ASHRAE FUNDAMENTALS SI CH 24-2013 Airflow Around Buildings.pdf

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1、24.1CHAPTER 24 AIRFLOW AROUND BUILDINGSFlow Patterns . 24.1Wind Pressure on Buildings . 24.3Wind Effects on System Operation. 24.7Building Pressure Balance and Internal Flow Control 24.9Physical and Computational Modeling 24.9Symbols 24.12IRFLOW around buildings affects worker safety, process andA b

2、uilding equipment operation, weather and pollution protec-tion at building inlets, and the ability to control indoor environmen-tal parameters such as temperature, humidity, air motion, andcontaminants. Specifically, wind causes variable surface pressureson buildings that can change intake and exhau

3、st system flow rates,natural ventilation, infiltration and exfiltration, and interior pres-sures. The mean flow patterns and turbulence of wind passing overa building can also lead to recirculation of exhaust gases into airintakes.This chapter provides basic information for evaluating windflowpatter

4、ns, estimating wind pressures, and identifying problemscaused by the effects of wind on intakes, exhausts, and equipment. Inmost cases, detailed solutions are addressed in other chapters.Related information can be found in Chapters 11, 14, 16, and 36 ofthis volume; in Chapters 31, 32, 45, 47, and 53

5、 of the 2011 ASHRAEHandbookHVAC Applications; and in Chapters 30, 35, and 40 ofthe 2012 ASHRAE HandbookHVAC Systems and Equipment.FLOW PATTERNSBuildings having even moderately complex shapes, such as L- orU-shaped structures, can generate flow patterns too complex to gen-eralize for design. To deter

6、mine flow conditions influenced by sur-rounding buildings or topography, wind tunnel or water channeltests of physical scale models, full-scale tests of existing buildings,or careful computational modeling efforts are required (see the sec-tion on Physical and Computational Modeling). As a result, o

7、nlyisolated, rectangular block buildings are discussed here. Englishand Fricke (1997), Hosker (1984, 1985), Khanduri et al. (1998),Saunders and Melbourne (1979), and Walker et al. (1996) reviewthe effects of nearby buildings.As wind impinges on a building, airflow separates at the buildingedges, gen

8、erating recirculation zones over downwind surfaces (roof,side and downwind walls) and extending into the downwind wake(Figure 1). On the upwind wall, surface flow patterns are largelyinfluenced by approach wind characteristics. Figure 1 shows that themean speed of wind UHapproaching a building incre

9、ases withheight H above the ground. Higher wind speed at roof level causes alarger pressure on the upper part of the wall than near the ground,which leads to downwash on the lower one-half to two-thirds of thebuilding. On the upper one-quarter to one-third of the building,windflow is directed upward

10、 over the roof (upwash). For a buildingwith height H that is three or more times width W of the upwind face,an intermediate stagnation zone can exist between the upwash anddownwash regions, where surface streamlines pass horizontallyaround the building, as shown in Figures 1 (inset) and 2. (In Figur

11、e2, the upwind building surface is “folded out” to illustrate upwash,downwash, and stagnation zones.) Downwash on the lower surfaceof the upwind face separates from the building before it reachesground level and moves upwind to form a vortex that can generatehigh velocities close to the ground (“are

12、a of strong surface wind” inFigure 1, inset). This ground-level upwind vortex is carried aroundthe sides of the building in a U shape and suspends dust and debristhat can contaminate air intakes close to ground level.The downwind wall of a building exhibits a region of low averagevelocity and high t

13、urbulence (i.e., a flow recirculation region)extending a distanceLr downwind. If the building has sufficientlength L in the windward direction, the flow reattaches to the build-ing and may generate two distinct regions of separated recirculationFig. 1 Flow Patterns Around Rectangular BuildingThe pre

14、paration of this chapter is assigned to TC 4.3, Ventilation Requirements and Infiltration.24.2 2013 ASHRAE HandbookFundamentals (SI)flow, on the building and in its wake, as shown in Figures 2 and 3.Figure 3 also illustrates a rooftop recirculation cavity of length Lcatthe upwind roof edge and a rec

15、irculation zone of lengthLr down-wind of the rooftop penthouse. Velocities near the downwind wallare typically one-quarter of those at the corresponding upwind walllocation. Figures 1 and 2 show that an upward flow exists over mostof the downwind walls.Streamline patterns are independent of wind spe

16、ed and dependmainly on building shape and upwind conditions. Because of thethree-dimensional flow around a building, the shape and size of therecirculation airflow are not constant over the surface. Airflowreattaches closer to the upwind building face along the edges of thebuilding than it does near

17、 the middle of the roof and sidewalls(Figure 2). Recirculation cavity height Hc(Figures 1 and 3) alsodecreases near roof edges. Calculating characteristic dimensions forrecirculation zones Hc,Lc, and Lris discussed in Chapter 45 of the2011 ASHRAE HandbookHVAC Applications.For wind perpendicular to a

18、 building wall, height H and width Wof the upwind building face determine the scaling length R thatcharacterizes the buildings influence on windflow. According toWilson (1979),R = (1)whereBs= smaller of upwind building face dimensions H and WBL= larger of upwind building face dimensions H and WWhen

19、BLis larger than 8Bs, use BL= 8Bsin Equation (1). For build-ings with varying roof levels or with wings separated by at least adistance of Bs, only the height and width of the building face belowthe portion of the roof in question should be used to calculate R.Flow accelerates as the streamlines com

20、press over the roof anddecelerates as they spread downward over the wake on the down-wind side of the building. The distance above roof level where abuilding influences the flow is approximately 1.5R, as shown inFigure 1. In addition, roof pitch also begins to affect flow when itexceeds about 15 (1:

21、4). When roof pitch reaches 20 (1:3), flowremains attached to the upwind pitched roof and produces a recir-culation region downwind of the roof ridge that is larger than that fora flat roof.If the angle of the approach wind is not perpendicular to theupwind face, complex flow patterns result. Strong

22、 vortices developfrom the upwind edges of a roof, causing strong downwash onto theroof (Figure 2). High speeds in these vortices (vorticity) cause largenegative pressures near roof corners that can be a hazard to roof-mounted equipment during high winds. In some extreme cases, thenegative pressures

23、can be strong enough to lift heavy objects such assidewalk pavers, which can result in a projectile hazard. When theangle between the wind direction and the upwind face of the buildingis less than about 70, the upwash/downwash patterns on the upwindface of the building are less pronounced, as is the

24、 ground-level vortexFig. 2 Surface Flow Patterns for Normal and Oblique Winds(Wilson 1979)Fig. 3 Flow Recirculation Regions and Exhaust-to-Intake Stretched-String Distances (SA, SB)Bs0.67BL0.33Airflow Around Buildings 24.3shown in Figure 1. Figure 2 shows that, for an approach flow angleof 45, strea

25、mlines remain close to the horizontal in their passagearound the sides of the building, except near roof level, where theflow is drawn upwards into the roof edge vortices (Cochran 1992).Both the upwind velocity profile shape and its turbulence inten-sity strongly influence flow patterns and surface

26、pressures (Mel-bourne 1979).WIND PRESSURE ON BUILDINGSIn addition to flow patterns described previously, the turbulenceor gustiness of approaching wind and the unsteady character of sep-arated flows cause surface pressures to fluctuate. Pressures dis-cussed here are time-averaged values, with an ave

27、raging period ofabout 600 s. This is approximately the shortest time period consid-ered to be a “steady-state” condition when considering atmosphericwinds; the longest is typically 3600 s. Instantaneous pressures mayvary significantly above and below these averages, and peak pres-sures two or three

28、times the mean values are possible. Althoughpeak pressures are important with regard to structural loads, meanvalues are more appropriate for computing infiltration and ventila-tion rates. Time-averaged surface pressures are proportional to windvelocity pressure pvgiven by Bernoullis equation:pv= (2

29、)wherepv= wind velocity pressure at roof level, PaUH= approach wind speed at upwind wall height H, m/s see Equation (4)a= ambient (outdoor) air density, kg/m3The proportional relationship is shown in the following equation,in which the difference psbetween the pressure on the building sur-face and t

30、he local outdoor atmospheric pressure at the same level inan undisturbed wind approaching the building isps= Cppv(3)where Cpis the local wind pressure coefficient at a point on thebuilding surface.The local wind speed UHat the top of the wall that is required forEquation (2) is estimated by applying

31、 terrain and height correctionsto the hourly wind speed Umetfrom a nearby meteorological station.Umetis generally measured in flat, open terrain (i.e., category 3 inTable 1). The anemometer that records Umetis located at heightHmet, usually 10 m above ground level. The hourly average windspeed UH(Fi

32、gures 1 and 3) in the undisturbed wind approaching abuilding in its local terrain can be calculated from Umetas follows:UH= Umet(4)The atmospheric boundary layer thickness and exponent a forthe local building terrain and ametand metfor the meteorologicalstation are determined from Table 1. Typical v

33、alues for meteorolog-ical stations (category 3 in Table 1) are amet= 0.14 and met= 270 m.The values and terrain categories in Table 1 are consistent with thoseadopted in other engineering applications (e.g., ASCE Standard 7).Equation (4) gives the wind speed at height H above the averageheight of lo

34、cal obstacles, such as buildings and vegetation, weightedby the plan-area. At heights at or below this average obstacle height(e.g., at roof height in densely built-up suburbs), speed depends onthe geometrical arrangement of the buildings, and Equation (4) isless reliable.An alternative mathematical

35、 description of the atmosphericboundary layer, which uses a logarithmic function, is given byDeaves and Harris (1978). Although their model is more compli-cated than the power law used in Equation (4), it more closely mod-els the real physics of the atmosphere and has been adopted byseveral codes ar

36、ound the world (e.g., SA/SNZ Standard AS/NZS1170.2 from Australia).Example 1. Assuming a 10 m/s anemometer wind speed for a height Hmetof 10 m at a nearby airport, determine the wind speed UHat roof levelH = 15 m for a building located in a city suburb.Solution: From Table 1, the atmospheric boundar

37、y layer propertiesfor the anemometer are amet= 0.14 and met= 270 m. The atmo-spheric boundary layer properties at the building site are a = 0.22 and = 370 m. Using Equation (4), wind speed UHat 15 m isUH= 10 = 7.8 m/sLocal Wind Pressure CoefficientsValues of the mean local wind pressure coefficient

38、Cpused inEquation (3) depend on building shape, wind direction, and influ-ence of nearby buildings, vegetation, and terrain features. Accuratedetermination of Cpcan be obtained only from wind tunnel modeltests of the specific site and building or full-scale tests. Ventilationrate calculations for si

39、ngle, unshielded rectangular buildings can bereasonably estimated using existing wind tunnel data. Many windload codes (e.g., ASCE Standard ASCE/SEI 7-10, SA/SNZ Stan-dard AS/NZS 1170.2) give mean pressure coefficients for commonbuilding shapes.Figure 4 shows pressure coefficients for walls of a tal

40、l rectangu-lar cross section building (high-rise) sited in urban terrain (Daven-port and Hui 1982). Figure 5 shows pressure coefficients for walls ofa low-rise building (Holmes 1986). Generally, for high-rise build-ings, height H is more than three times the crosswind width W. ForH 3W, use Figure 4;

41、 for H Cp exhaustand wind-opposed when the wind direction changes, causing Cp intakeCp exhaust. The effect of wind-assisted and wind-opposed pressuredifferences is illustrated in Figure 13.Example 3. Make a worst-case estimate for the effect of wind on the sup-ply fan for a low-rise building with he

42、ight H = 15 m, located in a citysuburb. Use the hourly average wind speed that will be exceeded only1% of the time and assume an annual hourly average speed of Uannual=4 m/s measured on a meteorological tower at height Hmet= 10 m at anearby airport. Outdoor air density is a= 1.2 kg/m3.Solution: From

43、 Table 2, the wind speed exceeded only 1% of thehours each year is a factor of 2.5 0.4 higher than the annual averageof 4 m/s, so the 1% maximum speed at the airport meteorological sta-tion isUmet= 2.5 4 = 10 m/sFrom Example 1, building wind speed UHis 7.8 m/s.A worst-case estimate of wind effect mu

44、st assume intake andexhaust locations on the building that produce the largest difference(Cp intake Cp exhaust) in Equations (9) and (10). From Figure 5, thelargest difference occurs for the intake on the upwind wall AB and theexhaust on the downwind wall CD, with a wind angle AB= 0. For thisworst c

45、ase, Cp intake= +0.8 on the upwind wall and Cp exhaust= 0.43 onthe downwind wall. Using these coefficients in Equations (9) and (10)to evaluate effective fan pressure pfan eff,pfan eff= pfan+ 0.8 (0.43)= pfan+ 44.9 PaThis wind-assisted hourly averaged pressure is exceeded only1% of the time (88 hour

46、s per year). When wind direction reverses,the outlet will be on the upwind wall and the inlet on the downwindwall, producing wind-opposed flow, changing the sign from +44.9Fig. 12 Intake and Exhaust Pressures on Exhaust Fan in Single-Zone BuildingaUH22-aUH22-Q2AL2-aUH22-Fig. 13 Effect of Wind-Assist

47、ed and Wind-Opposed Flow1.2 7.822-Airflow Around Buildings 24.9to 44.9 Pa. The importance of these pressures depends on theirsize relative to the fan pressure rise pfan, as shown in Figure 13.Minimizing Wind Effect on System VolumeWind effect can be reduced by careful selection of inlet andexhaust l

48、ocations. Because wall surfaces are subject to a wide vari-ety of positive and negative pressures, wall openings should beavoided when possible. When they are required, wall openingsshould be away from corners formed by building wings (see Figure11). Mechanical ventilation systems should operate at

49、a pressurehigh enough to minimize wind effect. Low-pressure systems andpropeller exhaust fans should not be used with wall openings unlesstheir ventilation rates are small or they are used in noncritical ser-vices (e.g., storage areas).Although roof air intakes in flow recirculation zones best mini-mize wind effect on system flow rates, current and future air qualityin these zones must be considered. These locations should beavoided if a contamination source exists or may be added