热湿地区采用的辐射冷顶板加置换通风系统secret.docx

热湿地区采用的辐射冷顶板加置换通风系统secret.docx

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热湿地区采用的辐射冷顶板加置换通风系统secret

Radiationalpanelcoolingsystemwithcontinuousnatural

crossventilationforhotandhumidregions

Abstract

Thispaperinvestigatesahybridcoolingsystem,utilizingwind-drivencrossventilationandradiationpanelcoolinginanofficesetting.Thecharacteristicsoftheindoorenvironmentareexaminedusingcomputationalfluiddynamics（CFD）simulation,whichiscoupledwitharadiationheattransfersimulation,andHVACcontrolinwhichthePMVvalueforahumanmodelinthecenteroftheroomiscontrolledtoattainthetargetvalue.Thesystemisdevisedwithanenergy-savingstrategy,whichutilizesstratifiedroomairwithaverticaltemperaturegradient.Thecooledairsettlesdownwithinthelowerpartoftheroom,whilethehotandhumidairpassesthroughtheupperregionoftheroom,sweepingouttheheatandcontaminantsgeneratedindoors.Thisstrategyisfoundtobequiteenergy-efficientintheintermediateseasonsofspringandautumninJapan.Evenunderhotandhumidoutdoorconditions,thehybridsystemcoupledwithradiationcoolingwouldbringsignificantenergysavingsarepossiblecomparedwithahybridsystemcoupledwithunderfloorair-conditioning.

1.Introduction

Wind-drivencrossventilationisconsideredtobeapromisingenergy-effectivestrategy,sinceitutilizesnature’spower.AdjustingtheindoorenvironmentbynaturalcrossventilationwasthetraditionalmethodinhotandhumidregionsofAsiaincludingJapan.However,inmodernbuildings,wherecertainty,reliability,andefficiencyarepreferred,theuseofwind-drivencrossventilationaloneisinadequate.

Ahybridairconditioningsystemwithcontrollednaturalventilation,oracombinationofnaturalventilationwithmechanicalairconditioningisconsideredtobeabletoovercomethedeficiencyofwind-drivencrossventilation,andhassignificantenergy-reducingeffects.Manystudieshavebeenconductedonnaturalventilationsystemsandhybridair-conditioningsystemsinEuropeandJapan。

InJapaninparticular,ahybridcoolingsystem,usingunderfloorairconditioningincorporatingnaturalventilationisprimarilyused。

Butmanystudieshavediscoveredthatradiationcoolingismoreeffectivethanconventionalair-coolingsystemsinmanyareas。

Inthisstudy,weadvocateahybridcoolingsystemusingnaturalventilationandradiationpanelcooling.Itisbasedontheconceptofutilizingbothnaturalventilationandradiationpanelcooling,aimingtointroduceoutdoorairintotheindoorspacesbycrossventilationandtoachievecomfortableindoorthermalconditionsbythepowerofnatureasfaraspossible.

Eventhoughitisimpossibleforahigheroutdoortemperaturetocoolaroombycrossventilation,outdooraircanstillbeintroducedandpassthroughtheupperpartoftheroom,expellingouttheheatandcontaminantsgeneratedindoors.Inthemeantime,withtheaidofaverticalthermalgradient,thelowerpartoftheroomcanbefurthercooledbyradiationcoolingpanelsinstalledinthelowerpartoftheroom.Thisstrategyisexpectedtobeenergy-efficientandtoprovideadequatethermalcomfortintheroom.Inthispaper,theconceptofahybridcoolingsystemisdescribedandthemeasuredresultsforthecaseswhereoutdoorairtemperatureandrelativehumidityare21℃at60%and30℃at70%areshown.

1.Conceptofradiationalcoolingsystemwithcontinuousnaturalcrossventilation

Fig.1isaconceptualdiagramofthesystemsuggestedinthisstudy.Thiscoolingsystemhasaventilationopeningintheupperpartoftheroomtoprovideventilationtotheindoorsbycrossventilationandtodischargeinternalheatgeneratedintheroom.Dehumidifyingradiationpanels

areinstalledinthelowerpartoftheroomtoregulateindoorhumidityandtoprovidecooling.Evenwhenoutdoortemperaturesbecometoohightocooltheindoors,heatandcontaminantsgeneratedwithintheroommaybedischargedbycrossventilationfromtheupperpartoftheroomwithoutsignificantlydisturbingtheoccupiedzone（ortaskzone）,andradiationcoolingpanelsinstalledinthelowerpartoftheroomwouldlocallycooltheoccupiedzoneonly.

Thustheproposedsystemdoesnotignorethepossibilityofadjustingtheindoorenvironmentusingnaturalpower,butmaximizesitsusetomaintaintheindoorenvironmentatacomfortablelevelandtosaveenergy.

3.OutlineofCFDanalysis

Analysisofroommodel

Thehybridcoolingsystemmaybeusedinbothresidentialandofficebuildings.Here,thecaseofanofficebuildingisexamined.TheofficesettingisshowninFig.3.Thedepthoftheroomis10.8m.Thewidthoftheanalyzedareaissetathalfofthe3.6mofficemodule（1.8m）,tomakeasymmetricalconfiguration.Hybridcoolingismodeledastheoutdoorairflowsintotheroomfromtheupperopeningofthewindow（0.5m×1.8m;Fig.3,left）,andisexpelledthroughtheopeningontheotherside（0.5m×1.8m;Fig.3,right）,whiletheradiationpanelcoolingisstilloperating.Theradiationpanelssetinfrontofthedesksintheroomserveaspartitioningpanels,andcanbeoperatedbelowthedewpointtemperature.Whereasinthecaseofahybridcoolingsystemcoupledwithunderfloorairconditioning,thecooledairissuppliedbydiffusersfromthefloorandreturnsthroughout-letairgrillsintheceiling.People,lighting,personalcomputers,solarheatgains,etctogenerateacoolingload,whichisremovedbynaturalcrossventilation,andradiationcoolingorunderfloorairconditioning.Ahumanmodelisplacedinthecenteroftheroomasthethermalcomfortsensor.

4.Results

4.1.Flowfields

TheflowfieldsforthedifferentcasesareshowninFig.7.InCase1（naturalventilation-radiationpanelcooling）,owingtotheconsiderabletemperaturedifferencesbetweentheindoorair（26.6◦C）andoutdoorair（21◦C）,thenegativebuoyancyeffectontheinflowingairisapparent.Theinflowingairflowsdownwardstothewindowside,andmixeswiththeindoorairinthelowerpartoftheroom.Thoughnotshownhere,theaircooledbytheradiationpanelsflowsdowntothefloor.

InCase2（naturalventilation-underflowAC）,theinflowingairshowsthesametendencyasinCase1.However,thisairmixeswiththeunderfloorACandascendstowardtheupperpartoftheroom.Owingtotheverticaltemperaturedifferenceintheroom,aclockwisecirculatingcurrentisgeneratedontheinnersideoftheroom.InCase3,sincethetemperaturedifferencebetweentheindoorandoutdoorair（30◦C）,issmall,thenegativebuoyancyeffectoftheinflowingairisnotapparent.Withthetallercoolingpanel,thermalplumesrisingfromthepersonalcomputerscanclearlybeseen（Fig.7（c））.Thisisbecausetheaveragetemperatureinthetaskzonedrops.ButinCase4,owingtotheverticaltemperaturedifference,theoutdoorairdescendstothefloor.

4.2.Temperaturedistributions

ThetemperaturedistributionsforthefourcasesareshowninFig.8.InCase1,theaverageindoortemperatureis26.6◦C,somewhathigherthanforCase2of24.4◦C.However,theverticaltemperaturedifferenceisslightatabout2◦C.InCase2,duetothesomewhatlowvelocity（0.8m/s）oftheunderfloorAC,thetemperaturenearthefloordropstoalmost20◦C.Asaresult,theverticaltemperaturedifferenceintheroomissteepatabout6◦C,especiallyintheinnerzoneoftheroom.Inbothcases,duetothedescendingcurrentsofincomingair,aratherlow-temperatureregionappearsnearthewindow.InCases3,theroomaveragetemperatureis29.5◦C.Therearevertical

temperaturegradientsintheroom,andahigh-temperatureregionisseenintheupperzoneoftheroombecausetheincomingoutdoorairstaysthere.InCase4,thetemperaturedistributionissimilartothatforCase2,butthetemperaturestratificationbecomescomparativelyclear.ComparedwiththeresultsforCase3,thehigh-temperaturezone（over30◦C）disappearsbecausethehotairiscooledbytheunderfloorairconditioning.

4.3.Relativehumiditydistribution

OnlytherelativehumiditydistributionforCase3isshowninFig.9.InCase1,sincetheradiationpanelsurfacetemperature（19.7◦C）ishigherthanthedewpointtemperature,nocondensationoccurs.Therelativehumidityintheroomairisabout45%overthewholespaceandindicatesarelativelyuniformdistributioncomparedtoCase2.InCase2,theaveragerelativehumidityisabout50%.

Aswiththetemperaturedistribution,asharphumiditystratificationisformed.InCase3,thesurfacetemperatureoftheradiationcoolingpanel（18.3◦C）islowerthanthedewpointtemperature,andcondensationoccursonthesurfaceoftheradiationpanel.Evenafterintroducinghigh-humidityoutdoorair,theaveragerelativehumidityintheroomreachesabout61%becausetheradiationcoolingpaneldehumidifiesthehumidoutdoorair.ComparedwiththeresultsforCase3,theaveragerelativehumidityinCase4（55%）dropsbecauseitiscontrolledbytheunderfloorairconditioning.

4.4.Ageofair（Fig.10）

Theageofthefreshoutdoorairiscalculatedbasedontheresultingflowfields.Thecalculationresultsarenon-dimensionalizedbythenominaltimeconstant.InCases1and2,tnis360s,andinCases3and4itis1200s.InCase1,youngairisobservedinthelowerpartoftheroombecausethefreshoutdoorairmovesintothebottomoftheroom.InCase2,thesametendencyisobservedneartheperimeterzonewithCase1.However,somewhatyoungerairisdistributedoverthewholespace,becausetheoutdoorairmixeswellwiththeindoorairduetotheaircurrentfromtheunderfloorAC.Inbothcases,theairintheinnerpartoftheroombecomesold,owingtothefreshoutdoorairmixingwiththeindoorair.

InCase3,duetotheoutdoorairflowintheu