1、2009 ASHRAE 421ABSTRACTThis paper deals with methods for control of the free convection flow around the human body, the aim being to improve the quality of the inhaled air for occupants at work-stations with personalized ventilation (PV). Two methods of control were developed and explored: passive -
2、 blocking the free convection development by modifications in desk design, and active - by local suction below the desk. The effectiveness of the two methods for enhancing the performance of PV was studied when applied separately and combined, and was compared with the reference case of PV alone. Th
3、e experiments were performed in a full-scale test room with background mixing ventilation. A thermal manikin with realistic free convection flow was used. The PV supplied air from front/above towards the face. All measurements were performed under isothermal conditions at 20 C (68 oF) and 26 C (78.8
4、 oF). The air in the test room was mixed with tracer gas, while personalized air was free of it. Tracer gas concentration measurements were used to identify the effect of controlling the free convection flow on inhaled air quality. The use of both methods improved the performance of PV and made it p
5、ossible to provide more than 90% of clean air for inhalation at a substantially reduced PV supply flow rate.INTRODUCTIONThe aim of personalized ventilation (PV) is to supply clean air to the breathing zone of each room occupant. Together with total volume ventilation, PV can provide supe-rior air qu
6、ality and can greatly reduce the risk of cross-infec-tion for occupants who spend a relatively long time at their workplace (Cermak and Melikov 2007). Individual control of the flow rate, temperature and direction of the supplied personalized air makes it possible to achieve a preferred microenviron
7、ment for each occupant. It has been documented that PV can significantly improve occupants inhaled air qual-ity and thermal comfort and can significantly decrease SBS symptoms (Kaczmarczyk et al. 2004, 2006). The performance of PV with regard to occupants thermal comfort and inhaled air quality depe
8、nds on the interaction of the flows in the vicinity of the human body, in most cases the personalized airflow, the free convection flow around the human body, the airflow generated by the background total volume ventilation and the flow of exhaled air. The personalized flow is typically a free jet i
9、ssued from a circular or rectangular opening or a nozzle. The first region of the jet, known as the potential core region, contains a core with constant velocity, low turbulence intensity and supply air unmixed with the polluted room air. A non-uniform velocity field at the air supply and a high ini
10、tial turbulence intensity that generates velocity fluctuations increase the mixing of the supplied clean air with the polluted surrounding room air and decrease the length of the potential core (Melikov 2004). The free convection flow is generated by the difference between the room air temperature a
11、nd the surface temperature of the human body. The greater the temperature difference, the stronger the free convection flow. The free convection flow develops from laminar, with low velocity at the lower legs, to turbulent, with relatively high velocity at the upper chest and the head region (Clark
12、and Toy 1975). Body shape and posture, room air temperature, clothing insulation, etc. define the mean velocity in the free convection flow, which may be as high as 0.25 m/s (49.21 fpm) at the head region, and the thickness of the boundary layer, which may measure 0.2 m (0.66 ft) or more (Homma and
13、Yakiyama 1988). This flow Improved Performance of Personalized Ventilation by Control of the Convection Flow around Occupant BodyZhecho D. Bolashikov Arsen Melikov, PhD Miroslav KrenekFellow ASHRAEZhecho Bolashikov is a PhD student, Arsen Melikov is an associate professor, and Miroslav Krenek is a m
14、asters student in the Department of Civil Engineering, Technical University of Denmark, Denmark.LO-09-038 2009, American 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
15、 reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.422 ASHRAE Transactionsinduces and transports air (as well as pollutants if present) from lower heights in the room to the breathing zone. There-fore the greater por
16、tion of the air that is inhaled by sedentary and standing persons in rooms is from the free convection flow (Brohus and Nielsen 1994). The background airflow is influenced by the location and type of air supply devices, supply airflow rate and tempera-ture, type and location of heat sources, etc. Th
17、e flow of exha-lation depends on the breathing mode (nose/mouth, mouth/nose, etc.), respiration flow rate (which depends on activity level, body weight), nose and mouth shape (different from person to person), body and head posture, etc. The interaction of the background flow with the free convectio
18、n flow is important for the heat loss from the human body. In order to avoid draught discomfort, present indoor climate standards (ISO 7730 2004, ASHRAE 55 2004) recom-mend low velocity (less than 0.2 m/s (39.37 fpm) in the occu-pied zone at the low range of comfortable room air temperature (20 C (6
19、8 oF) 24 C (75.2 oF). Under these conditions, the strength of the free convection flow may be equal to or even stronger than the strength of the background flow. The thermal plume generated above the human body by the free convection flow affects the room air distribution (Homma and Yakiyama 1988, Z
20、ukowska et al. 2008). The interaction of the personalized flow with the flow of exhala-tion determines the spread of bioeffluents and exhaled air between room occupants (Cermak and Melikov 2007). This study focuses on the performance of PV with regard to inhaled air quality. In this respect the inte
21、raction between the free convection flow and the personalized flow is of major importance. The interaction depends on many factors: strength of free convection flow and thickness of its boundary layer, velocity, turbulence intensity, direction and temperature of PV flow, body posture, shape and clot
22、hing design, etc. It has been documented that personalized flow directed against the face with a mean velocity higher than 0.3-0.35 m/s (59.06-68.89 fpm) is able to penetrate the free convection flow and provide 100% clean air. This however, may pose draught discomfort, especially at a relatively lo
23、w room air temperature (Melikov 2004). The risk of draught will decrease when the velocity of the personalized flow decreases, i.e decrease of the personalized flow rate when the air supply terminal device is not changed. This strategy will require a decrease of the strength and the thickness of the
24、 free convection flow at the breathing zone to enable its penetration by the personalized flow and to supply clean air for inhalation. However, methods for control of the free convection flow have not yet been devel-oped or studied.Two methods, passive and active, for controlling the free convection
25、 flow at the breathing zone were developed. The effect of these methods on the interaction of the personalized flow with the free convection flow and the resulting improve-ment of inhaled air quality was studied. The results are presented and discussed in this paper.METHODFull-Scale Test RoomThe exp
26、eriments were performed in a full-scale test room with dimensions 4.70 m 1.62 m 2.60 m (15.42 ft x 5.31 ft Figure 1 Floor plan of the test room and positioning of the sampling points: S1supply air, S2exhaust air; S3PV air for RMP; S4room air; and S5inhaled air.ASHRAE Transactions 423x 8.53 ft) (WLH)
27、. One workplace consisting of a desk with an air terminal device for PV, a chair and a seated thermal manikin was simulated in the room (Figure 1).Three fixtures (6 W (20.47 Btu/h) each) located in the ceiling provided the background lighting. The room itself was built in a laboratory hall, 70 cm (2
28、.3 ft) above the floor. The walls of the test room were made of particleboard and were insulated by 6 cm (0.2 ft) thick styrofoam. One of the walls was made from single layer glazing. Total Volume VentilationMixing type ventilation was used to condition the air in the test room. The air supply diffu
29、ser (a rotation diffuser) and the air exhaust diffuser (a perforated circular diffuser) were installed on the ceiling as shown in Figure 1. Supply air temperature and flow rate as well as exhaust flow rate were controlled during the measurements. Air humidity was not controlled but was measured as b
30、eing relatively constant (30% - 35%). The supplied air was clean (no recirculation was used). The supply flow rate was 12 L/s (25.42 cfm), which corre-sponded to an air change rate of 2.2 h-1. This flow rate provided good mixing in the room with relatively low velocity.Personalized VentilationThe ai
31、r terminal device of the personalized ventilation, named Round Movable Panel (RMP), was installed on the desk in front of the manikin. The RMP consisted of an arm, which was a lamp-like support enveloped in a flexible duct, and a hollow spherically shaped outlet (180 (7.1 in) with a honeycomb straig
32、htener at the end. It is described in detail by Bolashikov et al. (2003). A separate HVAC system was used to supply the person-alized air. The temperature and flow rate of the personalized air were controlled. The temperature of the personalized air was maintained constant to the set value by an ele
33、ctrically heated wire, coiled around the supply duct of the personalized air and controlled via a temperature sensor placed in the PV air terminal device. The humidity of the supplied personalized air was not controlled or measured, but was assumed to be close to that in the room (isothermal conditi
34、ons).Control DevicesTwo control methods, passive and active, were used to reduce the strength and the thickness of the free convection flow at the breathing zone. The passive control device did not require the use of energy, as opposed to the active control device. The two methods are referred to in
35、 the following as desk-based designs for control of the convection flow around the occupants body.Device for Passive Control The device for passive control was made of plastic card-board (10 mm (0.36 in) thick 630 x 360 mm (2.07 ft x 1.18 ft): length x width), placed in front of the manikin to block
36、 the gap between the abdomen and the front edge of the desk, and thus to prevent the warm air generated by the lower body (feet, legs, thighs) from moving upwards towards the breathing zone. Two designs were tested, one with a round front edge made to fit around the manikins abdomen “cut board” (Fig
37、.2a) and one with a straight front edge “straight board” (Fig.2b).Device for Active ControlThe device for active control aimed to reduce the convec-tion flow arising from the lower part of the body and prevent it from merging with the flow originating from the lower chest of the manikin. This device
38、 consisted of a box with 6 direct current (DC) ordinary PC fans of 1.4 W (4.78 Btu/h) nominal electric power (in 2 rows of 3 fans each). This device, named “suction box” was installed below the desk with its front edge in line with the edge of the table (Figure 3). The front and the rear groups of F
39、igure 2 Desk (4) with personalized ventilation (2) and passive control device (1) installed in front of thermal manikin (3): (a) passive control device with round shape“cut board”; (b) passive control device with straight front edge“straight board.”424 ASHRAE Transactionsfans could be operated separ
40、ately. The air sucked from the fans was exhausted in the test room more than 1 m (3.28 ft) away from the manikin in order to avoid possible disturbances of the personalized flow and the free convection flow. Thermal ManikinA thermal manikin with a surface temperature controlled to be the same as the
41、 skin temperature of an average person in a state of thermal comfort was used to resemble an occupant. The manikins body is shaped to resemble accurately the body of an average Scandinavian woman, 1.7 m (5.58 ft) in height. The manikin is made of a 3 mm (0.12 in) fiberglass coated polystyrene shell
42、and is divided into 23 segments. Each of these segments is equipped with heating and temperature measuring wiring controlled by a computer program so as to maintain a surface temperature equal to the skin temperature of a person in a state of thermal comfort at the actual activity level, and thus re
43、alistically to recreate the free convection flow surrounding the human body. The control of the manikin is described by Tanabe et al. (1994). Experimental ConditionsExperiments were performed under isothermal condi-tions, i.e. room air temperature equal to the personalized air temperature. The measu
44、rements were performed at two air temperatures, 20 C (68 oF) and 26 C (78.8 oF), i.e. the lowest and the highest comfortable room air temperature specified in the present thermal comfort standards (ISO 7730 2004, ASHRAE Standard 55 2004). The manikin, seated on an office chair, was positioned with i
45、ts abdomen at a distance of 0.1 m (0.33 ft) from the edge of the desk. During the measurements at 20 C (68 oF) the manikin was dressed in underwear, long elastic trousers, long-sleeved elastic pullover, light socks, leather shoes and light long-sleeved woollen sweater (1.2 clo). At 26 C (78.8 oF) it
46、 was dressed in underwear, shorts, t-shirt, light socks and train-ers (0.5 clo). The manikin was wearing a short hair wig just below the ear level.Most of the experiments were performed at three flow rates of personalized air (4, 6, 8 L/s) (8.47, 12.7, 16.94 cfm). The voltage to the fans in the suct
47、ion box was controlled to either 15V or 30V by a DC voltage alternator (the flow rate of the exhaust air was not measured). When either of the 2 groups of fans was running (rear or front), the other one was blocked. In all tested conditions, the RMP was positioned to supply personalized airflow towa
48、rds the middle of the face of the manikin (the symmetry axis of the outlet was pointing between mouth and nose). The distance between outlet and breathing zone was kept at 40 cm (1.31 ft).Tracer gas, Freon R134a, was used to simulate pollution in the room air. During the measurements a constant dose
49、 of tracer gas was supplied in the duct of the background ventila-tion system before the ceiling diffuser. After passing the plenum box and the rotation diffuser, the tracer gas was well mixed in the air supplied to the room. The personalized air was free of tracer gas. The tracer gas sampling and its concentration measure-ment was performed at 5 points by a real-time gas monitor based on the photo-acoustic principle of measurement. The positioning of the 5 sampling points is indicated in Figure 1. Point S4 was used to assess whether or not good mixing
