326 DIGITAL APTITUDES + OTHER OPENINGSBeyond Arrows: Natural Ventilation in a High-Rise Building with Double Skin Faade MONA AZARBAYJANI University of North Carolina at Charlotte INTRODUCTIONNaturally Ventilated Double Skin FacadesThe concept of DSF is not new and dates back to manyyearsagowhereincentralEurope;many houses utilized box-type windows to increase ther-malinsulation(Oesterle,2000).Thedouble-skin faadeisanarchitecturalphenomenondrivenby the aesthetic desire for an all-glass faade and the practical desire to have natural ventilation for im-proved indoor air quality in buildings. Until recently theuseofdouble-skinfacadeshadbecamemore popular in many high-rise buildings in Europe.A number of studies, research and several simulation programs have been done on employing the natural ventilation in buildings and thermal performance of double skin facades. Most of them have been carried out for solar chimneys -one way to increment natu-ral ventilation and to improve indoor air quality- and Trombewallspriortodoubleskinfacades.Mostof them found out the natural ventilation is possible in summer even for multistory buildings (Wong, 2006). Thepotentialofusingadoublefaadefornatural ventilation of the building in climates other than eu-rope has not been fully studied though. A number of interestinginvestigationsandfndingsarereported in the literature pertaining to passive ventilation in buildings with double-skin facades. Itwasfoundthatsignifcantenergysavingsare possibleifnaturalventilationcouldbeexploited through the use of a double-skin facade (DSF). For example,theLoyolaUniversityInformationCom-mons and Digital Library in Chicago integrated nat-uralventilationwithaDSFtocoolthebuildings. The results indicated that an integrated facade can reduce 30 percent of energy consumption (52 days operated in natural mode) (Frisch, 2005).Even though most of the research has been done intemperateclimateconditions,studieshavere-vealed a close link between natural ventilation de-sign and the DSF function. Grabe et al. (2002) de-velopedasimulationalgorithmtoinvestigatethe temperaturebehaviorandfowcharacteristicsof natural-convection DSFs through solar radiation. It was found that the air temperature increased near heatsourcesthatareclosetowindowpanesand shading device. Gratia and Herde (2007a, 2007b, 2007c)attemptedtolookatnaturalventilation strategies,greenhouseeffects,andtheoptimum positionofsun-shadingdevicesforDSFsfacing south in a northern hemisphere temperate climatic. They found that a suffcient day or night ventilation rate can be reached by a window opening, even if wind characteristics are unfavourable. METHODOLOGYNaturalventilationisan essential part of sustain-ablebuildingdesign.Energyconsciousdesigners harnessthecoolingcapacityofnaturalwindto increaseindoorthermalcomfortandultimately saveenergyforactive-spaceconditioning.Wind cancauseairmovementandperceptionofcool-ing, wind can bring in air of a different temperature andhumidity.Bynumericallysolvingaseriesof conservationequationsrelatedtomass,momen-tum,andenergy,computationalfuiddynamics (CFD) tools help designers predict detailed airfow forspecialdesigncasesandplanabuildingwith optimal natural ventilation. 327 BEYOND ARROWSTopreservethermalcomfortandreducecooling loads, natural cooling strategies can be incorporated into buildings. In this study, the behavior of natural ventilation in a new double skin facade (DSF) confg-uration will be studied. To that end, Fluent, compu-tational fuid dynamics software, was used to study the offce airfow path. Developing a CFD model is alengthyprocess.First,EnergyPlus-DesignBuilder was used to solve some boundary conditions, such assolarthermalenergy.Thetemperatureprofle and air velocity within the DSFs cavity and the in-ternal offce space were simulated by Fluent, and, as a result, the offces thermal comfort was calculated and presented. In this study, wind driven ventilation improvedwithstackeffectinthenobleconfgura-tion of DSF will be tested to see if it can maintain adequatecomfortduringsummerandspringtime in Chicago. The frst step would be study the ambi-ence which will be used as a boundary conditions in computational fudid dynamics tool. Initial studies of the macroclimate were carried out through Ecotect, whichallowedfortheeffcientvisualizationofthe local climatic conditions.AssumptionsSimulationswereperformedwithclimaticdata ofChicago(Midwayairport).Weatherdatawere recordedbytheWorldMeteorologicalOrganization and includes 12 actual months. Natural ventilation is possible during the shoulder season in Chicago. For this study, the months of May, June, September, and October were chosen. The frst stage of the air fow modeling is to con-structasimplemodeloftheinnovativeenclosure systemthecombinedshaft-corridorDSFinGam-bit. The numerical model is three-dimensional and themodelisbasedonacontrolvolumemethod. The geometric model of the entire building is con-structedusingamatrixofnumberstorepresent the points at which surfaces meet.Inthenextstage,theboundaryconditionswere solved,thegridsizerefned,andnumberofnec-essary iterations determined. The generated mod-elsolvedforwindvelocityalongwithbuoyancy forces. The model used to solve the iteration was bousinesqe in order to consider the buoyancy forc-es and in order to include wind force, the pressure at the surface boundary was calculated. In order to defne the external boundary condition, the airfow needed to be solved for the external wind. For this study, only wind direction which is perpendicular to the DSF has been considered.Wind Effects on High Rise BuildingIn assessing the effects of wind on buildings, it is importanttoconsiderthecharacteristicnatureof thewind.Thewindisturbulent,andthismeans speed varies with height. Vertical profles of mean wind speed for boundary layers are approximated by taking the wind speed tobeproportionaltotheheightraisedtosome powerapowerlawvariation(Davenport,1965) thesimpleexpressionwhichisusedextensively has the form :Vh=0.35Vmeth0.25WhereVhisthelocalwindspeedatheight,h, andVmetisthemeteorologicalwindspeed.Based on this formula, wind has been calculated for spe-cifc times that the CFD analysis was performed. Thewindspeedfromthemeteorologicaldatacor-responds to the wind speed at 10 m height in open country.Sincethebuildingislocatedinanurban environment,theappropriatewindproflewasas-sumed to be (0.35 and 0.25 are the constants which depends on the train in the vicinity of the buildings).WIND PRESSURES The distribution of wind pressure around a building dependsverycloselyuponthelocalvariationin windvelocitywhichthebuildingproduces.Inac-cordance with the elementary pressure-velocity re-lationship, the pressure distribution is represented by a dimensionless pressure coeffcient :=Where,= wind pressure, Pa = wind speed at specifed height, This formula has been used to calculate the boundary condition for the inlet and outlet pressure in Fluent.328 DIGITAL APTITUDES + OTHER OPENINGSThe Geometry of the Cfd Model Thenewconfgurationtaketheadvantagesofthe strategiesincludeventilationdrivenbydifferent combinations of wind and external stack.The most distinguishingfeatureofthisconfgurationisits coolingstacktoweringoverthesouthsideofthe building. This confguration combined both two shaft and corridor type through the buildings facade. The coolingstacksallowforfurtherventilationonhot, stagnantsummerdayssothebuildingcanalways remain cool within reasonable comfort levels.Figure 1 shows how air fows through the chimney and provide ventilation inside each offce module. Air gap inlet draw in fresh air at a low level and di-rect the fresh air into the room. The air exhausted throughtheoutletatthehigh-levelgapofinner pane. Multi-storey chimneys suck exhausted air via a bypass opening at the top of the inner level. The vertical height of the glass chimney creates stron-ger uplift forces due to increased stack effect.Theeffectivenessofventilationdrivenbythermal buoyancy, or stack effect, is determined by the in-letairtemperature,heightbetweentheinletand outletopenings,sizeoftheseopenings.Figure2 shows the section of themodules and the confgu-ration of openings inlet and exhasut.MODEL DESCRIPTIONThefrststageofCFDmodelingistoconstructa seven-storymodulewithgeometricaldimensions of27x7m,with3.5mceilingheightand1.5m cavity corridor in front of the offces in the Gambit. The simplifed single skin facade of the model has openings on panes with 6mm thick glass. The DSF constructionhasoneopening(inlet)attheouter paneandtwoopenings(airinletandexhaust)at the inner pane. The model is constructed in 3-D in Gambit as shown in the fgures below. Figure 1. Intuitive diagrams Natural Ventilation and DSF (section of the new confguration)Figure 2.Vertical cross section of the new confguration of DSF329 BEYOND ARROWSCFD MODEL BOUNDARY CONDITIONSThisstudyisgoingtomodellevelsfromthesix-teenthtothetwenty-thirdstoriesofahigh-rise building. In the new confguration, the DSF has a ventilated shaft, which is 1.5x1.5 m with two open-ingsonthelowerandhighchimneylevels.This study introduces a shaft to improve the possibility ofnatural-ventilationstackeffecttoextractheat from the offces and improve airfow rates required to reach thermal comfort level within the interior. TheresultsofFluentmodelofthissevenstory blocklookedatthevelocityprofle,airfowpat-terns, temperature within the double skin, and the internal offce space. To calculate thermal comfort, the boundary conditions for wind velocity, external temperature and relative humidity were set to the rangessimilartoChicagoclimaticconditionsand are presented in the Table 1.SIMULATION RESULTS DEMONSTRATION For the analysis of airfow and temperature in DSF andadjacentspace,thecombinedshaft-corridor DSF has been generated and airfow patterns and temperature profle within the DSF has been illus-trated for specifc times of the year. Based on those data, the level of thermal comfort within the space will be predicted. Cavity and Room Air TemperatureThe location of the chimney openings in this shaft-corridor type in relation to the chimney exhaust will haveaneffectontheindoorthermalcomfortand airfow velocity. It is a fact that the higher the ex-haust opening is located from the inlet, the stronger stack effect within the air gap. This effect will then pull more air from offce spaces to circulate through-out the building. The other factor that impacts on air velocity are inlet and outlet sizes. Due to the ventu-Figure 5.Cavity temperature gradientFigure 3. Model boundary settingsFigure 4. As displayed in fuent analysis of room temperature profle330 DIGITAL APTITUDES + OTHER OPENINGSry effect, the smaller the size, the more the velocity. Figure 4 illustrates the rooms temperature gradient, which is clearly lower than the outside temperature of20C.Thereissometemperaturevariationas thecavityisventilated.Theroomairtemperature increases towards the top as shown in Figure 4. Figure5clearlyillustratesanincreaseinthe gradienttemperaturefromtheinlettotheoutlet wherehigher-surfacetemperatureswerereached due to accumulated heat and buoyancy effect. Asillustratedinpreviousfgures,theinteriorair temperatureisslightlyhigherthantheexterior inthehalfoftheroomclosertothecavityand also in the half upper part. The temperature goes from19Catthebottomopeningtoabout19.9 C at the top opening. Figure 6 illustrates how the DomainDomain material Air at 20C, 1 atmReference pressure 100,000 Pa (atmospheric pressure)Reference temperature 295 KSourcesBuoyancy modelBoussinesq-calculates airfow from temperaturedifference rather than density difference.Buoyancy reference temperatureOutdoor air temperature.Gravitational acceleration-9.81 m/s in the y-direction.Boundary ConditionsSide, top, bottom, & front domain boundaryconditionsOpenings with deduced air velocities, externaltemperature = outdoor air temperature at infowonly, external pressure at atmospheric pressure.Back boundary conditions Adiabatic solids (concrete) with depth = cavityDepth, glass: standard 6 mm clear glass.Heat source (external glass)11.43 W/Heat source (internal glass)7.93 W/Walls heat25 W/Velocity inlet 5 m/sCavity DetailsCavity size 3.5 m high by 7 m wide by 1.5 deep.Cavity external facade Internal plate with surface temperatures on bothsides is calculated based on heat source. Cavity internal facade External plate with surface temperature base on heat source.Shaft and opening detailsShaft size 1.5 m deep,1.5 long, 24.5 m highDSF opening size for inner pane, air gap size (inlet and exhaust).300 mmTable 1. Model assumptions and inputs required by fuent331 BEYOND ARROWStemperaturestratifcationrangesfromlowonthe foor to highest close to the ceiling. Figure6 shows the cavity air temperature near the interiorglazing.Thetemperaturegoesfrom18C at the bottom opening to about 19.5 C at the top opening.Thestackairtemperatureincreasesto-wards the top of the chimney in a fairly linear pro-gression, as shown qualitatively in Figure 7. All these fgures depict the cavity model with 20 C outdoor air temperature and 480 watts/m2 incident solar en-ergy obtained from the weather data. The interior air temperature is slightly higher than the lower half of the chimney air in average 1C per foor.Atemperaturecontourstudyhasshownthatthe offce spaces lower foors have lower internal tem-peratures compared to the higher foors. As we in-crease the shaft height, the upper foors would be hottest and probably uncomfortable for occupants during the summer month. It is interesting to dis-cover that the offces mid-portion for all the foors arehavinghighertemperaturescomparetothe front part. This could be due to the airfow pattern shown in the next section.Airfow The air velocity through the cavity is due to buoy-ancyandwindforces,anditisquitehigh.Fig-ure8showsthebuildingsairvelocitymodel. Thevelocityinthismodelrangesfrom7m/sin-letto2.3m/soutletofthechimney.Theinfow from the external screen is 1 m/s and the internal screeninfowvelocityis0.45m/s,whiletheex-haust airfow is 0.46 m/s on average. The exhaust air velocity to the chimney averages 1.3 m/s. The greatestvelocityisneartheinlettothechimney and exhaust from the stack. As shown in the fgure below,thevelocitiesareincreasedrelativetothe one-story high cavity. Figure9showsthesectionofairfowattheopen-ing of one story horizontally. Velocities are greatest near the opening, when the air is forced through the smaller area. In the back of the room, air velocities are high as they move toward the exhaust to get out from the stack. As illustrated in the fgure below, air velocityinthechimneyishigherclosetotheback wall of the chimney, and it is generally laminar when driven only by buoyancy without wind effects. Figure 6.Cavity temperature profle-output from fuentFigure 8. Model of air velocity vector in the building Figure 7. Chimney air temperature profle332 DIGITAL APTITUDES + OTHER OPENINGSFigure 10 shows the horizontal velocity profle in the cavityandchimney.Thevelocityrangesfrom0.46 m/sawayfromthesurfaceand0.97m/snearthe opening. The greatest velocities are near the cavity openingswherethewindimpactstheairfowrate. Figure 15 shows velocity vectors at the openings of thechimneycavitymodelasanalyzedinFigure1 Velocities are greatest near the openings, when air is forced through the smaller area. In this model, the maximum velocity at the inlet is about 11 m/s. Other thanattheopenings,airfowthroughthechimney well is around 5 m/s and is driven by both wind and stack effect.A small turbulent fow forms at the stack inlet and outlet, and stack airfow is generally laminar when it is only driven by buoyancy. Figure13showsaverticalprofleofairvelocity vectors to the stack from the cavity. The velocity is quite high and it extracts hot stuffy air from offces Figure 10.Horizontal velocity gradientFigure 9. Horizontal velocity proflesFigure 11. Model of air velocity close to the inlet openingFigure 12. Airfow patterns and velocity at the chimneys outlet openingFigure 13. Air velocity vectors in the cavity and chimney333 BEYOND ARROWStothestackandoutdoors.Theairfowislikely tobemoreeffectiveinextractingheatwhenthe wind velocity is high and the temperature gradient occurs in the shaft increase the buoyancy effect. Figure 13 shows how air comes inside the chimney, whichisthenextractedfromtheoffcesthrough the chimney opening.Figure 14 shows the section of air-fow path inside thecavityandshaftsandhowairfromtheoffce outletisdirectedoutside.Theairisbasicallyin-ducedtofowupwardsbyabuoyancyeffectcre-ated by the accumulated heat. Figure 15 illustrates howthefowreacheshighervelocitieswithinthe inlet and outlet where pressures are higher. As de-picted, fow tends to be turbulent near the glazed surfaces and openings where forces are higher. The airfow inside the cavity is much higher than what is measured by typical examples in the literature.The airfow inside the room is illustrated in fgure 15. The fgure shows an air-movement trend from laminar close to the wall boundaries to turbulent in the rooms center. The airfow inside the rooms is less than 1 m/s and more than 0.1 m/s. FINDINGS AND ANALYSISTheresultsforasouth-facing,combinedshaft-corridorDSFwithexternalwindvelocityof5m/s and50percentairhumidity,respectively,are tabulated in Table 2. Results for the combined shaft-corridor DSF type as shown in Figure indicate that the DSF air gap size of 300 mm gives a comfortable resultfortheseparticularconditionsinanatural ventilated space. The offce spaces lower foors would generate the lowest operative temperature due to the stack ef-fectprovidedbytheDSFconfguration.Thishas enhancedthenaturalventilationstrategytopro-vide better internal thermal comfort condition.Thereisaninternaltemperaturedifferenceof1 CfortheDSFmid-foor,whichcouldbedueto slowerinternalairvelocitygenerated.Thesouth-facingDSFconfgurationproducedan82percent acceptability limit for the 0.3 m opening for exter-naltemperature23C,accordingtotheThermal EnvironmentalConditionsforHumanOccupancy from ANSI/ASHRAE Standard 55-2004. Figure 16 shows the PMV ranges for naturally ven-tilated spaces with 80 percent acceptable limits in-dicated for the third foor in May. This graph shows Figure 14. Velocity vectors in the roomTable 2. The simulation results are illustrated in the table below: Table 3. Calculated air velocity in the offce room: 0.5 meter from the windows and 1 meter from the foor. SimulationFloor Level Temp C Air Velocity Radiant Temp RH % PMVOT 3200.2822.9340.70.1820.22 522.40.1622.5347.30.2920.3 May 724.80.1133.6445.590.3720.56 323.70.202751.10.520 5240.1937.5050.680.6621.30 June 724.60.1527.1956.600.7621.09 323.40.2726.350.30.424 523.70.2026.2557.400.5425.38 Sep 724.20.1325.9856.350.7125.20 3220.2723.336.4-0.32 22 522.30.2023.442.40.1122.80 Oct 722.80.1924.645.40.2322.04 K RNG Turbulence Model Run DetailsModel Results Model Grid number Run time iterations T cavity inlet and exhaust Airflow rate( ) 5,000,000 72 hours 100013.2102.5 Figure 15.Average PMV index frst foor months under study334 DIGITAL APTITUDES + OTHER OPENINGSthattherearebroaderacceptabletemperature rangesfornaturallyventilatedspaces.Airmove-mentsdetermineconvectiveheatandmassex-changeofthehumanbodywiththesurrounding air.IntheChicagoclimate,highairvelocitieswill increasetheevaporationrateattheskinsurface, whichresultsinacoolingsensation.Therecom-mended upper limit of indoor air movement is usu-ally 0.8 m/s for human comfort; such air velocity permits the interior space to be 1-2 degrees higher than the human comfort temperature to maintain desirable a comfort level (Hien et al, 2005).It was found that the combined shaft-corridor DSF willgenerateastrongstackeffectwithintheair gap, which, in turn, will pull more air out from the offce space through rear-wall vents. The tempera-ture generated within the offce space is then desir-able and close to the human comfort requirement. The airfow pattern created will have a good ven-tilation effect with cool air coming from the vents, and across and above the internal space, and dis-charge through the inner pane high level opening. CONCLUSIONThe simulation of the combined shaft-corridor DSF was carried out seven story stack high to investigate the thermal behavior of this new design facade and evaluate the processes involved in thermal comfort. Fluent was used to simulate turbulent airfow. A pro-cess was also described to use pre-determined wind pressurevaluesatthecavitysopeningstomodel a DSF cavity in a CFD model. This method reduces computationtimethatwouldbeneededtoana-lyzewindeffectsalongwithbuoyancyforces.CFD modelswereusedtodeterminevelocitiesatvari-ous points on the building for wind velocity of 5 m/s based on the weather data. Based on airfow model-ing of the combined shaft-corridor DSF, it was found that the stack encourages buoyancy and induces air movement. Convective forces inside the cavity could be used to promote air extraction from the room, al-though it is needed to promote air movement within the room to release the excess heat. The CFD results appear to confrm the designs ef-fectivenessintwoways.First,theairfowfollows theceilingandexitsthroughthechimneyopen-ings,whichsuggeststhatthecoolnightairwill effectivelydrawtheheatoutoftheceilingslab. Thesecondoutcomeoftheairfowstayingnear the ceiling is the avoidance of high air velocities in theoccupiedzoneduringtheday.Thisstudyhas shown that the combined shaft-corridor DSF has a possibility of providing acceptable internal thermal comfortthroughnaturalventilationstrategyina hot summer continental climate. REFERENCESAfonso,C.,andOliveira,A.(2000).ASolarchimneys: Simulationandexperiment.EnergyandBuildings, 32(1), 71-79. A.GDavenport.1963proceedingsoftheconferenceon windeffectsonbuildingsandstructures,vol1. HMSO ,1965Arons, D. (2000). Properties and Applications of Double-SkinBuildingFacades.MScthesisinBuilding Technology,MassachusettsInstituteofTechnology (MIT), USA. Web address: http://libraries.mit.edu/docsAzarbayjani,M.Energyperformanceassessmentofa naturally ventilated combined shaft-corridor DSF in anoffcebuildinginChicago,ARCC2011,Detroit MI. Azarbayjani, M. Integration of natural ventilation in high-risebuildingwithdoubleskinfaade,BESS2011, Pamona, CA. Champagne,C.(2002).ComputationalFluidDynamics andDoubleSkinFacades.Assignmentforthe ArchitecturalEngineeringComputerLabs, PennsylvaniaStateUniversity,USA.Webaddress: http://www.arche.psu.edu/courses/ae597J/Champage-Hw1.pdf Gratia, E., and De Herde, A. 2004. Natural ventilation in a double-skin facade. Energy and Buildings, 36(2), 137-146Hamza,N.,andUnderwoodC,2005.CFDsupported modelingofdoubleskinfacadesinhotarid climates,9thInternationalIBPSAConference Building Simulation 2005, Vol 1, Montreal (Canada). 365-372.Hensen, J. 2003.Paper Preparation Guide and Submission Instruction for Building Simulation 2003 Conference, Eindhoven, The NetherlandsHien,W.N.,Liping,W.,Chandra,A.N.,Pandey,A.R., &Xiaolin,W.(2005).Effectsofdoubleglazed facadeonenergyconsumption,thermalcomfort andcondensationforatypicaloffcebuildingin Singapore. Energy & Buildings, 37(6), 563-572. Macdonald,A.Melikov,Z.Popiolek,P.Stankov(2002), Integrating CFD and building simulation, Building and Environment 37, pp. 865-871.Mitchell,J.W.,Beckman,W.A.1995.Instructionsfor IBPSAManuscripts,SEL,UniversityofWisconsin, Madison, WI. Navvab,M.,2005.Fullscaletestingandcomputer simulationofadoubleskinfaadebuilding.9th International IBPSA Conference Building Simulation 2005, Vol 2, Montreal (Canada). 831-838.Oesterle,E.,Leib,R.D.,Lutz,G.,Heusler,B.(2001). Doubleskinfacades:integratedplanning:building physics, construction, aerophysics, air-conditioning, economic viability, Prestel, Munich.Safer,N.,WoloszynM.,RouxJ.J.,KuznikF.,2005, Modelingofthedoubleskinfacadesforbuilding 335 BEYOND ARROWSenergysimulation:radiativeandconvectiveheat transfers.9thInternationalIBPSAConference Building Simulation 2005, Vol 2, Montreal (Canada). 1067-1074.Stec, W., and Passen D., 2003. Defning the performance of the double skin faade with the use of the simulation model. 8th International IBPSA Conference Building Simulation2003,Vol2,Eindhoven(Netherlands). 1243-1250. Torres,M.,AlavedraP.,GuzmanA.,2007,Doubleskin facades-CavityandExterioropeningsDimensions for Saving energy on Mediterranean climate. IBPSA Proceedings of Building Simulation 2007, 198-205.Wong,P.C.,Prasad,D.,Behnia,M.(2008).Anewtype of double-skin facade confguration for the hot and humidclimate.EnergyandBuildings,40(10),pp. 1941-1945. Zllnera,A.Wintera,E.R.F.andViskanta,R. (2002)Experimentalstudiesofcombinedheat transfer in turbulent mixed convection fuid fows in double-skin-faades.International Journal of Heat and Mass Transfer, 45(22), 4401-4408.