adv-tutorial-text

11.ModelinganAirborneElectronicsSystematAltitudeKeywords–Avionics,Altitude,Parametrics,Non-ConformalMeshing,Functions1.1IntroductionTherearenumerousmethodsforcoolingelectronicssystemsandthechoiceoftechniqueisgenerallyafunctionofthedesign,application,reliabilityrequirementsandpowerdissipation.Foravionicelectronics,onecommonapproachistouseambientair,orchilledairductedoverthesealedchassisofthesysteminacoldwall.HeatfromthecomponentsconductsthroughthePCBtothecardguidesthatareclampedtothechassisandremovedviathecoldwall.Thistechniquealsoremovesanyrisktotheactualelectronicsarisingfromdust,moistureetc…intheairflow,andthusincreasestheoperatingreliability.Operatingconditionsofsuchequipmentvaryconsiderablyduetothedensityvariationofairfromsealeveltocruisingaltitude.Asthealtitudeincreases,thedensitydecreases,andconsequentlythemassflowratesofthefan(s)arealsoreduced.Inthisexerciseacold-wallcooledavionicsunitwillbemodelledandthecoolingefficiencyevaluatedatbothsealevelandaltitude.ThisexerciseassumestheuserhashadalreadycreatedsomemodelsinIcepak,andinparticularhasexperienceinmeshingassembliesseparately,andalsoinsettingupparametricsimulations.1.2OpeningtheProjectUnpackandloadthemodelcalledavionics_box.tzr.Thisisthebasiccoldwallgeometrywithtwoparametricrunsdefined.Thefirstassumesauniformloadingof30WperPCB,givingatotaldissipationof450Wfor15cards.Thisisapplieduniformlytothecardedgesonbothsidesofthechassisgiving15Wpercardoneachside.Anadditional30Wisappliedtothefrontoftheunitoppositethefantorepresentanyadditionalloadfromadditionalequipment.Heatcanonlybeconductedoutoftheunit,sotheinteriorairisnotmodelled,andreplacedbyahollowblock.Recallthatwithinahollowblocknoequationsare2solved,andnomeshisgenerated,sothismethodologyreducestheproblemdowntoaworstcasescenario.Itremainstoaddtwoheatsinkstorepresentthefininginthecoldwallsonthesidesoftheunit.1.3AddingtheFinsTheeasiestwaytoaddthefinsistogeneratetwoheatsinks,oneforeachofthecoldwallducts.Name–Heatsink.1StartPosition:(2,2,19)mmPlane:Y-ZDimensions:YLength183mmZlength307mmBaseheight0mmOverallheight10mmFlowdirectionZChangethetypeto‘detailed’anduse‘extruded’fintypeFinspecifiedbyCount/ThicknessCount50Thickness0.15mm–effectivethicknessonlycheckedName–Heatsink.2StartPosition:(256,2,19)mmPlane:Y-ZDimensions:YLength183mmZlength307mmBaseheight0mmOverallheight-10mmFlowdirectionZChangethetypeto‘detailed’anduse‘extruded’fintypeFinspecifiedbyCount/ThicknessCount50Thickness0.15mm–effectivethicknessonlycheckedPlaceeachheatsinkintoitsownassembly,faninanassemblyandchoosetomesheachassemblyseparatelyandapplyappropriateslackvalues.Note:thesecondheatsinkcouldbemadebycopyingthefirstandthenusingthealignmenttoolstore-positionit.31.4ModelingtheeffectsofAltitudeOpenthematerialstabunderthemodeltree.Twomaterialshavebeenaddedtothisproject–Air_0MandAir_10000m.UndertheProblemsetupgotoDefaultsettingsandassignavariabletothematerial(rememberingtoprefixitsnamewitha“$”),andmakethistakediscretevalues–“Air_0m”and“Air_10000m”.Thesecanbeassigned,bygoingtotheRunOptimizationMenu,andmakingsurethattheoption-“ParametricTrialsBycolumn”isactive.Goingto“DesignVariables”panelthevaluescanbeassigned.NowedittheFaninthemodel.Ataltitudethemassflowrateandinlettemperaturewillbeverydifferentfromthesameflowrateatsealevel.Assigntwoparametersfortheinlettemperatureandthemassflowrate.Thetemperatureparametershouldtakethevalues20and-10ºC.Andthemassflowrate0.023and0.0066kg/s.Applythesesettings,andresetthetrials.Checkthatthecorrectcombinationofparametersforsealevel,and10Kmhavebeenassigned.Itshouldbenotedthatitisthemassflowrateoftheair,andnotitsvolumetricflowratethatgovernstheamountofcooling.Thevolumeflowrateisconstantoveraltitude,sothemassflowratemustbechangedtoreflectthedifferentdensities.Definesomepointstomonitortemperatureetc…Inadditioncreatethreefunctions,onetoreporttheglobalmaximumtemperature,andtheothertwotocalculatethepressuredropacrosseachoftheheatsinks.Finallyrunthetrials.Finallyanalysethetwosimulationstocompareresults.Generatesomeparticletracksfromthefanthroughthesystem.Createasummaryreportforbothrunsforthetemperaturesofthesourcesandforthemassflowratethroughthefan.1.5Furtherwork(1)Currentlythecardguidesarearrangedsuchthattheairflowisperpendiculartothem.Sotheaircoolingthesectionofthechassisimmediatelyadjacenttoaslotispre-heatedbythepreviousslots.Thatis,theairflowingnexttoslot4say,hasbeenpre-heatedbyslots1-3.Loadthepackedmodelcalledavionics_parallel.tzr.Thishasbeenmodifiedsothattheairflowsparalleltothecardguidesinstead.4Runthemodelwithuniformpowerdissipationandassesswhethertheoverallperformanceoftheunitimproves.Whatotherchangesshouldbemade?(2)Inthemodelfortheheatsinks,thefinsweremodelledwiththeeffectivethicknessoptionchecked.Thisisequivalenttomodellingthemas2-Dthinconductingplates–thatistheoptiontomodeltheeffectivethicknessonlywaschosen.Youmaywanttoconsiderrunningthemodelagainwiththisoptionturnedofftoassesswhetherthiswasavalidmodellingdecisioninthisinstance.Underwhatcircumstancesisthisapproachallowable?Whatmodificationsorsimpleadditionalsimulationscou