Impact of Open Windows on Room Air Flow and Thermal Comfort

Transcription

1 Internatinal Jrnal f Ventilatin Vlme 1 N Impact f Open Windws n Rm ir Flw and Thermal Cmfrt Per Heiselberg, Erik Bjørn, Peter V. Nielsen Hybrid Ventilatin Centre, albrg University Shngaardshlmsvej 57, DK-9 albrg, Denmark bstract In the design f natral ventilatin systems, there is a wide range f pssibilities with regard t the selectin f windw type and the psitining f windws in the facade. Each windw type has niqe characteristics, which affect air flw and thermal cmfrt cnditins in the ccpied zne. cmbinatin f varis windw types and façade lcatins in a rm shld be capable f imprving thermal cmfrt and minimising draght risk. This paper describes the reslts f labratry investigatins in a mck-p f an ffice space with the prpse f investigating the impact f different pening strategies n thermal cmfrt cnditins in the ccpied zne. The reslts shw that different windw pening strategies reslt in qite different air flw and thermal cmfrt cnditins. The cnditins are a reslt f a mltivariable impact, and ths detailed descriptins f the flws invlved are qite cmplex. Key wrds: windws, natral ventilatin, air distribtin, air jet, thermal cmfrt 1. Intrdctin With natral ventilatin, fresh air is spplied thrgh penings in the bilding envelpe. These penings are ften windws, bt ther types are als sed, sch as trickle vents r lvers. The driving frces cnsist f the pressre differences develped by wind and thermal byancy. Openings are ften placed either clse t the ceiling and/r clse t the ccpied zne. Ultimately, cmfrt cnditins will depend n the flw characteristics and lcatin f ventilatin penings. In the case f single sided ventilatin, tw-way air flw exists thrgh the pening. This is driven either by thermal byancy, trblent wind flctatins r a cmbinatin f bth. De t the relatively lw pressre differences and (dring winter) lw tdr temperatres, the incming air falls twards the flr and stratified flw alng the flr is established. This characteristic has been shwn bth experimentally (Heiselberg et al (1), Nielsen et al (), Eftekhari (1995) and Zeidler and Fitzer (1998)) and by nmerical predictins (Gan ()). In the case f crss ventilatin, ne-way air flw exists thrgh the pening. This is typically driven by thermal byancy, in a mlti-strey r passive stack bilding, r by wind pressre, in a single flr space. In practice, cmbinatins f natral ventilatin mechanisms are ften sed and natral frces can als be fan-assisted. De t the larger pressre differences f crss flw ventilatin, the mmentm f inlet air is larger and jet flws are created in the rm, see Heiselberg et al (1). This paper describes the reslts f crss flw ventilatin measrements thrgh windw penings lcated clse t ceiling level. The verall aim f this research is t establish a scientific basis n which t develp intelligent strategies fr the cntrl f windw penings t ptimise thermal cmfrt in the ccpied zne. The reslts f a series f fll-scale experiments, perfrmed in an ffice mck-p (with real windws) based n a real-life test case, are described. The experimental gals are: T bserve and characterise the air flw patterns created in the space; T btain characteristic vales fr flw frm windw penings, which allw the flw t be calclated by existing semi-empirical air flw mdels; T bserve differences between different pening strategies, fr instance pening windws in different psitins r in different nmbers t ptimise thermal cmfrt; T estimate thermal cmfrt cnditins in the ccpied zne nder different cnditins and with different pening strategies. 91

2 P. Heiselberg, E. Bjørn, P. V. Nielsen. Labratry Setp W Operatin f ne f the inner windws (leaving the remaining windws clsed); The experiments were cndcted in a labratry test Operatin f all fr windws tgether. rm. This test rm was partitined with plywd walls, see Figre 1, t create a mck-p f an The cnfigratin represents a crner windw ffice rm with dimensins width = 4.9 m by where ne side is blinded by the adjacent walls. length = 4.9 m by height = 3.8 m and an inslated On the ther hand the W cnfigratin has cld rm. The cld rm can be cled t adjacent windws n bth sides and hence cld abt C t simlate tdr cnditins, while the simlate an arbitrary windw in a lng rw. The ffice rm can be kept at nrmal rm cnfigratin cvers the fll width f the temperatre with a flr heating system. rm, see Figre 1. The degree f windw pening is defined by the pening length L pen, which is the distance between the inner side f the (mvable) windw frame, and the ter windw cnstrctin, see Figre. In each windw cnfigratin (, W and ), this crrespnds t a minimm pening area, measred as the minimm crss-sectinal area the air mst pass thrgh. The pening lengths, L pen and crrespnding gemetrical areas sed are shwn in Table 1. ir was spplied t the cld rm via a ventilatin inlet. The spplied air flw rate was measred by sing an rifice plate pressre drp accrding t the DIN 195 Standard. Care was taken t ensre that this inlet did nt have a direct inflence n the air flw thrgh the windws. n exhast was placed in the back f the ffice rm. This arrangement made it pssible t cntrl the pressre difference and the flw rate acrss the windw facade by valves in the spply and exhast dcts. Figre 1. The strctre f the fll-scale test rm. The façade mainly cnsists f a windw system divided int 16 separate sectins with a ttal width f 4 m and a ttal height f.5 m. The bttm rw f fr sectins cnsists f manally cntrlled side-hng windws. These penings were nt sed in the present stdy. The tp rw f fr sectins cnsists f bttm hng windws pening int the ffice rm, see Figre. The length f each windw is 1. m and the height is.45 m. They are lcated.4m frm the ceiling. The windws are atmatically cntrlled and can be pened either individally r all at the same time. The windw pening cnfigratin was arranged in three different ways; these being: Operatin f ne f the end r tmst crner windws (leaving the remaining windws clsed); The pressre drp (and hence air flw rate) acrss the façade was determined by pressre measrements sing pressre taps mnted beside the windws lcated 1cm belw the ceiling. Experiments were perfrmed at different pressre drps acrss the façade. Pressre differences as lw as.1 Pa were sed, and as high as 6 Pa (bt nly fr very small penings). Befre these measrements were made, the leakage characteristics f the cld rm were measred. This was necessary t crrect the measred air flw thrgh the windws (i.e. by allwing fr cld rm leakage). The leakage frm the cld rm was fnd t be 1m 3 /h at 5 Pa. Bth isthermal and nn-isthermal experiments were perfrmed. In the nn-isthermal experiments, the small (cld) rm was cntinsly ventilated by cld air at a flw rate f 15 m 3 /h t keep the cld rm temperatre stable. s a large part f this is recirclated air, the actal air flw rate thrgh 9

3 Internatinal Jrnal f Ventilatin Vlme 1 N the windw was determined frm the previsly measred pressre characteristics f the windw. Differences in air temperatre between the cld rm the ffice rm f C (isthermal), 7 C, 1 C and 14 C were sed. 3. Measrements Smke-tests f nn-isthermal flw were cndcted t estimate the penetratin length f the cld inlet jet. The penetratin length was defined as the length frm the windw t the place where the jet leaves the ceiling and flws dwn int the ccpied zne. ir velcities in the warm rm were measred with 4 channel Dantec 54N1 ht sphere anemmeters, which were calibrated fr the actal flw directins. Pressre differences were measred with a micrmanmeter (Frness Cntrls Limited mdel FC51). Velcity prfiles were measred belw the ceiling with ht sphere anemmeters in rder t characterise the inlet flw thrgh the windw penings. In cases and W, velcity prfiles were als measred at an angle frm the windws in rder t characterise the inlet jets frm the trianglar side penings. Measrements f the velcity in the dwnward air flw nder nn-isthermal cnditins were carried t with ht sphere anemmeters at 1.8 m abve the flr, which was defined as the entry f the ccpied zne. Fr these measrements, 1 anemmeters were placed n a rack with a mtal distance f.5 m. The rack was mved abt between measrements t btain mre measring pints, in rder t catch the maximm velcities. 4. Reslts and Discssin 4.1 ir Flw Pattern in the Office Rm Smke tests shwed that, in the case f crss ventilatin with fresh air entering thrgh the bttm-hng windws placed jst belw the ceiling, there are basically 5 pssible air flw patterns in the rm, see Figre 3. ir flw pattern () nly ccrs at very lw pressre drps (.1.5 Pa) and lw tside temperatre, where the flw is lw-trblent. This is a s-called creeping flw. s trblence is intrdced with increasing pressre difference and flw rate, a recirclatin bbble is created belw the windw (air flw pattern (B)). This is very similar in natre t the flw described in Heiselberg et al. (1995), where it is shwn that the size f the recirclatin zne is a fnctin f the rchimedes Nmber. It is cnceivable that an bstacle, fr instance the windwsill r a table, will distrb the flw and pssibly create lcal discmfrt. In the Figre. Pictre f bttm-hng windw. Smke visalisatin f the inlet jet, shwn fr pening areas f =.9 m (tp) and.35 m (bttm) fr cnfigratin W. Principle fr measrement f the gemetrical pening area. Table 1: Opening lengths and gemetrical areas in experiments L pen (mm) win (m ) W

4 P. Heiselberg, E. Bjørn, P. V. Nielsen cases f air flw patterns () and (B), cld, fresh air flws alng the wall, hits the flr, and mves hrizntally acrss the rm in a stratified, displacement-like flw, see Nielsen (1994), Heiselberg (1994) and Gan (). The characteristics f the stratified flw will determine thermal cmfrt and the risk f draght. Stråletype [-] E5 D4 C3 B =,71 m =,89 m =,179 m =,3 m =,4 m 1 Figre 3. Pssible air flw patterns in rm.,,1,,3,4,5,6,7 r [-] t higher pressre differences ( p > 4-6 Pa) and/r higher tdr temperatres ( T < 5 K), the flw pattern frm the windw establishes itself as a free jet (air flw pattern (C)), and flws int the rm fllwing a dwnward trajectry, see Kestel (1955) and Etheridge and Sandberg (1996). With frther increases in air flw rate, the free jet attaches t the ceiling de t the Canda effect (air flw pattern (D)). The penetratin depth is f interest, as is the maximm velcity at the entering pint f the ccpied zne. gain, this is a well-described phenmenn, see Nielsen et al (1987), bt the specific characteristics fr these windws are nt knwn. t high tside temperatres ( T < K) and/r high pressre differences and air flw rates ( p > -3 pa) the jet attaches t the ceiling ver its entire length, fllws the back wall, and enters the ccpied zne alng the flr (air flw pattern (E)). ls in this case, it is f interest t knw the characteristics f the jet t predict thermal cmfrt and draght risk. Figre 4 shws the type f air flw pattern verss rchimedes Nmber. The rchimedes Nmber is defined by: r β g h = T where β is the thermal expansin cefficient [/K], g is the gravitatinal acceleratin [m/s ], h =C d /L is the effective pening height f the windw [m], T is the temperatre difference between inside and tside [ C], = Q/C d =( p/ρ).5 is the calclated Figre 4. Type f air flw pattern verss rchimedes Nmber. inlet velcity [m/s] and L is the ttal length f the fr windws. The air flw pattern depends n the rchimedes Nmber f the flw, bt als n the gemetrical area f the windw pening (and pssibly als ther parameters). The air flw frm a small pening drps int the ccpied zne in sitatins with smaller rchimedes Nmbers than the air flw frm large penings. Prbably the lcal gemetrical cnditins als have an impact, see Figre. This is becase the air flw frm the windw is hrizntal and des nt reach the ceiling fr small pening areas, while fr large pening areas the air flw is directed pwards and therefre mre readily attaches t the ceiling. 4. ir Flw Capacity - Discharge Cefficient, C d The air flw rate thrgh a windw pening can be determined by: Q = v c c () (1) where c is the minimm crss sectin area f the flw thrgh the pening [m ] and v c is the air velcity f the flw thrgh this area [m/s]. The tw qantities can be related t knwn nes by: c = Cc (3) v = C v (4) 94 c v the

5 Internatinal Jrnal f Ventilatin Vlme 1 N where C c is a cntractin cefficient, the the wall. Figre illstrates hw the gemetrical gemetrical pening area [m ], C v is a velcity pening area is estimated. cefficient depending n frictin in the pening and v the is the theretically btainable velcity when n The vlmetric air flw rate is determined frm the frictin is taken int cnsideratin [m/s]. v the can be inlet spply rate and the pressre drp acrss the determined by: façade, after crrecting fr exfiltratin (i.e. leakage p vthe = frm the cld rm). The exfiltratin was abt -15% (5) f the vlme flw rate thrgh the windw with the ρ highest vales fr small pening areas. The abslte vale f the discharge cefficient is therefre ncertain where p is the pressre difference acrss the especially at small pening angles. pening [Pa] and ρ ο is the density f incming air [kg/m 3 ]. Eqatin () can be rewritten as: Figre 5 shws the discharge cefficient as a fnctin f pening area fr the three different windw pening p Q = Cd (6) cnfigratins. The reslts are fr 5-7 different degrees ρ f pening, and fr tw temperatre differences (i. e. C (isthermal) and 1 C). It can be seen that the where C d is the discharge cefficient defined as the discharge cefficient is nt a cnstant bt is dependent prdct f the cntractin (C c ) and the velcity (C v ) n windw cnfigratin, pening area and cefficients. The discharge cefficient is a characteristic temperatre difference. Fr windw cnfigratins parameter fr a specific windw and takes bth the and W, the vale f the discharge cefficient cntractin and the frictin lss in the windw pening decreases fr increasing pening area. Hwever the int accnt. ndersen (1996) cntains a mre vale is rather cnstant and clse t the vale f C thrgh discssin f inlet cefficients. d =.7, which is ften sed. Fr windw cnfigratin, the vale f the discharge cefficient varies Based n measrements f the gemetrical pening mch mre and increases fr increasing pening area area, the vlme flw rate thrgh the pening and appraching a vale clse t 1. The vales fr reverse the pressre difference acrss the pening, the flw fr windw cnfigratin shw the same discharge cefficient is calclated frm trend, bt are lwer, and apprach the vale f.8 fr Eqatin (6). The gemetrical pening area is fnd large pening areas. Fr this windw type, the fr the narrwest passage f the flw, which is very temperatre difference has a minr inflence. difficlt t estimate becase f the cmplicated gemetry f the frames, see Figre. It is especially The reslts shw that, besides the pening area, the difficlt at small pening angles where the lcal gemetrical and air flw cnditins have a ncertainty is high, becase leakages alng all ther large impact. In windw cnfigratins and W sides f the pening accnt fr a relative large part the air flws in at bth the tp and the sides f the f the pening area. The windws are bilt int a pening, see Figre. In windw cnfigratin wall with a thickness f.4 m and in, sme cases,, the distance between the individal penings the narrwest passage is nt within the windw is nly mm, and mst f the air flws in at the pening itself, bt between the windw pening and 1, 1, 1, 1,,8,8 Cd [-],6,4,,, inflw, inflw, 1C, tflw,,1,,3,4,5 real [m] Cd [-],6,4,,, 1C W W, 1C,,1,,3,4,5 real [m] Figre 5. Discharge cefficient C d as a fnctin f pening area and temperatre difference fr the three different windw cnfigratins, W, and 95

6 P. Heiselberg, E. Bjørn, P. V. Nielsen tp f the pening. Therefre, fr the inflw f air in case, the pening frms a kind f fnnel reslting in high C d vales. Fr tflw, the air is nt gided thrgh the pening and lwer C d vales are seen, as fr and W. 4.3 Isthermal ir Flw frm Bttm Hng Windws Under isthermal cnditins, air flw pattern E was bserved in all experiments. Fr small pening areas, the jets were initially hrizntal, and attached t the ceiling de t the Canda effect. Fr large penings, the air was directed twards the ceiling, see Figre. In windw cnfigratins and W, the flw frmed a three-dimensinal wall jet alng the ceiling. It is assmed that the maximm velcity in the jet can be described by (Rajaratnam 1976): x = K a (7) x + x where x is the maximm velcity at the distance x frm the inlet [m/s], = Q/C d =( p/ρ).5 is the calclated inlet velcity [m/s], =C d is the effective pening area f the windw [m ], K a is a cnstant related t the pening, x is a virtal rigin f the jet [m], p is the pressre difference between the rms [Pa], ρ is the density f the incming air [kg/m 3 ] and Q is the air flw rate thrgh the windw [m 3 /h]. The definitin f implies that the pressre lss thrgh the pening is zer and therefre the velcity cefficient is assmed t be 1.. This is nt the case. Hwever, fr a sharp edged pening, the cntractin cefficient will be in the range f.6.65 while the velcity cefficient will be in the range f.94 t.98, see ndersen (1996). The inflence f the frictin lss is therefre small cmpared t the cntractin. The same relatin between cntractin and pressre lss is nt necessarily tre fr the windw investigated. Hwever, since the discharge cefficient in mst cases is between.7 1., the pressre lss mst be lw and the inflence f the assmptin n the vales f the inlet velcity and effective pening area is limited. In windw cnfigratin, the flw frmed a tw-dimensinal wall jet alng the ceiling. It is assmed that the maximm velcity in the jet can be described by (Rajaratnam 1976): x h = K p (8) x + x where h =C d /L is the effective pening height f the windw [m], K p is a cnstant related t the pening, L is the length f the pening. The measrements fr windw cnfigratins and W shwed the characteristic behavir f a 3 dimensinal wall jet. Velcity prfiles f the wall jet were measred at eight different distances frm the inlet. The characteristic cnstants f the jet were determined graphically. The distance t the virtal rigin (x ) and the K a vale are shwn in Table and Figre 6, respectively. The measrements with all fr windws pened, windw cnfigratin, shwed the characteristic behavir f a dimensinal wall jet, see Eqatin (8). Velcity prfile measrements were carried t fr different pening areas. In Table, the area f the pening and the matching height are listed, as well as the fnd distance t the virtal rigin. The estimated K p vales frm the measrements are shwn in Figre 6. Kp, Ka [-] 7, 6, 5, 4, 3,, 1,, W 1-4 W 1 W,1,,3,4,5 rea [m ] Figre 6. Characteristic jet flw cnstants K p and K a as a fnctin f pening area fr the three different windw cnfigratins, W, and The jet characteristics depend n the pening area. The velcities in the ccpied zne will, therefre, nt nly depend n the air flw rate bt als n the necessary pening area, which depends n the available pressre difference. 96

7 Internatinal Jrnal f Ventilatin Vlme 1 N,5,5,5,45 Pressre Difference 1 Pa,45 Pressre Difference Pa,45 Pressre Difference 5 Pa Maximm Velcity (m/s),4,35,3,5,,15,1,5 W Maximm Velcity (m/s),4,35,3,5,,15,1,5 W Maximm Velcity (m/s),4,35,3,5,,15,1,5 W,,,1,,3,4,5 ir Flw Rate (m3/s),,,1,,3,4,5 ir Flw Rate (m3/s),,,1,,3,4,5 ir Flw Rate (m3/s) Figre 7. Predicted maximm velcities in the ccpied zne fr windw cnfigratins, W and as a fnctin f air flw rate and pressre difference The maximm velcity in the ccpied zne, rm, is in the reverse flw clse t the flr at a distance f /3L frm the windw pening. rm can be estimated as 7% f the reference velcity L, which is the velcity in an ndistrbed wall jet f length L frm the windw pening, see Hestad (1975). The predicted maximm velcities in the ccpied zne are shwn in Figre 7 as a fnctin f air flw rate fr three different pressre differences. It is seen that windw cnfigratin reslts in the lwest air velcities and the highest level f thermal cmfrt. If the rm is ccpied by -3 persns, the necessary air flw rate fr prviding acceptable indr air qality will be less than.5 m 3 /s. This can be achieved witht draght prblems with any windw cnfigratin. Predicted Velcity (m/s),6,5,4,3,,1,,,1,,3,4,5,6 Measred Velcity (m/s) Figre 8. Cmparisn f predicted and measred maximm velcities in the ccpied zne fr windw cnfigratin In sitatins needing passive cling, in which there are small temperatre differences between tside and the rm, mch higher air flw rates can be prvided by windw cnfigratin witht risking draght prblems. Figre 8 shws a cmparisn between predicted and measred maximm velcities in the ccpied zne fr windw cnfigratin fr three different cmbinatins f windw pening and pressre difference. It shws a reasnable crrespndence between the predictins by the develped semiempirical mdel and measrements. 4.4 Thermal ir Flw frm Bttm Hng Windws Under nn-isthermal cnditins, the inlet flw frm a bttm hng windw, cnfigratins and W, can frm a three-dimensinal thermal wall jet alng the ceiling, air flw pattern (D) in Figre 3. The penetratin depth f the cld wall jet was measred fr different effective pening areas, a nmber f different pressre differences and fr three different temperatre differences. Dring each experiment, the pressre difference between the rms was recrded as well as the air temperatre in a nmber f psitins in each rm. The penetratin depth was determined by adding smke t the air tside the windw, see Figre 9. Each smke experiment was videtaped fr dcmentatin. s described by Grimitlin (197) and Nielsen et al. (1987), the penetratin depth is expected t fllw Eqatin (9): x s + x y = K a K sa T (9) where x s is the penetratin depth [m], K sa is a cnstant related t the type f rm and heat srces and T is the temperatre difference between inside and tside [ C]. 97

8 P. Heiselberg, E. Bjørn, P. V. Nielsen T 5, 4,5 4, 3,5 3,,5, 1,5 1,,5, win win =.9 m win =.9 m win =.173 m 1 3 x + x win T 7, 6, 5, 4, 3,, 1,, win win =.9 m win =.77 m win =.119 m win =.17 m win =.35 m W 1 3 x + x win Figre 9. T win verss the relative penetratin length fr windw cnfigratins and W. Table : Distance t the virtal rigin, X, K a and K p vale fr windw cnfigratins, W and verss pening area. Cnfigratin Cnfigratin W Cnfigratin win [m ] x [m] K a K a K sa win [m ] x [m] K a K a K sa win [m ] H [m] x [m] K p K p K sp Under nn-isthermal cnditins, the inlet flw frm a bttm hng windw, cnfigratin, can frm a tw-dimensinal thermal wall jet alng the ceiling. The penetratin length can be described by the fllwing eqatin, see Nielsen and Möller (1987): x s + x = K sp K p T h h 3 (1) where K sp is a cefficient dependent n rm dimensins, lcatins f thermal lads etc. The pint f separatin r the penetratin length ccrs when the frces f byancy becme mre inflential than the frces f inertia. The smke tests shwed a highly nsteady flw with variatins in the pint f separatin f p t 1 m dring the mintes bservatin perid. In Figres 9 and 1, the vale f characteristic cefficients is the reciprcal f the slpe f the linear regressin line that has been added. Fr windw cnfigratins and W there is a reasnable crrelatin, while it is mre nclear fr cnfigratin. It is seen that the cefficients, K a K sa and K p K sp, depend n the windw cnfigratin and the pening area. The characteristic cefficients f the thermal wall jets fr windw cnfigratins, W and are shwn in Table. 98

9 Internatinal Jrnal f Ventilatin Vlme 1 N T h w in =,71 m w in =,89 m T h w in =,3 m w in =,4 m x + x h x + x h Figre 1. ( / Th ) /3 verss the relative penetratin length fr windw cnfigratin Predicted max. velcity (m/s),7,6,5,4,3,,1, T = 7 P =, Pa C W,,1,,3,4,5 ir Flw Rate (m3/s) Predicted max. velcity (m/s),7,6,5,4,3,,1, T = 14 C P = 4,9 Pa W,,1,,3,4,5 ir Flw Rate (m3/s) Figre 11. Predicted maximm velcities in the ccpied zne fr windw cnfigratins, W and as a fnctin f air flw rate fr tw cmbinatins f temperatre and pressre difference. The maximm velcity in the ccpied zne, rm, will ccr when the wall jet separates frm the ceiling and flws int the ccpied zne. The maximm velcity is calclated as the velcity in an ndistrbed wall jet f the length f (L+(H-1.8m)) frm the windw pening. The predicted maximm velcities in the ccpied zne are shwn in Figre 11 as a fnctin f air flw rate fr tw cmbinatins f temperatre and pressre difference. It is seen that all three windw cnfigratins reslt in the same air velcities in the ccpied zne and that here is a high risk f draght. ls there is a risk f draght even if the air flw rate fr prviding acceptable indr air qality is less than.5 m 3 /s. Figre 1 shws a cmparisn between predicted and measred maximm velcities in the ccpied zne fr windw cnfigratins and fr pening areas and different cmbinatins f temperatre and pressre difference. It shws a reasnable crrespndence between the predictins by the develped semi-empirical mdel and measrements, despite the very nsteady air flw cnditins. Predicted Velcity (m/s),7,6,5,4,3,,1, w in =,9 m, w in =,9 m, w in =,173 m, w in =,89 m, w in =,4 m,1,,3,4,5,6,7 Measred Velcity (m/s) Figre 1. Cmparisn f predicted and measred maximm velcities in the ccpied zne fr windw cnfigratins and. 99

10 P. Heiselberg, E. Bjørn, P. V. Nielsen 5. Cnclsins The air flw in a natrally ventilated rm with bttm hng windws psitined clse t the ceiling will assme ne f five main air flw patterns. These patterns will be dependent n pening area, temperatre and pressre difference. T describe the air flw capacity f an pening, the discharge cefficient, C d, is ften sed. This cefficient is nt cnstant and the reslt shws that, in additin t the pening type and area, the lcal gemetrical and air flw cnditins have a large impact. Therefre, the discharge cefficient is nt very sefl as a characteristic cefficient since it incldes the inflence f several very different effects cvering gemetry, flw cntractin, pressre lss, lcal air cnditins, temperatre effects, etc. It is necessary t try t separate these effects in rder t imprve the estimatin f air flw thrgh prpse prvided penings in the bilding envelpe. The measred air flw characteristics shwed that tw f the air flw patterns established cld be described by traditinal flw element thery. Hwever, the characteristic cefficients f the pening were nt cnstant bt depended n pening area, the lcal gemetrical cnditins and the lcal air flw cnditins. This makes the se f a semiempirical mdel t predict air flw and cmfrt cnditins mch mre cmplicated and the amnt f measred data needed t develp the mdel very large. The reslts shwed that, fr isthermal (small temperatre differences) flw cnditins, the risk f draght was very lw in the ccpied zne even at high air flw rates. Natral ventilatin is therefre very sefl fr passive cling when tdr temperatres are clse t the interir temperatres. t high temperatre differences, air spply reslted in a high risk f draght in mst cases. References ndersen KT. (1996) Inlet and tlet cefficients, Prceedings f ROOMVENT '96, 5th Int. Cnf. n ir Distribtin in Rms, Ykhama, Japan. Eftekhari MM. (1995) Single-sided natral ventilatin measrements, Bilding Serv. Eng. Res. Technl., 16, N. 4, pp 1-5. Etheridge D and Sandberg M. (1996) Bilding ventilatin: thery and measrement, Jhn Wiley and Sns. Gan G. () Effective depth f fresh air distribtin in rms with single-sided natral ventilatin, Energy and Bildings, 31, pp Grimtlin M. (197) Zlftverteilng in Rämen. Lft- nd Kältetechnik, Nr. 5. Heiselberg P. (1994) Stratified flw in rms with a cld vertical wall, SHRE Transactins, 1, part 1, pp Heiselberg P, Overby H and Bjørn E. (1995) Energy efficient measres t avid dwndraft frm large glazed facades, SHRE Transactins, 11, part, Jne Heiselberg P, Svidt K and Nielsen PV. (1) Characteristics f air flw frm pen windws, Bilding and Envirnment, 36, pp Hestad T. (1975) design prcedre fr air terminal devices based n thery, fll-scale experiments and experience, Tekniska Meddelanden nr. 83, Inst. För Uppvärmnings- ch Ventilatinsteknik, KTH, Stckhlm, Sweden. Kestel. (1955) Paths f hrizntally prjected heated and chilled air jets, HVE Transactins. Nielsen PV and Möller ÅT. (1987) Measrements n byant wall jet flws in air cnditined rms, Prceedings f "ROOMVENT 1987" Internatinal Cnference n ir Distribtin in Ventilated Spaces. Nielsen PV. (1994) Stratified flw in a rm with displacement ventilatin and wall-mnted air terminal devices, SHRE Transactins, 1 Part 1, pp Nielsen PV, Dam H, Sørensen LC, Svidt K, and Heiselberg, P. () Characteristics f byant flw frm pen windws in natrally ventilated rms Prceedings f the 7 th Internatinal Cnference n ir Distribtin in Rms, Reading, UK, 9-1 Jly,, Elsevier Science Ltd pp Rajaratnam N. (1976) Trblent jets, Elsevier, msterdam. Zeidler O and Fitzner K. (1998) Investigatin f the impact f natral ventilatin thrgh windws n thermal cmfrt, Prceedings f ROOMVENT 98, Stckhlm, Sweden, Jne 14-17,, pp