TAB LE OF CONTENTS T0-98-15-3 (RP-897)

AIVC 11881

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Measuring Adjacent Building Effects on Laboratory Exhaust Stack Design

David J. Wilson, Ph.D., P.E. MemberASHRAE

ABSTRACT

Ian Fabris

Current methods for designing exhaust stack height and exit velocity are based on avoiding contamination of the roof, walls, and nearby ground surface of the building on which the stack is located. Usually, no account is taken of the effect of adjacent buildings that add turbulence and increase disper­sion if they are located upwind and may be contaminated them­selves if they are downwind of the emitting building. To account for these adjacent building effects, ASH RAE Research Project 897 used water-channel simulation of the atmosphere to eval­uate rooftop contamination in more than 1, 700 different configurations of adjacent building height, width, spacing, stack location, stack diameter, height, and exit velocity. Exhaust stacks on scale models of flat-roof buildings were tested using fluorescent dye tracer illuminated by thin laser light sheets, with digital video images to measure dilution in the exhaust plume at roof level air intake locations. The results show that high stacks and exit velocities that represent good design on an emitting building can be less effective and are sometimes counterproductive in reducing contamination of the roof of a nearby adjacent building. The implications of the study for developing practical stack design guidelines are discussed in this paper.

INTRODUCTION

Many exhaust stacks from laboratory fume hoods and industrial exhausts are not actually designed but instead are simply specified to have the minimum height, typically about 7 ft (2.1 m), that will meet local fire and building code require­ments. For stacks that are designed by making exhaust-to­intake dilution calculations, the effect of adjacent buildings in changing the rate of plume dispersion and deflecting plume trajectories is only accounted for by rough approximations.

Mark Y. Ackerman, P.E. Member ASHRAE

For example, the current ASHRAE design method in chapter 15 of the 1997 ASHRAE Handbook-Fundamentals (ASHRAE 1997) specifies that no credit for stack height should be given when the plume passes over a downwind building or rooftop obstacle that is higher than the stack. Denying credit for stack height below obstacles is based on wind tunnel experiments by Wilson and Winkel ( 1 982).

Studies of the effect of buildings on plume dispersion have focused primarily on the added turbulence and flow downwash that affect ground level concentrations from a stack located downwind of the building. Wind tunnel measurements by Robins and Castro (1 977a, 1977b), Huber ( 1 989, 1 991), Huber and Snyder (1982), Huber et al. (1991), and Thompson (1993) all focused on the effect of a single isolated upwind building affecting a ground-based stack. The effect of the plume from an isolated upwind stack impinging at roof level on a downwind building was investigated in a wind tunnel study by Wilson and Netterville (1978). None of the existing studies dealt with the important question of how a closely­spaced adjacent building will influence the exhaust gas disper­sion from a roof-mounted stack on another building.

OBJECTIVES

ASHRAE RP-897 carried out a systematic study of sixteen adjacent and emitting building configurations to deter­mine how adjacent buildings should be accounted for in labo­ratory stack design. The research project had four distinct objectives:

1. To use scale models of emitting and adjacent building pairs to catalogue the effect of adjacent buildings over a wide range of stack heights, exhaust velocity to windspeed ratios, stack locations, and emitting/adjacent building configura­tions.

David J. Wilson is a professor.Ian Fabrisis a graduate student, and Mark Y. Ackerman is a faculty service officer, Department of Mechanical Engineering, University of Alberta, Edmonton, Canada.

THIS PREPRINT IS FOR DISCUSSION PURPOSES ONLY, FOR INCLUSION IN ASHRAE TRANSACTIONS 1998, V. 104, Pt. 2. Not to be reprinted in whole or in part without written permission of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, NE, Atlanta, GA 30329. Opinions, findings, conclusions, or recommendations expressed in this paper are those of the author(s) and do not necessarily reflect the views of ASH RAE. Written questions and comments regarding this paper should be received at ASHRAE no later than July 10, 1998.

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2. To use the results of this catalogue of building dilution measurements to recommend stack desigR procedures that improve plume dispersion (and reduce intake contamina­tion) when an adjacent building is present.

3. To compare measured dilutions with dispersion calcula.! tions using the computer models ISC3 and SCREEN3 developed by the Environmental Protection Agency (EPA 1995a, i 995b) and the ASHRAE dilution model from chap� ter 15 of the 1997 ASHRA.E Handbook-F1111damentals.

4. To prepare a commentary on the suitability of existing models for predicting exhaust-to-intake dilution in the pres­ence of ��jacent liuildings and to evaluate the suitability of availability of design standards such as ANSI/A/HA Sum­dard 29.9-1992 which states that re-entry of fume-hood exhaust should be avoided at nearby buildings as well as on the emitting building. -

In this paper, only the first two objectives will be . addressed: to describe the experimental measurements of dilu­tion and to recommend methods for improving stack designs that account for adjacent building effects. The detailed discus­sion of the performance of EPA and ASHRAE dispersion models is presented in the final report of RP-897 (Wilson et al. 1998).

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The experiments of Wilson and Winke l (1982) suggest that there may be no benefit from higher stacks when the emit­ting building roof is enveloped by the flow recirculating zone bchjnd an adjacem upwind bu i lding. WJ1yll,thc �djacent build- /. iog i downwind, I.he plume from a high stack ejected at high exhaust velocity (to avoid contaminating the emitting building root) may rise and impinge directly on the roof level intakes. if the adjacent building js higher tp� the emilting.buil1din�. The mea i1rements reported here show that both lhese imde� sirable situations· do occur and 'cause large increases in contamination of roof level intakes.

BUILDING AND STACK CONFIGURATIONS

The building configurations used in the tudy are shown in Figure l, where they are presented in six different subcat­egories. The two factors used for categorizing adjacent build­ing effects are (1) whether the upwind or downwind building is emitting the exhaust and �2� the relative roof heights of Lhe two buildings.:Figure· shows the definitions of step-across, step-up,-and step-down roof levels.

All the data shown here is for the single-width buildings (2.5 H wide). Normalized dilution plots for the double-width buildings show the same genhal trends as the single width buildings and are presented in the final report of the project (Wilson et al. 1998).

For each of the building configurations, 54 combinations of stack lbcatio�, stack height; arld exhaust velOcity were

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tested. Three stack locations: 0.25 H from upwind and downwind roof edges and in the center of the roof.

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Three stack heights of h, = 0.175 H, h, = 0.25 H, and h, = .0.5 H to simulate stack heights of7 ft, 10 ft, and 20 ft on a nominal emitting building with height H = 40 ft. Six values of exhaust velocity ratio M = W,!UH of 1.0, f:5, 2.0, 3.0, 5.0, and 8.0, with equal exhaust and ambi­e_n.t air densities Pe = Pa• to produce a non-buoyant momentum jet from the uncapped stack.

These measurements were made using stacks with inside diameter d = 0.10 in. (2.54 mm). This d/H = 0.05 stack simu­lated a nominal diameter of d = 2 ft (0.61 m) for building �eight of H= 40 ft ( 12.2 m) in the nomin�l'240: I scale. For the

;.sixteen configurations in Figure l, 1728 .combinations were measured for roof level concentrations. To confirm that the results from the d = 0.05 H stack could be applied to other stack diameters using appropriate scaling factors, a series of 21 tests were performed to repeat selected configurations

_'u.sing a stack diameter of d = o.q�5 H, half the size of the stan­dard stack.

Model buildings were constructed by machining solid blocks of aluminum to emitting building height H = 2.0 in.

_ (50.8 mm) a.mi equal widths and lengths of 5.0 in. (101.6 mm). : Several auxiliary blocks were constructed so that groups of · bfocks could be arranged to create the 1.5 H and 2. 0 H high and 5.0 H wide double-width buildings. One of the single-width blocks was hollowed out, and friction-fit adjustable height

tacks were installed in a 0.25 in. (6.35 mm thick roof. Flex­ible plastic tubes c<onnccted each stack lo a manifold valve and a ·bank Of fi�e r0La1neters to cover the now needed for the M = 1.0 to 8.0 range.

'J $urface concentrations for plumes jmpinging on building walls were also investigated using model buildings with glass root: anq walls with mirrors inounted iriternaJTy at 45° angles to allow the downward looking video camera-to see through the glass roof and out the glass building wall. One of the laser light sheets shown in Figure 1 was realigned to a vertical orien­tation just in front of the building wall, and plume impinge­ment concentration were recorded. These ..yall impingement measurem"nts produced. another 97.2 . coinpi11i;iJions. The re ult· for these building wall impingement studies are not reported here but may be found in the final project report {Wilson et al. 1998).

SCALE MODEL SIMULATION OF ATMOSPHERIC DISPERSION

Both wind tunnels and water channels can be adapted to produce simulated atmospheric turbulence for dispersion experiments. With both available, the present study chose to use water-channel simulation because of its superior ability to produce full-field video images of plume cross sections using th,in lase{, lig�t sheets.1�o illuminat� the entir� �oof of scale model buildings. Simulating,the st�ck e){haustgas with water allowed us to accurately mix known concentrations of fluo­rescent dye tracer to produce the exhaust concentration Ce in the water-channel experiments.

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'- I ;1 I • l •• , ol I .•Figure 1 ·•Adjacent building configurations te6'/ in water channel ·im11latio11.,B11i�(ii11g widtf1� ofi.5B anri ?-0 f;l 1(!$1f!;dfor

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buildings. . , ·, 1 • .:

Simu'lalion of atm�spheric diffusi�n. from .siacks ne,ar buildings requires the following factors to

' be considered: ' ' ·'-. J Corrvct simulation of atmospl:ieric turbulence intensity and eddy scales in the .�imulated app.1;oach wind profile.

Correct flow reattachment and wake recirculating regions for the model buildings.

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Correct similarity of exhaust gas momentu111, buoyancy, · ' 'velocity profllei; and turbu lence fothe emergi'rig flow from

rhe laboratory stack. 1 '' • ; '" ··' ,, ..

. } . Coµ-eqt scaling of '!"in4-tunnel or water.channel <:Qocentra­.. tion data lO an appropriatcJull-scale atmospheric averaging time. This, averaging tirµ,e is the time du.ring, which the pll\ine .is alloyted to IIle\ill*r back and forth to prodµce the tim!!-f!Yeraged .receptor. copcen.tration.

Boundary Lt:1v.er Si1T1ulation ; : ,

The water,ch'anncl in FiJtltre 2 v.!hs specifically designed 10 model atmosplieric'cllsper'sion. Tlie 16 ft (4.9 rT\) l�ng test sectioh1allows develo'p'n�enl ofthe large cddy"stmctur� '(ypi­cal of the lower 300 n:ooo ru)' of a neutrally .stallie athio� spheric boundary fa'yel':: Flow processing; ' to. acnieve this simulation began with �·fiow-blockagc screen \Viti\ variable area openings OJ{th'e downsi�eiu side {?f the flow straigtiten�r' at th water:lch.ah nel enti'ance.'This blilckage scre�n l1el��d LO rapidly de�diop a wind boundniy lay(Hprorlle.tjycl1hi1gfog 1r{e i.nlet'velociLy profilefrori\ oa�'tb�t wast\ frt'oNn wH_frhcight c� a closer.approxirl1:alion of'ihe ihcreasii1�'i;��ed y,;iUfhci'ghi in1 the developed boundary layer. After passing 6ver a 10�':,;,,a1i' \o generate cross-stream '\(Orti�e�, the ,ilow developed natu-· - · ' . "

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rally over .a plate covered wit\! uni fomily distributed nylon pins that were designed ro .producelln urban rklughness bound­ary layer With a ltig�law roughness scaling length in U oc hi (z/ z0)'of z0=L60 rnm ir\ �e water channel, equivalent to a full-

.. .. . . . ' scale roughness length of z" = 380 mm m the 240: l scale <}l • . \ t1 ' \ "• • t 1

which the exp�rimi:nt w.ere t?,nducted. J'he .lll!!an }'.elocity profile in Figure 3 was fit wilh the power law U ex z0·26 lY.PiCIJI.

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figure} Wq(er-Q,ia11miL �newr veloc,ir,y ,cmd t11rb11le11ce · .u: . p(Qfile�·1 vitf1 �10. br�ifd!ng presem .. A;f.odel .tea/ft.

·"" ·:· : , .;e1�1itti11g �J1ildin_g /ieig�!'·. H1'f 5,0.8 111111,(�.0 in.)

in 'approximate ?'fP: 1 sf:alt::, 1• • • 1

of a low-rise urban area (Irwin 1979). In the absence of build­ing models, the water-channel' speed at model building height, H = 2.0 in. (50.8 mm), was maintained at UH= 0.57 ft/s (174 mm/s). At this speed, the building Reynolds number was ReH = U HH/v = 7700 for a water temperature of 59°F (15°C).

In his review of building flow simulations, Snyder (1993) performed experiments that supported the Reynolds number scaling recommendations of Robins and Castro (1977a). For a turbulent boundary layer wind profile approaching a model building, they found that a correct flow simulation was obtained for Reynolds numbers ReH > 3000 to 4000. Our experiments were carried out at a Reynolds number twice this minimum requirement

One widely accepted reference for atmospheric dispersion simulation is Snyder (1981), updated in Snyder (1993). Snyder recommends that for correct turbulence intensity and scale simulation, the Reynolds number formed using the surface fric- · tion velocity u* and log-law roughness length scale z0 should be u.z,jv > 2.5. Using the measured value u. = 19.9 mm/s, from the single-component laser doppler velocity measurements shown in Figure 2, our friction velocity Reynolds number was 23, a factor' of 10 larger than the required minimum.

Flow Reynolds numbers in the water-channel boundary layer are several thousand times smailer than their full-scale atmospheric counterparts. These low Reynolds numbers in the laboratory flow require that the roughness elements used iri the water channel be much larger and more d,ensely packed than would be found in a full"scale atmospJ1eric flow. Jn �rder to allow the boundary layer Lo fprget this exaggerated roughness height, a smooth su1rface the iength of 4.0 H was placed in front of the model buildfo.gs, as shown iri Figure 1. Measurements were tnade at roof height to confirm that this short readjust­ment length had a negligible effect on wind speed at roof height in the absence of buildings.

Stack Exhaust Simulation

The correct full-scale sfack Reynblds number cannot be matched in the scale model. Stainless-steel tubes with inside diameters ofd = 0.05 in. (1.27 mm) and d = 0.1 in. (2.54 mm) were used to simulate the 1.0 ft (305 mih) and 2.0 ft(610 mm) diameter stacks in 240: 1 scale. For exhaust velcicity ratios rang­ing from W,!UH = 1.0 to 8.0, the lar:ge diameter 0.1 irt. model st�cks had laminar flow, with Reynolds number Wd div = 400 to 3200. Equivalent full-scale stacks would have Reynolds numbers of 200,000 to 1,600,000 with fully turbulent flow.

No attempt was made fo induce turbulent flow in the small diameter model stacks. Instead, a correction was applied to adjust the model scale flow velocity to produce the same exhaust momentum flux 'that would occur in a flat turbulent flow profile emerging from a full-sqaie exhaust stack: A momentum flux correction fac.tor, a:= 2.0 'for l:uninar flow was used when calculating M, the density-weighted exhaust velocity we to windspeed u H ratio: I

T0-98-15-3 (l'iP-897)

:8ACK 'fO PAGE ONE

M = o.s(�)0.5 J:Ve

Pa UH (1)

where Pe and Pa are.the exhaust and atmospheric densities. For the' flat (uniform) velocity profile that would be expected for highly turbulent flow in full scale, a = 1.0. All values of M shown in the figures are full-scale equivalent values calcu­lated using

w M = 1 414 •· !llotle/

. U.H, motlol ' , ( (2)

where Pe and Pa for the water were used to simulate both atmo­spheric air and exhaust gas in the model.

To determine the effect of laminar exhaust flow on plume Lrajectory and turbulenL mixing, a series of experiments were carried out using a'Jarger cale stack of 0.35 in. (8.8 mm) eject­ing fiuorescerit dye into the laser light sheet. These tests, with a stack Reynolds n.i.Jmber of 1600, showed that the emerging jet underwent a rapid transition to turbulence within ten diam­eters after exiting with velocity ratios ranging from M = 1.0 to 4.0. When a turbulence generator was inserted in this larger model stack to produce a flat turbulent exit profile, the entrain­ment and mixing into the bent-over jet was ·indistinguishable from its laminar counterpart at the same momentum-adjusted veiocity ratio M after the plume h�d .traveled ten diameters from the stack tip. These experiments confirmed that momen­tum-adjusted laminar exhaust flows in the model stacks provide an accurate siinulatiort of jet mixing, plume rise, and dispersioni.

Plume Spread Simulatio� . The final step '.in as�essing the a�curacy pf scale model

simulation was to measu�e crosswind. plume spread iJJ, the absence of buildings to determine the appropriate concentra­tion averaging time that was being simulated in the water chaimeL This full-scale concehtratioh averagirig time cannot be determined by using a scale factor to adjust the concentra­tioh sampling time used to collect 'water-channel data: In the water channel, the glass side'wails prevertt the large cross­wind plume meandering that occurs for averaging times longer than several minutes in the atmosphere.

The effective averaging tinl.t( for the water channel was determined by me�suring crosswind plume spread O'y in the Wat�r channel using 100 second averages of 1000 frarp.es of digital plume images of fluorescent dye illuminated by l.aser light s.heets. A streamlined isokinetic injector .tupe 0.135, in. (3.45 111m) ,i!lSide diamete� ejected dye tr(!cer para,llel to ,the flO\� �I building heigl1� z = H:=o.2.0 in. (50.8 mm) above. the �ur(;tce . with no building mqdels present. Figure 4,. shqws J1'\easured plume spread over the rangl( C?f down\yind.distances dH where building models would be tested. The data shows that an effec:ti�� i�itial sout;c'� �ize 0'0 a4ds linyfll'�Y t() thy tu�bulence induc�d spread:

(3)

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1,00 ,----�--.....---...,,-----.----'.,----,---,

0.80

lf�tk.SGu1�.sm�� Surfac• 'V pok'ii1d�tm Q polntadup&'bHll'I

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0.00 '-----'----'---''----__,__--�-�--� 0

Figu;e4.

2 4 6 X/H . i!',

Crosswind plum� spread �! a/H_ 1vi1h. 110

b11ildl11tp- for pl11111�s froih w1 isoki11etic fluorescen.t dye tracer :w1irce injected m height H mu( location x � · 0 · wh0�re the emitting building will be p�sitioned. '

Here, x' is the distam:�e x' = (l-:X�,�ck) from the stack, With the origin x = 0 at the point wher� ,the upwind wall of the emit­Ling bui lding is located :(see.Figl�re 2). Ini the absence of build­ings, we found·A>;,,,0t1c1 � 0.080 � q P4��veraged over several different rescs. '

Full-scale urban dispersion measurements of McElroy and Pooler (1968) were used by Briggs' to develop 60-minute averaging time plume spread equations foray and Gz for EPA model ISC3 and SCREEN3. For the neutrally table ca e simulated here, these functions are linear in the first 1500 ft ("' 500 m) (rom,.�he �ource, �vith Ay,aimos ;_;; 0.16 f9r 60-minute averages in the neutrally stable,alm�sphere (class D) simu­lated in the water c)\annel (EPA 11'95a). Using the widely-' ti • • accepted 0.2 power law to adjL1st�;vnlues for changes in aver-aging time 'm•g• ' . · �

(4)

For A)',morlol"' 0.08, Ay.mmos = 0. 16, and tav8,atmo.< = 60 min, we calculated from.Equation 4that1111,g,,,;�d•/"' '1.9 min. Using this indirect method we were able lo interpret our water chan­nel measurements as representing two-minute av�rages in the full-scale atmosphere. 1

���py .. it i.� wprth no�ing ��a� tlle. lin�ar r�_l.!tlio1�.of,Figll�e 3 between plumi; spread and downwind di lance allows· the bif!1�.

ing1¥o�e1 ex:R�°:�cn�� t? ti1e �c1�1eCJ\\� 9� scaled down as long as the �1ze of the two bu _1ldir�gs �ogether d9�s not exceed

• • • .., , •• # •• i ,, ') the range of about 500 m where the linear spread function is

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valid. This means that thelscale model buildings with length L = 2.5 H can be used1to represent.building heights a factor.of 4 higher and lower, . ranging from about H = 10 ft (3 ,05 m) to H = 160 ft (48.8 m) rather than the.single nominal building size of H = 40 ft (12.2m) chosen for the researc:ili: project.

In the following discussion, we will refer to .a:n H = 40 ft building height with stack heights hs = 7 ft, lO·ft, arid 20 ft above the roof. These nominal values will help us visualize a typical full-scale situation, bui1in fact may be scaled up and down by about a factor of fou'r to. ;represent other buildings with the same stack to building height ratios h/Jl.:

DILUTION MEASUREMENTS ,, ., , , WITH Fl.UORES<;;ENT DYE TRACEFJ · .

''· Rooftop concentration measutements were made using disodium fluorescein dye ·as"a tracer in the w�ter injected from the stacks, as shown in Figure 2. 'The dye was illuminated"by laser light sheets generated by a 4 W argon ion laser and recorded using a video camera looking tlown through the surface of the water at tlie building rooftops.' Digital ima:ges of the, plume were obtained using a· 10-bit (1024 step) frame grabber board to directl:Y avenige 1000 frames .. acquired at 10 frames pet1seccind over:a.100 �econd sampling time.,• ,

The single beam from the 4 W argon ion laseii was split into two beams that were <:arried, by fiber�optic: cables to the water channel-where they were processed through converging and cylindrical lenses to produce·a thin light sheet about 0.04 in. (1.0 mm) thick diverging at ·about a, 10° angle over the building , rooftops. 1 The \juilding models and . surrounding ground surface plate.<> were painted flat black to minimize r-ejlections.. ,) ·

, ;. ; · " A black and whiteJ�CD vidoo·camera with a 3:1 zooin

lens was used to record· video images of the. plume. Because the camera look:ed through the water surface (see Figure 2},• a plastic float th1at spanned ,the widthi of the water channel wQs anchored upstream to remove surface ripples over the building models. The camera lens was fitted with a yellow filter Lo remove the blue-green laser light reflections and allow only the green-yellow dye fluorescence to be recorded. " ' . 11.: . d� . ,. " y,de� Image Cfil ibratje>n.,. " ;

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Each of the 480 by 640•pixels in the frame grali>ber .array were individually calibrated1by placing an op�n-bottomed bpx over the models and filling it with concentrations of fluoroo­cent dye solution. Digital images over the model·roofs then· gave; individual pixel calibration values.:<llfi:concentration ,¥S. intensity counts on the frame grabber. By including the model· bJJ:ildings, direct compensation'. :was. obtainad for. surface refleGtiwis. J'he three, stack-11 •on the emitting building JbOf passed through the laser sheet and cast shadows iand prdduc:ea bright reflections. These shadows were too dark to be corrected by the1ditect.t'alibration technique and are visible in dilution contour:p!g_t (Figi.1r,e 5)'. The .di�utiqn data points near the stacks we1:e obs�ured by reflections and are ·removed from the graphs (Fi�ure g).

To;g8-1$3 (RP-897)

Calibration of the camera and frame grabber produced a linear. intensity-concentratiqn response over

'a 250: 1 range of

concentrations• in the calibration box. This linear response allowed us to use real-time averaging of the pixel intensities for the 1000-frame sample, followed by c'onversion of the average intensity to avf)rage concentration;

The comers of the erµitting building prbvided a .scale reference to c;onvert pi.xels into position distances on the aver­age video image: The width and length of each pixel element was about 0.02 in. (0.5

·mw or 0.01 H) �m .the model rqof. This

high spatiatresolution was found.to.be.unnecessary, and only every fifth pixel was recorded in the final data arrays. This resulted in each of the single width building .rooftops (2.5 ;H long and wide) having a dillltion measurement array of 50 x 50 points on the roof surfa".e giving 250q measurements on each building roof to define confours of constant concentration and profiles o.f mi�i�um diluticm. . · . '

. •' :-' ' ,, . .

NORMALIZED plLUTION FUNCTIONS The dilution'factor Dis defined !\S the rl).tio of exhaust to

receptor co�centiation, D = C/C, where C is the pollutant concentration �t any receptor a11d Ce.is the pollutant conc'en-. tration in the exhaust gas. This dilutton, dept;nds op the volume flow rate from the stack and the ability qf the atmosphere to disperse poHution, characterized by .variables .such as wind­speed and verticai find cros.swind plume �preadsdn order to present the ,results of our measurements i11 !l.form,that can.be used for stack de�i�. we normalized,these .ineasured dilutions in a form that allows them to be adjusted.to a yatjety of full­scale atmo�pqeric v.:ii;id _cqnditions and varying ratios of exhaust �elocity to windspeed. The appropriate normalizing factor was derived by co11si<l,ering roof level co11centration for the.Gaussian concentration prqfile for a plume dispersing in homogeneous turbµlence over a roqf.with the,plµme refleCtf1d from the hnperviqus roof.sur:face (Panofsky and Dutton 1984).

l. . ' i· · C0Q0 (-(It,+ 6/i)

J . . Croof = 'ft.U <J <J exp 2 , ,'

" c 1 �· · ?-crz . (5)

where Croof,is the roof level concen,tration on plume center­line, Uc is the effective wind velocity for plume convection, and Ca is the concentration of.po�luta�t (in,ttie same units ppm or 1µg/1113 as _Civof). in the.tot�[ exhaust ,v 9lu1ne flow rate. Qe. The plume spreads cry and 0'2 .in the crosswind and vertical qirections vary, with distance· x' from the stack. The stack height i� hs with a fi11al p�ume rise �� above the top of.the

$� .

. .

.

The rninim,um (i.e., plume centerline) dilution \s defim:d· as Dmin = C/C.roof Rearranging· Equation 5· to define the nonnalizt;d·dilutic;m, , '

(6)

BACK TO PAGE ON:

where the building height H has been used to normalize the dilution to be consistent with normalizing distance as x/H. The wind speed UH is observed at emitting building height z = H above ground with no buildings present.

For a non-buoyant momentum jet, Briggs (1975, 1984) gives the final momentum rise height as

tlh = 3.0 Md, (7)

where the density-weighted velocity ratio M is defined in Equation 1. We used Equation 7 to define the normalized exhaust velocity ratio Mdldmf with the reference !iiameter equal to the stack tested, dref = 0.05 H. For all the measure­ments shown in this paper, d = d,e1and the normalized exhaust velocity to windspeed velocity ratio was equal to the actual ratio M. DILUTION FOR UPWIND EMITTING BUILDING STEP-ACROSS ROOFS

Figure 5 shows dilution contours for a flat-roof building at a low exhaust velocity M = 1 and a higher exhau�t velocity of M = 3, bolh for the medium (0.25 H) stack height, equiva­lent to hs = 10 fl on an H = 40 fl high. building. At the low exhaust velocity, the plume is down washed into the roof recir­culation region at the leading edge . of , the building and

Figure's

'

Dnino. 2

UHH

'°""'

M=1 ....

.... • I I.DO

' ; """

...

o.n•

M=3

Diluti01:i coi1to11 rs 011 the flat-roof bttilding (step-across roof, 110 �ap) for the medium J 0 Jc stack height (hlH = 0.25)\�ith exhaust veloci'ry 'ro windspe�d ratios' of M = 1 <111 M = 3. ' ·

.I '1 . - '

1

BMD612B h,•10'

. , · .

,• .

l :llM M=3

§,00 ....

, ...

0.50

O.lf?

: � • . .. I

Figure 6 Dilution cantaim'for step-across roof with 1.0 'El gap between b11ildi1ig°'lfor the ;11edi1111l I 0 ft stack'

. , height .,(h/H = 0.25) with exhaust velocity to windspeed ,ratios ofM ;= 1 and M = 3.

" produces: significant concentrations upwind to. the roof edge. Note_ the· distortion' in the •dilution contours 'cause'd by light sheet shai:iows fiom'the truee· stacks.

i i ; J • ; i . Figure 6 -�how�; th� S!lm� st�c� he�ght �s Figu,re 5 but wi.th

a gap.of 1 .0 H bet-.v,een the buildtngs. Alc;mg t�e plµme center­line, the dilution increases in the gap but then decreases as material trapped in the gap is sucked up over the roof of the downwind adjacent building.

Figure 7 presents the' normalized minimum dilution on the plume centerlirid01'show the'effect of gap width between the buildings. The data points in Figure 7a for_M::;:: l.O and 3.0 are the same as shown in Figure 6.

. ; " "':.. . -r . ._._

Reference Building Dilution Measurements

In all the normalized dilution plots, we have included lines showing the normalized dilution n;ieasured on the long flat-roofed reference building (Figure I )i _Mell!iure�e�1ts fro.1�· this building give a basel ine with which.1,��,.00PJP.ate �dj�c;_e,nl building effects at each exhaust velocity ratio. Although measurements we're made for· six velocity ratios; forcla:rity .wt; have shown only three.

8.

.Q 100.00 2 Q 1 '" E 0 z

10.00

BACK TO PAGE ONE

.,.__ l!mlltlng building -0.10 .._._ _ _,_ __ _._--L_.___,__,_ __ _._ __ _,__ _ __.

0 1 2 3 4 5 6

1000.00

c 100.00 _Q � l5 ] .� .. E 10.00 0 z

O�N� . :r: .

=> '. 0 . 1 .00

I "\• 1 !

, X/H Distance from Emitting Building Edge (a) � l»'l:*f•

' ... J

' ... .. .. ! �! .

..

I ,._ emltUng buidlqg -

• Mdld,.,=1 � tfiL cirJ . ,._ Mdld,,1=3 --

• Mdld•I"" . . h,JH=0.25

-- ,.,, bldg

... . ·•······

____ .. ,___ __ .•.

' •

, . •!"'•• buldlno-.,_ • '!"

7

0.10 '---��-��-..__.___._ __ _.__ __ ..__-=-"' ' L I 0 1 2 3 4 ' S" 6 7

XIH, Distan,ce f(om ,fi'mitting Bltilding Eqge . _ , : (b) Figure 7 Effect of gap between buildings (symbols) and

no gap (solid lines) references with step-across roof for medium I 0 ft stack height (h/H = 0.25) on the upwind emitting building.

Effect of Gap Betw�en Buildings for Step-Across Roof

Comparing Figure (a and 7b with the reference building data: we see t�?t th.e gap J:1etw�en the buildings has a negligible

. effect on difutioi1 for slep-acrqss roof levels . Differences betwee·n 1 ie liiies and tllc dat� syqJbo1s can be iil1ribute<1 10 the ±30% rqn-to-run random variability between centerline : concentrations from the I 000- frame averages. Because we are plotting normalized dilution, th-is run-to-run repeatability includes uncertainties in water channel speed UH and in the exhaust'flow, ·Q�; along with the untfortainties inherent ' in quantitative video imaging.

T0-98'1 5-3 (RP-897)

c: .Q 'S a al ·"' o; E 0 z

. "'

10.00

. Ot::r 0 ::> l,00

MDfl Ul/12111,. + Md/d,.,•1

i :

' .

"' Mdld,.,"3 • Md/dm•B

-- rvl bld11. h,tH=0.26

. . . . . . . . . . . . . ·._.:_�-----"-�----. . .. . . .

. .

. . . . . .. . . . .

... i erTih.-t.lng buildfng -:----<-I o.io o�_,__--='o.-=-s --...1.1 ___ 1.L.s ___ 2!_ __ 2L.s __ _J XII-I Distance from Emitting Building Edge

Figures · Effect of locating a stack far from upwind roof edge. Sol.id lines for reference building are for stack 1 located 0.25 H from upwind roof edge (see Figure 1 ).

Stack Location: Central vs. Upwind Edge

Comparing the measured dilution in Figure 7, we see an interesting effect when the stack is located near the roof edge in Figure 7a vs. the central stack shown in Figure 7b. At the lowest exhaus�,velocity ratio M = LO, the near-edge stack has dilution 2 to 5 time� less than the central stack for the first stack heights downwind. This effect disappears at the higher exhaust velocity ratios of M = 3.0 and '8.0.

The adverse e'ffe.ct 'Of mounting a stack 'near the upwind epge of the ·n)9f is even more apparent in Figure 8, which compares the stack location near the upwiii.d edge of the refer-

r '

' ·-

.f 1 ·· H

BACK TO PAGE ONE

ence building roof with data .taken on a downwind emitting building with no gap, placing the stack at x = 2. 75 H from the upwind edge of the roof. For the reference building, stack 1 is only x = 0.25 H from the upwind roof edge (Figure 1). An important point for designers to note is that the measured dilu­tion in Figure 8 from the edge-mountd:i stack on ¢.e reference building (solid lines) is a factor of 2 to 10 less than for the same stack located far from the upwind roof edge (data points).

.Using the flow visualization measurements of Wilson (1979), the 1997 ASHRAE Handbook-Fundamentals, chap­ter 15, Equation 23 gives the height of the roof edge recircu­lation cavity as He = 0.22 H0.67y0·33, where Yls the crosswind width of the building. For the single building widths reported here, Y = 2.5 H and the height of the roof edge recirculation cavity was He = 0.30 H. The top of the stack with hs = 0.25 H in Figure 2.8 ended inside this edge recirculation cavity, and rooftop dilution factors were considerably reduced. This dramatically illustrates the disadvantage of locating a stack in the region of high wind velocity caused by the flow acceler­ating over the roof edge near a zone of flow recirculation.

These observations lead to a clear recommendation to designers: avoid locating stacks near the edge of the roof where high windspeed can deflect the plume into the roof edge recirculating cavity.

STEP-UP f'.IOOF LEVEL �ITH EMITIING BUILDING UPWIND

Figure 9 shows the pluine from a low level stack on an upwind building impinging on the wall ofa higher downwind adjacent building with its step-up roof level. From Figure 9, it is easy to imagine that the worst case for contaminating the downwind building roof woqld be, a high .exhaust velocity that carries the plume trajectory to the. level of the downwind roof. At the other extreme, a ·plume with very low exhaust velocity would remain close to the roof level of the emitting building and be swept around'the side·s of the adjacent building with

· increased plume spread

' • I

Figur'!. 9 Plume trajectory and spread from an upwind:emittirig 'btiildirig over a downwindadjacent building with a step- : up roof.

T0-98-1 5-3 (RP-897).

� . . ' =

:::; ' on dOwnwtnd �ocfr

�:EJ M=1 ' : · I ,.;

,,

on upwind roof

' ..

on upwind roof

D M=8

on downwind roof

on downwind roof . ., , ...

I

,_.

._,

Figure lo Dilution contours on step�up roof 2H adjacent building for a short 7 ft stack height (h/H =

0.175) with exhaust velocity to win(i,sP,eed ratios ofM = 1, M = 3, and M = 8. -

only a small fraction being carried:over the roof. This situation would minimize adjacent building roof contamination at the expense of contaminating the roof of the emitting building.

The dilution contourd in Fi&ure l O clearly show the diffi­culty faced Qy Sl�tC'k designers With high adjacent dQWny,;ind bui ldings. At low _exhaust velocity M = LO, the exhaust plume from the central stack J?roduces very low dilution (high concentration? areas on I.he eipili.ing building, but passes around the sides ?f the a9jace.rit I?�ild!.Og lcavi;ng its roof uncontami nated . At a moderate exhaust velocity ratio M =

' • : ... ' ' , . \ '' · "it ;4 � ' 3.0, about the same leve l o(contammation occurs on lhe emit-ting anf!,fl�jl\C.�l\l pui lding � .<?fs. At l�!'l high�s\ex.h�usl Vf(loc.­ity, .¥ =:= �.p . t�� m<;>P.1entullJ jqi .cause ti� pl1.1me to rise high abo,v� th11e itting.R�1 i ldiu ' o !l�ay)l.Y conl�mi;na.te the roof of the adJ.ace'?\t1,bu i ld�ng. At M_ r::18.0, t!Je Ngll cRll!<!<!Jlrnlion (low

10

SACK TO PAGE ONE

dilution);plume cor� is easily v.�sibfo as i t is carried over the edge of the adjacent quilding roof. The effect of.the diverging streamlines in the decelerating flow approaching the high adjatent building is readily apparent in the broadening of the plume di�tion'contours a$ they ap�roach the adjacenJ build-ing.

' . . Figu.res l la and ll b show the a9mbined effect of stack height and.-exhaust velocity in producing worst-case dilutions on the adjacent rooftop. To test our hypothesis that the worst­�flSe copdition occurs when the centerline: of the undisturbed plume trajectory is at the height of the adjacent roof, use the final momentum jet rise from Equation 7 to calculate the point '· . : . ' . , • · . \'. '

.. . . . . '·

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0.10 .__ _ __.__.__,___.___.__.___._ _ __, __ __._ _ __. 0 1 z 3 4 5 6 XIH, Dls'\'�ce from �:im1ng Building E�ge 7

Figure 11 dEfject ofstack-height on minimum dilution for a -, . . �t�P-llP, r���f." 1a 2 H atVa'{e_lll bu{l.ding wit?, 1.0

. H '!}�p for s)1ort 7 ft (h/H = O. 1 75) '4nd wll 20 ft '(h/H I� o.,s) <.:e111er sta"¢1fs Ofl th� 11pi·vind emitting . b!tilt1t11c: ,_ · ·• . ' I • \ ... .. • • , , , ,, •• ' t I

T0-98-1q-3 (RP-697)

M

1 .0 3.0 8.0

TABLE 1 Calculated Impact Heights for Plumes'

Striking a 2H Adjacent Building

Figure llah/H = 0.175 Figure llbh/H = 0.5 &imnac/H Equation 9 &;mnac/H Equation 9

-0.68 -0.35 '-'-0.38 -'--0.05 Observed Worst Case

+0.38 Observed Worst Case +0.70 impact for the plume trajectory on • the downwind adjacent building .as 1 •

) ' llzimpact = (H + h,. + 3Md) - H0d} ' (8)

where &impact is the height at which an undisturbed plume will pass above the roof of an adjacent building roof level Hadj' where H is the height of the emitting building, hs is the stack height, and 3Md is the final momentum jet rise from Equation 7. If we normalize by H,

(9)

The height �impact will be negative if the .undisturbed plume Lrajectory. shown as a dashed l ine in Figure 9, strikes the adjacent building wall below the roof. Note that Equation 9 includes no adjustment for the upward deflection of the plume trajectory, shown in Figure 9. Table 1 shows values of &impact calculated for the configurations in Figures l la and l l b. Considering the simple assumptions inherent in Equation 9, it does a remarkably good job ·of predicting the worst-case exhaust velocity ratio for both short and tall stacks as the value of &impact closest to zero.

1000.00 -----�-�--�-�---.------,, • "' - .

-- ref, bldg, � 100.00 -

2l � .!:I 7U � 10.00 ;:-

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I •• .... ._ • :z ..

_ ........ .. -· ..

.. ,... ::.

0.10 L----L-'--.l--'---1.--..1----'---'----' 0 1 2 3 4 5 6 .. X/tn Dls!ance from Emitting 'Building Edge•

figilre 12 Effect of gaR between buildings (r:ompare with · ' Figure ) lb) on minimum dilutfonfor a srep·up

1vof on a 2 H adjacent b1Lilding with a shore center 7 ft stack (il.IH :::: 0. 175) on the upwind emitting building.

T0'-98-15-3 (RP-8971

BACK TO PAGE ONE

The higher downwind adjacent building affects the dilu­tion on the emitting building roof in unpredictable ways. In Figure 1 l a, the dilution close to the stack is a factor of 2 to 4 times larger than the reference building case at high exhaust velocity with a gap between the emitting and adjacent build­ings. In contrast, comparing Figure 12 to Figure l l a shows that when the gap between the buildings is closed, the dilution on the emitting building roof becomes a factor of 5, smaller than the reference · building case when the exhaust velocity ratio is high. This reversal ofoffects when the gap is removed suggests that there is still a strong need for case-specific stud­ies using wind tunnel and water-channel scale models for crit-ical situations.

'

When there is a gap between buildings, the location of the stack does not appear to be critical. Figure 13 shows the effect of stacks locatep at the upwind and downwind edge of the emitting building. Both produce about the same normalized

c: 0 100.00 • ... � .. .. 15 "" -� ;;; I§ 10.00 -0 z , ....... .

•IJl'"•·�··•••..a.u�

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C �I 1.00

0.10 '-'---'----"--''---.1----'--"--__,'----'--__J 0 1 2 3 4 5 6 XIH Distance from Emitting Building Edge (a)

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0.10 �--'----'-'-L--L-_.__._ __ .._ _ _._ _ __, 0 1 � 3 . . . :4 ' 5 _ , , ' 1 , 6 X/H Dl�tance from Emitting Building Edge " (b) . .

Figure 13 Effect of stckk locdtidn I vs: 3 (upwind'edge vs. downwind edgr!) oh 'rrilnimum dilution for a step�

I up roof 011 a 2 H adjacent buildi11¥ wi h 1.0 H gap ' an i a sho'rt 7 ftJ fll/H = 0. 175) stack Oil the I upwind emitting building.

1 1

j . ..

roof edge recirculation

cavity

building wake .; , . recirculation cavity

. · • : ·:

BACK TO PAGE ONE

increased plume spread

in wake

Figure 14 Plume trajectory and spread from an upwind emitting building over a downwind adjacent building with a step-down roof

.

dilution on ilie roof of the adjacent building, and there is only a modest increase in dilution over the emitting building roof caused by the presence of the adjac�nt building.

E�ITTING BUILDING UPWIND ,. WITH STEP-DOWN ADJACENT ROOF

The t;ffect of a step-dowl} building located downwind from the emitting building is shown in Figure 14, The dilution contours in Figure 15 indicate that the plume !continues to spread in a relatively normal fashion even though it has been entrained by the wake recirculation behind the emitting build­ing. Measurement$ of �rO'swind plume spread cr,, on the downwind bui lding roof were compared to mea ured spread over tl1c reference building.and fom1d.to differ by no more tlrnn a few percent. This surpn�ing result suggests that the effects of'a step-dowh roof din be' expressed of in terms "of increased vertical -�lume spre�d· crz tombined With a step-down in the loc:ation ofthe roof receptor. ' ' :

BAR01133 11,•T

M=3 '\ ' I

10.0

5.0

2.D

1.D

... �111111 o.2 I • " . ,

D.1

Figure 1 6 shows that the' net effect for a steS:.ctown build­ing is that the dilution is always greater on its roof than for an equivalent reference building with a flat roof. In addition, Figure 16 shows that there is virtually no effect of havi11g a gap between the upwind emitting building and the downwind step­down adjacent building. These same results occurred for the other stack heights and exhaust vetocities -tested; .

• 1 �· ., �:: As a general rule-of-thumb, ignoring the step0-down roof

with an upwind emitting bu ilding \�ill result in a consetvative design that underestimates actual di lution by factor bf 2 to 5 on the step-down roof.

t . DOWNWIND EMITTING BUILDING WITH ST�P-UP ··

ROOF FROM LOWER ADJACENT BUILDING J I 1 � , -� ; { • ( , '

Figure 17 shows schematically the effect of an upwind buj,1W,q�,.thilt\!\dds1 f>HFQulen\:e and, .helps .c\eflect s�reamJipt;s . . . ·, before they arrive at the emitting building. This makes the

l'.4

Figure 15 Dilution contours for step-down roof level 0.5 H ·

'ttdjacent building with a short. 7 ft central stack heigh( '(h/H = 0.175) with exhaust velocity to windspeed ratios ofM = 1 and M = 3.

downwiii.d buildiii.g look shorter and reduces the size of its roof edge recirculation cavity.

Figure 1 8 shows that the effect of a lower upwind adjacent \,uild,h;1gjs ahyays helpfu�, p�oducing significant it'icreases in dilution compared to the flat roof reference build·ing. This was

1000.00 .,,----..--.,..---�---.----.---..-----,,

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0.10 �-__,_...._ _ _,___,__.__ _ __, __ __._ __ ..._ _ __, 0 1 2 3 . $ 6 X/H Distance from Emitting Building Edge (b)

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c 100.00

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. . . . . .. . . . . .

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0 0.6 1 1 .5 2 2.5 XIH Distance from Emlttlng Building Edge . (b) Figure 16 Effect gap between buildings on minimum

dilution for a step-down 0.5 H adjacent building with a short 7 ft. center stack height (b/H 0. 1 75) on the up�ind emitting building.

Figure 18 /]!feet of stack height on minimum dilution for a step-up roof with short 7 ft 01/H = 0.175) (1,.nd . tall 20 ft (b/H = 0.50) edge stacks- , on - the ,downwind emitting building.

Actual Plume

' I

l I • "

added turbulence� may increase ·

plume spread

upwind building reduces size of roof eqge ,rei;:irculati�m cavity ;

" '

. . . ..

Figure 17 Plume- trajectory and spread over an emitting building with a step�up roof located downwind from tlie adjiu:ent J ' ,. building. " , . 1 '

' f3

I+---- Actual -I cavity length

...

Figure 19 Plume trajectory and spread of exhaust from a central stack on a step-down roof The exhaust is trapped in the flow recirculation cavity.

true even for the high stack in Figure 18b that extended well above the roof edge recirculation caviiy.

STEP-DOWN EMITTING BUILDING WITH HIGHER UPWIND ADJACENT BUILDING

Figure 19 shows the trapping and recirculation of the exhaust gas plume from a short st&fk on a step-down down­wind emitting building. The conventional model of these trapped plumes is thafthey fill the entire crosswind and; verti­cal wake behind the building with a relatively uniform concen­tration. The dilution contours in Figure 20 provide a surprising contradiction to this conventional view. At the low exhaust velocity ratio M=l .0, the plume from the short stack is trapped

BAJ0t138

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�1147

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· · · ! '.

.. . .. ·�

10.0

.,,

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1.0

0,1

Figure 20 Dilution dont<Yurs from a'short 7 ft (h/IL= 0; 175) stack 011 a step-dOWfl roof At an exhaust velocity tD uii'ri(fspeeil railoti M !d''i'�lfe'pllli1i�1 i'i trc;pped

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14

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BACK TO PAGE ONE

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. . . . . . . . . . . ' . . . . . . .

"'

a+:c o; rp"\cio

.. . .. .. . . . . . . . . .

.. .

. �. '. i : o.1,0 o"-.--'-o ... .s---',---,.!-5-. ;-: -.-'-,---'._-s --,-'a, : .

X/11 Distance from Emitting Building Edge • · • (b) .. ;

' · : I : � ' j.' ' I ' 1,

Figure 21 Effect of 1 .0 H gap between buildings on: minimum dilwio11 from a short 7 ft (h/H =

. · 'o. 775) . edg ,. siack·roii tfi"-e• enifrttng buflaihg 1 • • f loc,ated <i,()W'l-}'.findJrarr 1 , the 2 JI,. adjacent

kuilding. . . . ; •)· • . , ,

and carried upwind fi'orri its central stack location. However; the dilulion contourii 'c'lo nb(rl!I lie c(ossw'i1'lll wake unlforlii!y and tfi�·piume tieha'ves1rttorelike poinL ·fotlice plume w illl l;t

virtual ·orig in shifted 1�pwli1d. At 'M�3.0, · the exhausL J�t is )..,, . . , .,

lrOf\� enough tq pen�trate �e c:av1,ty, ,<;m9 .al t�ough the plume is r�P,idly di,spers�d w roof lev�l, n'on�.'i'f il app,i;:ars upwj1.1;?�9� th�.�tac� Joc.:�tio1�. Figure. 2 1 s��ws that tncre ls .v.irt�ia)ly. np effctct, �n dilµ,lion of,x�moyjngJhe gap between .the,µ,y9 b,uild� ing& i{ e).'ji,aust is, .trappt;d in the recirculation. ,re,gion for b9th cases . ..., .1 i ! �.: ·. i, :::. ' ' I · ! ' : t. \ , l ! ·.i , ' ·, , · ·.• ! ,· . ,n , : 1 •

The major advantagy of increasil')g ex,baust velo�·ty is \o provide inilial dilution b0y jct entrain�;ent. Th� infti�j dilu1io1; o:'catl· ed ·b . at1'ei'hraiMnen1 h1to a bentlover Oiornentum jet

f � • .. • • • • ' 1 I J f I . • .) ' I . , ' . •

. . • • l • ; ! was 1d�riv�? 9� . W!! :on �nd. �ai;ib O?i94) a�d1 . is1 p ,�d. i,1.1 ��

if9l}S�/U1/{Hl1i!f{h90�-:---:f:u11'da:.� 1fn \��{�· ph_q..p 1r 1,5, �qu�T Lion 1. 7. Here we adapt it for use in a recirculation cavi ty by

...

using an effective windspeed, u., in the cavity rather than the undisturbed approach-flow windspeed UH, to write

w. D0 = 1 + 1 3.0-U . s

(10)

Because Us < UH in the recirculation cavity, W/Us > 1 , and the initial "1" term can be neglected in almost all cases, and, for two different exhaust velocities w.1 and w.2, we have

Doi w. t M1 Do2 = We2 = M2 (11)

With this approximation, we expect the M = 8.0 data to have about 8 times the dilution of the M = 1 .0 data in Figures 21a and 2lb. The actual measured ratio is about'a factor of 6 close to the stack.

Figures 22a and 22b show how stack position and stack height affect the fraction of the plume that is trapped in the cavity." The measured concentrations upwind of the central stack location show that part of the plume remains trapped in the recirculation cavity even for the high stack hJH=0.5 at the highest exhaust velocity M=8.0. However; there is a factor of 100 increase in roof level dilution using the high stack at the high exhaust velocity (see Figure 22b) compared to the lowest stack at the slowest exhaust velocity (see Figure 22a). This result suggests that the factor of 8 expected increase in initial dilution from increasing the exhaust velocity is enhanced by allowing the high stack to have 90% of its exhaust escape the recirculation cavity and avoid contaminating the emitting bujlding roof. This result emphasizes the need-for de igners to use the highest stacks and lhe highest exhaust velocities possi­ble to reduce roof ievd contamination even for stacks that are trapped within recirculation cavities from high upwind build­ings.

CONCLUSIONS AND OESIGN GUIDELINES •Measurements of normalized exhaust dilution on emit­

ting and adjacent building rooftops are consistent with common-;sense deductions frq!}l flow patterns and �ecircula­tion zones that cai;i ,be s�etch�d aro4.nd the pairs of, buiJdfngs, The effect of adjacent l;>uildin.gs on dilution c11n be easily seen by d �rect compari�on with "lea urement on an equivalent flat roofed long reference building.

·

· The gap between an emitting and adjacent building had a negligible effect on dilution except for tlfo case of an upwind emitting 'building with a higher downwind adjacent building; Iil this 'cas'e, the downwind adjacent building blocks ·the flow over the emitting building roof and can dramatically decrease the roof level dilution on the upwind emitting building.

Guideiines for Stack Designers • . � • _ • . . 1 ! , I

In spite of the fact that plume lt:ajeccodes anq turbulent dispersion around pairs of emitting and adjacent buildings are extremely compl icated, the\� are some general guidel ines to aid designer/; in avoiding contamination M roof level Intakes.

' '. 1 ·

Cl J: . :I:

BACK TO PAGE ONE

� Mdld..i•t I & Md/d-·3 • Mdllf,..,a.B

- 191.bldg.

. . . . . • • • • t

t 0 ::::> 1.00 . . . . . ... ... . ..

• • • • • • • • • I

---- emtt''lngi bulldlng 0.10 --�--_,___.__..__ _ _._ __ --'----'

0 0.5 . 1 1.5 2 2.5 X/H' Distance from Emlltlng Building Edge

(a)

+ Mdli:l,..•1 I -C15 "'- Md/dr..-l e Mdld..f•8

- ret:tfdQ · · · · · ·

· . .

. . . . . . . .

. . . · · · · · · I • • • I

N I

a :

. . . . . ..

� omitting bul1c:J1ng -----........ 0.10 �-�--�--'----'----'----'-----'

0 0,5 1 1.5 2 2.5 XIH D}s!anca fr<!m Emitting Building .Edge (b)

Figure 22 Effect of stack height on minimum dilution from a step-down roof to a downwind emitting building with short 7 ft (h/H = 0. 175) and tall 20 ft (h/H = 0.5) stacks. Plumes from both stacks are trapped i� the recirculation cavity.

Design�rs shoµld avoi� locating stacks near the edge of a roof, w.here the high �indspeed in the flow accelerating over the roof edge can deflect the plume into the roof edge recirculation cavity and reduce dilution by factors of 2 to 10. See Figures 7 and 8.

With the emitting boil<Jiii.g upwind, a lower step-down roof adjacent building will filways have higher dilution on the step-down roof than would occur on a flat roof at the emit­ting building heig�t. �gnonng the step-dQwn in roof level will prodµce conservative designs. See Fig.ore 16.

If the lower �djacent buil�ing is upwlnd . of the emitting b�iiding, it "{ill block the. flow approaching the emitting bujlding,.ptoducing lowe� velciciµes and recirculation cavi­ti�s on the emitti�g bui�dil).g roof and inc;teas,i�g dilution by

15

factors of 2 to 10 on the emitting building roof. See Figure 1 8.

Designers should use increased exhaust velocity to produce jet dilution when the plume will be trapped in the recircu­lation cavity from a high upwind adjacent building. This initial dilution is directly proportional to exhaust velocity, and increasing exhaust velocity by a factor of 5 will give an extra factor of 5 in dilution everywhere on the emitting building roof, including regions within the flow recircula­tion cavity. Using both a high stack and high exhaust veloc­ity in these situations helps the exhaust jet escape from the upwind building recirculation cavity.

When the adjacent building is higher than the emitting building, designers should try to avoid placing air intakes on the adjacent building at heights above the roof level of the emitting building. Then, use the highest stack and exit velocity possible on the emitting building.

ACKNOWLEDGMENTS This research was supported by joint funding from

ASHRAE RP-897 and a Natural Sciences and Engineering Research Council of Canada Individual Research Grant to DJW.

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Briggs, G.A. 1984. Plume rise and buoyancy effects, Chapter 8. Atmospheric Science and Power Production, D. Randerson (ed.) U.S. Dept. of Energy DOEffIC-27601 : 327-366.

Huber, A.H. 1989. The influence of building width and ori­entation on plume dispersion in the wake of a building. Atmospheric Environment, 23: 2019-21 16.

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Huber, A.H. 1 99 1 . Wind tunnel and Gaussian plume model­ing of building wake dispersion. Atmospheric Environ­ment, 25A : 1 237- 1 249.

Huber, A.H., and W.H. Snyder. 1982. Wind tunnel investiga­tion of the effects of a rectangular-shaped building on dispersion of effluents from short adjacent stacks. Atmo­spheric Environment, 16: 2837-2848.

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Huber, A.H., S.P. Arya, S .A . Raiala, and J .W. Borek. 1 99 1 . Preliminary studies o f video images o f smoke disper­sion in the near wake of a model building. Atmospheric Environment, 25A: 1 199- 1209.

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Robins, A.G., and LP. Castro. 1977a. A wind tunnel investi­gation of plume dispersion in the vicinity of a surface­mounted cube-Part I: Flow field. Atmospheric Envi­ronment, 1 1 : 291 -298.

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Snyder, W.H. 1 98 1 . Guidelines for fluid modeling of atmo­spheric diffusion. Environmental Protection Agency, Research Triangle Park, North Carolina, Technical Report EPA-600/8-81-009.

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Thompson, R.S. 1993. Building amplification factor for sources near building: A wind tunnel study. Atmo­spheric Environment, 27 A: 23 13-2325.

Wilson, D.J. 1 979. Flow patterns over flat roofed buildings and application to exhaust stack design. ASHRAE Trans­actions 85(2): 284-295 .

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Wilson, D.J., and D.D.J. Netterville. 1978. Interaction of a roof level plume with a downwind building. Atmos. Environment 12: 105 1 - 1 059.

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T0-98-15-3 (RP-897)