1、OR-05-8-1 Air Distribution in Rooms Generated by a Textile Terminal-Comparison with Mixing and Displacement Ventilation Peter V. Nielsen, PhD Fellow ASHRAE Mads Sjnnichsen ABSTRACT Experiments with air distribution in rooms generated by a low impulse textile terminal are compared with the air distri
2、- bution obtained by mixing ventilation and displacement venti- lation. The air distribution in the room is mainly controlled by buoyancy forces from the heat sources, although thejlow from the textile terminal can be characterized as a passive displace- mentjlow with a downward direction in areas w
3、ithout thermal load. A variant of the system is supplied with horizontal jets from openings in the textile terminal to generate a mixingflow in the room. Both systems are compared with mixing ventila- tion based on a wall-mounted difuser and with displacement ventilation with a wall-mounted low velo
4、city difuser. All systems are tested in the same room with the same heat load consisting of two manikins, each sitting at a desk, two PCs, and two desk lamps, producing a total heat load of 480 W In addi- tion, cases with a single workplace are also tested. The design of the air distribution system
5、is in all four cases based on flow elements for the difuser, a maximum velocity assumption, and a critical vertical temperature gradient in the room. The characteristics of the air distribution systems are addressed by analyzing the acceptable conditions for the supplyjlow rate and the temperature d
6、iference for the different systems. The paper shows that an air distribution system based on textile terminals is able to generate comfortable velocity and temperature conditions at the same thermal load as can be obtained by both a mixing ventilation system with a wall- mounted difuser and a displa
7、cement ventilation system with a low-velocity wall-mounted difuser. Claus Topp, PhD Associate Member ASHRAE Heine Andersen The comparison is extended by considering both the local discomfort caused by draft rating and the percentage of dissat- isJied due to the temperature gradient when this is rele
8、vant to the systems. The draft rating is very low for the low impulse system (textile terminals), and the temperature gradient is also low because of the high level of room air mixing. INTRODUCTION The aim of an air-conditioning system is to remove excess heat in a room and replace room air with fre
9、sh air to obtain a high air quality. It is not sufficient to remove heat and contam- inated air; it is also necessary to distribute and control the air movement in the room to create thermal comfort in the occu- pied zone. Most air distribution systems are based on mixing venti- lation with ceiling
10、or wall-mounted diffusers or on displace- ment ventilation with wall-mounted low velocity diffusers. New principles for room air distribution are introduced with the textile terminals, which can be arranged as low impulse systems. Figure 1 shows the layout of a low impulse system with a passive and
11、an active downward-directed flow from the textile terminal, respectively. The right illustration in Figure 1 shows a textile terminal with a number of small openings that generate horizontal flow to increase the width of the down- ward flow from the diffuser and to improve mixing in situa- tions whe
12、n heated air is supplied to the room. The heat sources in the room control the flow in a large area, and a displacement effect will only be present in some parts of the room. The effect is especially pronounced if heat sources are located outside the downward-directed flow from the terminals. Peter
13、V. Nielsen is a professor and Mads Sennichsen and Heine Andersen are students in the Department of Building Technology and Struc- tural Engineering, Aalborg University, Copenhagen, Denmark. Claus Topp is with the engineering firm, Lindab A/S Comfort, Farum, Denmark. 02005 ASHRAE. 733 Textile termina
14、ls have been used for many years in the catering and food processing industry because they are easy to clean and because condensation on fabric surfaces is prevented. Textile terminals are now increasingly used as comfort ventilation in large offices as call centers and in other areas with a high co
15、oling load. This paper addresses four air distribution systems in all, namely, two low impulse systems based on textile terminals with and without horizontal jet flow (see Figure 1) and mixing ventilation from a wall-mounted terminal and displacement ventilation from a wall-mounted low velocity diff
16、user (see Table 1). The design strategies are discussed for all four systems. The supply flow rate qo and the temperature differ- ence AT, between return and supply are chosen as design parameters. The local discomfort caused by draft rating and the dissatisfied due to the temperature gradient are a
17、lso addressed. System Supply Return Mixing ventilation End wall-mounted Return opening below sup- Displacement ventilation End wall-mounted End wall-mounted below ply terminal ceiling Low impulse textile tenni- Ceiling-mounted End wall-mounted at floor na1 level TEST ROOM Figure 2 shows the full-sca
18、le rooms and the location ofthe diffuser for mixing ventilation, the diffuser for displacement ventilation, and the textile terminal at the ceiling. The dimen- sions of the room are in accordance with the requirements of the International Energy Agency Annex 20 work with length, width, and height eq
19、ual to 4.2 m, 3.6 m, and 2.5 m. The return openings have different locations, as indicated in Figure 2 and described in Table 1. Load, one person Load, two persons W W 240 480 240 480 240 480 Figure 1 Low impulse systems without and with horizontal control jets. Low impulse textile termi- na1 with h
20、orizontal con- trol jets Figure 3 shows the textile terminal, which is designed as a half cylinder (d = 3 15 mm) located close to the ceiling. The textile terminal with horizontal control jets has the same loca- tion and it has four groups of openings on both sides of the terminal. Every group of op
21、enings consists of 42 openings each with a diameter of 4.5 mm covering a distance of 475 mm of the textile terminal. They are distributed at a distance of 525 mm along the terminal. Figure 4 shows the furnishings and the heat load of the room (office room layout). The heat load consists of two PCs,
22、two desk lamps, and two manikins, producing a total heat load of 480 W. The room dimensions, the furnishings, and the heat load are identical to earlier test cases with mixing and displacement ventilation (see Jacobsen et al. 2002 and Nielsen et al. 2003). Therefore, it is possible to make a direct
23、comparison between four different air distribution systems. Some experiments are also carried out with a single workplace in the room. In this case the workplace is located symmetri- cally in the room. The temperature distribution is measured along three vertical lines and in supply and return with
24、type-K thermocou- ples connected to a data logger. The velocities are measured along a line at the height of 1.8 m (height of the occupied zone). The measurement line is moved down along the whole length of the room. Velocities are also measured at head, chest, and foot height of the two thermal man
25、ikins and on the two tables. All velocity measurements are made by 18 hot sphere anemometers connected to a multichannel flow analyzer. Ceiling-mounted End wall-mounted at floor levei LOW IMPULSE SYSTEM-THE DESIGN GRAPH The thermal load is constant in all experiments. The flow rate has been varied b
26、etween 0.017 m3/s and O. 10 m3/s with a variation of temperature difference AT, between 11 K and down to 2 K. The room temperature is 22.9“C in all the exper- iments. The room temperature in mixing ventilation and low impulse ventilation is the average temperature of the occupied zone, while the roo
27、m temperature in displacement ventilation is the air temperature at the height of 1.1 m. 240 l- 734 ASH RAE Transactions: Symposia Figu Ire 2 The upper sketch shows the full-scale room with location of diffusers and return openings for mixing ventilation and displacement ventilation (hatched areas).
28、 The lower sketch shows the full- scale room equipped with a textile terminal (without and with horizontal control jets) as well as the location of the return openings. Three important parameters are considered in the design of room air distribution. The parameters are the air velocity (draft), the
29、vertical temperature gradient, and the asymmetry in the mean radiant temperature. The air velocity can either be the velocity u, when it penetrates the upper boundaries of the occupied zone (1.8 m above the floor), the velocity uol close to the ankles of the manikins, or u I in head height of the ma
30、ni- kins. The similarity principles show that any dimensionless velocity in the room can be given as a unique function of the Archimedes number if the flow in the room is a fully devel- oped turbulent flow (high Reynolds number flow-see Thti and Goodfellow 2001). The similarity principle is used to
31、obtain a formulation ofthe velocity level for u, uo1, and u1 l. The Archimedes number Ar is given by where , g, d, and AT, are the thermal expansion coefficient, gravitational acceleration, diameter of the textile terminal, and temperature difference between return and supply flow, respectively; u,
32、is the supply velocity given as a face velocity qda,; qo is the flow rate to the room, and a, is the total area of the diffuser. Figure 3 Textile terminal for the low impulse system. Figure 4 Furnishings and heat load in the full-scale room. Figure 5 shows the measured dimensionless velocities uOc u
33、ol and uoll are velocity at ankle level and head level, respectivey. mixing effect. This diffuser is especially developed for situa- tions where heating can be a part of the load and according to the manufacturer it can be used for inlet temperatures up to 3 to 5 K above room temperature. Figure 6 s
34、hows the design graph for the two low impulse systems. The graph is based on the maximum allowable flow rate q, to the room and on the maximum temperature difference between return and inlet. The values are restricted to a level where uol and u1 are less than O. 15 ds to avoid draft. The draft at an
35、kle and head height causes the restrictions indicated in Figure 6. The experiments show uol and u1 equal to O. 15 m/s, corresponding to a u, value close to 0.2 ds. It can thus be concluded that u, equal to 0.2 ds should be the maximum entering velocity in the occupied zone for rooms ventilated by te
36、xtile terminals in the case considered here. It is also a requirement that the temperature gradient dT/dy should be less than 3 Wm, but this will not add further restric- tions to the q, - AT, graph because the gradient is very small in all measurements. The comfort requirements (draft and temperatu
37、re gradi- ents) do not give restrictions to the level of supply flow rate q, or the temperature difference AT, (see Figure 6). The sup ly flow rate qo could for practical reasons be limited to O. 1 m /s, corresponding to an air change rate of 10. As the graph shows, the temperature difference is res
38、tricted to 12 K but the producer recommends a maximum difference of 6 K. The design graph shows that the highest heat load can be handled with the low impulse diffuser and two workplaces in the room. The limiting design parameter in this case is the velocity at head height, u1 l. The velocity at ank
39、le height, uol, is the limiting velocity in the room layout with one workplace. The textile diffuser with horizontal control jets creates a strong mixing in the room, which causes a lower design load. P AToOCI 12 10 8 6 4 2 O 0.00 0.02 0.04 0.06 0.08 0.10 0.12 qo m3is1 Low impulse with control jets
40、(I manilan) -.?ow impulse (2 mannequins) Figure 6 Design graph for the low impulse system without and with control jets. The low impulse system without controljets is shown for both one and two manikins in the room. LOW IMPULSE SYSTEM- THE LOCAL DISCOMFORT The design models discussed in the previous
41、 section can only give the limits for the operation of the air distribution system. It is necessary to consider thermal comfort for all flow rates if the system has to be optimized. The thermal environment often shows temperature gradi- ents, velocity gradients, different turbulence levels, and an a
42、symmetric radiant temperature distribution. The local discomfort, which is the result of this environment, is found from measurements of the local values of air temperature, air velocity, and turbulence level and from measurements of surface temperatures or asymmetric radiant temperatures (see Fange
43、randLangkilde 1975,Olsenetal. 1979,Fangeret al. 19891, and Toftum et al. 19971). The number of dissatisfied because of draft, the draft rating (DR), is used as a measure of local discomfort. The draft rating (DR) is defined as d(I, - 24) (34-tu)(u-0.05)062(0.37uTu+3.14), (2) DR = e where tu, u, and
44、Tu are ambient air temperature, air velocity, and turbulence intensity, respectively (see Toftum et al. 19971). Variable d is a factor that is dependent on direction. Equation 2 will take account of low velocities (below 0.3 5 ds) and it is therefore more sensitive than the air difision 736 ASHRAE T
45、ransactions: Symposia DR% 18 I 1 ATO OC 4-1 O! I O O,O2 0,04 O,% 0,oS 0,l 413 qo */SI -e- Termiid with controljets Figure 7 Draft rating versusJlow rate. One manikin in the room. performance index (ADPI) (see ASHRAE 1997 for both indices). Figure I shows the percentage of dissatisfied because of dra
46、ft, DR, with one manikin in the room. The draft rating is based on the velocity measured at the height of 0.1 m. It is shown that the textile terminal without control jets has the lowest draft rating for all flow rates. The draft rating for the textile terminal with horizontal control jets is about
47、5% higher for all flow rates because the momentum flow from the control jets generates a higher velocity level in the occupied zone. The draft rating is nearly independent of the flow rate in all measurements. The measurements are made at constant heat load, which means that the buoyancy force is la
48、rge at small flow rates and small at large flow rates. This effect may lead to a relatively constant velocity at floor level where the largest draft rating is measured. The measurements show small temperature gradients and a low level of asymmetric radiation. This could be explained by the high mixi
49、ng of air in the room. COMPARISON BETWEEN MIXING VENTILATION, DISPLACEMENT VENTILATION, AND LOW IMPULSE VENTILATION The design graphs for mixing and displacement ventila- tion are also based on the air velocity and the vertical temper- ature gradient, and the variation in AT, and q, both have to be O 0,02 0,OJ 0,06 0.08 0,l qo Ii3/S Mixing ven ir ln tr on Dtsplncoment ventilnttoii Air qirnlitv Figure 8 Design graph for a low impulse system without control jet, mixing and displacement Ventilation. q, is assumed to be larger than 0.02 m3/s be
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