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UA336027821 About Us » Industry Focus Technical Benefits Business Benefits Tips & Tricks Contact Us Posted By LEAP CFD Team on Apr 22, 2014 | 1 comment Engineers who are tasked with designing heating, ventilation and airconditioning (HVAC) systems for buildings will need to assess the indoor air quality to ensure optimum health and comfort for occupants and meet minimum regulatory requirements. Generally a HVAC CFD analysis will take into account variables such as air temperature, relative humidity, air species concentrations and velocity. Additionally, CFD engineers can use ANSYS CFD to solve for the local "mean age of air" (MAA) to assess the air quality within an indoor environment. By examining MAA across the habitable space within a building, engineers can quantify the air change effectiveness (ACE) of their ventilation system and confirm that their design meets NABERS/GreenStar regulations. How can I plot the Mean Age of Air in ANSYS CFDPost? Usually designers are interested in local distributions, therefore it is useful to plot the ACE as the ratio between the nominal time constant and the age of air or its inverse. Within ANSYS CFDPost, it is possible to calculate the ACE at a specified height with a simple expression defined using CEL. This allows us to quantify the area occupied by air with an age exceeding the mean value across a reference surface (typically located at breathing height, ie. 1m above floor). According to many regulatory standards, the area exceeding the target value must be less than 5%. The setup for the Normalised Age of Air variable (expressed as Age of Air/Nominal Time Constant). Note that when solving using FLUENT, a variable called "Scalar 0" will be present in the variable list. Its dimensions can be displayed by defining a new expression such as AgeofAir = Scalar 0 [s] and then creating a new variable which by definition is equal to AgeofAir. Clipping the Age of Air to a specific range of values This is useful to quantify the size of any pockets of air that may exceed our target MAA value (ie. the value prescribed by regulations). In order to visualize Home Featured Tips & Tricks: Calculating the Mean Age of Air for HVAC simulations in ANSYS CFD Search for: Search Subscribe to email addr Sub Leading Engineering Australia: 1300 88 2 New Zealand: 09 97 Visit LEAP Website f CFD, FEA, CAD, CAM products, plus our tra schedule. Follow Recent Popular Exc adv Win Eng usin CFD Apr 1 Usi pre gen and Tips & Tricks: Calculating the Mean Age of Air for HVAC simulations in ANSYS CFD

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  • 5/23/2015 ComputationalFluidDynamics(CFD)BlogLEAPAustralia&NewZealand|Tips&Tricks:CalculatingtheMeanAgeofAirforHVACsimulationsinA

    http://www.computationalfluiddynamics.com.au/tipstrickscalculatingmeanageofairwithcfd/ 1/7

    UA336027821

    About Us Industry Focus Technical Benefits Business Benefits Tips & Tricks Contact Us

    PostedByLEAPCFDTeamonApr22,2014|1commentEngineerswhoaretaskedwithdesigningheating,ventilationandairconditioning(HVAC)systemsforbuildingswillneedtoassesstheindoorairqualityto

    ensureoptimumhealthandcomfortforoccupantsandmeetminimumregulatoryrequirements.GenerallyaHVACCFDanalysiswilltakeintoaccount

    variablessuchasairtemperature,relativehumidity,airspeciesconcentrationsandvelocity.Additionally,CFDengineerscanuseANSYSCFDtosolvefor

    thelocal"meanageofair"(MAA)toassesstheairqualitywithinanindoorenvironment.ByexaminingMAAacrossthehabitablespacewithinabuilding,

    engineerscanquantifytheairchangeeffectiveness(ACE)oftheirventilationsystemandconfirmthattheirdesignmeetsNABERS/GreenStarregulations.

    HowcanIplottheMeanAgeofAirinANSYSCFDPost?

    Usuallydesignersareinterestedinlocaldistributions,thereforeitisusefultoplottheACEastheratiobetweenthenominaltimeconstantandtheageofair

    oritsinverse.WithinANSYSCFDPost,itispossibletocalculatetheACEataspecifiedheightwithasimpleexpressiondefinedusingCEL.

    Thisallowsustoquantifytheareaoccupiedbyairwithanageexceedingthemeanvalueacrossareferencesurface(typicallylocatedatbreathingheight,

    ie.1mabovefloor).Accordingtomanyregulatorystandards,theareaexceedingthetargetvaluemustbelessthan5%.

    ThesetupfortheNormalisedAgeofAirvariable(expressedasAgeofAir/NominalTimeConstant).

    NotethatwhensolvingusingFLUENT,avariablecalled"Scalar0"willbepresentinthevariablelist.ItsdimensionscanbedisplayedbydefininganewexpressionsuchasAgeofAir=Scalar0[s]andthencreatinganewvariablewhichbydefinitionisequaltoAgeofAir.

    ClippingtheAgeofAirtoaspecificrangeofvalues

    ThisisusefultoquantifythesizeofanypocketsofairthatmayexceedourtargetMAAvalue(ie.thevalueprescribedbyregulations).Inordertovisualize

    Home Featured Tips&Tricks:CalculatingtheMeanAgeofAirforHVACsimulationsinANSYSCFDSearchfor:

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    Excitingadvances inWindEngineeringusing ANSYSCFDApr14,2015

    Using CFD topredict flowgenerated noiseand other

    Tips & Tricks: Calculating the Mean Age of Air for HVAC simulations in ANSYS CFD

  • 5/23/2015 ComputationalFluidDynamics(CFD)BlogLEAPAustralia&NewZealand|Tips&Tricks:CalculatingtheMeanAgeofAirforHVACsimulationsinA

    http://www.computationalfluiddynamics.com.au/tipstrickscalculatingmeanageofairwithcfd/ 2/7

    theairwhichisolderthanthisnominalvalue,wecreateanisoclipforavaluegreaterthanthetargetvalue,appliedtoaplanelocatedataspecifiedheight

    abovethefloor(typically1metre).

    Theresultshouldlooksimilartotheoneshownintheimagebelow.The"holes"areregionswheretheMAAvaluesareaboveorbelowthethreshold

    specifiedinyourisoclipsetup(inthiscase,anythingbelow1200secsandabove1300secs).

    AnisoclipfortheAgeofAirVariable.

    Itisthenstraightforwardtocreateanexpressionthatquantifiesthe%areaoftheisocliptothefullareaoftheplane.Typicallyengineersaimtoensurethat

    thisdoesnotexceed5%(accordingtotheACE0.95criteria).InthisexampletheCELexpressionwouldbewrittenas:

    Ratio=area()@IsoClip1/area()@Plane3

    HowtocliptheAgeofAirVariabletothedesiredvalues

    HowdoIcalculatemeanageofairinmyANSYSCFDsimulation?

    Themeanageofair(MAA)isnotautomaticallyincludedasapredefinedvariableingeneralpurposeCFDsoftware,suchasANSYSCFXandANSYS

    Fluent.Itcanbedefinedinthepreprocessingstageasanadditionalvariableusingthemethodsoutlinedbelow.Thissectionprovidessomebackground

    andfurtherdetailsonhowtoincludetheMAAvariableinbothANSYSCFXandANSYSFluent.

    Somebackground...

    Themeanageofair,denoted ,isobtainedbysolvingthefollowingadvectionequation:

    Thederivativewithrespectoftimeofthesourceis1sothattheintegralitselfistime.Whenintegrated,theagescalarwillequaltheresidencetime.

    InANSYSCFDsolvers,scalarquantitiesaremultipliedbythedensity inthetransportequations,sothatisinthisparticularcasetheequationtouse

    is:

    Thetermontherighthandsideoftheequationisknownasthesourceterm,andithastobetakenintoaccountwhenimplementingthemeanageofair

    variableinthesimulation.

    AccordingtotheACE0.95criteriaformodernbuildingregulations,forinstance,thedisplacementairwithintheenclosedenvironmenthastoprovideanair

    changeeffectiveness(ACE)higherthan0.95foratleast95%ofthenetlettablearea(NLA)whenmeasuredinaccordancewithASHRAE1291997.

    aeroacousticeffectsNov6,2014

    Smart gasappliancemanufacturersuse rising gascosts to theircompetitiveadvantageOct29,2014

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  • 5/23/2015 ComputationalFluidDynamics(CFD)BlogLEAPAustralia&NewZealand|Tips&Tricks:CalculatingtheMeanAgeofAirforHVACsimulationsinA

    http://www.computationalfluiddynamics.com.au/tipstrickscalculatingmeanageofairwithcfd/ 3/7

    AddinganadditionalvariableinCFXPre

    thesetupforthecreationoftheAgeofAirVariable

    ThesetupoftheAgeofAirVariableintheFluidModelstab

    Airchangeeffectivenessisdefinedastheratiobetweenthenominaltimeconstantandtheaverageoftheageofairatareferenceheight

    Thenominaltimeconstantistheratiobetweentheairvolumeofaspaceorbuildingdividedbytherateofoutdoorairsupply,whichistheincoming

    volumetricflowrate.Meanresidencetime

    Thereferencesurfaceistakenat1[m]abovethefloortoconsiderthebreathingzoneoftheenclosedspace.

    ACEgivesameasureofthedistributionofairatthebreathingheightandavalueclosetounityindicatesthattheairdistributionwithinthezonehas

    reachedperfectmixing.AccordingtotheGBCAsGreenStarrequirementsofIEQ2version2,awelldesignedceilingairdiffusionsystemwillachievean

    ACEofnearunity(between0.8and1.2)atdesignairflowrates.AdisplacementventilationsystemtypicallyachievesanACEofgreaterthan1.2.

    Howtodefinethevariable"AgeofAir"inANSYSCFX:

    Tocalculatetheageofair,itisnecessarytocreateanewvariableundertheExpression,FunctionsandVariablestreeinCFXPre.Inthisexamplethenew

    variablehasbeennamedAgeofAir.Ithasunitsofseconds.

    CFXsolvesatransportequationforthenewlycreatedvariable.ThisisshownintheFluidModelstabofthedefaultdomain,undertheAdditionalVariable

    Models.Astheageofairdoesnotdiffuse,itisnotnecessarytodefineaKinematicDiffusivityforit.Note,thatinANSYSCFX,turbulentdiffusionisincluded

    bydefault.

    Asthetransportequationisoftype'specific',itisnecessarytoaddasourcetermequaltothefluiddensity.Thisisdonebycreatingasubdomain(here

    namedAoA),undertheDefaultDomainanddefiningthe'Source'tobedensityasbelow.

    modellingmodelswallfunctions

  • 5/23/2015 ComputationalFluidDynamics(CFD)BlogLEAPAustralia&NewZealand|Tips&Tricks:CalculatingtheMeanAgeofAirforHVACsimulationsinA

    http://www.computationalfluiddynamics.com.au/tipstrickscalculatingmeanageofairwithcfd/ 4/7

    OncecreatedthesubdomainisdisplayedintheDefaultDomaintree

    Thesetupforthesourceinthesubdomain

    DefiningaUserDefinedScalar(UDS)

    ThelaststepinANSYSCFXistosetavalueforAgeofAiratanyboundaryconditionswhereaircanenterthedomain.Inthisexample,theAgeofAiratthe

    diffuserandattheexhaust(foranyreturnflow)hasbeensetto0[s].Itisalsopossibletospecifynonzerovaluesifrecycledairisused.

    Howtodefinethevariable"AgeofAir"inANSYSFluent

    Intheprevioussectionwe'vedescribedthemethodtoincludeanadditionalvariable(intheformofatransportequation)forcalculatingtheageofairin

    ANSYSCFX.Similarly,inANSYSFLUENTwecancalculatetheageofairthroughuseofauserdefinedscalar(UDS).Thisinvolvescreatesauserdefined

    scalarfortheageofairthensolvingitsequationafterasolutionfortheflowfieldhasbeenobtained.

    ThefollowingUDFcomputesthediffusivitytoaccountforturbulentdiffusion(whichisnotimplicitlydefinedasinCFX)andthesourceforthemeanageof

    air.ThisfunctioncanbeexecutedasaninterpretedorcompiledUDF.

    #*******************************************************************

    #include"udf.h"

    #include"prop.h"

    #ThisdefinetheAoAdiffusivity

    DEFINE_DIFFUSIVITY(mean_age_diff,c,t,i)

    {

    returnC_MU_EFF(c,t)/0.7

    }

    #Thisdefinesthesource

    DEFINE_SOURCE(mean_age_source,c,t,dS,eqn)

  • 5/23/2015 ComputationalFluidDynamics(CFD)BlogLEAPAustralia&NewZealand|Tips&Tricks:CalculatingtheMeanAgeofAirforHVACsimulationsinA

    http://www.computationalfluiddynamics.com.au/tipstrickscalculatingmeanageofairwithcfd/ 5/7

    ThesetupofthesourcetermfortheUDS

    DefinitionoftheUDSattheboundaries

    TheUserScalar0intheSolutionControlspanel.ByselectingitthesolverwillcalculateasolutiononlyfortheUDS.

    DefinitionoftheUDSDiffusivityinthematerialdialogbox

    {

    realsource

    source=C_R(c,t)

    dS[eqn]=0

    returnsource

    }

    #********************************************************************

    AftertheUDFisinterpretedorcompiled,thenamespecifiedintheDEFINEmacroargument(e.g.,mean_age_diff)willbecomevisibleandselectableinthe

    Create/EditMaterialsdialogboxinANSYSFLUENT

    Thenextstepistodefinethescalarinthevolumeoffluidrepresentingthesolutiondomain,asshownintheFigurebelow.

    Finallyitisnecessarytosetaboundaryconditionforthescalaratallinletsandoutlets.Inthisexampleuds0scalarvalues=0attheinlet(but,asinCFX,

    canbesetatanonzerovalueifdesired).ThesetupforthenewvariableformeanageofairinFLUENTisnowcomplete.

    ThisposthasaimedtoprovideanoverviewoftheageofairparameterinCFD,andsometipsonhowtocreateandpostprocesstheresultsformeanage

    ofairinANSYSCFD.CFDsimulationsofthistypeareincreasinglybeingperformedbybuildingdesignersandengineerstobothmeetstringentbuilding

    codesandhelpdifferentiatethesustainabilityandliveabilityaspectsoftheirdesign.Tothiseffect,ANSYSCFDprovidesengineerswithavaluabletoolto

    assessthelevelsofthermalcomfortforbuildingoccupants,byexaminingparameterssuchastheairtemperatureandradiantheatloads,aswellasatool

    toensureadequateairchangeeffectivenessbysolvingandassessingthemeanageofair.ForfurtherinformationortodiscussyourownCFDproblem

  • 5/23/2015 ComputationalFluidDynamics(CFD)BlogLEAPAustralia&NewZealand|Tips&Tricks:CalculatingtheMeanAgeofAirforHVACsimulationsinA

    http://www.computationalfluiddynamics.com.au/tipstrickscalculatingmeanageofairwithcfd/ 6/7

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    Author:LEAPCFDTeam

    Post a Reply

    Gregory CouerbeNOVEMBER7,2014

    WithANSYSCFXSolver,iftheAdditionalVariableisdefinedas"Volumetric"insteadofas"Specific"thentheSourcetermistobesetto"1"

    (insteadtobeequaltothedensity).Thedefinitionisthereforesimpler.

    Typethetext

  • 5/23/2015 ComputationalFluidDynamics(CFD)BlogLEAPAustralia&NewZealand|Tips&Tricks:CalculatingtheMeanAgeofAirforHVACsimulationsinA

    http://www.computationalfluiddynamics.com.au/tipstrickscalculatingmeanageofairwithcfd/ 7/7

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