1、458 2008 ASHRAE ABSTRACT This paper summarizes the results of the research studyconducted for the ASHRAE Technical Committee (TC) 4.3 onstack ganging. The purpose of the research was to quantify thebenefits of ganging (i.e., placing next to one another) exhauststacks to increase the throw or rise of
2、 exhaust plumes, therebyreducing adverse air quality impacts without directly mani-folding the exhausts or using induced air systems. The researchused wind tunnel modeling to develop a database of plume riseobservations for various ganged stack configurations. Theplume rise measurements were analyze
3、d to determine a plumerise enhancement factor based on the relative positions of theganged stacks. The results of this research can be used to opti-mize the layout of ganged stacks such that maximum plume risecan be achieved for all wind directions.INTRODUCTIONThis paper summarizes the results of re
4、search project1167 conducted for the ASHRAE Technical Committee (TC)4.3. TC 4.3 is concerned with ventilation, infiltration, airflowaround buildings, and the reentry of exhausts, including theirinteractions with indoor air quality, HVAC system perfor-mance, and energy consumption. This study improve
5、s thestate-of-knowledge in two of the areas of TC concern, namelyavoiding the reentry of exhaust while at the same time mini-mizing energy and equipment costs (i.e., fan sizes and stackheights). The importance of the work is mentioned in the studywork statement specified by the TC: Improved dispersi
6、on could still be achieved withoutmanifolding if the exhaust stacks were ganged (grouped)closely together so that the plumes would quickly mergeinto a single plume with a combined momentum.However, little is known about the maximum separationdistance between stacks or the effects of various arraypat
7、terns on the ability of the plumes to combine. To illustrate the importance of stack ganging, one firstneeds to know that concentrations downwind of a stack arerelated to the plume rise via an exponential relationship. So,small changes in plume rise can have a large effect on theresulting roof top,
8、or side wall, air intake and ground-levelconcentrations. Further, since plume rise is proportional to exitvelocity times mass flow raised to some power, ganging is ofmost benefit for low volume flow and/or low exit velocitystacks placed near a high flow stack. To avoid high concentra-tion levels due
9、 to these stacks, designers often have to increasethe stack height, use an induced air fan system, or design amanifold system that combines the flow from many low flowstacks to yield one high volume flow stack. One of the earliest attempts to calculate plume rise frommultiple nearby (i.e., ganged) s
10、tacks was performed by Briggs(1974, 1975, 1984). Using the database of Carpenter et al.(1968), Briggs was able to perform a series of plume risecalculations for multiple stacks. The observed data consistedof plume rise measurements at various Tennessee ValleyAuthority (TVA) power plants with one, tw
11、o, three, four, ornine equally spaced buoyant stacks operating in an along windconfiguration during periods of stable stratification. Briggsfound that when the plumes merge, the combined plume risecould be calculated by summing the buoyancy flux values foreach stack and then inputting the total valu
12、e into the plume riseequation he developed for buoyant plumes. The relationship,in its simplest terms, increased the plume rise by a maximumvalue of N1/3, where N is number of merged stacks. BriggsThe Effect of Ganging on PollutantDispersion from Building Exhaust StacksR.L. Petersen, PhD J.D. Reifsc
13、hneiderMember ASHRAER.L. Petersen is a principal at CPP, Inc., Fort Collins, CO. J.D. Reifschneider is a project engineer at CPP, Inc., Fort Collins, CO.NY-08-0562008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions,
14、Volume 114, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 459(1984) has a good summary of some of the past work onenhanced plume rise due to ganged st
15、acks. Of particular interest for this evaluation are the past stud-ies by Overcamp and Ku (1980, 1988). Overcamp looked atthe merging of two, three, and four identical buoyant plumesusing a wind tunnel. He found maximum plume rise enhance-ment when the stacks were in line with the wind direction and
16、little enhancement when the stacks were perpendicular to thewind direction. He saw the buoyancy flux for two, three, andfour stacks completely combine for distances out to eight-stack diameters, the maximum distance evaluated, when thestacks were in line with the wind direction. No added buoy-ancy f
17、or two, three, or four stacks was found at two-stackdiameters, the closest distance evaluated, when the stackswere perpendicular to the wind direction. He performed somelimited sensitivity tests with the stacks placed at 30 and 60degrees with stack spacings equal to two-stack diameters. At30 degrees
18、, the plume buoyancy from the two stackscompletely combined, but at 60 degrees, the enhancement wasinsignificant.More recently, Macdonald et al. (2002) looked at themerging of two identical buoyant plumes using a water flume.He found somewhat similar findings as other researchers;namely, maximum enh
19、ancement was achieved when thestacks were in line with the wind direction, and little enhance-ment was observed when the stacks were perpendicular to thewind direction. He saw the buoyancy flux for the two stackscompletely combine for distances out to nine-stack diameterswhen the stacks were in line
20、 with the wind direction. No addedbuoyancy was found at three-stack diameters, the closestdistance evaluated, when the stacks were perpendicular to theflow. He performed some limited sensitivity tests with thestacks placed at 18 and 34 degrees with stack spacings greaterthan 10 diameters. At 18 degr
21、ees some enhanced plume risewas observed, but at 34 degrees the enhancement was insig-nificant.Most of the work in the literature was for buoyant plumesexcept for the work of Gregoric et al. (1980) who examined theplume interaction of seven closely spaced jets in a cross flow.He observed the largest
22、 increase in plume rise when the stackswere in line with the wind direction and the least plume risewhen the stacks were at 45 and 90 degrees to the flow. Hisresults suggest that the plume rise increase for momentum-dominated plumes can be treated in a similar manner as buoy-ant plumes. The past lit
23、erature provided limited information on plumerise enhancement for stack spacings of less than two- to three-stack diameters. This is the region that was focused on in thisstudy, not only due to lack of data, but because most ASHRAEapplications will have stacks spaced in relative close proxim-ity. He
24、nce, the purpose of this research is to quantify the bene-fits of ganging exhaust stacks to increase the throw or rise ofexhaust plumes, thereby reducing adverse air quality impactswithout directly manifolding or combining the exhausts. Thestudy defines how closely the stacks need to be to get theen
25、hanced plume rise effect and how they should be orientedwith respect to wind direction. The ultimate purpose of thisresearch is to provide designers with specific guidance onwhere to place a lower flow stack in relation to a higher flowstack to achieve the maximum plume rise benefit. Alternately,thi
26、s research will help designers gang many low flow stacks toachieve maximum plume rise.THEORETICAL CONSIDERATIONSPlume Rise Predictions for Single Stacks2003 ASHRAE HandbookHVAC Applications, Chapter44 (2003), currently only provides the following equations forcalculating plume rise (h):(1)where(2)is
27、 the final rise above stack top due to momentum, and(3)is the negative rise due to stack wake effects and occurs whenVe/Uh3.0, hdis equal to zero. is equalto zero for a capped stack and 1.0 for a non-capped stack.The above equations are only valid for momentum-domi-nated exhaust at the point of fina
28、l rise. It should also be notedthat Equation 2 is a simplified version of a more complicatedequation developed by Briggs (see Equation 7 below). Equa-tion 2 was derived based on the assumption that the entrain-ment constant, , discussed later, is equal to 0.6, and. It should also be noted that Equat
29、ion 2 does notinclude a downwind distance variable. For this evaluation,plume rise as a function of downwind distance is important,and the ASHRAE equations above will not suffice. In addi-tion, if the exhaust stream is buoyancy dominated, the equa-tions presented in ASHRAE are not appropriate. The m
30、ost widely used equations for predicting plume riseare referred to as the Briggs equations (Briggs 1974, 1975,1984). Briggs developed these equations starting with thebasic equations for conservation of continuity, momentum,and buoyancy. To develop analytical equations, he assumes theplume has a top
31、 hat distribution and that plume growth isproportional to distance along the plume trajectory times anentrainment constant. Using these assumptions, Briggs devel-oped the following plume rise equations for neutrally buoyantplumes (i.e., exhaust and ambient density are nearly identical)under neutrall
32、y stratified ambient conditions:hhshrhd+=hr3.0deVeUh-=hdde3.0 VeUh()=jUhU* 15=460 ASHRAE Transactions(4)where(5)is the momentum flux(6)is the jet entrainment coefficient(7)is the final plume rise, and(8)is the well known logarithmic wind profile question. Thedimensionless factor, Uh/ U*, is composed
33、 of velocity at stackheight, Uh, and the friction velocity, U*. The zovalue is thesurface roughness of the site due to objects such as trees,crops, or buildings. For this study, zowas equal to 0.005 m(0.016 ft), and hswas 15.24 m (50 ft). Substituting into Equa-tion 8 gives Uh/ U*= 20.The following
34、is the equation Briggs developed whenbuoyancy effects dominate the rise:(9)where(10)is the buoyancy flux(11)(12)is the final rise.For buoyant plumes, the total plume rise can be calculatedusing the following equation:(13)As can be seen, the addition of buoyancy greatly compli-cates the plume rise ca
35、lculation process. If one neglects buoy-ancy and only accounts for momentum, the plume rise will beunderstated, and conservative results will be obtained. Most ofthe work in this study dealt with neutrally buoyant plumes.Sensitivity tests were conducted with buoyant plumes to see ifthe same concepts
36、 regarding stack ganging would apply.Plume Rise Predictions for Ganged StacksTo obtain ganged stack plume rise predictions, one onlyneeds to define the total Fmand/or Fbterms for the gangedstack arrangement and use the value(s) in the appropriateequations above. The following relationships were used
37、 in thisstudy to compute total momentum or buoyancy as appropriate:(14)(15)In the above equations, the subscript o denotes the stackthat is being enhanced by the neighboring stacks, 1 through n.The variables A1and B1will be a function of the location andpossibly the stack momentum or buoyancy. The e
38、xperimentalprogram discussed later was designed to determine the A andB values to be utilized in the above equations. It should benoted, that B values were not needed for this study, as momen-tum dominated the plume rise for all cases evaluated. This wastrue for even the cases that simulated heated
39、plumes.At this point, it is important to note some importantaspects about the accuracy of the A and B values. For simpli-fication purposes, consider two ganged momentum-dominatedstacks. If the two stacks momentums add completely, A = 1.If the two stacks do not interact, A = 0. To determine A, plumer
40、ise measurements are made for an individual stack and theganged stacks. The increase in plume rise is then used to backcalculate the A value. Lets assume Fm,o= Fm,1. Using Equation 4, one wouldexpect the plume rise to change by 21/3or by a factor of 1.26.Next, assume the error in the plume rise meas
41、urement is 2%.In that case, the maximum plume rise increase factor could be asmuch as 1.31 (i.e., one measurement 2% high and the other 2%low), which in turn would mean the A value is 1.25 (1.313 1)versus a maximum possible value of 1.0. For this reason, themaximum value for A was limited to 1, even
42、 though the best-fitanalysis, discussed later, may have shown an A value greaterthan 1. A greater than 1 value is most likely due to experimentalerrors on the order of 2% or more.Wind Tunnel Simulation of Plume RiseAn accurate simulation of the boundary layer winds andstack gas flow are essential pr
43、erequisites to any wind tunnelstudy of plume rise from exhaust stacks. The similarityrequirements can be obtained from dimensional argumentsderived from the equations governing fluid motion. A detaileddiscussion on these requirements is given in the EPA fluidhrm,min3Fmxj2Us2-13hfm,=FmTaTs-Ve2 d24-=j
44、13-UhVe-+=hfm,0.9 FmUhU*12Uhj-=UhU* 2.5ln hszo()=hrb,min 1.6 Fbx2Uh313hfb,()=FbgVeD2Ta()4Ts-=hfb21.425Fb34Us- for Fb55=hrhrm,3hrb,3+()13=Fm total,Fmo,A1Fm 1,A2Fm 2,A3Fm 3, AnFmn,+=Fbtotal,Fbo,B1Fb 1,B2Fb 2,B3Fb 3, BnFbn,+=ASHRAE Transactions 461modeling guideline (Snyder 1981). For this study, plume
45、 risein a low ambient turbulence setting was simulated. To model neutrally buoyant plume trajectories, the veloc-ity ratio, R, and density ratio, , were matched in model andfull-scale. These quantities are defined as follows:(16)(17)whereUh= wind velocity at stack top (m/s or fpm)Ve= stack gas exit
46、velocity (m/s or fpm)= stack gas density (kg/m3or lb/ft3)= ambient air density (kg/m3or lb/ft3)In addition, the stack gas flow in the model was designedto be fully turbulent upon exit, as it is in the full scale. Thiscriteria is met if the Stack Reynolds number (Res= dVe /vs) isgreater than 670 for
47、buoyant plumes and greater than 2000 forjets (Arya and Lape 1990). Trips were installed inside themodel stacks to increase the turbulence level in the exhauststream prior to exiting the stack.For the buoyant plume cases, the following parameterswere matched in model and full-scale:momentum ratio, Mo
48、(18)and buoyancy ratio, Bo(19)where(20)is the Froude number.Using the above criteria and the source characteristicsshown in Table 1a, the model test conditions were computed forthe stacks under evaluation. The relevant dimensionless param-eters for each case are summarized in Table 1b. Table 1b show
49、sthat the Exhaust Reynolds numbers did not meet the criteriarecommended in Snyder (1981); therefore, trips were installedin the stacks to ensure fully turbulent flow upon exit (hence,similar plume trajectories). Most cases evaluated had an infi-nite Froude number (i.e., neutrally buoyant plumes). Twobuoyant plume cases were evaluated as indicated by Froudenumbers of 6.75 and 68.1. The ambient flow was also neutrallys
