particles in straight ventilation ducts. The test particles were seeded in multiple layers. An experimental method was developed and employed to determine the particle resuspension rate attributed to lift-off into air over time.
Fig. 1. Sketch diagram of the testing method for straight ventilation duct.
Fig. 2. Sketch diagram of the testing straight ventilation duct.
Our previous study showed that the particle concentration along a straight duct is an exponential distribution [19], thus we can fit the distribution and calculate the integration of the particle concentration along the straight ventilation duct by the particle concentration upstream and downstream. Defining
Zx2
2. Experimental methodology
Cðx;tÞ dx x1
2.1. Experiment design
C ðtÞ ¼
As the resuspension rate is widely used to characterize the particle resuspension processes [17,18], the resuspension rate herein is defined by
~ ¼ C C ðtÞ ðx1 ;tÞ Cðx2 ;tÞ
ðx2 x1 Þ
Particle resuspension flux from surface to air flow mg=m2 $s R ¼ Particle mass load of the surface mg=m2
The experimental approach is based on the particle mass/ number balance principle under the combined effect of inflow, outflow of particles, particle deposition flux and resuspension flux in the test section. Consider the straight ventilation duct as Fig. 1 shows. The particle mass balance in the test section with length X gives that the particle mass increment equals to the net particle mass flow rate between upstream and downstream and the resuspended particle flux minus deposited particle flux.
1 0 x Z2 Zx2 d@ A Cðx;tÞ WHdx ¼UWH Cðx1 ;tÞ Cðx2 ;tÞ þ Rðx;tÞ L0 Wdx dt x1
x1
Zx2
(4)
(1)
Zx2 RðtÞ $X ¼
Rðx;tÞ dx
(5)
x1
One can transform Eq. (2) into
XWH
dC ðtÞ ~ þ XWL $R XWV $C ¼ UWH$C 0 ðtÞ ðtÞ ðtÞ d dt
(6)
For a short enough time interval, Eq. (6) can be further transformed as follow:
C ðtÞ C ðtTÞ ~ þ XWL $R XWV $C ¼ UWH$C XWH 0 ðtÞ ðtÞ ðtÞ d T (7)
Vd Cðx;tÞ Wdx x1
(3)
ð2Þ
The values of the air speed, U, the ventilation duct width and height, respectively, W and H, the length of the test section, X, and
S. Wang et al. / Building and Environment 53 (2012) 119e127
2.2. Experiment setup
Table 1 Experimental conditions. Air relative Turbulence Air Air Initial mass speed temperature humidity (%) intensity (%) load L0 (g/m2) (Mean value S.D.) U (m/s) (Mean value S.D.) ( C) 3.8 5.1 6.3 7.1 8.0 8.8
21.06 19.52 18.92 21.22 18.74 18.34
0.2% 0.6% 0.4% 0.3% 0.5% 0.4%
121
20.0 20.0 20.0 20.0 20.0 20.0
1 1 1 1 1 1
40 40 40 40 40 40
3 3 3 3 3 3
0.5 0.5 0.6 0.6 0.6 0.7
2% 3% 7% 4% 6% 6%
Fig. 3. Volume percentage distribution of the testing particles.
the initial mass load, L0, can be measured directly. Vd is the particle deposition velocity onto the duct surfaces, and it was obtained from the experimental results of Sippola and Nazaroff [20]. By continuous measurement of the particle concentrations both up- and downstream of the test section, the average particle resuspension rate Rt from the inner surface of the test section can be determined according to Eq. (7) as a function of time. With the proposed methodology, we can measure the particle concentration changes upstream and downstream of the test section in a ventilation duct to estimate the continuous change of particle resuspension rate over time. This approach allows us to avoid measuring the mass load change that usually requires high resolution measurement of mass load for many times, which is significantly influenced by the operation conditions.
The experimental setup is illustrated in Fig. 2. The horizontal stainless steel ventilation duct has a 0.25 0.25 m size of crosssection, which is typical in a real ventilation duct and is 4.0 m in length. A turbulent grid located upstream of the duct was employed to produce fully developed turbulent air flow. The test section was 0.2 m long and 0.25 m wide, and it was placed 2.0 m downstream of the grid. During the experiments, the wind velocities were controlled by a variable frequency device (VFD) connected to the fan upstream of the turbulent grid. The experiments were conducted at six air speeds: 3.8, 5.1, 6.3, 7.1, 8.0, and 8.8 m/s, which are typically found in straight ducts of actual ventilation systems [21]. The turbulence intensity was measured by the Hot Wire Anemometer and Turbulence Intensity LSI BSV107 (LSI-LASTEM Ltd., Milan, Italy) with an accuracy of 4%. The measured turbulence intensities (mean values) corresponding to each air speed are 0.5%, 0.5%, 0.6%, 0.6%, 0.6%, 0.7%, as shown in Table 1. During the tests, the fan was turned on after particles had been settled down. Time zero is defined by the moment when particle concentration in the air flow rised, which implies occurrence of the particle resuspension. Each test lasted for 180 s. The temperature and the relative humidity of the air during the tests were 20.0 C 1 C and 40% 3%, respectively. They were controlled by the same air handling unit during all experiments. The polydispersed test particles made of calcium salt were deposited onto the floor of the test section. The particles deposited onto the surfaces formed multiple layers. The density of the testing particles was 2540 kg/m3 with a standard deviation of 0.15. It was measured by using the AccuPyc 1330 pycnometer (Micromeritics Instrument Corporation, Norcross, GA, USA). The size distribution of the testing particles was measured by the Mastersizer 2000 particle size analyzer (Malvern Instruments Ltd, Malvern, UK) with an accuracy of 0.1%. The measured size distribution of the testing particles was stable and the volume percentage is shown in Fig. 3. They were nearly spherical and in the size range of 0.25e10 mm in mass median diameter with a lognormal distribution. The initial mass load of particles on the test section was measured by the Hangping JA1003 microbalance (ShangHai Balance Ltd., Shanghai, China) with an accuracy of 0.01 g. The mass of particles in each size group was calculated with the aforementioned size distribution and density of the testing particles. The initial particle mass load on the test section was at the order of 20 g/ m2, which was close to the threshold limit values for dust accumulation amount in ducts in China [22]. The experiments were
Fig. 4. Repeated experimental results (air speed U ¼ 8.0 m/s).
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Fig. 5. Measured particle resuspension rate against time.
conducted with six initial particle mass loads (mean values): 21.06, 19.52, 18.92, 21.22, 18.74, and 18.34 g/m2. The information of experimental conditions is summarized in Table 1. Grimm dust monitor 1.209 (GRIMM Technologies, Inc., Douglasville, GA, USA) was used for the measurements of the particle concentrations. It measures and records the concentrations of particles in fifteen different size ranges (0.3e0.4 mm, 0.4e0.5 mm, 0.5e0.65 mm, 0.65e0.8 mm, 0.8e1.0 mm, 1.0e1.6 mm, 1.6e2.0 mm, 2.0e3.0 mm, 3.0e4.0 mm, 4.0e5.0 mm, 5.0e7.5 mm, 7.5e10 mm, 10e15 mm, 15e20 mm, and 20.0 mm) with a reproducibility of 2%. The monitor has been calibrated by the manufacturer before the measurements. The particle concentrations were measured at both ends of the test section in the wind duct following the
isokinetic sampling procedure. The measuring nozzles were connected to the instrument alternatively every 6 s by an automatic switch valve so that the particle concentrations of each end (upstream and downstream) were measured alternatively by the same instrument. Therefore, the particle concentrations were determined for consecutive integrated period of every 12 s. Our experiments focus on the initial 180 s. This is shorter than what has been reported by other researchers [23]. However, such a time scale may be important when considering the period when the ventilation system is just started to run, during which the particle resuspension may be dominant [17,18]. Due to the number limitation of the dust monitor, we could only measure the particle concentration at one certain point for
S. Wang et al. / Building and Environment 53 (2012) 119e127
123
Fig. 5. (continued).
each test. To get the average particle concentration in one certain cross section (e.g., the cross section in upstream) of the ventilation duct, we virtually divided the measured cross section into 9 equivalent areas, and the particle concentrations at these 9 areas were measured respectively in 9 repeated tests. Then the average particle concentration in this cross section could be calculated with the 9 measured values. We measured the average particle concentration in upstream and downstream by the automatic switch valve as mentioned above. The turbulence intensities between the 9 measurements were controlled at a certain value by the turbulent grid, where the standard error (S.D.) of the 9
times measured turbulence intensities is less than 7% (see Table 1). 3. Results and discussion The experiment for the case corresponding to an air speed of 8.0 m/s was repeated for three times. For a clear presentation, only the repeated experimental results for three particle size groups are shown in Fig. 4. In these three experiments, the measured particle resuspension rates were close to each other. Thus the measurement method was deemed stable to give repeatable results.
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Fig. 5. (continued).
The uncertainties shown in Figs. 4e6 represent the minimum and maximum values of measured particle resuspension rates, which were determined based on the uncertainties of particle concentration, initial particle mass loading and air speed measurements. 3.1. The dependence of particle resuspension rate on time The measured particle resuspension rate vs. time is shown in Fig. 5. For each experiment, the particle resuspension rate was
observed to be stable for the first 60 s followed by a rapid drop. The time dependence of particle resuspension rate in some previous studies were described as a negative exponential [24,25], whilst others proposed an inverse relationship between resuspension rate and time [26,27]. Our experimental results, however, showed otherwise. It is likely that most theoretical analyses and experimental studies mentioned above were for monolayer dust that deposited on the surfaces, whereas the experiments herein were for multilayer dust. For the case of multilayer dust deposited on the surfaces, particles in the bottom layer are attached to the duct
S. Wang et al. / Building and Environment 53 (2012) 119e127
125
Fig. 5. (continued).
surface by strong forces (e.g., Van der Waals force between the particles and surface) and, theoretically, it is difficult for them to resuspend. However, particles in the upper layers were attached to the surrounding particles by much weaker adhesion forces. For example, the Van der Waals force exerted on the particles in the upper layers (particle between particle mode) is weaker than that exerted on the particles in the bottom layer (particles between duct surface mode) [9], and they were expected to be subjected to constant adhesion, lifting and drifting forces. Consequently, the rate of particle resuspension is expected to remain constant until it reaches the bottom layer of particles, when the resuspension rate dropped abruptly. This is also confirmed by the visual observation
during the experiments. This may experimentally verify the “loose particles” concept mentioned by Mortazavi [28]. He pointed out there might be loose particles in the dust layer when particle resuspension started and it is easier for these loose particles to resuspend than those adhered to the duct surfaces in lower layers. The measured results confirmed the “loose particles” concept indirectly. On the other hand, the measured particle resuspension rates were much smaller than some data in the literature (e.g. 107 to 103 s1 [17,29]), although they fall in the range of 1013 to 104 s1 as previously reported by others [30e36]. This can also be explained by the fact that our experiments focused only on the
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Fig. 6. Measured particle resuspension rate against air speed and diameter in ventilation duct.
particles lifted into air flow while the previous studies (of larger resuspension rate) measured the particle removal rate owing to all the three mechanisms. The difference is likely attributed to the measuring errors of the experiments. We measured the particle resuspension owing to lift off in this measurement because we focused on the particle mass balance principle in the test section, where the control volume is the air within the test section. However, the previous studies measured the particle resuspension rate by weighing the mass load change of particles on the surfaces in the test section, which considered the combined effects of particle sliding off, rolling off and lifting off. The difference also implies that the amount of particles sliding and rolling off was much larger than lifting. This is consistent with previous theoretical analyses [28,34]. It is suggested that more particles slide and roll off than being lifted into air flow. 3.2. The dependence of particle resuspension rate on the particle size and the air speed The measured particle resuspension rates against air speed and particle diameter in the ventilation duct are shown in Fig. 6 (particle resuspension rates vs. particle diameters are also shown in Fig. 5). The experimental data confirmed the theoretical analysis that particle resuspension or reentrainment strongly depends on the particle size. The resuspension rate increases with the size of the particles as indicated in our experiments, which is consistent with previous experimental [30e36] and theoretical studies [9,28,35]. The measured particle resuspension rate also increased with the air speed when it was low, which is also consistent with previous studies [30e36]. However, the resuspension rate decreased when
the air speed reached a certain value (ranging from 7.1 to 8.8 m/s corresponding to the studied particle size, see Fig. 6 and Table 2). And the value depended on the particle size. Consequently, there is a peak value, ranging from 9.2 1011 s1 to 3.7 109 s1, for the particle sizes of 0.4e10 mm. We define the air speed corresponding to the maximum resuspension rate (the peak value of the resuspension rate) as the scale air speed. The peak values of the particle resuspension rates and the scale air speeds corresponding to the particle sizes are listed in Table 2. These results indirectly show that the larger the particles, the easier to be resuspended. Generally, the scale air speed decreases with the increase of the particle size. When the air speed exceeds the scale air speed, the particle resuspension rate did not increase as expected. The methodology presented in this paper is based on the measured concentration changes upstream and downstream of the test section; it cannot reflect the change of the mass of particles on the surfaces of ventilation ducts caused by particles sliding and rolling off. Therefore, the experimental results may imply that particles are more likely to slide or roll off than being lifted into the high-speed air flow. Other mechanisms may cause such a complicated phenomenon. Further theoretical analysis is needed for a better understanding the mechanism. 3.3. Limitations of current work Although the experiment methodology was novel and some of the experimental results differed from those previously reported, there are some limitations in this study. First of all, more experimental data is needed with different temperature and humidity of the air as well as the materials of the particles and duct surfaces before a more general conclusion can be made.
Table 2 The peak values of the particle resuspension rates and the scale air speeds corresponding to the particle size. Particle size (mm)
0.3e0.4 0.4e0.5 0.5e0.65 0.65e0.8 0.8e1.0
9.24 Peak values 1011 of R (s1) Scale air speed (m/s) 8.8
1.17 1010 8.8
1.51 1010 8.8
2.13 1010 8.8
1.0e1.6
1.6e2.0
2.0e3.0
3.0e4.0
4.0e5.0
5.0e7.5
7.5e10.0
2.96 1010 3.05 1010 1.19 109 8.37 1010 1.16 109 1.95 109 2.33 109 3.66 109 8.8
8.8
8.8
8.8
8.0
8.0
7.1
7.1
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We used one dust monitor to measure the particle concentration at two locations (up- and downstream of the test section), there was time lag for the two measurements inevitably, while it demands simultaneous measuring theoretically. The standard sampling time of the dust monitor is 6 s in its “fast mode”. The response time was not listed in the manual and it was only mentioned that the monitor measure particle concentration in a real time. Therefore, it is deemed to be reasonable to ignore the effect of the response time. During our experiments, however, there was a 6-s gap between the measurements of up and downstream concentrations. This can be improved by simultaneous measurement of up- and down-stream particle concentrations in the future. Despite the limitations, this study does suggest a simple method to measure the particle resuspension rate in ventilation ducts, which avoids measuring the mass load change that requires high resolution measurement of mass load for many times. 4. Conclusions In this study, an experimental approach was developed to study the resuspension rate of particles lifted into air flow. Without laborintensive measurement of particle mass load on the surfaces, the particle resuspension rate against time could be measured by only measuring the concentrations of the airborne particles at both ends of the test section continuously. This method concerned the case where polydispersed particles deposited in multiple layers on top of a straight ventilation duct. Based on the experiments and analyses within this work, the following conclusions may be drawn. (1) By using polydispersed particles deposited on a horizontal solid surface, one can determine the particle resuspension rate attributed to lift-off excluding sliding and rolling. The experimental data confirmed the theoretical analysis that larger particles were easier to be lifted and that a higher air speed in a certain range (7.1e8.8 m/s) increased the resuspension rate for the particles in the size range of 0.4e10 mm in diameter. (2) The measured particle resuspension rates were much smaller than data in literature because our method was for measuring the resuspension rate due to lift-off only. The experimental results suggested that more particles slided and rolled off rather than being lifted into air flow. A high air speed (higher than 7.1e8.8 m/s corresponding to the particle size of 0.4e10 mm) may prevent the particles from being lifted into air flow as more of the particles may be removed by rolling and sliding off. (3) The particle resuspension rate increased as the air speed increased until it reached a certain value (ranging from 9.2 1011 s1 to 3.7 109 s1 for the particle size of 0.4e10 mm), when it started to drop quickly. The maximum resuspension rate depended on the particle size. (4) The particle resuspension rate was independent of time during the first 60 s and then decreased suddenly under the conditions tested. Acknowledgement This work was sponsored by the National Natural Science Foundation of China (Grant No. 50908127). We are much appreciated that Prof. Wai-tin (Daniel) Chan provided the dust monitor for measurements. References [1] Pope CA. Review: epidemiological basis for particulate air pollution health standards. Aerosol Sci Tech 2000;32:4e14.
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