ASHRAE FUNDAMENTALS IP CH 20-2017 Space Air Diffusion.pdf

1、20.1CHAPTER 20SPACE AIR DIFFUSIONIndoor Air Quality and Sustainability. 20.2Terminology 20.2Principles of Jet Behavior. 20.3Symbols . 20.8OOM air distribution systems are intended to provide thermalR comfort and ventilation for space occupants and processes.Although air terminals (inlets and outlets

2、), terminal units, fan-coilunits, local ducts, and rooms themselves may affect room air diffu-sion, this chapter addresses only air inlets and outlets and their directeffect on occupant comfort. This chapter is intended to presentHVAC designers the fundamental characteristics of air distributiondevi

3、ces. For information on naturally ventilated spaces, see Chapter16. For a discussion of various air distribution strategies, tools, andguidelines for design and application, see Chapter 57 in the 2015ASHRAE HandbookHVAC Applications. Chapter 20 in the 2016ASHRAE HandbookHVAC Systems and Equipment de

4、scribes thecharacteristics of various air inlets, outlets, fan-coil units, chilledbeams, air curtain units, and terminal units, as well as selection toolsand guidelines.Room air diffusion methods can be classified as one of the fol-lowing as shown in Figure 1: Mixed systems produce little or no ther

5、mal stratification of airwithin the space. Overhead air distribution is an example of thistype of system.Fully (thermally) stratified systems produce little or no mixingof air within the occupied space. Thermal displacement ventila-tion is an example of this type of system.Partially mixed systems pr

6、ovide some mixing within the occupiedand/or process space while creating stratified conditions in the vol-ume above. Most underfloor air distribution and task/ambient con-ditioning designs are examples of this type of system.Local temperature and carbon dioxide (CO2) concentration havesimilar strati

7、fication profiles.Air distribution systems, such as thermal displacement ventila-tion (TDV) and underfloor air distribution (UFAD), that deliver airin cooling mode at or near floor level and return air at or near ceil-ing level produce varying amounts of room air stratification. Forfloor-level suppl

8、y, thermal plumes that develop over heat sourcesin the room play a major role in driving overall floor-to-ceiling airmotion. The amount of stratification in the room is primarily deter-mined by the balance between total room airflow and heat load. Inpractice, the actual temperature and concentration

9、 profile dependson the combined effects of various factors, but is largely driven bythe characteristics of the room supply airflow and heat load config-uration.For room supply airflow, the major factors areTotal room supply airflow quantityRoom supply air temperatureDiffuser typeDiffuser throw heigh

10、t (or outlet velocity); this is associated withthe amount of mixing provided by a floor diffuser (or room con-ditions near a low-sidewall TDV diffuser)For room heat loads, the major factors areMagnitude and number of loads in spaceLoad type (point or distributed source)Elevation of load (e.g., overh

11、ead lighting, person standing onfloor, floor-to-ceiling glazing)Radiative/convective splitWhether pollutants are associated with heat sourcesThe preparation of this chapter is assigned to TC 5.3, Room Air Distribu-tion.Fig. 1 Classification of Air Diffusion Methods20.2 2017 ASHRAE HandbookFundamenta

12、ls 1. INDOOR AIR QUALITY AND SUSTAINABILITYAir diffusion methods affect not only indoor air quality (IAQ)and thermal comfort, but also energy consumption over the build-ings life. Choices made early in the design process are important.Programs such as U.S. Green Building Councils (USGBC 2013)Leaders

13、hip in Energy and Environmental Design (LEED) v4 rat-ing system, which was originally created in response to indoor airquality concerns, now include prerequisites and credits for increas-ing ventilation rates and improving indoor environmental quality.These program requirements are sometimes achieva

14、ble by follow-ing good room air diffusion design principles, methods, and stan-dards (see Chapter 57 of the 2015 ASHRAE HandbookHVACApplications).ANSI/ASHRAE Standard 62.1 provides a table of typical valuesto help predict zone air distribution effectiveness. For example,well-designed ceiling-based a

15、ir distribution systems produce near-perfect air mixing in cooling mode, and yield an air distributioneffectiveness of 1.0. Displacement ventilation and underfloor airdistribution (UFAD) systems have the potential for values greaterthan 1.0. More information on ceiling- and wall-mounted air inletsan

16、d outlets can be found in Rock and Zhu (2002). Displacementsystem performance is described in Chen and Glicksman (2003).ASHRAEs (2013) UFAD Design Guide discusses UFAD in detail.More information on ANSI/ASHRAE Standard 62.1 is available inits users manual (ASHRAE 2010).2. TERMINOLOGYAspect ratio. Ra

17、tio of length to width of opening or core of agrille.Attached jet. A supply air jet drawn to a surface, parallel to thedirection of airflow and caused by the Coanda effect.Axial jet. A supply air jet with a conical discharge profile.Centerline velocity. Maximum velocity of an air jet at anygiven cro

18、ss section perpendicular to the direction of airflow.Coanda effect. Effect of a moving jet attaching to a parallelsurface because of negative pressure developed between jet andsurface.Coefficient of discharge. Ratio of area at vena contracta to freearea of opening.Core area. Area of a register, gril

19、le, or linear slot diffuser per-taining to the inside of the frame or border.Diffusion. Distribution of air into a space.Distribution. Moving air to or in a space by an outlet discharg-ing supply air.Draft. Current of air, when referring to localized effect (gener-ally, the unwanted local cooling of

20、 the body caused by air move-ment) caused by one or more factors of high air velocity, lowambient temperature, or direction of airflow whereby more heat iswithdrawn from a persons skin than is normally dissipated.Drop. Vertical distance that the lower edge of a horizontallyprojected airstream descen

21、ds between the outlet and the end of itsthrow.Effective area. Net area of an outlet or inlet device throughwhich air can pass; equal to the free area times the coefficient ofdischarge.Entrainment. Air drawn into an air jet because of the pressuredifferential caused by the airstream discharged from t

22、he outlet.Entrainment ratio. Volumetric flow rate of total air (supply airplus entrained air) at a given distance from an outlet divided by thevolumetric flow rate of supply air.Free area. Total minimum area of openings in an air outlet orinlet through which air can pass.Free jet. An air jet not obs

23、tructed or affected by walls, ceiling,or other surfaces.Induction. Movement of space air into an air device.Induction ratio. Volumetric flow rate of induced air divided byvolumetric flow rate of primary air.Inlet. A device that allows air to exit the zone (e.g., grilles, reg-isters, diffusers)Isothe

24、rmal jet. An air jet in which supply air temperatureequals surrounding room air temperature.Linear jet. A supply air jet with a relatively high aspect ratio.Neck area. Nominal area of duct connection to air outlet orinlet.Nonisothermal jet. An air jet in which supply air temperaturedoes not equal su

25、rrounding room air temperature.Occupied zone. The volume of space intended to be comfortconditioned for occupants (see ANSI/ASHRAE Standard 55).Outlet. A device discharging supply air into the space (e.g.,grilles, registers, diffusers). Classified according to location andtype of discharge.Outlet ve

26、locity. Average velocity of air discharging from anoutlet.Primary air. Air delivered to an outlet or terminal device.Radial jet. A supply air jet that discharges 360 and expandsuniformly.Spread. Divergence of an airstream in a horizontal and/or verti-cal plane after it leaves an outlet.Stratificatio

27、n height. Vertical distance from floor to horizontalplane that defines lower boundary of upper mixed zone in a fullystratified or partially mixed system.Stratified zone. Zone in which air movement is entirely drivenby buoyancy caused by convective heat sources. Typically foundin fully stratified or

28、partially mixed systems.Supply Air. Air delivered into a zone from an outlet.Terminal velocity. An arbitrary specified centerline air veloc-ity at a distance from an outlet.Throw. The distance from the centerline of an outlet perpendic-ular to a point in the mixed airstream where the velocity has be

29、enreduced to a specified terminal velocity (e.g., 50, 100, 150, or200 fpm), defined by ASHRAE Standard 70.Total air. Combination of supply air and entrained air at agiven distance from an outlet.Vena contracta. Smallest cross-sectional area of a fluid streamleaving an orifice.Outlet Types and Charac

30、teristicsStraub and Chen (1957) and Straub et al. (1956) classified outletsinto five major groups (the subgrouping was added in 2017 and wasnot part of the original research):Group A1. Outlets mounted in or near the ceiling that dischargeair horizontally (Figures 2 and 3).Group A2. Outlets dischargi

31、ng horizontally that are not influ-enced by an adjacent surface (free jet; Figure 4).Group B. Outlets mounted in or near the floor that discharge airvertically in a linear jet (Figure 5).Group C. Outlets mounted in or near the floor that discharge airvertically in a spreading jet (Figure 6).Group D.

32、 Outlets mounted in or near the floor that discharge airhorizontally (Figure 7 and 8). When used in fully stratified systems(TDV), these outlets use low discharge velocities; in mixed sys-tems, they use higher discharge velocities.Group E. Outlets that project supply air vertically downward(Figures

33、9 and 10). These outlets When used in partially stratifiedsystems (e.g., laminar flow outlets, TDV), these outlets use lowdischarge velocities; in mixed systems (e.g., air curtain units, otherdownward directed ceiling devices, etc.), they use higher dischargevelocities.Space Air Diffusion 20.33. PRI

34、NCIPLES OF JET BEHAVIORAir Jet FundamentalsAir supplied to rooms through various types of outlets can bedistributed by turbulent air jets (mixed and partially mixed sys-tems) or in a low-velocity, unidirectional manner (stratified sys-tems). The air jet discharged from an outlet is a primary factora

35、ffecting room air motion. The jet boundary contours are not welldefined and are easily affected by external influences. Baturin(1972), Christianson (1989), and Murakami (1992) have furtherinformation on the relationship between the air jet and occupiedzone.If the supply air temperature is equal to t

36、he ambient room airtemperature, the air jet is called an isothermal jet. A jet with aninitial temperature different from the ambient air temperature iscalled a nonisothermal jet. The air temperature differentialbetween supplied and ambient room air generates thermal forces(buoyancy) in jets, affecti

37、ng the jets (1) trajectory, (2) location atwhich it attaches to and separates from the ceiling/floor, and (3)throw. The significance of these effects depends on the ratiobetween the thermal buoyancy of the air and jet momentum.If an air jet is not obstructed or affected by walls, ceiling, orother su

38、rfaces, it is considered a free jet. When outlet area is smallcompared to the dimensions of the space normal to the jet, the jetmay be considered free as long asX 1.5 (1)whereX = distance from face of outlet, ftAR= cross-sectional area of confined space normal to jet, ft2Fig. 2 Example Airflow Patte

39、rns of Outlet Group A1Fig. 3 Example Airflow Patterns (Nonisothermal) of Outlet Group A1Fig. 4 Example Airflow Patterns (Isothermal) of Outlet Group A2Fig. 5 Example Airflow Patterns (Nonisothermal) of Outlet Group BAR20.4 2017 ASHRAE HandbookFundamentals Jet Expansion Zones. The full length of an a

40、ir jet, in terms of themaximum or centerline velocity and temperature differential at thecross section, can be divided into four zones:Zone 1 extends from the outlet face, in which the velocity andtemperature of the airstream remains practically unchanged.Zone 2 is a transition zone, with its length

41、 determined by the typeof outlet, aspect ratio of the outlet, initial airflow turbulence, etc.Zone 3 is a zone of jet degradation, where centerline air velocityand temperature differential decrease rapidly. Turbulent flow isfully established and may be 25 to 100 equivalent air outlet diam-eters long

42、. The angle of divergence is well defined. Typically, freeair jets diverge at a constant angle, usually ranging from 20 to 24,with an average of 22. Coalescing jets for closely spaced multi-ple outlets expand at smaller angles, averaging 18, and jets dis-charging into relatively small spaces show ev

43、en smaller angles ofexpansion (McElroy 1943). The angle of divergence is easilyaffected by external influences, such as local eddies, vortices, andsurges. Internal forces governing this air motion are extremelydelicate (Nottage et al. 1952a).Zone 4 is important because, in most cases, the jet enters

44、 the occu-pied area in this zone. Distance to this zone and its length dependon the velocities and turbulence characteristics of ambient air. In afew diameters or widths, air velocity becomes less than 50 fpm.Centerline Velocities in Zones 1 and 2. In zone 1, the ratio Vx/Vois constant for a given o

45、utlet and ranges between 1.0 and 1.2, equal tothe ratio of the centerline velocity of the jet at the start of expansionto the average initial velocity. The ratio Vx/Vovaries from approxi-mately 1.0 for rounded entrance nozzles to about 1.2 for straight pipedischarges; it has higher values for diverg

46、ing discharge outlets.The aspect ratio (Tuve 1953) and turbulence (Nottage et al.1952a) primarily affect centerline velocities in zones 1 and 2. As-pect ratio has little effect on the terminal zone of the jet when Hoisgreater than 4 in. This is particularly true of nonisothermal jets.When Hois very

47、small, induced air can penetrate the core of the jet,thus reducing centerline velocities. The difference in performancebetween a radial outlet with small Hoand an axial outlet with largeHoshows the importance of jet thickness.When air is discharged from relatively large perforated panels,the constan

48、t-velocity core formed by coalescence of individual jetsextends a considerable distance from the panel face. In zone 1,when the aspect ratio is less than 5, use the following equation forestimating centerline velocities (Koestel et al. 1949):Vx= 1.2Vo(2)In zone 2, the ratio Vx/Vobegins to decrease.

49、Experimental evi-dence indicates that, in zone 2,(3)whereVx= centerline velocity at distance X from outlet, fpmVo= Vc/CdRfa= average initial velocity at discharge, fpmVc= nominal velocity of discharge based on core area, fpmCd= coefficient of discharge (usually between 0.65 and 0.90)Rfa= ratio of free area to core areaHo= width of jet at outlet or at vena contracta, ftKc2= centerline velocity constant, depending on outlet type and discharge patternX (1/Kc2Ho)1/2= distance from outlet to measurement of centerline velocity Vx, ftCenterline Velocity in Zone 3. In zone 3, cen