ASHRAE LO-09-029-2009 Air Distribution Effectiveness with Stratified Air Distribution Systems《分层气流组织系统中气流组织效能的研究》.pdf

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1、322 2009 ASHRAEThis paper is based on findings resulting from ASHRAE Research Project RP-1373.ABSTRACTStratified air distribution systems such as Traditional Displacement Ventilation (TDV) and Under-Floor Air Distri-bution (UFAD) systems have been known to provide better indoor air quality. This stu

2、dy examined the influence of several key design parameters on air distribution effectiveness by using a validated CFD program. The parameters studied were space type, diffuser number, supply air temperature, cooling load, return location, total airflow rate, and secondary heating system. Six indoor

3、spaces were investigated to develop a database: classrooms, office spaces, workshops, restaurants, retail spaces, and auditoriums. The air distribution effectiveness at breathing zone was at 1.1 1.6 for offices, classrooms, restaurants and retail shops, and 1.6 2.0 for workshops and auditoriums. The

4、 spaces with a high ceiling such as workshops and auditoriums had higher air distribution effectiveness than those with a low ceiling. Thus, the stratified air distribution systems are better for spaces with a high ceiling. The air distribution effectiveness for the TDV and UFAD with low throw heigh

5、t was similar and was higher than that of UFAD with high throw height and mixing ventilation. A database was established containing 102 cases of the para-metric study results. With this database, the investigation iden-tified the six most important parameters to follow in developing a set of correla

6、tion equations for calculating air distribution effectiveness through statistical analysis. The air distribution effectiveness calculated by the equations was mostly within 10% of that for the corresponding case in the database.INTRODUCTIONStratified air distribution systems such as Traditional Disp

7、lacement Ventilation (TDV) and Under-Floor Air Distri-bution (UFAD) systems are becoming popular because they can create better indoor air quality (Chen and Glicksman 2003, Bauman and Daly 2003). This is because they supply fresh air directly to the occupied zone at a temperature slightly lower than

8、 that of the air in the room. Due to the thermal buoy-ancy, the cold but fresh air can stay in the lower part of the room. In many cases, contaminant sources in the room are associated with heat sources, such as occupants, equipment, etc. The thermal plumes generated by the heat sources bring the co

9、ntaminants to the upper part of the room since the exhausts are typically located at or near the ceiling level. Thus, the contaminants can be extracted directly through the exhausts without mixing with the fresh air. In addition, the thermal plume from an occupant induces the fresh air from the lowe

10、r part of the room to the breathing level of the occupant. The air breathed by the occupant is rather clean. This has been further confirmed by our recent investigation reported in a companion paper (Lee et al. 2009).The ventilation performance of the stratified air distribu-tion systems has been ta

11、ken into consideration by the ASHRAE standards through the air distribution effectiveness. For example, Table 6-1 of ANSI/ASHRAE Standard 62.1-2004 (ASHRAE 2004) defines the minimum required amount of outdoor air, Vbz, delivered to the space (or zone) for control-ling contaminant concentration. Tabl

12、e 6-2 of the standard defines zone air distribution effectiveness, Ez, for different air distribution configurations. The outdoor airflow required at Air Distribution Effectiveness with Stratified Air Distribution SystemsKisup Lee Zheng Jiang, PhD Qingyan Chen, PhDStudent member ASHRAE Fellow ASHRAE

13、Kisup Lee is a PhD candidate and Qingyan Chen is a professor in the Department of Mechanical Engineering, Purdue University, West Lafay-ette, Indiana. Zheng Jiang is a partner of Building Energy and Environment Engineering LLP, Lafayette, Indiana.LO-09-029 (RP-1373) 2009, American Society of Heating

14、, 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 written permission.ASHR

15、AE Transactions 323the zone (usually through the supply diffusers) is determined as Vbz(from Table 6.2) divided by Ez. Thus, the zone air distri-bution effectiveness plays an important role in determining the minimum required amount of outside air for a space. The stratified air distribution systems

16、, such as TDV systems and low-height-throw under-floor air distribution (L-UFAD) systems, are assigned with Ez= 1.2 in cooling mode. The low height throw is defined as a situation in which the air velocity from a supply jet decays to less than 0.3 m/s (60 fpm) at a height of 1.35 m (4.5 ft) above th

17、e floor. When the systems are used for heating, the Ezdrops to 0.7 with ceiling return or1.0 with floor return. High-height-throw under-floor air distri-bution systems (H-UFAD) are assigned with Ez= 1.0, where the air velocity from the supply jet is still higher than 0.3 m/s (60 fpm) at a height of

18、1.35 m (4.5 ft) above the floor. It appears that these coefficients are not affected by space layout, load distribution in the space, supply airflow and temperature, number or type of diffusers, etc.Early research found that many parameters play an impor-tant role in the performance of the TDV and U

19、FAD systems. The occupancy patterns (Rock et al. 1995), system types (Akimoto et al. 1999), supply air temperature and thermal load (Di Tomaso et al. 2001, Xu et al. 2001, and Lin et al. 2005), and airflow rates and inlet locations (Xing et al. 2001 and Lin et al. 2005) were found to be such paramet

20、ers. These conclusions are consistent with those found by Yuan et al. (1999) for TDV systems. Kobayashi and Chen (2003) revealed that diffuser types are crucial for ventilation performance. Sherman and Walker (2008) indicated that the location of sources of contaminants could lead to different conta

21、minant distribu-tions. However, although these studies are useful, they are fragmented. The systems studied by one researcher could be different from those studied by another. It is hard to make a direct comparison. Some of the studies were limited to the same system or the same space layout. Theref

22、ore, it is difficult to extend these results to the general design of TDV and UFAD systems for various kinds of spaces.It is important to systematically study the impact of these parameters on the ventilation performance of the stratified air distribution systems. This is because more new offices, c

23、lass-rooms, restaurants, retail shops, workshops, and auditoriums at present are using these systems in the United States. Previ-ous studies (Akimoto et al. 1999; Chen and Glicksman 2003) have implied that the required minimum amount of outdoor air for displacement ventilation in these buildings can

24、 be smaller than that for mixing ventilation due to the high effectiveness in ventilation. Bauman and Daly (2003) indicated the same for the UFAD ventilation systems. This has been acknowledged in ASHRAE Standard 62.1-2004 but with a fixed Ezvalue. Obtaining an accurate Ezvalue for stratified air di

25、stribution systems may, in many cases, justify reduction of the minimum outside air required to be supplied to a space under peak cool-ing load conditions without compromising indoor air quality. This, in turn, will reduce capacity and energy consumption of cooling systems, and further reduce greenh

26、ouse gas emis-sions. Therefore, it is desirable to develop an equation for calculating Ezvalue in design practice.The equation to be developed should take into consider-ation the impact of space layouts that are likely to use the strat-ified air systems as well as the parameters that have been shown

27、 to be important according to the literature. The space layout should include offices, classrooms, restaurants, work-shops, retail shops, and auditoriums. According to the litera-ture review above, the parameters should be diffuser type, diffuser number, supply airflow rate, supply air temperature,

28、heat source strengths, return outlet locations, and heating or cooling operating modes. This investigation reports our effort to create a database of air distribution effectiveness values for the TDV and UFAD systems and to develop a set of equations of air distribution effectiveness from the databa

29、se through statistical analyses.RESEARCH METHODSEvaluation of ventilation system performance can use different parameters, such as ventilation effectiveness and air distribution effectiveness. Ventilation effectiveness, Ev, is a description of an air distribution systems ability to remove internally

30、 generated pollutants from a building, zone or space. In Chapter 27 of the ASHRAE Fundamental Handbook (ASHRAE 2005), ventilation effectiveness is defined as(1)where Ev= the ventilation effectivenessCe= the contaminant concentration at the exhaustCs= the contaminant concentration at the supplyCb= th

31、e contaminant concentration at the breathing zoneHowever, HVAC design engineers do not have knowledge of or control of actual pollutant sources within buildings, so the ventilation effectiveness may change dramatically if the pollutant source is moved slightly from one location to another. Therefore

32、, this study used air distribution effective-ness. The definition of air distribution effectiveness is the same as the definition for ventilation effectiveness, but the contaminant source is assumed to be uniformly distributed in the entire indoor space. (2)where E is the air distribution effectiven

33、ess and C is the contaminant concentration at the location where the air distri-bution effectiveness is determinedBy using the averaged C value in the breathing zone, the E becomes Ez, the averaged air distribution effectiveness in the breathing zone.To develop a database of air distribution effecti

34、veness for various space layouts and under different thermal and flow EvCeCsCbCs-=ECeCsCCs-=324 ASHRAE Transactionsconditions, two approaches are possible: experimental measurements and computer simulations by CFD. Our companion paper (Lee et al. 2009) has discussed the pros and cons of the two appr

35、oaches. Obviously, CFD is becoming more reliable, more user-friendly, less expensive, and faster compared with traditional mock-up tests. Thus, our investiga-tion used the validated CFD program discussed in our companion paper to create the database.The development of the database used six different

36、 types of spaces: classrooms (Fig. 1(a), office spaces (Fig. 1(b), workshops (Fig. 1(c), restaurants (Fig. 1(d), retail spaces (Fig. 1(e), and auditoriums (Fig. 1(f). For each type of space, there is a reference case, which is with the L-UFAD system under typical summer cooling conditions. Table 1 g

37、ives the size and the key thermal and flow boundary conditions for the reference cases. For each type of indoor space, this investigation varied the thermal and flow parameters so that a total of 17 cases was stud-ied as shown in Table 2. The standard conditions were those shown in Table 1. Variatio

38、ns were made on the parameters, which may have a major impact on the air distribution effective-ness according to the literature review. When changing a param-Figure 1 Indoor space types used to create the air distribution effectiveness database: (a) classroom, (b) office, (c) workshop, (d) restaura

39、nt, (e) retail shop, and (f) auditorium.ASHRAE Transactions 325eter, it is usual to reduce by 15-40% or increase by 15-40%, depending on the space type and possible variation that one would find in reality. All the variations of the parameters are in bold letters in Table 2. The diffuser number for

40、the reference case was the one recommended by diffuser manufacturers, which must have a flow rate within the range specified in the product catalog. Variation 1 means using fewer diffusers and variation 2, more diffusers than for the reference case. The supply airflow rate and supply air temperature

41、 must be varied at the same time since the cooling/heating load is the same. This implies that variation 1 used a lower airflow rate and a lower temperature and variation 2, a larger flow rate and higher temperature than those for the reference case for cooling. Obvi-ously, the supply airflow rate a

42、nd air temperature are inter-related, so they were counted as one parameter. When the heat source strength was increased or decreased, the supply airflow Table 1. Size and Key Thermal and Flow Conditions Used in the Reference Case for Each Type of SpaceSpace TypeDimensionm (ft)Total Cooling Load Sup

43、ply Air Flow RateACHSupply AirTemperatureC (F)Diffuser Numberw (Btu/h) w/m2 (Btu/hft2)Classroom11.7 9.0 3.3(38.3 25.9 10.8)5,840(19,930)51.5(16.25)6.017.2(62.9)16Office4.2 4.8 2.43(13.7 15.7 7.9)1,528(5,215)70.4(22.20)8.016.5(61.7)3Workshop15.0 12.0 4.5(42.9 33.5 14.7)13,705(46,765)70.7(22.30)4.517.

44、0(62.6)22Restaurant15.0 15.0 3.0(49.2 49.2 9.8)15,484(52,830)64.0(20.18)6.017.0(62.6)22Retail shop10.0 14.0 3.0(32.8 45.9 9.8)8,553(29,180)56.9(17.95)5.517.0(62.6)16Auditorium30.0 20.0 8.0(98.4 65.6 26.2)36,204(123,530)54.7(17.25)3.017.5(63.5)74Table 2. Parametric Study Matrix for Each Type of Indoo

45、r SpaceCaseNumberDiffuserTypeDiffuserNumberSupply Airflow or Air TemperatureHeatSourcesReturnLocationOperatingModeREF L-UFAD Standard Standard Standard Ceiling CoolingCase 2 TDV Standard Standard Standard Ceiling CoolingCase 3 H-UFAD Standard Standard Standard Ceiling CoolingCase 4 L-UFAD Variation

46、1 Standard Standard Ceiling CoolingCase 5 L-UFAD Variation 2 Standard Standard Ceiling CoolingCase 6 L-UFAD Standard Variation 1 Standard Ceiling CoolingCase 7 L-UFAD Standard Variation 2 Standard Ceiling CoolingCase 8 L-UFAD Standard Standard Variation 1 Ceiling CoolingCase 9 L-UFAD Standard Standa

47、rd Variation 2 Ceiling CoolingCase 10 L-UFAD Standard Standard Standard Ceiling HeatingCase 11 L-UFAD Standard Standard Standard Side wall HeatingCase 12 TDV Standard Standard Standard Ceiling HeatingCase 13 TDV Standard Standard Standard Side wall HeatingCase 14 H-UFAD Standard Standard Standard Ce

48、iling HeatingCase 15 L-UFAD Standard Standard Standard Ceiling 2ndHeatingCase 16 TDV Standard Standard Standard Ceiling 2ndHeatingCase 17 H-UFAD Standard Standard Standard Ceiling 2ndHeating326 ASHRAE Transactionsrate remained unchanged but the supply air temperature was adjusted accordingly to main

49、tain the same room air tempera-ture. The change of the return outlet location was straightfor-ward, as shown in Table 2. The operating mode indicated the system could operate not only in cooling mode but also in heat-ing mode. Under the heating mode, the supply air temperature was higher than that of the room air. Thus, the ventilation system may not always create a stratified condition as one could always find in cooling mode. The “secondary heating” refers to the second