1、Brandon E. Boor is a graduate student, Jeffrey A. Siegel is an associate professor, and Atila Novoselac is an assistant professor in the Department of Civil, Architectural and Environmental Engineering, The University of Texas at Austin, Austin, TX. Development of an Experimental Methodology to Dete
2、rmine Monolayer and Multilayer Particle Resuspension from Indoor Surfaces Brandon E. Boor Jeffrey A. Siegel, PhD Atila Novoselac, PhD Student Member ASHRAE Member ASHRAE Member ASHRAE ABSTRACT Resuspension is an important source of particles in the indoor environment. Particles deposited on various
3、indoor surfaces can be re-entrained by a passing fluid stream, thereby elevating airborne particle concentrations. This can lead to human inhalation exposure and respiratory problems such as lung inflammation in asthmatic and sensitive individuals. This paper presents research related to the develop
4、ment of an experimental methodology to investigate particle resuspension of monolayer and multilayer particle deposits exposed to a variety of flow conditions. Material samples were seeded with monodisperse fluorescent tracer particles (3 and 10 m) and polydisperse Ultrafine Arizona Test Dust (ATD)
5、(1 to 20 m). The seeded samples were then exposed to a controlled test flow in a micro-scale wind tunnel, where the air velocity was modified to represent a broad range of flow conditions found in the indoor environment. A fluorescence stereomicroscope was used to precisely count the number of resus
6、pended fluorescent particles. Preliminary experiments have shown that particle resuspension is significantly greater for multilayer deposits compared to monolayer deposits. These findings will help bridge the gap between previous wind tunnel experiments and theoretical models that are limited to mon
7、olayer particle deposits and actual deposits in the indoor environment in which particles may be deposited in layers. INTRODUCTION The impact of particle resuspension on indoor environmental quality has been investigated in recent years. Thatcher and Layton 1995 demonstrated that resuspension associ
8、ated with walking in and out of a room could significantly increase the airborne concentration of particles with diameters greater than 5 m. Ferro et al. 2004 further demonstrated that common human activity indoors, such as walking, folding blankets, dusting, and making a bed, could lead to a substa
9、ntial increase in the indoor concentration of 2.5 and 5 m particles. Recent studies by Qian and Ferro 2008, Rosati et al. 2008, Qian et al. 2008, Oberoi et al. 2010, and Cheng et al. 2010, have verified that particle resuspension due to human activities elevates indoor particle concentrations. Howev
10、er, resuspension due to the operation of heating, ventilation, and air conditioning (HVAC) systems has not been as thoroughly investigated to assess its contribution to indoor particle concentrations and human inhalation exposure. Batterman and Burge 1996 examined the emissions of various pollutants
11、 from contaminated HVAC components. They found that dust particles, including fibers and metal degradation products, could be released from HVAC components during their operation, such as the shedding of particles from in-duct filters. Prezkop et al. 2004 analyzed particle loading on filter fibers a
12、nd determined that resuspension of deposited particles can reduce the fiber deposition efficiency by 10 to 20 percent. While analyzing particle deposition in ventilation ducts, Sippola and Nazaroff 2004 found that tracer fluorescent particles (5 LV-11-C053434 ASHRAE Transactions2011. American Societ
13、y of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAES prior written permis
14、sion.m and larger) used in their experiments would resuspend at velocities between 5 and 10 m/s. Lastly, a recent study by Krauter and Biermann 2007 demonstrated that fungal spores could be resuspended by airflow within ventilation ducts. Previous fundamental resuspension studies have characterized
15、the resuspension of particles under controlled test flow conditions in a wind tunnel (e.g., Wu et al. 1992; Nicholson 1993; Ibrahim et al. 2003; Gomes et al. 2007; Ibrahim et al. 2008; Jiang et al. 2008). These experiments typically involve smooth surfaces, such as glass, and large monodisperse part
16、icles ( 10 m in diameter) deposited in a sparse monolayer, where there is no particle to particle contact. Theoretical models have also been developed for monolayer deposits, which aim at elucidating the interaction between a particle and surface under various environmental conditions and surface ch
17、aracteristics (e.g., Ibrahim et al. 2003; Ahmadi and Guo 2007). As demonstrated in Ibrahim et al. 2003 and Kim et al. 2010, these models generally agree well with published experimental data for monolayer deposits. Indoors, particles may be deposited in clusters and on top of one another to form mul
18、tilayer deposits, where particle to particle contact becomes significant. Visible dust accumulations on the floor or on the surface of a ventilation duct are examples of multilayer deposits. However, to the authors knowledge, the actual geometry of real indoor particle deposits has never been studie
19、d. Several theoretical models have been developed to investigate particle resuspension from multilayer deposits, including: Lazaridis and Drossinos 1998; Friess and Yadigaroglu 2002; and Nitschke and Schmidt 2009, with a focus on aerosols associated with the nuclear industry. However, no experimenta
20、l data exists to validate these models or elucidate the impact of the deposit (monolayer versus multilayer) on resuspension. The objective of this research is to investigate how resuspension varies between monolayer and multilayer deposits on real indoor surfaces and to develop a reliable experiment
21、al methodology that can be expanded in the future to investigate the impact of numerous environmental variables, such as relative humidity and the physical arrangement of the multilayer deposit. EXPERIMENTAL METHODOLOGY DEVELOPMENT An experimental methodology was developed to determine the resuspens
22、ion of particles from indoor surfaces exposed to controlled flow conditions in a micro-scale wind tunnel. The focus of this paper is on galvanized sheet metal, which was selected to represent a common HVAC duct material used in residential and commercial buildings. The velocities studied were 2.5 m/
23、s, 5 m/s, 7.5 m/s, 10 m/s, 12.5 m/s, 15 m/s, 25 m/s, and 40 m/s (8.2, 16.4, 24.6, 32.8, 41, 49.2, 82, and 131.2 ft/s, respectively). These velocities represent a wide range of flow conditions that may exist in ventilation systems during their operation. Generation of Monolayer a volatile chemical th
24、at easily evaporates, permitting the particles to dry quickly, and does not degrade the particles. The diluted solution was then placed in a three-jet Collison Nebulizer, manufactured by BGI, Inc. Filtered, pressurized air supplied by the laboratorys compressed air system is directed into the Collis
25、on Nebulizer at 20 psig, where it disperses between three jets into the diluted solution. Isopropyl alcohol droplets are subsequently generated, carrying the fluorescent particles with the effluent air stream. Because a residual electrostatic charge can accumulate on the particles with the glass jar
26、 of the nebulizer, the particle stream was passed through a TSI Kr-85 Aerosol Charge Neutralizer. The neutralized particle stream is then directed into the seeding chamber, as shown in Figure 1a. The seeding chamber for the monolayer fluorescent particles has a volume of 50 L and is internally lined
27、 with grounded aluminum tape to minimize particle loss to deposition on the sidewalls (Figure 1a.). A small DC voltage fan ensures the chamber particle 2011 ASHRAE 435concentration remains uniform, which was subsequently verified by assessing the seeding density uniformity amongst the samples (seedi
28、ng density coefficient of variance amongst samples was below 5%). The galvanized sheet metal samples were placed at the bottom of the chamber. Each sample was 4.5 cm by 4.5 cm in size and thoroughly cleaned with isopropyl alcohol to minimize surface contamination and residual electrostatic charges.
29、The surface characteristics of the samples were analyzed using a Dektak 6M stylus profilometer and the average surface roughness was found to be 4.31 m (standard deviation of 2.05 m for 10 samples), approximately an order of magnitude greater than that of polished glass. A steady-state particle conc
30、entration was reached after an injection period of 15 minutes (the nebulizer discharge produces an air exchange rate of 6 h-1), after which the particles were deposited via gravitational settling overnight. The seeded samples were then placed in a conditioning chamber prior to wind tunnel exposure,
31、where the relative humidity was recorded with a HOBO data logger and remained at an average of 65% 3%. Although moisture in the air generates a capillary force between the particle and sample surface as the water condenses, this variable was not investigated in this research. Kim et al. 2010 found t
32、he effect of relative humidity to be small in the range of 18 to 67%. Figure 1 Particle seeding procedures: (a.) particle seeding chamber for fluorescent particles (0.35 x 0.35 x 0.4 m) and (b.) particle seeding chamber for ATD (0.41 x 0.41 x 0.45 m) To generate the multilayer deposit, two seeding c
33、hambers and three seeding stages were required. Firstly, a surface layer of 10 m fluorescent particles was deposited in a sparse monolayer employing the aforementioned seeding method. The purpose of the surface layer was to ascertain how the first layer of particles behave when particle to particle
34、contact existed with the upper layers. The seeded samples were then placed in a second seeding chamber (Figure 1b.), where they were seeded with a multilayer deposit of polydisperse (1 to 20 m) ISO 12103-1 A1 Ultrafine ATD. ATD was chosen over latex and silica microspheres and potassium chloride par
35、ticles because it is both inexpensive and easily generated in large quantities. An improvised aerosolizing chamber was developed in which roughly 20 g of ATD was contained and an impinging jet of filtered air aerosolized the powder, which was then evenly dispersed through small inlets of the aerosol
36、izing chamber into the well-mixed seeding chamber (Figure 1b.). The ATD dust load was measured using gravimetric methods and found to be approximately 28 g/m2, similar to that studied in the chamber experiments by Qian and Ferro 2008 (20 g/m2). The heavy dust load and the visible thickness of the de
37、posit ensured the existence of a multilayer deposit. Lastly, the samples were then seeded with a monolayer of 3 m particles on the canopy of the existing multilayer deposit. Figure 2 shows the unseeded and seeded galvanized sheet metal sample with ATD and fluorescent particles on the canopy and surf
38、ace layers. The canopy layer was used to assess the impact of the multilayer deposit and particle to particle contact on the absolute resuspension fraction when compared to the monolayer experiments. The surface and canopy layers were distinguished by the different fluorescent dyes used for the 3 an
39、d 10 m particles. 436 ASHRAE TransactionsFigure 2 Galvanized sheet metal sample: (a.) unseeded sample (4.5 x 4.5 cm), (b.) sample seeded with ATD (4.5 x 4.5 cm), (c.) fluorescence microscope image of the canopy layer of 3 m particles (1676 x 1252 m) and (d.) fluorescence microscope image of the surf
40、ace layer of 10 m particles (8623 x 6443 m) Development of a Micro-scale Wind Tunnel In order to study the impact of various flow conditions on particle resuspension, wind tunnels are commonly used, most of which employ a fan, located downstream of the seeded sample, as the driving mechanism for flu
41、id flow. Because airborne particle concentrations will be sampled for the multilayer deposit experiments, this setup severely limits the sampling capacity because the entire effluent air stream cannot be directed into a particle sampling device. To resolve this issue, a new, micro-scale wind tunnel
42、that employs filtered, pressurized air was developed with the aid of computational fluid dynamics (CFD). The wind tunnel has a rectangular cross section that is 5 cm wide by 1.25 cm tall to accommodate the small samples used in this research and is 20 cm in length. The wind tunnel was built using cu
43、stom laser cut 0.25 in. acrylic sheets and assembled with an acrylic adhesive to ensure an air tight assembly. In order to generate high velocities above the material sample for a reduced volumetric flow rate, a wall jet was created via a 1 mm by 5 cm rectangular nozzle positioned immediately upstre
44、am of the sample, as shown in Figure 3. The wind tunnel was modeled in the CFD program ANSYS Airpak, where the Reynolds Average Navier Stokes (RANS) 2-Equation k- Renormalization Group (RNG) turbulence model was employed. A very fine, unstructured hexa-mesh of over 450,000 cells was generated and re
45、fined near areas of interest, such as the jet discharge and immediately above the sample. Convergence was achieved in approximately 5000 iterations and confirmed by a flattening of the residual curves and velocity monitoring points positioned above the sample. The wall jet was found to produce a ver
46、y uniform discharge over the sample and exhibited the characteristic profile for turbulent plane wall jets. An example of the flow field for the 40 m/s case examined in this paper is presented below (side view, plane at the center of the sample). Air velocity measurements were also taken with a pito
47、t tube/micromanometer and a one-dimensional hot-wire anemometer to confirm the CFD results. Figure 3 ANSYS Airpak model of the micro-scale wind tunnel 2011 ASHRAE 437Figure 4 CFD velocity contour plot of the turbulent plane wall jet for 40 m/s case presented in this paper Application of Fluorometric
48、 Methods to Determine Absolute Resuspension Fractions To determine the absolute resuspension fraction, , for the monolayer particle deposits and the surface and canopy layers of the multilayer deposit, a Leica MZ16FA fluorescence stereomicroscope equipped with a charge-coupled device (CCD) camera wa
49、s used. The microscope and camera, along with morphometry analysis, were used to determine the seeding density, , defined as the number of particles in a given area. is defined as the change in seeding density before and after the seeded sample is exposed to a given flow condition in the wind tunnel, divided by the initial seeding density, as shown in Equation 1. varies between 0, in which there is no detectable resuspension, and 1, for maximum resuspension. The red 3 m and green 10 m fluorescent particles were each detected using a different fluorescent
