ASHRAE OR-05-5-5-2005 Effects of Source Type and Location on Contaminant Dispersion in a Displacement Ventilated Room《在位移通风室内里的源的类型和位置对污染物色散的影响》.pdf

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1、OR-05-5-5 Effects of Source Type and Location on Contaminant Dispersion in a Displacement Ventilated Room Guoqing He, PhD Student Member ASHRAE Xudong Yang, PhD Member ASHRAE ABSTRACT This research studies the effects of source type and loca- tion on contaminant dispersion and exposure in a dsplace-

2、 ment ventilated room. A full-scale environmental chamber is used to measure the airflow and contaminant distributions in a mockup ofice setting. A point source is positioned at four diferent locations to examine thesensitivity of thecontaminant distribution to source locations. This is followed by

3、the expo- sure measurements in the same room with an area of contam- inant sources on the floov. Experimental data are used to validate a computationalfluid dynamics (CFD) model, and the CFD program is further applied to simulate the contaminant dispersion with more area sources. Results show that t

4、he source type und location affects the exposure distributions for both point source cases and area source cases. Unusually high exposures are observed locally in the vicinity of the point source, while this is unlikely to happen in aveu source cases. It is also shown that even when the Contaminant

5、source is ut floor level (i.e., unfavorable source location), displacement ventilation can stillgenerate slightly lower concentration at or below the breathing zone compared to completely mixing ventilation. INTRODUCTION In indoor environments, contaminant sources can exist in a multitude of types a

6、nd locations. There are many local sources as well as area sources in buildings. Examples of local sources include smoking, cooking, washing machines, craft activities, dish washers, cleaning products, caged pets (e.g., hamsters, birds), cosmetic products, bioeffluents from occu- pants, etc. Floorin

7、g materials and surface finishes represent large area sources. Not only do source locations and types vary Jelena Srebric, PhD Member ASHRAE throughout a building, but also the sources are typically time- dependent. A building ventilation system may not perform the same way in removing different con

8、taminants; therefore, it is important to understand the performance of a ventilation method subject to different source conditions. Pollutant dispersions from fixed local point sources (Yuan et al. 1999; Srebric and Chen 2002; Brohus and Nielsen 1996; Hagstrm et al. 2002) or a small area source (Hag

9、strm et al. 1999; Cheong et al. 2003) have been widely studied using both experimental measurements and computational fluid dynamics (CFD) modeling. Brohus and Nielsen (1996) stud- ied the effect of source height on personal exposure and found that when the source is passive (not associated with hea

10、t sources), the perceived personal exposure varies significantly with the elevation of the source. However, the study concen- trated only on the elevation ofthe source. Exposure evaluation from a single source may be useful to identi6 critical zones, but it may not be able to give aglobal evaluation

11、 of the contam- inant distribution. In another study, Hagstrm et al. (1999) investigated the effect of uniformity of heat sources and contaminants on pollutant distributions. They found that the nonuniform distribution of heat and contaminant sources has a remarkable influence on the contaminant rem

12、oval efficiency and that the influence depends on the air supply method. Recently, He et al. (2004) compared the volatile organic compound (VOC) dispersion from a large floor source using several ventilation methods. The study confirms that both ventilation method and source types and locations can

13、signif- icantly affect local contaminant distributions, even in a room with “mixing” ventilation. This paper presents a study of the effect of source type and location on exposure in a displacement ventilated room. Guoqing He was a graduate research assistant and Xudong Yang is an assistant professo

14、r in the Department of Civil, Architectural and Envi- ronmental Engineering, University of Miami, Coral Gables, Fla. Jelena Srebric is an assistant professor in the Department of Architectural Engineering, Pennsylvania State University, University Park, Pa. 646 02005 ASHRAE. 2 Z I 5 Figure 1 Configu

15、ration of the mockup ofice with displacement difuser (1-tables, 2-person simulators, 3-lamps, 4-computers, kabinets, 6- displacement difuser, 7-exhaust, 8-window). Displacement ventilation has received a lot of attention due to its unique characteristics. Unlike conventional ventilation systems, whi

16、ch create a mixing condition in a room, displace- ment ventilation generates a stratified distribution of temper- ature and concentration. When the internal heat is the major cooling load, as suggested to be the situation for displacement ventilation (Sandberg and Etheridge 1996), the concentration

17、stratification may be formed at the thermal stratification height (Bjorn and Nielsen 2002). The contaminant exchange between the two zones (above and below the stratification height) is influenced by the thermal plumes generated from the internal heat sources. Both local point sources and area sourc

18、es at different locations are considered in this study. A point source is posi- tioned at four different locations to examine the sensitivity of the contaminant distribution to source locations. This is followed by the exposure study in the same room with sources on the floor, a vertical wall, and f

19、our walls. METHODS A 5.16 x 3.65 x 2.26 m full-scale environmental chamber is used to conduct measurements of concentration field result- ing from different contaminant sources. It represents the indoor environment of a two-person office, as shown in Figure 1. A displacement supply diffuser (0.54 x

20、1.1 O m) is placed on the floor against the west wall, and the exhaust diffuser (0.44 x 0.44 m) is located at the center of the ceiling. Heat sources within the office include two person simulators (66 W and 43.2 W), two computers (87.6 W and 74.4 W), and four lights (25 W each). The total internal

21、heat gain is 371.2 W. In addition, there are two tables and two cabinets representing the office furniture. Pollutant source is simulated by releasing a tracer gas (SF,) from a different location (point source) or the entire floor (area source). x- II I0 I- - I l I l Simulator I 1 - - pde3iic SurceS

22、e - Human Simulator 2 Figure 2 Point source and measurement pole locations (Top view, where C3 represents measurement pole position, 0 represents point source location). The experimental data from the floor source case are used to validate a CFD model. The validated CFD program is further applied to

23、 simulate more cases with different area sources in the room. This approach saves time and effort put into the experiments, while sufficient data can be obtained from the numerical simulations. Point Source Experimental Conditions Four different locations of a point source are chosen, as shown in Fi

24、gure 2. Locations 1 and 2 are on opposite sides of human simulator 1 with the source at the same height as the top surface of the human simulator (I. 1 m above the floor). Loca- tion 3 is at the centerline of the room and 0.2 m above the floor, and Location 4 is in the north part of the room, 1 m fr

25、om the centerline and 0.2 m above the floor. Therefore, point sources 1 and 2 are associated with the heat release from human simu- lators and represent so-called active sources, while point sources 3 and 4 are not close to a heat source and represent “passive sources.” Tracer gas (1.01% SF,) is int

26、roduced through !LI in. PTEF tubing. The source strength is 0.337 mg/s, the same for all four test cases. The spatial distributions of temperature, velocity magnitude, and concentration are measured at four vertical poles, which are marked as Pole1 to Pole 4 in Figure 2. On each pole, six to seven d

27、ata points are collected for concentra- tion, temperature, and velocity magnitude along the height (z direction). For each experiment, the data are collected after the system has operated for more than 24 hours to achieve a steady-state condition. Hot-sphere anemometers are used to measure the tempe

28、rature and velocity magnitude distributions in the room. The measurement range of the anemometers is from 0.05 to 0.5 ds. The repeatability is 0.01 ds, or 32%. Ther- mocouples are also used to measure the temperature of the ASHRAE Transactions: Symposia 647 Airflow Rate Supply Air (m3/s) Velocity (m

29、/s) 0.0562 3.0 chamber walls. The thermocouple system has an overall measuring accuracy of *0.3”C. The tracer gas concentration is sampled and analyzed by a multi-gas monitor (1302 moni- tor) and analyzer (1309 sampler). This system has a detect threshold of 1 O” ppm and 1 % repeatability. The air e

30、xchange rate is set through an automatic control system that regulates fan speed of the HVAC system. The ventilation rate is further calibrated using the mass balance of SF, based on the SF, injection rate and the measured inlet and outlet concentra- tions. The supply air is 100% outside air with a

31、very low background concentration of SF, ( The source is on the south wall away from the occupant. The wall is also a heat source. The plume generated by the south wail prevents to some extent the transport of the contaminant horizontally into the occupied zone. The global airflow pattern of displac

32、ement ventilation does not encourage contaminant transport from the upper level to the lower level. N Y (b) N Four-Wall Source. In this case, all four vertical walls are assumed to be the source, except the window on the east wall. Figure 7 presents nondimensional concentrations at three sections: x

33、 = 1.5 m, x = 4.23 m, andy = 1.83 m. Compared with the single-wall source case, the four-wall source case has a more uniform concentration distribution. Occupant 2, which experiences a low exposure in the single- wall source case, is exposed to a relative higher concentration. Figure 7c shows the st

34、ratification in the room where the down- ward airflow meets the upward flow in the near wall region. Such stratification is not generated with the presence of the thermal boundary. The plume, however, has a positive effect in improving the local air quality around the occupants, as indi- cated by Fi

35、gures 7a and 7b. Table 2 gives the simulated concentrations atz = 0.9 m for the three area sources studied, and Figure 8 compares the aver- aged dimensionless concentrations at different levels in the room. All three cases show a clear stratified concentration distribution. The contaminant could be

36、brought up to a higher level in the room by thermal plumes generated by heat sources. The floor area source case appears to be the worst case regard- ing air quality in the occupied zone due to its low source loca- tion. Nevertheless, its concentration at or below the breathing level is still slight

37、ly lower than the exhaust concentration. CONCLUSION (4 This paper discusses the effect of source type and location Figure 7 Simulated iso-concentration contours on exposure in a displacement ventilated room. Both point (dimensionless) for the four wall case. (a) x = sources and area sources are used

38、. The results lead to the 1.5 m (occupant I), (b) x = 4.23 m (occupant 2), following conclusions. () y = (center Of the (d) four 1. When the source is a concentrated point source, the expo- sure is more sensitive to the source locations and whether or walls as source, exclude the window ASHRAE Trans

39、actions: Symposia 65 1 N 1 .o O .8 O .6 o .4 o .2 o .o -o- fio o r couth th wall -I- fo u r w a Ils 0.4 0.6 0.8 I .O 1 .2 C Figure 8 Comparison of vertical distribution of averaged concentrations (C, dimensionless) for three area source cases. C = (c-cJ/(c,-cJ, c is local concentration (mg/m3), cs i

40、s inlet concentration (mg/m3), ce is outlet concentration (mg/m3). z = z/H, z is vertical location (m), H = 2.26 m is room height. C = 1.0 represents a perfect mixing of contaminant in the room. not the source is associated with a heat source. In the studied point source cases, the dimensionless con

41、centrations at the breathing level are about 13% to 85% higher than the concentration at the room center, depending on the source locations. High exposure is observed locally in the vicinity of the point source, while this is not observed in area source cases due to the uniformity of the resulting c

42、oncentration field. Exposure at a certain point in the ventilated room also depends on how the source is distributed. The source height and its connection with heat sources are crucial. 2. 3. ACKNOWLEDGMENTS This research was financially supported by the U.S. National Institute for Occupational Safe

43、ty and Health (NIOSH) through a SERCA Award (Grant No. 1 KO1 OH00 190). We thank Professor Qingyan (Yan) Chen for his generous support to this study. REFERENCES Bjorn, E., and P.V. Nielsen. 2002. Dispersal of exhaled air and personal exposure in displacement ventilated rooms. Indoor Air 12:147-164.

44、Brohus, H., and P.V. Nielson. 1996. Personal exposure in dis- placement ventilated rooms. Indoor Air 6: 157-167. Chen, Q. 1995. Comparison of different k-E models for indoor airflow computations, Part B. Numerical Heat Transfer 28:353-369, 652 Chen, Q. 1996. Prediction of room air motion by Reynolds

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48、if- ferent ventilation schemes. Ph.D. dissertation, Depart- ment of Civil, Architectural, and Environmental Engineering, University of Miami, Coral Gables, USA. He, G., X. Yang, and J. Srebric. 2004. Removal of contami- nants released from room surfaces by displacement and mixing ventilation: Modeli

49、ng and validation. Submitted to Indoor Air. Hu, B., G. He, J. Srebric, and X. Yang. 2004. Critical simula- tion parameters for accurate CFD predictions of con- taminant dispersion from indoor point sources. Submitted to The Annals of Occupational Hygiene. Patankar, S.K. 1980. Numerical Heat Transfer and Fluid Flow. Washington, DC: Hemisphere. Sandberg, M., and D. Etheridge. 1996. Building Ventilation: Theory & Measurement. New York: John Wiley & Sons. Srebric, J., and Q. Chen. 2002. Simplified numerical models for complex air supply diffusers. HVAC&R Research Yang, X., and Q. Chen. 2