ASHRAE LO-09-084-2009 Experimental and Numerical Study of Airflows in a Full-Scale Room《在全尺寸房间中气流的实验和数值研究》.pdf

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1、2009 ASHRAE 867ABSTRACT This paper describes the experimental and numerical investigation of airflows in a full-scale ventilated room. The Volumetric Particle Streak-Tracking Velocimetry (VPSTV) was used to measure the three spatial components of air velocities at three ventilation rates: 3 ach, 8.6

2、 ach, and 19.5 ach (Air Change per Hour). Reattachment length and jet penetration length measured at seven different ventilation rates ranging from 3 ach to 100 ach were also reported. The data obtained from the experiments were used to validate different numerical models, including turbulence model

3、s based on the Reynolds-Averaged Navier-Stokes (RANS) method and the Large Eddy Simulation (LES) with dynamical subgrid model. It was observed that one primary central recirculation vortex was formed in the middle of the room and one secondary small vortex existed near the left bottom corner. The si

4、zes and posi-tions of the two vortices varied with the ventilation rates: the central vortex became fuller and moved towards the center of the room with increasing ventilation rates; on the other hand, the secondary vortex became smaller and moved toward the left bottom corner. The reattachment leng

5、th and jet penetration length showed a strong dependence on the ventilation rates and became relatively constant once the ventilation rate reached a threshold value of 19.5 ach. The LES generated the best predictions for the three ventilation rates while Reynolds Stress Model (RSM) predictions were

6、closest to measurements among the RANS models. The outcome of the study will allow scientists to gain a better understanding of airflows and be useful in designing better ventilation systems that will improve the air quality and human health in indoor environments, such as offices, aircraft cabins,

7、and other working environments. INTRODUCTIONIn recent years, indoor air quality has gained more and more attention as people begin to realize that indoor air quality is important to their health and comfort (Zhang 2005). In addi-tion, intensive animal production and animal welfare in confined animal

8、 buildings have raised the issues related to indoor environmental control such as reduction of cold down-drafts, preserving thermal conditions and reduction of contaminants (Bennetsen 1999). The thermal comfort of animals and people are affected not only by temperature, but also air velocity (Boon 1

9、978). Furthermore, the transport and distribution of particulate matter and gaseous pollutants are greatly affected by airflow patterns, especially turbulence. Thus, it is important to study the air distribution, turbulent characteristics, and contaminants distribution in the ventilated rooms. It is

10、 worthwhile to briefly mention why airflows in venti-lated rooms are important from the theoretical point of view. First, ventilated room airflows exhibit almost all the simple and complex flow phenomena that can possibly occur in incompressible flows, such as eddies, secondary flows, three dimensio

11、nal flow characteristics, separation and reattach-ment, complicated particle motions, instabilities, transition, and turbulence. Second, the flow domain for a given ventilated room is unchanged and thus investigations are facilitated over the whole range of Reynolds numbers from zero to infinity. On

12、e of the most interesting aspects of fluid mechanics is the inherent instability of viscous flows and the related transition from laminar flow to turbulence with increasing Reynolds numbers. Given an appropriate disturbance, the flow charac-teristics, topological structures, symmetries or even time

13、dependent nature in a mechanically ventilated room may be Experimental and Numerical Study of Airflows in a Full-Scale RoomJianbo Jiang, PhD Xinlei Wang, PhDAssociate Member ASHRAE Member ASHRAEYigang Sun, PhD Yuanhui Zhang, PhDMember ASHRAE Fellow ASHRAEJianbo Jiang is a postdoctoral fellow at Mone

14、ll Chemical Senses Center, Philadelphia, PA. Xinlei Wang is an associate professor, Yigang Sunis a senior research engineer, and Yuanhui Zhang is a professor in the Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, Urbana, IL.LO-09-084 2009, American

15、Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior writt

16、en permission.868 ASHRAE Transactionschanged (Van Dyke 1982). Airflows in a ventilated room provide an excellent case for studying the behavior of the flow field dominated by three-dimensional vortical structures and undergoing unstable transitions. The co-existence of low air velocities and high ai

17、r turbu-lent intensities makes the measurement difficult in the venti-lated rooms. Due to the secondary airflows resulting from thermal buoyancy around the sensor, thermally based sensors such as Hotwire are not suitable for low velocity measure-ments in indoor rooms. For most of the existing air ve

18、locity instruments, the inadequacy of being able to measure many locations simultaneously in a room limits their uses. Further-more, velocity measurements with Hotwire and Laser Doppler Velocimetry (LDV) in the separated flow regions where instantaneous flow reversal occurs may be subject to errors

19、caused by velocity bias (Adams and Eaton 1988). In order to understand airflow patterns and turbulent characteristics in large rooms, simultaneous and non-intrusive measurements are needed. In recent years, Computational Fluid Dynamics (CFD) along with theoretical and experimental tests has become a

20、 powerful tool in the study of room airflows due to the increase in the computing power. Direct Numerical Simulation (DNS), which solves the entire Navier-Stokes equations directly, is theoretically the best tool to investigate the airflows and related quantities. But due to the formidably high cost

21、s, its use is limited to very low Reynolds number flows and is unfeasible in real engineering problems such as room airflows in the fore-seeable future. On the other hand, Reynolds-Averaged turbu-lent models based on Navier-Stoke equations (RANS) are widely used in engineering applications due to th

22、eir simplicity and low cost with comparably accurate results. The limitation related to the RANS models is that most of them are only applicable to fully developed turbulent airflows. In fact, airflows in real ventilated rooms, such as an office, are usually not fully developed turbulent flows due t

23、o their low ventilation rates. It has been the belief in recent years that the Large Eddy Simulation (LES), which is between DNS and RANS, is a promising tool in the study of room airflows, especially those at low ventilation rates.Fully developed turbulent indoor airflows have been stud-ied extensi

24、vely during the past several decades. Different RANS models have been evaluated, but there is no complete evaluation of different RANS models in the same configura-tion (Voigt 2001). In addition, the effects of different inlet boundary conditions, the sidewall effects on the indoor airflows have not

25、 completely been understood (Chiang et al. 2000; Lee et al. 2002). On the other hand, only a few studies have been devoted to the indoor airflows at low ventilation rates (airflows are in the transitional regimes). There is a lack of experimental data in the non-fully turbulent indoor airflows. The

26、applicability of the RANS models in non-fully turbulence airflows is also not well understood (Davidson et al. 2000). The present study attempts to fill this gap.MATERIAL AND METHODSExperimentsTo provide a real-time full-field view of airflow as an aid in understanding the complexities and provide c

27、orroborating evidence for the flow simulation results, a series of experi-ments using Volumetric Particle Streak-Tracking Velocimetry (VPSTV) were conducted in the Room Ventilation Simulator (RVS). The schematic of the RVS is shown in Figure 1. It consists of an insulated outer building (L W H = 12.

28、2 m 9.1 m 3.6 m or 40 ft 30 ft 12 ft) and an inner room (L W H = 10 m 7 m 2.4 m or 33 ft 23 ft 8 ft). Through the use of its own heating, ventilating, air conditioning and humidifier/dehumidifier, temperature and relative humidity controls, the conditions in the outer room can vary in range (-27C, 3

29、8C or -17F, 100F) and (20%, 90%) respectively. The inner room can be adjusted to different room sizes by use of modular walls and is divided into two parts in the present experiments. One is a test room with a dimension of 5.5 m 3.7 m 2.4 m (or 18 ft 12 ft 8 ft, L W H) and the other is a camera room

30、 for image acquisition with a dimension of 5.5 m 2.5 m 2.4 m (or 18 ft 8.2 ft 8 ft). One sidewall of the test room is made of glass in order to permit optical access and all remaining walls were painted with flat-black paint to form a good optical background. The independent HVAC system for the inne

31、r room was used to provide constant conditioned supply air and the conditions around the inner room were maintained at the same temperature. The walls, ceiling and floor were built with plywood and batt fiber glass was used for insulation. A sketch of the test room is shown in Figure 2. Both the inl

32、et and outlet spanned the whole width of the test room and were put on the opposite walls. Two glass slits were installed on the two end walls (where the inlet and outlet were located with the purpose of transmitting the illumination light). The VPSTV system was set up in the RVS and consisted of tw

33、o group projector lamps, two bubble generators and two digital cameras (Figure 3, Sun et al. 2001, 2004; Sun 2007). An air delivery system was used to provide ventilation air during the experiments. The system consisted of a flow rate measurement chamber, a centrifugal fan and a frequency controller

34、. This system was calibrated using a fan test chamber designed and based on ASHRAE Standard (1985). Based on calibration curves, airflow rates can be obtained from any pressure drop, which is monitored using a manometer. The mean inlet air velocity was measured with a single-probe Hotwire anemom-ete

35、r (Model 8330, TSI Inc., St. Paul, MN) which has a detec-tion limit of 0.025m/s (or 5 ft/min). In the experiments, 300W projector lamps were chosen as the lighting source due to the efficiency in illumination, stronger light intensity, and rela-tively even volume illumination. Sixteen 300W (1024 BTU

36、/hour) projector lamps were installed on the outside two oppo-site walls of the inner room, with eight lamps on each side. A vertical glass window in the middle of each side wall was used to transmit the illumination light. Forced-air cooling systems ASHRAE Transactions 869were installed near the la

37、mps for circulating air around the bulbs, thus, preventing heat accumulation. To further reduce the heat gain from the light beam radiation on the airflow, the lamps were switched on only when photos were taken. Thus, isothermal conditions were well maintained during the measurements.Helium-filled s

38、oap bubbles were chosen as tracer parti-cles due to their good neutral buoyancy. A SAI Model 5 Bubble Generator with two heads made by Sage Action Inc. (1995) was used to generate the helium filled soap bubbles. The diameters of the bubbles were in the range of 1.3 mm to 3 mm (or 0.004 ft to 0.01 ft

39、) while film thickness was in the range of 0.1 m to 0.3 m (or 3.3e-7 ft to 9.8e-7 ft). Two digital cameras were selected as the image-capturing equipment in the experiments. Their resolution was 3072 2048 pixels. The distance between the cameras was about 5 m (or 16 ft) and they Figure 1 Schematic o

40、f room ventilation simulator (RVS) (Zhang 1991).Figure 2 Schematic drawing of the test room (H = 2.4 m 8 ft, W = 3.7 m 12 ft, L = 5.5 m 18 ft, h = 0.05 m 0.16 ft, h1= 0.325 m 1.07 ft, t = 0.2 m 0.66 ft, t1 = 1.29 m 4.23 ft.870 ASHRAE Transactionswere 5.2 m (or 17 ft) away from the center plane of th

41、e flow field. The angle between the two cameras optical axes was 40 due to the limitation of the camera wide-angle lens, although an angle of 90 was preferred. More details about the princi-ples of VPSTV are described in Sun et al. (2001, 2004) and Sun (2007).Data saved in the camera memory cards we

42、re first trans-ferred to the computer in the format of JPG files. Then the image processing software WIP (whole image processing) developed by the Bioenvironmental Engineering (BEE) group at the University of Illinois at Urbana-Champaign was used to process the JPG files (Wang 2005). Several steps w

43、ere involved in this image processing, including image prepro-cessing, discriminating simple and crossover streaks, and crossover streak recognition (Wang 2005). Once the image was properly processed, the centers, lengths, and angles of the streaks were obtained. After the raw image data were extrac

44、ted during the image processing, the second image processing software VPSTV-BEE was used for post-processing, includ-ing system spatial calibration, image paring and 3-D velocity computation (Sun 2007). Interpolation was adopted for a clear view of the flow patterns (Sun 2007). The maximum measure-m

45、ent relative error was estimated to be below 30%.A series of measurements were conducted to investigate airflows in the full-scale room. Three ventilation rates of 3 ach, 8.6 ach, and 19.5 ach were selected to represent a wide range of ventilation rates from smaller rooms such as laboratories, opera

46、ting rooms, and airplane passenger cabins to larger animal production buildings (ASHRAE 2001). The corre-sponding Reynolds numbers (based on inlet mean velocity and inlet height) were 753, 2158, and 4895, which lied in the tran-sitional and turbulent regions (Nielsen et al. 2000). All measurements w

47、ere conducted under isothermal conditions and a summary of the measurements is provided in Table 1.Numerical MethodsGoverning Equations for Large Eddy Simulation. Assume the flow is isothermal and incompressible. The conti-nuity and Navier-Stokes equations, in dimensionless and conservative form are

48、:(1)(2)where are the velocity components, is time, is pressure. The variables are non-dimensionalized by the maximum mean inlet velocity and the inlet height . The Reynolds number is . The basic idea of LES is to separate the turbu-lent field into large energy containing scales that would be solved

49、explicitly, and small scales that would be modeled. Applying a spatial filter (“-”) to continuity and momentum equations would generate the following equations:(3)(4)where,is or , and is the filter. is the sub-grid stress (SGS) and needs to be modeled.Eddy viscosity assumption is commonly used in LES subgrid model and takes the following form:(5)where The most basic subgrid model developed by Smagorinsky (1963) and Lilly (1966) uses the following form:(6)where and , is the volume of the cell. The commonly used value of is 0