1、低溫換熱器涌傳遞特點吳蘇月（中國重慶大學動力工程學院，重慶，400044袁秀芳，李遠瑞，彭亮（中國重慶大學動力工程學院，重慶，4000442007年1月9日收入通過分析運行在低于環境溫度的低溫換熱器的涌傳遞過程，提出了涌傳遞總效率的概念，并得到了涉及涌傳遞總效率相關量的表達式，如熱交換單元數，冷流體到熱流體熱能率等。以順流，逆流，交叉流為例，給出它們涌傳遞總效率的數字結果，并將涌傳遞總效率與熱傳遞總效率進行比較。關鍵詞：低溫換熱器，涌傳遞總效率，熱傳遞總效率1 .引言低溫換熱器主要用于制冷和低溫產業，將熱量從一種流體傳遞到另一種流體，并具有極高的重要性。在低溫換熱器中涌傳遞不可避免地伴隨著熱傳

2、遞。另外，在工程中我們通常關注的是某一確定流體增加的涌。這篇文章為工程中低溫換熱器的運行評價提供了參考，并幫助工程師設計更好的低溫換熱器。2 .低溫換熱器涌傳遞總效率T5加圖1.換熱器中典型的熱交換考慮低溫換熱器對流換熱典型的熱量交換（如圖1所示），假設熱量傳遞和流動過程處于穩定狀態，不考慮換熱器軸向熱傳遞，粘性損失和熱損失。另外，假設流體的物理特性為常數。對于一個工作在環境溫度以下的換熱器，熱流體把熱量傳給冷流體，冷涌增加,但冷流體吸收熱量，冷涌下降，即熱傳遞和涌傳遞的方向是相反的。增加的涌是熱流體冷涌的增加。體積為dFx的微元體的熱和涌傳遞速度為dQ二kdFxTx二cphdTh(1)dE二

3、kexdFxTx(2)當忽略壓力涌損失，體積為dFx,質量流量為Gh的微元的涌傳遞速度為dE二VhCPh(T¥TTh-1)dTh(3)結合上面的等式，工作在環境溫度以下的換熱器的當地涌傳遞總效率是kexk(T'X；/Th-1)(4)由換熱器的能量平衡，可得到逆流換熱時熱流體溫度Th隨著Fx的變化。并引入了三個影響換熱器的非維度因素：熱流體與冷流體熱量速度比率R二GcCpc/GhCph,熱傳遞效率心二(Teo-Tci)/(Thi-Tei),傳熱單元數NTU二kF/GcCpc,且L3R肥TUi1(5)k.ReeNTU(1R)-R(1e)eNTU、R)F*1其中F*Fx/F,eTe

4、i/Thi,coT!i/Thi平均涌傳遞總效率定義是keF；1oFkexTxdFx(6)結合等式(5)和(6)A拽j，R(LC)CoLn1:.R(1C)k-：R(1.e)工作在低于環境溫度的換熱器順流和交叉流(熱流體混合，冷流體不混合)的當地涌傳遞總效率是酶.一母小1k.(Rc_1)_R(1e)e-NTU(1-R)F*1I)kex"co-1r.*、RX*（*NTU.1）kc一（1-c）e其中，X*二x/L,（x是熱流體的流動方向，L是換熱器的長度）對于順流流三和交叉流流動的低溫換熱器的平均涌傳遞總效率和逆流流動是樣的，只是對于不同的流動形式有著不同的表達式。3 .結論及討論表二給出了

5、不同流動形式低溫換熱器ke/k隨著NTU和R的變化。如圖2所示,當R或NTU為定值，ke/k會隨著NTU或R的增加而增加，當NTU或R更大些時，ke/k趨于一定值。當R或NTU為常數時，對于不同的流動形式，一般逆流的ke/k最大，交叉流次之，順流最小。低溫換熱器順流流動或交叉流流動時，kex/k隨NTU的變化和ke/k隨NTU的變化相同。但對于逆流，當NTU>4時，ke/k會隨NTU慢慢減小。同時，ke/k會隨著R的增加而增加并接近一定值。但當R偏大或偏小時相對的影響程度不同。0.40d-片0£1counterflow/TrtI79flnvv圖2.不同流動形式ke/k隨NTU和

6、R的變化鳴謝這項研究工作由CQCST3然科學基金計劃提供資金支持。參考文獻1.M.Yilmaz,ONSaraSKarsli,Int.J.Exergy,1(4):278-94(2001).2.D.Shrivastava,T.Ameel,ASMEInternationalMechanicalEngineeringCongress&Exposition,2002.WorldSuientifir:WWW.world$*citritIItiternaiionaJJcurnalofModernPhysicsBVol.21r13&19/即。了)3的A35C3©WorldScienl

7、iilcPublishingCcmpanyEXERGYTRANSFERCHARACTERISTICSONLOWTEMPERATUREHEATEXCHANGERSS.Y.WUCollegeqfPowerEngineeringChongqingUniversity,Chongqing400044,ChinaX.F,YUAN；Y,RLiandLFENGCoftgPowerEngineering,Chqngq'gUniwrifyfC'hMgqMg400()44,ChinaReceived9January2007Byanalyzingexergytransferprocessofthel

8、owtempernfure：hearuxchangersnperafingbelowthe號unmundingteinperature,th已conceptnfexergytransfercoefficient汰putforwardandtheuxprcs士ion%whichinvolvingrLcviintvariablesfortheExergytransfercoefficient,theheal,transferuni糖riuckiberandnheratioofcoldtohotfluidsbearoi附cityrate,etc,hederived.Takingihe質面品Iflow

9、,counterflowandcrossflowlowtemperatureheatexchangersexamples.IhenumericresultofexergytranfeircoefficienlaregivenandLhecomparisonofexergytninsfercoefficientwithheattninsfcircoefficientis.analyzed.Keywords:Lowtcmpcralurc-heatexchangers;exergytnsnsfcrcoefficient:;beartransferc-ocfficknt.LIntroduclionLo

10、wtemperatureheatexchangersarccucnsivclyusedinrcfrigcralionandcryogenicindustriestotransferheatfroinonefluidstreamtoanother,andareofconsiderableimportance.Heartransferisinevitablyaccompaniedwithexergytransferinalowtemperatureheatexchanger.Inadditiononealwaysfocusestintheamountofgainedexergyforacertai

11、nfluidinilicengineering.ThispaptrprovidesreferenceofLhcrrnotiynamicperformancecvaluciliononlowlemperaiurehealexchangersinengineeringandagistsengineersiodesignbetterlowicmraturcheatexchangers2. ExtrgyTransferCoenicienii>nLowTemperatureH電4tExchangerri.l.Typicalt)?atirarx住rinh聞exchanger.3503脂口1s.Y.鼠

12、al.CnniiiiJerthelypiealhealirantifiirinalowtemperaturehealexchangerofcounterflow(asshowninFigJ).Assuinethattheheattransferandflowprocessareinsteadystate.Theaxi;ilheatconductionhviscuusdissipationandhentlossesofhealexchangerarenutconsidered.Inaddition,thefluidsphysicalpropertiesareassumedtobeconstant

13、.ForeiheatexchangerwhichOperEitesbelowthesnrrouTidingiemperaiurtfTihuhoifluidgivesouthealtocoldfluidand(hecoldexergyincreases,bunihecoldfluidabsorbshca(andlliccnldexergydccreasts."IhaLisTLhedircctinnfiofhealiransfcrandextrgytraniferarccontrary.Tliegainedexergyislheincreaseofcoldexergyofhotfluid

14、.JheheatandexergytransferrateoveradifferentialelementofvolumedFTartgivendQ=3£M=-0/d。d£=#*；MWhenihtpressureexergyIoskkignored,theexergytransferratecfhotfluidwithmassratehGh,overadifferentiaelementofvolumedFaisgivenbyRycombiring±eaboveequations+theIdc3Iexergytransfercoefficiemcfheatexch

15、angeropcralinghclowlhesurroundingtemperatureif；(4)Byenergybiilanteoftheheatexchanger,thebulktemperaturevariationotthehotfluidforeounierflow.丁補alongtheF、canbeobtained.Andthreenon-dimi：nionalperformanceparametersofheatexchangerareintroducedasfollows:ratioofhotlocoldfluidsheatcapacityrateR=Gvc(i(/Glcfi

16、.heattransfereffectivenesse=(7.一7-Tf),heattransferunitsnumberNTU=kt/Gc,theniIk一二的千用十取i一)c-XwhereF*=FJF,c=TL，"=Themeanexergy-transicrcftefficicntisdefinedasfellows.小拜"(6)CombiningEq隊andk_銀(l-c)+t7口In:二解->)T-r/?(l-r)ThelocalexergytransfercoefficientforparallelflowfindcrossHow(hotfluidmixe

17、dandcoldfluidnotmixed)heLitexchangersoperEitingbelowthesurruuTidingteinperalureart.(m)1k一(屋.十】)十一(1_£1*""一"*EienyyTninferCharacteritic&onLowTtmpEraturEiHeatExchangers350-5%=%.E1kc+Q-c)ft-1>whereX-x!Ltx*theGowdirccinn。hocfluidandListhelengthofhealexchanger).Themeanexergy(ra

18、nskrcoefficienttorparallelflow；indcrossflowlow(tmperalun;heatcxchaniirKarcthesame紙LhalforenunterRow.Andt：卜解dificrcnLexpresfiinnsfordifferentflowpaHems.3. R鶴ul恒and口版us對on臺Fig.2givesthevariationofkjkwithNTUandRfordifferentflowpanernslowtemperatureheatexchangers.AsshowninFig.2,whenRorNTUisconsiant,kjki

19、ncreaseswiththeincreaseofNT口erRandgraduallyapproachesaconstantvaluewhenNTUor/?isgreater.FordifterentflowpatternsTthekjkofcourierflowheatexchangsrisgenerallythegreatest,lhecrossflowhealexchangerisfollowed弱ndheparalleltlowhc4texchangeristhekavwhenRorNTU認canstant.ThevarialionofkfJfkwithNT1'Uforparalle