Chapter 1. Introduction

Transcription

1 Chaptr 1 Introduction On of th major causs of poor indoor air quality at som facilitis is th sporadic occurrnc of xhaust ringstion at frsh air intaks. Univrsity, hospital and industrial laboratoris as wll as manufacturing facilitis ar particularly suscptibl to this phnomnon sinc thy mit a wid rang of toxic and odorous chmicals. Th ffct on workr halth and comfort is substantial. Unfortunatly, th stat-of-th-art has not bn sufficintly advancd to allow building nginrs to apply appropriat dsign critria to avoid this problm for nw construction or to hlp allviat it for xisting buildings. Consquntly, numrous incidnts of poor air quality hav bn rcordd and documntd. Th first phas of th study, commissiond by IRSST in 1996 [Stathopoulos t al. (1999)] providd a significant amount of information on th disprsion of plums mittd from buildings in an urban nvironmnt. Exprimnts wr prformd on two buildings for a limitd numbr of wind dirctions; corrsponding tsts wr also carrid out in a wind tunnl. Th rsults indicatd that th us of high vlocity xhaust stacks dos not guarant adquat plum dilution at all locations on th roof, and th xhaust momntum ratio, M (i.. th ratio of xhaust vlocity to wind spd), was found to significantly influnc th dilution of a tracr gas mittd from a stack. Both currnt ASHRAE dsign formulas and wind tunnl modlling may, at a givn location, undrprdict th maximum concntration of a contaminant. 1-1

2 This rport dscribs th scond phas of th study, which commncd in January Th main goals of th study wr: 1. Evaluation and improvmnt of th two most commonly-usd modlling tools (mpirical dsign formulas [.g. ASHRAE] and wind tunnl simulation) for th assssmnt of concntration of rooftop stack xhaust at various locations, and 2. Dvlopmnt of dsign guidlins to assist nginrs in th propr slction of location, gomtry and functional charactristics of rooftop stacks for various cass by rvising th currnt modls whn appropriat. Th study includd both fild tsts and wind tunnl xprimnts. A sris of fild tracr gas xprimnts wr carrid out on a 3-story building in Montral using a portabl fan as th mission sourc. Th influnc of th following factors on short-rang plum disprsion wr invstigatd : 1. location of th stack rlativ to rgions of flow sparation; 2. stack hight; 3. M-valu 4. atmosphric turbulnc (associatd with roughnss of upwind trrain); 5. rooftop structurs, 6. a tallr upwind adjacnt building; In th fild study, sulfur hxafluorid, SF 6, was mittd from a variabl spd fan locatd on th roof. Exprimnts wr carrid out for two xhaust spds (7.5 m/s, 17.5 m/s) and two stack hights (1 m, 3 m). Tracr gas concntrations wr obtaind at 15 locations using th 1-2

3 air sampling mthodology dvlopd in th first phas of th study. Four stack locations wr chosn for th study. Fild tsts prformd with th stack at locations 1 and 2 providd data for cass with high upstram turbulnc. Stacks 3 and 4 wr usd for tsts with a tall upwind building. Stack 4 was also usd for an opn ftch cas with typical suburban roughnss. A paramtric study was carrid out in th wind tunnl to provid mor dtaild information rgarding th influnc of stack hight and M on plum disprsion.. 1-3

4 Chaptr 2 Litratur Rviw Currnt standards for building vntilation systms rcommnd that rooftop stacks b dsignd such that thir missions do not contaminat frsh air intaks of th mitting building or narby buildings. This may rquir xtnding th hight of th stack, h s, or incrasing its xit vlocity, w. Th ffcts of h s and w on concntration distributions on building surfacs hav bn invstigatd using wind tunnl or watr channl simulations. In most of ths studis, plum disprsion was valuatd for a singl isolatd building modl. Schulman and Scir (1991) carrid out a wind tunnl study to invstigat th influnc of stack hight and xit vlocity on th disprsion of missions from a rooftop stack. Th xprimnts wr prformd with an isolatd low-ris building with a stack in th cntr of th roof. Figur 2.1 shows th influnc of stack hight on normalizd dimnsional concntration, C*=CU H /Q m, whr C is th rcptor concntration, U H is wind spd at building hight and Q m is th mass flow rat of tracr gas. Th rsults ar givn for a typical xhaust vlocity, xprssd in trms of th xhaust momntum ratio, M=w /U H. In this cas, M=3, which is gnrally associatd with modratly strong winds. Th rsults show that incrasing h s from 0 to 4.6 m causs C* to rduc by a factor of 100 nar th stack. Howvr, at th lward dg of th building, th incras in stack hight producs only a marginal bnfit. Th 4.6 m stack has a C* rduction factor of

5 Figur 2.2 shows th ffct of M on C* for a short stack (h s = 1.5m). Th data show that nar th stack, C* can b rducd by a factor of 100 by incrasing M from 1.5 to 5. Nar th lward dg of th building th ffct of M is rducd; C* is rducd by a factor of 10 as M is incrasd from 1.5 to 5. Th potntial for rducing indoor pollutant concntrations using hiddn (wall) frsh air intaks was invstigatd in a wind tunnl study carrid out by Ptrsn and LCompt (2002). Th study providd a mthod for prdicting concntrations at hiddn intaks basd on standard disprsion modls (ASHRAE, Gaussian). Howvr, sinc th rsults wr obtaind with an isolatd building modl, furthr tsts ar rquird to dtrmin th applicability of th mthod for cass whn othr buildings ar narby. Mrony t al. (1999) valuatd disprsion and flow pattrns around various building shaps using wind tunnl xprimnts. Flow visualization xprimnts dmonstratd that flow rcirculation zons ar intrmittnt and consquntly, th dirction of flow at a rooftop location may chang frquntly from upwind to downwind. Th authors suggst that th inaccuracy of CFD disprsion prdictions is du, in part, to th absnc of intrmittncy in th computr modl. Wilson t al. (1998) invstigatd th ffct of h s and w on th disprsion of building xhaust in a sris of watr channl xprimnts. In addition to an isolatd low-ris building, tsts wr also prformd for cass in which an adjacnt building was upwind or downwind of th mitting building. 2-2

6 Rsults obtaind with a tallr upwind building showd that th lward wall of th upwind building may xprinc low dilution valus (high concntrations), dpnding on stack location, stack hight and xhaust vlocity. Figur 2.3 shows masurmnts of minimum dilution on th lward wall of th adjacnt building du to xhaust from a 2.1 m stack nar th lading dg of th mitting building. Th dilution valus ar xprssd in normalizd form: D min Q /U H H 2, whr Q is th xhaust volum flow rat and H is th hight of th mitting building. Th rsults indicat that, for all M, th lowst dilution valus occur nar th top of th lward wall. Dilution incrass significantly with incrasing M; an incras in M from 1.5 to 8 causs normalizd dilution to incras by approximatly a factor of 10. On th roof of th mitting building, dilution did not vary significantly with distanc from th stack. Th avrag dilution on th roof was approximatly qual to th minimum dilution masurd on th wall of th adjacnt building. Th study found that stacks should not b locatd nar building dgs and frsh air intaks should not b placd on th lward wall of a building if missions from a lowr downwind building ar toxic or odorous. Furthrmor, a lowr adjacnt building upwind of th mitting building tnds to incras dilution on th mitting building roof, whras in th cas of a highr upwind building, incrasing xhaust vlocity is mor bnficial than incrasing stack hight. 2-3

7 Fluid modling studis hav dmonstratd th bnfits of high xit vlocitis and incrasing stack hight in rducing pollutant concntrations at critical rcptors. Howvr, fild studis hav shown that vn with high xit vlocitis and modratly high stacks, pollutant concntrations may b unaccptably high at particular locations [Wilson and Lamb (1994), Gorgakis t al. (1995) Saathoff t al. (2002)]. Svral factors may account for th occasional poor prformanc of rooftop stacks. Ths factors includ th location of th stack rlativ to rgions of flow sparation and flow r-attachmnt, th prsnc of rooftop irrgularitis such as pnthouss and high upstram turbulnc. It is important to validat th rsults of fundamntal fluid modling studis with full-scal data. Although most flow faturs (.g. wak siz, rattachmnt lngths tc.) undr nutral atmosphric conditions can b accuratly simulatd in wind tunnls and watr channls, it is ncssary to dtrmin th limitations of fluid modling with rspct to plum disprsion. Rlativly fw studis hav compard wind tunnl concntration data with fild data for narfild diffusion cass (i.. rcptors within 50 m of a stack). This is on of th most difficult fluid modling applications, sinc th plum charactristics may b snsitiv to a numbr of local factors (building wak ffcts, th position of th stack rlativ to rooftop rcirculation zons or dlta-wing vortics, stack Rynolds numbr tc.). On th othr hand, for far-fild applications, plum charactristics ar much lss snsitiv to ths factors. 2-4

8 Higson t al. (1994) conductd fild tracr gas xprimnts with a stack at varying distancs upwind of a small rotatabl building and compard th rsults with wind tunnl data. Thy found that th maximum concntrations wr gnrally ovrstimatd in th wind tunnl tsts; th minimum concntrations wr undrstimatd. This suggsts that th wind tunnl plum was narrowr than th fild plum du to th absnc of largscal turbulnc in th wind tunnl. Svral studis by th authors hav valuatd th accuracy of wind tunnl disprsion masurmnts [Stathopoulos t al. (2002), Saathoff t al. (2002)]. For th most part, th rsults indicat good agrmnt btwn wind tunnl and fild data. Th wind tunnl concntration valus wr usually within a factor of two of th fild valus. Th accuracy of th wind tunnl gnrally incrasd as stack-rcptor distanc incrasd. 2-5

9 Effct of stack hight (M=3.0) [Schulman-Scir (1991)] 0.1 C* (m -2) hs=0 hs=1.5m hs=4.6 m distanc from stack (x/l) Figur 2.1 Effct of stack hight on normalizd concntration [from Shulman-Scir (1991)] 2-6

10 Effct of M (hs=1.5m) [Schulman-Scir (1991)] C* (m-2) M=1.5 M=3 M= distanc from stack (x/l) Figur 2.2 Effct of M (xhaust momntum) on normalizd concntration [from Shulman-Scir (1991)] 2-7

11 H Prdictd by modl M =1 M = 8 Figur 2.3 Normalizd dilutions on th downwind wall of 2H high building with a downwind mitting building of hight, H. Buildings sparatd by a gap siz, H [from Wilson t al. (1998)] 2-8

12 Chaptr 3 ASHRAE Disprsion Modls In th prsnt study, xprimnts can b dividd into two typs: 1. th mission sourc xposd to th approaching flow (opn ftch) 2. th mission sourc in th wak of a tall building. Various modls hav bn dvlopd for stimating narfild dilution of plums mittd from rooftop stacks for opn ftch situations. Two such modls ar rcommndd in ASHRAE (1999) and ASHRAE (2003) and ar dscribd blow. Th accuracy of ths modls will b valuatd in Chaptr 5 using fild data obtaind in th prsnt study. For th cas of a tall building upwind of an mitting building, dilution stimats ar rquird for rcptors on th adjacnt building lward wall, as wll as th roof of th mitting building. To dat, an accptabl dilution modl for this cas has not bn dvlopd [.g. s Wilson t al. (1998)]. In addition to dilution modls that provid quantitativ stimats of plum disprsion, ASHRAE (2003) also provids a gomtric mthod to prdict th liklihood of a plum making contact with a critical rooftop rcptor. This mthod, which is qualitativ in natur, is dscribd blow. 3.1 Th ASHRAE Gomtric Dsign Mthod 3-1

13 ASHRAE (2003) provids a gomtric stack dsign mthod for stimating th minimum stack hight to avoid plum ntrainmnt in th flow rcirculation zons of a building and its rooftop structurs. Dimnsions of th rcirculation zons ar xprssd in trms of th scaling lngth, R, which is dfind as: R = B s 0.67 B L 0.33 (3-1) whr B s is th smallr of upwind building hight or width and B L is th largr of ths dimnsions. Th dimnsions of flow r-circulation zons that form on th building and roof-top structurs ar: H c = 0.22R (3-2) X c = 0.5R (3-3) L c = 0.9R (3-4) L r = 1.0R (3-5) whr H c is th maximum hight of th roof rcirculation zon, X c is th distanc from th lading dg to H c, L c is th lngth of th roof rcirculation zon, and L r is th lngth of th building wak zon. Not that th hight of th wak zon is qual to th hight of th structur. Figur 3.1 shows th rcirculation zons for a typical building. Th dsign mthod assums that th boundary of th high turbulnc rgion is dfind by a lin with a slop of 10:1 xtnding from th top of th lading dg sparation bubbl. Th location of th plum rlativ to th rcirculation zons is dtrmind by taking into account plum ris du to xhaust momntum and assuming a conical plum with a slop of 5:1. 3-2

14 Th ffctiv hight of th plum abov th roof or rooftop structur is: h = h s + h r h d (3-6) whr h s is stack hight, h r is plum ris and h d is th rduction in plum hight du to ntrainmnt into th stack wak during priods of strong winds. It should b notd that h s is th hight of th stack tip abov th roof minus th hight of rooftop obstacls (including thir rcirculation zons) that ar in th path of th plum. Plum ris, which is assumd to occur instantanously, is calculatd using th formula of Briggs (1984): h r = 3βd (w /U H ) (3-7) whr d is th stack diamtr, w is th xhaust vlocity, U H is th wind spd at building hight and β is th stack capping factor. Th valu of β is 1 for uncappd stacks and 0 for cappd stacks. To account for th stack downwash causd by low xit vlocitis, whn w /U H < 3.0, Wilson t al. (1998) rcommndd a stack wak downwash adjustmnt h d, which is dfind as, h d = d (3.0- βw /U H ) (3-8) For w /U H > 3.0 thr is no stack downwash (h d = 0). 3.2 Dilution Modls for an Opn Ftch A numbr of smi-mpirical modls hav bn dvlopd for prdicting minimum dilution (D min = C /C max ) of xhaust from rooftop stacks, whr C is th xhaust concntration and C max is th 3-3

15 concntration at a roof or wall rcptor on th plum cntr-lin. Th ASHRAE Applications Handbook [ASHRAE (2003)] rcommnds a Gaussian dilution modl that was dvlopd using data from watr channl xprimnts of Wilson t al. (1998). In an arlir vrsion of th Handbook [ASHRAE (1999], minimum dilution modls formulatd by Wilson and Lamb (1994) and Halitsky (1963) wr rcommndd. Of ths, th Wilson and Lamb modl provids a mor accurat lowr bound of dilution, basd on wind tunnl and fild cas studis [Ptrsn and Wilson (1989), Stathopoulos t al. (2002)]. Th Halitsky modl has bn shown to b ovrly consrvativ in most cass. In th currnt study, th accuracy of th Wilson-Lamb and ASHRAE (2003) dilution modls will b valuatd using data obtaind in fild tsts conductd with an opn ftch Th Wilson-Lamb Modl Th Wilson-Lamb modl, hraftr dsignatd as WL, is basd on a prvious dilution modl for flush vnts (h s = 0) drivd from wind tunnl data obtaind with isolatd building modls [Wilson and Chui (1985, 1987), Chui and Wilson (1988)]. In this modl, minimum dilution along th plum cntr-lin is givn by: D min = (D o D d 0.5 ) 2 (3-9) whr D o is th initial dilution at th location and D d is th distanc dilution which is producd by atmosphric and building-gnratd turbulnc. Th formulas for D o and D d rcommndd in ASHRAE (1999) ar: D o = βM (3-10) 3-4

16 D d = B 1 S 2 /MA (3-11) whr B 1 is th distanc dilution paramtr, S is th strtchd string distanc from stack to rcptor and M is th ratio of xhaust gas vlocity, w, to th man wind spd at th building hight, U H. Th paramtr, β, is th stack capping factor and is st qual to 1.0 for uncappd stacks. Th paramtr, B 1, is st at a constant valu with th magnitud dpndnt on th location of th rcptors. Dilution data obtaind in a fild study [Wilson and Lamb (1994)] and a wind tunnl study [Wilson and Chui (1987)] indicat that B 1 is strongly affctd by th lvl of atmosphric turbulnc in th approaching flow. Th ffct of upstram turbulnc on th distanc dilution paramtr is approximatd by th following formula: B 1 = σ θ (3-12) whr σ θ is th standard dviation of wind dirction fluctuations in dgrs and varis btwn 0 and 30. Th modl suggsts that distanc dilution has two componnts -- th dilution du to building-gnratd turbulnc and that du to atmosphric turbulnc. It assums that D d is significantly nhancd by atmosphric turbulnc. For an urban nvironmnt, ASHRAE (1997) rcommnds a typical valu of σ θ =15, which givs a valu of for th atmosphric componnt of th distanc dilution paramtr, B1= Thus, mor than 50% of D d is assumd to b du to upstram turbulnc Gaussian Dilution Modl (ASHRAE 2003) 3-5

17 Th Gaussian dilution modl rcommndd in ASHRAE (2003) is basd on a sris of xprimnts carrid out in a watr flum by Wilson t al. (1998). Th modl prdicts worst-cas dilution at roof-lvl, D r, assuming that th plum has a Gaussian (bll-shapd) concntration profil in both th vrtical and horizontal dirctions, as shown in Figur 3.2. It should b notd that D r is th prdictd dilution on th plum cntr-lin and thus, corrsponds to D min obtaind using th WL modl. Th roof-lvl dilution for a plum at hight, h, at a rcptor distanc, X, from th stack is givn as: 2 U σ H y σ z h D r = 4 xp 2 (3-13) w d d 2σ z whr U H is th wind spd at th building hight, d is stack diamtr, w is th xhaust spd and σ y and σ z ar th plum sprads in th horizontal and vrtical dirctions, rspctivly. Th hight of plum abov th roof or rooftop structur, h, is dtrmind using Eq. 6. Th quations for σ y and σ z ar th quations usd in th ISCST disprsion modl, which was dvlopd by th U.S. EPA [EPA (1995)], adjustd from a 60 minut avraging tim to a 2 minut avraging tim using th 0.2 powr law applid to both vrtical and crosswind sprads. Th plum sprad formulas ar basd on watr channl data of Wilson t al. (1998), which ar assumd to hav a full-scal quivalnt avraging tim of 2 minuts. Th cross-wind and vrtical sprads ar givn by th quations, 3-6

18 σ d y t X d avg 0.2 o = 0.071( ) + (3-14) 2.0 σ d σ d z X d o = (3-15) σ d whr t avg is th concntration avraging tim in minuts, and σ o is th initial sourc siz that accounts for stack diamtr and for dilution du to jt ntrainmnt during plum ris. Th formula for σ o /d is: σ d o 2 w w.5 [ 0.125β β ] = U H U (3-16) H whr β is th rain cap factor, also dscribd prviously: β=1 for no rain cap and 0 if th rain cap is prsnt. Th Gaussian dilution modl (Eq. 13) should not b usd whn th plum hight, h, is lss than th maximum hight of th roof rcirculation zons that ar in th path of th plum. This critical hight is rfrrd to as h top and is shown in Figur 3.3. For cass in which th plum hight is gratr than h top but lss than th hight rquird to scap all critical rcirculation zons (h valid in Figur 3.3), th physical stack hight should b st at 0 whn calculating h [ASHRAE (2003)]. 3-7

19 Figur 3.1 Dsign procdur for rquird stack hight to avoid contamination [from Wilson (1979)] 3-8

20 Figur 3.3 Flow rcirculation rgions and xhaust-to-intak strtchd-string distancs [from ASHRAE (2003)] 3-9

21 Figur 3.2 Coordinat systm showing Gaussian distributions in th horizontal and vrtical dirction [from Turnr (1994)] 3-10