WIND TUNNEL EXPERIMENT ON THE EFFECT OF WIND ON SMOKE EXHAUST SYSTEMS FOR A HIGH RISE BUILDING

Transcription

1 WIND TUNNEL EXPERIMENT ON THE EFFECT OF WIND ON SMOKE EXHAUST SYSTEMS FOR A HIGH RISE BUILDING K.C. Chug Mechaical Egieerig, Natioal Yuli Uiversity of Sciece ad Techology Touliu City, Yuli 4, Taiwa ABSTRACT Natural or mechaical smoke exhaust systems are supported to istall i tall buildigs for removig hot smoke from fire i Taiwa s buildig code. Normally, the mechaical smoke exhaust volumes are desiged accordig to floor area or give a assiged value of 1 m /mi. A opeig area of m or m is commoly used for atural smoke exhaust vet i Taiwa s buildig code. However, the wid effects o tall buildigs are ot icluded i Taiwa s buildig code or fire safety regulatios. The smoke exhaust systems are strogly iflueced by wid effects, especially, for tall buildigs. Therefore, a experimetal study usig a wid tuel was coducted to ivestigate the fire pressure ad wid iduced pressure aroud tall buildigs. Improvemets of the desig of the smoke exhaust systems of tall buildigs were submitted accordig to the test results. A height-to-wide ratio 4:1 model buildig was adapted to measure wid pressure i a wid tuel with a 4 m x. m cross sectio area. Three importat smoke exhaust system desig parameters, buildig height, smoke temperature ad wid agles are adopted to aalyze the wid tuel test results. The study will preset useful ad practical desig limitatios of atural/mechaical smoke exhaust system for differet buildig heights. KEYWORDS: Smoke exhaust system, Smoke vet, Tall buildig, Wid tuel INTRODUCTION The desig of smoke exhaust systems for tall buildigs is simple i cocept but complex i practice because of the may ucertaities which ifluece the values that eed to be decided for the desig parameters ivolved. Smoke flow i buildigs is due to small differeces i air pressure caused by the preseces of the fire, the larger pressure differeces caused by mechaical smoke exhaust fas, ad those caused by the atural wid pressure. It is importat for smoke exhaust systems to maitai a pressure differece with the eviromet withi limits, so that the forces may drive hot smoke to act freely or egate it. The limits of smoke mometum eed careful defiitio; failure of the system ca be obtaied as beyod these limits, although failure may ot be complete i some circumstaces. Butcher et al. 1 measured the maximum pressure differece caused by eclosure fire at the top of a door is almost Pa. The pressure o the fire side of a door will be iflueced by the leakage of the compartmets surroudig the fire space. However, most buildigs are sufficietly leaky ot to geerate a greater pressure for the fire. For a tall buildig, the smoke exhausted through a vet or staircase may have a strog buoyat force caused by the fire that will exhaust hot smoke outside aturally. The atrium may be treated as a tall buildig i some aspects. I a tall atrium fire, the mass etraimet rate ito the risig plume icreased rapidly with the height of rise of the plume, a higher smoke layer height would result i a higher smoke exhaust rate. Some practical limitatios to the use of vet through the atrium are submitted by Morga. There is maximum mass flow rate of 1 to kg/s ad/or miimum smoke layer temperature of o C above the vet. These limitatios ca be applicable to either a static vetilatio system or a mechaical vetilatio system. Based o the estimatio of the mass etraimet rate of the balcoy spill plumes, they suggested that oe or aother limit is usually reached whe the height of rise above the fire room opeig exceeds 8 to 1 m. Whe the tall buildig or atrium has a certai height the wid pressure may have positive or egative effects o the smoke exhaust systems. Copyright Iteratioal Associatio for Fire Safety Sciece

2 The wid pressure was recogized as a importat desig parameter for a successful atural/mechaical smoke exhaust system for a tall buildig. Wid actig o the surface of a simple rectagular buildig will geerate a positive pressure o the side ormal to the flow ad egative pressures o the other three sides ad the top surface. For example, a icidet wid speed of 18 m/s will give a omial stagatio pressure of Pa. The actual pressure will deped o the distributio of pressure coefficiets over the surfaces of a tall buildig. The variability of the effects of wid pressure o fire disasters ca be foud o some reports 4,. I some cases, staircases may be full of hot smoke ad ot available for occupat s evacuatio whe the wid pressure is greater tha the hot smoke buoyacy force i the staircase. It seems that the pricipal effect of wid o the buildig is to geerate horizotal movemet of air. The pressure due to wid effect is much larger tha the pressure differece due to the volume output of a fire ad the buoyat force betwee smoke ad ambiet air. Therefore, the wid effects shall be cosidered i desigig the smoke exhaust system for tall buildigs. WIND PRESSURE ON BUILDING The speed of the wid passig over a poit o the groud will vary with height because of the surface drag provided by the surface of the groud dowwid of the poit. The variatio with height is expressed as a power law whose expoet is selected with respect to the ature of the terrai 7. The equatio may be expressed as: V w = a [1] V m kz where V w is the wid speed (m/s), V m is the meteorological wid speed at 1 m (m/s), κ is the coefficiet relatig to wid speed to height, z is the buildig height (m), ad a is the expoet relatig wid speed to height. For precisio predicated the wid speed at local area, actual wid speed of Taiwa city from weather bureau are used to obtai the κ ad a values from regressio. Oce the profile of the wid at a locatio has bee established, this ca be modeled i wid tuel ad the the flow directios over, ad pressures developed o, the surfaces of scale models ca be observed ad measured. EXPERIMENTAL FACILITIES Usig wid tuels to measure wid pressure o model buildigs ad mass-trasport were ivestigated sice 19. Wid tuel studies at the Natioal Physical Laboratory ad at Colorado State Uiversity revealed that physical modelig of wid effects requires a properly simulated boudary-layer flow. This fidig was reiforced by compariso of mea pressure measuremets at the Wid Laboratory, Techical Uiversity of Demark, o a 1: scale model i a wid tuel with field measuremets o the full-scale buildig 8. A closed-circuit low-speed wid tuel was used to measure the wid pressure coefficiet o a model buildig i this examiatio. Wid Tuel Facility The tests were coducted i a boudary-layer retur type wid tuel whose cross-sectio is m high x 4 m wide, as show i Fig. 1. The airflow velocity is adjustable i a rage - m/s. Wid is simulated usig calibrated roughess o the tuel floor for scales ragig from 1/ to 1/. A axial fa with a diameter of 1. m ad 1 adjustable blades is drive by a 1 kw electric motor mouted i the fa acelle. A maximum free stream wid speed of 1 m/s ca be obtaied at the etrace of the test sectio. A rotatio pa is assembled o the floor of wid tuel to simulate differet wid directio of the object. A 4:1 aspect ratio acrylic resi buildig model was placed o

3 the rotatio pa. At first, the buildig scale model was set to perpedicular to the wid directio ad the pa was rotated at agles of,,, 9, 1, ad 1 respectively. The wid speed was measured with combied vae/temperature probes of 1 mm diameter. These probes are appropriate to measure wid speeds up to m/s, yet the ideal wid speed rages from 4 to 4 m/s. All probes were calibrated ad give, cosiderig their accuracy which is. m/s, the same value. The Reolds umber is approximated as x 1 costatly durig the test processig. No. No. 1. Scree 4. Adjustable ceilig. Hoeycomb. Scree. Rotatable pa. Fa 1 Test Sectio 4 FIGURE 1. Schematic of the experimetal wid tuel Istrumetatio Experimetal results of the wid tuel tests are the pressure coefficiet o the model buildig at differet heights ad wid agles. The wid pressure coefficiets were measured by a electric pressure scaer which may sca samples per chael per secod. A total of eight levels were desiged for recordig the wid pressure coefficiet at six poits of each level. The dimesios of the buildig model are 1.8 m x.4 m x.7 m ad have 1 levels. Each level drills three holes ad the diameter of each hole is 1. mm. A total of 1 pressure holes are preset at each level. Durig the experimets, readigs from the electroic pressure scaer were cotiuously sampled at 1 Hz for at least miutes. Also, a costat temperature aemometer was coupled with a hot wire probe to measure the wid speed. Therefore, the performace of the smoke exhaust system due to wid pressure coefficiet at differet buildig heights ca be easily evaluated. RESULTS AND DISCUSSION The complete wid pressure coefficiets were recorded durig the tests. Table 1 lists the values for o. 1 to o. at lowest level ad o. to o. 4 at highest level aroud the model buildig. The higher level of model buildig the greater positive/egative pressures may be obtaied aroud the model buildig. The greatest wid pressure was foud at the widward surface where the wid agle is zero. Negative pressures were measured at both left ad right had sides of model buildig. The largest egative pressure was recorded at the top level of the ormal surface. The actual pressure will deped o the distributio of pressure coefficiets over the widward side reduce laterally ad above the poit of maximum stagatio pressure but will reduce less below this poit because of the dowward flow of air.

4 TABLE 1. Wid tuel test pressure coefficiet at zero degree wid agle Normal surface Back surface Left surface Right surface Number o model buildig surface No.1 No. No. No. No. No Smoke Exhaust System for Residetial Area The existig Taiwa s buildig code has two categories about smoke exhaust system for tall buildigs. Oe is for residetial areas, ad aother is for elevator lobby areas. Either atural or mechaical smoke exhaust systems are available for tall buildigs i Taiwa. I buildig code asked the mechaical smoke exhaust volumes must be 1 m /mi ad/or larger tha per meter square floor area for oe cube meter smoke exhaust volume. Therefore we ca examie the performace of smoke exhaust system usig this critical value. Neglectig the cotractio coefficiet of the opeig the pressure differece may be calculated as the followig equatio; m Δ P = [] ρa where m is the smoke exhaust mass (kg/s), ρis the smoke desity (kg/m ), ad A is the smoke vet size (m ). Whe the area of smoke exhaust vet was.4 m ad.48 m the pressure differece will be.4 Pa ad 1.41 Pa, respectively. Comparig the values with the pressure differece by wid tuel, a useful evaluated criterio may be obtaied from Fig.. Smaller smoke exhaust vet offers greater smoke exhausted mometum, thus, mechaical smoke exhaust systems ca work efficietly i higher buildig heights. However, the mechaical smoke exhaust vets are still strogly recommeded to istall o the roof because the roof has the largest egative pressure differece. The smoke exhaust effective buildig height moved from m to 1 m whe the area of smoke exhaust vet reduced from.4 m to.48 m. Vets are frequetly used for atural smoke exhaust system i residetial areas for tall buildigs. I Taiwa s buildig code the atural vet must be istalled withi 8 cm beeath the ceilig. Iside the fire compartmet the relatioship betwee hot smoke temperature ad pressure differece was reported by Tamura 9. Followig the smoke temperature a atural height may be obtaied from equatio []. Z Pi Po = [] ( ρ ρ ) g i o where Z is the atural height of smoke vet/widow, P is the pressure, ρis the desity (kg/m ), i represets the fire room, ad o represets outdoor. 4

5 Pressure Differece(Pa) m.4 m Wid Pressure Buildig Height(m) FIGURE. Variatio of pressure differece with buildig height at ormal wid directio (zero wid agle) for residetial area Whe Z is less tha the height of bottom of vet from floor (H b ), it meas that the pressure iside fire room is greater tha outside. Therefore the hot smoke may flow through the vet automatically. Coversely, for the Z larger tha the height of top of vet from floor (H t ), the hot smoke may ot able to flow through the vet due to the wid pressure. A schematic diagram was show o Fig. for a size of.8 m x. m vet istalled 8 cm beeath the ceilig. Fig. 4 demostrates the relatioship betwee hot smoke temperature ad atural height of vet with differet buildig heights. It is clear whe the smoke temperature is greater tha o C, hot smoke may flow through vets to outside at 1 m buildig height because the buoyacy of hot smoke provided larger pressure tha the buoyacy of wid. The smoke flowed outside aturally oly at less tha m the buildig height at smoke temperature less tha o C. At the begiig of fire, the hot smoke was ot able to flow outside at the buildig height greater tha m due to wid pressure. Therefore, the fire suppressio equipmets, such as spriklers were required for tall buildigs to cotrol the fire at the begiig of burig. I the regulatios, the vet size must be greater tha % of floor area of the smoke compartmet. It is appropriate for most 1 m height of buildigs to use atural smoke systems to exhaust hot smoke after the smoke temperature is higher tha o C. Smoke Exhaust System for Elevator Lobby Area Aother importat area applied for smoke exhaust system is elevator lobby/stairwell area which provides a safety evacuative route for occupats. I Taiwa s buildig code, 4 m /sec smoke exhaust volume is required for mechaical smoke exhaust system at lobby area. The compariso results are illustrated i Fig.. For size of.8 m ad 1. m vets the critical buildig height are m ad 11 m respectively for mechaical smoke exhaust system workig effectively uder the zero degree wid directio. The higher buildig height reduced the smoke exhaust volume through smoke exhaust vet. Therefore, istallig the smoke exhaust vet o the roof is the most effective way for mechaical smoke exhaust systems for tall buildigs because of the egative pressure differece o roof.

6 Z 8 cm cm i o Z. m i o i o Z H b i:iside o:outside H t H H t Z < H b b Z < Z Ht FIGURE. The flow directio at differet atural heights for a atural vet/widow 8 7 Z 4 (m) Temperature( ) 1m m m 4m m m 1m m Ht Hb FIGURE 4. Variatio of atural height with smoke temperature for differet buildig heights I Taiwa s buildig code, the atural smoke exhaust vet/widow shall be located at half the height of floor with miimum size of m of elevator lobby area. I practice, the locatio of atural vet iflueces the smoke exhaust performace greatly. Thus, two differet locatios of atural smoke vet/widow are ivestigated i this study. As show i Figs. (a) ad (b), 7 ad cm are used to compare the smoke exhaust performace o the same wid effect respectively. At the same smoke temperature the effective buildig height for smoke exhaust system may rise from m to m whe the vet locatio move up from 7 cm to cm dow below the ceilig. The locatio of smoke exhaust vet is strogly recommeded to be istalled ear the ceilig as possible for highest atural smoke exhaust performace.

7 Pressure Differece(Pa) m 1 m Wid Pressure Buildig Height(m) FIGURE. Variatio of pressure differece with buildig height at ormal wid directio (zero wid agle) for elevator lobby area (a) 8 7 Z 4 (m) Begiig of Fire Temperature( ) 1m m m 4m m m 1m m Ht Hb (b) 8 7 Z 4 (m) Begiig of Fire Temperature( ) 1m m m 4m m m 1m m Ht Hb FIGURE. The flow directio at differet atural heights for a atural vet/widow at (a) 7 cm dow below ceilig ad (b) cm dow below ceilig 7

8 Desig of Natural Smoke Exhaust Vet/Widow I Taiwa s buildig code the atural smoke exhaust vet/widow is always closed at ormal time. The vet/widow shall be opeed whe the fire sigal coected to the vet/widow. Of course, the vet/widow also ca be opeed by maually. However, o exact opeig width is specified by regulatios. Basically, the opeig width of atural smoke vet/widow will ifluece the performace of smoke exhaust performace. Therefore, a wid tuel experimetal program was coducted to examie the pressure differece iside ad outside vets/widows for differet opeig widths. Four differet opeig agles of vet/widow, 1,, ad 9 degrees were used for the experimetal program. Behid the vet/widow, a air supply fa provided appropriate air volume through air duct agaist the air directio of wid tuel. Therefore, the pressure differece may be adjusted betwee the frot ad the rear of vet/widow. Test results are depicted i Figs. 7 (a) ad (b) for m/s ad m/s air velocity of wid tuel, respectively. Followig the Taiwa s weather data, the pressure differece may also stad for the buildig height techically. Accordig to the test data, the atural smoke exhaust vet/widow ca have effective fuctio for the opeig agle greater tha degrees. However, for higher wid velocity or less hot smoke buoyacy the vet/widow may eed wider opeig agles. I Fig. 7 (b) whe the air velocity of wid tuel icreased to m/s the 9 degree opeig agle case did ot have the maximum smoke exhaust performace. It is believed that some degree of obstacle i frot of smoke vet/widow may provide better smoke exhaust performace for higher wid velocity cases. Whe fire occurred i tall buildigs, the to degrees opeig agles are suggested for a successful ad effective atural smoke exhaust vet/widow system. Smoke Exhaust Volume(m/sec) (a) (b) Smoke Exhaust Volume(m/sec) Pressure Differece(Pa) Pressure Differece(Pa) 1 9 FIGURE 7. The relatioship betwee smoke exhaust volume ad pressure differece (buildig height) for (a) m/s velocity i wid tuel ad (b) m/s velocity i wid tuel 8

9 CONCLUSIONS May aspects will ifluece the smoke exhaust system of tall buildigs, such as buoyacy force, mechaical vetilatio, stack effect, ad wid effect etc. The wid effect will play a importat role o atural smoke exhaust systems of tall buildigs. The wid tuel test results provide useful data for evaluatig the performace of smoke exhaust system i tall buildigs of Taiwa uder the existig buildig code. For cosiderig the wid pressure aroud buildigs some useful coclusios may provide i this study. (1) For residetial areas the smaller vet size may provide higher exhaust smoke velocity that will provide higher pressure to overcome the wid pressure. The wid pressure will icrease with the buildig height, therefore, the smoke exhaust vets are recommeded to istall upo roof for better smoke exhaustio efficiecy. () For a smoke vet/widow beeath 8 cm of ceilig ad the smoke temperature at o C, the smoke ca exhaust through out the buildig at m buildig height. Notwithstadig, whe the smoke temperature icreases to o C the smoke ca exhaust through out the buildig at 1 m buildig height. () For the elevator lobby area, the locatio smoke exhaust vet/widow was discussed i this paper. Whe the vet/widow locatio moves from 7 cm to cm dow below the ceilig the effective buildig height that hot smoke may exhaust through the vet/widow shifts from m to m. The atural smoke exhaust vet/widow istall ear the ceilig as possible may obtai optimal smoke exhaust effect. (4) I cosiderig the desig of atural smoke exhaust vet/widow, the to degrees opeig agles are suggested for a successful ad effective atural smoke exhaust vet/widow for tall buildigs. REFERENCES 1. Butcher, E.G., Fardell, P.J. ad Clarke, J., Pressurizatio as a Meas of Cotrollig the Movemet of Smoke ad Toxic Gases o Escape Routes, Symposium of Movemet of Smoke o Escape Route i Buildigs, HMSO, Lodo, pp.-41, Morga, H. P., Ghosh, B. K. ad Garrad, G., Desig Methodologies for Smoke ad Heat Exhaust Vetilatio, Buildig Research Establishmet Report BR8, Buildig Research Establishmet, Garsto, UK, Kadola, B.S., Effects of Atmospheric Wid o Flows through Natural Covectio Roof Vets, Fire Techology,, 17-1, May Best, R. ad Demers, D.P., Ivestigatio Report o the MGM Grad Hotel Fire, LS-4, Natioal Fire Protectio Associatio, Quicy, MA. Jauary Klem, T.J., Fire Ivestigatio Report: First Iterstate Bak Buildig of Califoria Fire, Los Ageles, CA, Natioal Fire Protectio Associatio, Quicy, MA. May Marchat, E.W., Effect of Wid o Smoke Movemet ad Smoke Cotrol Systems, Fire Safety Joural, 7, -, Dalgliesh, W.A. ad Boyd, D.W., Wid o Buildigs, Caadia Buildig Digest 8, , May Cermak, J.E., Wid-tuel Developmet ad Treds i Applicatios to Civil Egieerig, Joural of Wid Egieerig ad Idustrial Aerodyamics, 91, -7,. 9. Tamura, G.T., Smoke Movemet ad Cotrol i High-rise Buildigs, Chapter 4, NFPA,