1、2009 ASHRAE 395ABSTRACTThe interaction of the personalized airflow supplied from ceiling mounted nozzle (diameter of 0.095 m (0.312ft) with the thermal plume generated by a seated thermal manikin with the body size of an average Scandinavian woman and its impact on the body cooling was studied. Expe
2、riments were performed in a test room with mixing ventilation at numerous conditions comprising four combinations of room air temperature and personalized air temperature (23.5C (74.3F) / 21C (69.8F), 23.5C (74.3F) / 23.5C (74.3F), 26C (78.8F) / 23.5C (74.3F), 26C (78.8F) / 26C (78.8F), four airflow
3、 rates of the personalized air (4 (8.48), 8 (16.95), 12 (25.43), 16 (33.91) L/s (cfm) and positioning of the manikin directly below the nozzle (1.3m (4.265ft) distance between the top of manikins head and the nozzle). The asymmetric exposure of the body to the personalized flow was studied by moving
4、 the manikin 0.2m (0.656ft) forward, backward and sideward. The blockage effect of the unheated manikin on the personalized airflow distribution, studied at the case 23.5C (74.3F)/23.5C (74.3F), was clearly observed 0.2m (0.656ft) above the top of manikins head where the centerline velocity was redu
5、ced to about 85% under all personalized airflow rates. The neutral level, Xnl, defined as the distance from the nozzle where the impact of the thermal plume on the velocity distribution in the personalized airflow was observed, increased from 0.8m (2.625ft) to 1.1m (3.609ft) with the increase of the
6、 airflow rate. Above 16L/s (33.91cfm) the personalized airflow was able to completely destroy the thermal plume. In comparison with the reference case without personalized airflow, the manikin based equivalent temperature for the head decreased with the increase of the airflow rate from -1C (-1.8F)
7、to -6C (-10.8F) under 23.5C (74.3F)/21C (69.8F) case and from -0.5C (-0.9F) to -4C (-7.2F) under 26C (78.8F)/26C (78.8F) case, which are the two extreme cases among the four cases studied. The personalized airflow was least efficient to cool the body when the manikin was moved forward. INTRODUCTIONP
8、ersonalized ventilation aims to provide clean and cool air in the vicinity of the breathing zone of human body and thus to improve inhaled air quality. Occupants thermal comfort is also improved, especially at relatively high room temperatures (Gong et al. 2005, Kaczmarczyk et al. 2006). Most often,
9、 personalized ventilation systems (PV) with desk mounted air terminal devices (ATD) have been studied (Faulkner et al., 1999, Tszuzuki et al. 1999, Melikov et al. 2002, Kaczmarc-zyk et al. 2004, etc.). The idea of ceiling mounted PV ATD provides more flexibility in arranging the furniture in the occ
10、upied zone. It also improves the indoor aesthetics because extended air ducts for transporting of clean and cool air to different workstations are not needed. Occupants may be provided with individual control of the PV airflow rate in order to obtain preferred micro-environment in term of thermal co
11、mfort and indoor air quality. The cooling effect of ceiling mounted nozzle depends on the PV airflow rate and the temperature of the PV supply air (Yang et. al. 2008). The size of the nozzle (its diameter if it is circular nozzle) and the initial airflow conditions at the exit define the size of the
12、 target area in contact with the body. The system can be regarded as one kind of individual spot cooling system by vertical air jet, which can provide occupants with acceptable thermal comfort conditions (Azer et al. 1971, 1972; Azer and Nevins 1974; Olesen and Nielsen 1980, 1983; Ma and Qin 1991; M
13、elikov et al. 1994, 1994a). At constant airflow Performance Evaluation of Ceiling Mounted Personalized Ventilation SystemBin Yang Arsen Melikov, PhD Chandra Sekhar, PhDStudent Member ASHRAE Fellow ASHRAE Fellow ASHRAEBin Yang is a doctoral student in the NUS-DTU Joint PhD program and Chandra Sekhar
14、is an associate professor in the Department of Build-ing, National University of Singapore (NUS), Singapore. Arsen Melikov is an associate professor in the Department of Civil Engineering, Technical University of Denmark (DTU), Denmark.LO-09-036 2009, American Society of Heating, Refrigerating and A
15、ir-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 written permission.396 ASHRAE Transactionsra
16、te, the increasing of the nozzle outlet diameter will reduce air velocity of the personalized airflow but will increase the size of the target area of the jet (target area is defined as the cross section of the jet where it first meets the occupant) while decreasing nozzle diameter will increase air
17、flow velocity at the target area but will decrease its size, i.e. the airflow will not cover occupants body (Melikov et al. 1994a). Human body will affect the personalized airflow as an obstacle and with the generated thermal plume. The characteristics of the thermal plume generated by human body de
18、pend on the body posture (standing and seated), clothing design and thermal insulation, type of the chair, surrounding air and radiant temperature, etc. (Hyldgaard 1998, Zukowska et al. 2007, 2007a). The interac-tion of the personalized airflow with the thermal plume gener-ated by the human body is
19、of major importance for the body cooling. It may also affect the air distribution pattern in the entire space. In reality, people at workplaces will move and may not be located directly below the nozzle. The asymmetric exposure of the body, i.e. the changes in positioning of the thermal plume will a
20、ffect its interaction with the personalized airflow, which will result in a non-uniform cooling of the body. Thus the whole body cooling effect of the personalized airflow will decrease while the local cooling of some body parts may increase leading to draught discomfort. These effects need to be st
21、udied for proper design of ceiling mounted personalized ventilation. The interaction of the personalized airflow from ceiling mounted PV nozzle with the thermal plume from human body and its whole body and local cooling effect was studied under different conditions. The impact of occupants movement
22、on the airflow interaction and thus on the body cooling was explored. The results are presented and discussed in this paper.It may be noted that CFD tools could be employed in stud-ies involving room air distribution. A validated CFD model is useful for parametric variation studies in which refine g
23、rids become essential, especially for simulating thermal plume around human body. However, this is beyond the scope of the present paper.METHODSExperimental Set-UpExperiments were conducted in a field laboratory (4.7m (15.42ft)5.4m (17.71ft)2.6m (8.53ft) specially designed for studying PV systems. A
24、n air distribution system was modi-fied to provide personalized air to a nozzle mounted at the ceil-ing in the middle of the room (Figure 1). The nozzle with a parabolic contracting profile was designed to generate vertical downward circular free jet with initial diameter of 0.095m (0.312ft) and uni
25、form velocity profile. The nozzle was connected to a circular duct with its larger diameter of 0.16m (0.525ft). The exit of the installed nozzle was located at the height of 2.5m (8.202ft) above the floor. The ambient cooling in the PV lab was managed by a mixing ventilation system supplying airflow
26、 from four swirl diffusers symmetrically mounted on the ceiling. One return air grill was located at the upper part of one side wall and a completely ducted return route was used.Figure 1 Personalized ventilation (PV) laboratory.ASHRAE Transactions 397Measuring InstrumentsA thermal manikin consistin
27、g of 23 individually controlled body segments with the size of an average Scandi-navian woman was used to resemble human body with the generated thermal plume. A comfort control mode was used, i.e. the surface temperature of the manikin was controlled to be the same as the skin temperature of an ave
28、rage person in state of thermal comfort under the exposed environmental condi-tions (Tanabe et al., 1994). The control system allows for read-ing of the surface temperature of each body segment and the supplied heat power. The manikin was dressed with long sleeve shirt, trousers, underwear, socks an
29、d shoes at about 0.7clo (0.1085m2K/W) thermal insulation and was seated in an office chair with thermal insulation of approximately 0.15-0.2 clo (0.02325-0.031 m2K/W). Multi-channel low velocity thermal anemometer with five omni-directional velocity probes (SENSOR-ELEC-TRONIC HT-400) was used to per
30、form measurements of velocity field in the personalized airflow. The velocity measurement range of the system was 0.05m/s (0.164fps) to 5m/s (16.4fps), with the accuracy of 0.03m/s (0.098fps). In order to obtain accurate velocity results, all probes were calibrated before the measurements in a low v
31、elocity wind tunnel. A rake of five velocity probes placed beside each other horizontally (Figure 1) was used to measure velocity distribu-tion at eight cross sections of the airflow above the manikin as schematically shown in Figure 2. For the two cross sections close to the nozzle outlet, 5 points
32、 were selected. For other cross sections, velocity measurements at 9 points were performed. Another single channel low velocity thermal anemometer with omni-directional probe (SWEMA 300) was used for measuring velocity at nozzle outlet. The accuracy of the instrument was 0.03 m/s (0.098fps) and it w
33、as cali-brated by the manufacturer. Each of the velocity measure-ments lasted 3 minutes.Experimental ConditionsThe distance between nozzle outlet and the head of the seated manikin was 1.3m (4.265ft). The nozzle was selected according to the jet theory (Awbi, 2003) to provide airflow with diameter o
34、f 0.35-0.5m (1.148-1.64ft) at the target area, i.e. a airflow covering the head/shoulders of the seated mani-kin. Depending on the supply airflow rate the potential core region of the generated personalized airflow was 0.25-0.45m (0.82-1.476ft) long.Experiments were performed at four different perso
35、nalized airflow rates (4, 8, 12 and 16 L/s) 8.48. 16.95, 25.43 and 33.91cfm. The supply temperature of personalized airflow was controlled at 21C (69.8F) and 23.5C (74.3F). The room air temperature was controlled at 23.5C (74.3F) and 26C (78.8F). As a result, four temperature combinations were achie
36、ved: 23.5C (74.3F) / 21C (69.8F), 23.5C (74.3F) / 23.5C (74.3F), 26C (78.8F) / 23.5C (74.3F), 26C (78.8F) / 26C (78.8F). The isothermal case 23.5C (74.3F) / 23.5C (74.3F) was selected to explore velocity distribution under the nozzle without manikin, with unheated manikin and with heated manikin. Av
37、erage velocity and Reynolds number at nozzle exit, centerline velocity at target area without manikin under this isothermal case are listed in Table 1. Figure 2 Distribution of omni-directional thermal anemometer probes.Table 1. Personalized Air Velocity at Nozzle Exit and Target Area under Differen
38、t Personalized Airflow RatesPersonalized Airflow Rate (L/s, cfm) Exit Velocity (m/s, fps) Re Number Target Velocity (m/s, fps)4 (8.48) 0.55 (1.804) 3393 0.24 (0.787)8 (16.95) 1.22 (4.003) 7526 0.39 (1.28)12 (25.43) 1.52 (4.987) 9377 0.61 (2.001)16 (33.91) 1.96 (6.43) 12091 0.8 (2.625)398 ASHRAE Tran
39、sactionsThe Reynolds number is defined as follows:(1)(2)Whereu = exit velocity (m/s, fps)d = characteristic length (diameter of nozzle outlet) (m, ft)= kinematic viscosity (m/s, ft2/s) = dynamic viscosity of the fluid (Ns/m, lbm/fth) = density of the fluid (kg/m)Furthermore, experiments with manikin
40、 moved 0.2m (0.656ft) forward, backward and sideward were performed to simulate occupants movement in reality at workplace and to study its impact on the airflow interaction. All temperature combinations under different personalized airflow rates were explored. Some results, which include 16 L/s (33
41、.91cfm) personalized airflow rate under different temperature combi-nations and isothermal case 23.5C (74.3F)/23.5C (74.3F) under different personalized airflow rates were utilized to demonstrate the change of local cooling effect.Data AnalysesThe measurements allowed for determination of the blocka
42、ge effect and the neutral level in the airflow. The block-age effect was caused by unheated manikin, which can be only regarded as an obstacle for the personalized airflow. The neutral level, Xnl, was defined as the vertical distance from the nozzle to the point where the impact of the thermal plume
43、 on the velocity distribution in the personalized airflow begins to be observed, i.e. the vertical distance from the nozzle to the point where the difference in velocity distribution across the airflow with and without the presence of heated manikin was observed.In order to quantify the cooling effe
44、ct of the thermal envi-ronment which the manikin was exposed to, sensible heat loss measured from each body segment as well as from the whole-body of the manikin was transformed into a parameter named manikin-based equivalent temperature. The manikin-based equivalent temperature was defined as the t
45、emperature of a uniform enclosure in which a thermal manikin with realistic skin surface temperature would lose heat at the same rate as it would in the actual environment (Nilson et al. 1999). The difference of manikin based equivalent temperature with and without personalized air supply (Teq) calc
46、ulated for each body segment and for the whole body was used as an evaluat-ing index. The manikin was calibrated before the experiments. RESULTSAirflow InteractionComprehensive measurements of the velocity field across the personalized airflow at eight different cross sections were performed in the
47、case of free vertical jet without manikin, with unheated manikin and with heated manikin. Mean velocity profiles measured at selected cross sections without manikin, with unheated manikin and with heated manikin were compared for exploring the blockage effect and the neutral level. The mean velocity
48、 profiles measured at selected cross sections with a heated manikin at 4 L/s (8.48 cfm) is shown in Figure 3. Velocity profiles similar to those shown in Figure 3 were obtained for the remaining airflow rates of 8, 12 and 16 L/s (16.95, 25.43 and 33.91 cfm) and similar results have been observed. At
49、 the two cross sections close to nozzle outlet, centerline velocities are only slightly lower than the velocity at nozzle outlet because they were within the core region. The comparison of the results show that at 4 L/s (8.48 cfm) the blocking effect of the unheated manikin on velocity distribu-tion in the personalized airflow was observed after approxi-mately. 11.58 initial nozzle diameters, i.e. approximately. at distance of 1.1m (3.609ft) from the exit of the nozzle and 0.2m (0.656ft) from ma
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