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1、FLuent-FLuent-换热器的相变模换热器的相变模拟计算拟计算 UGM 2001MotivationDemonstrate the use of Fluent to model phase change in heat exchangersProcesses of interestuCondensationuEvaporationuBoiling Illustrate how to model one such process through use of a detailed exampleShell-and-tube condenserProvide motivation for u
2、sers to begin developing models of their own devices2Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001OutlineProblem DescriptionShell-and-tube condenseruPure vapor condensationuNon-condensable gasesModeling ApproachPorous mediumHeat and mass transfer modelingModel Implem
3、entationUser-Defined Functions and User-Defined MemoryResultsSteam condenser with non-condensable gasesCommercial chiller condenser3Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Description of ProblemShell-and-tube condenser 4Company Confidential Copyright 2001 Fluent
4、 Inc. All rights reserved. UGM 2001Goals of CFD ModelingCondenser performance characterized by heat and mass transfer rateCFD allows evaluation of factors affecting heat and mass transfer in condenseruTube bundle configurationtTube arrangementtNumber of passestLocation of inlet portstBafflesuPressu
5、re dropuVelocity fielduNon-condensablestLocation and configuration of purge systemResults allow identification of potential design improvements5Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Film Condensation ProcessDriving potential for condensation is the temperature
6、 difference between vapor and cooling waterDriving potential variation caused by Pressure dropRise of cooling water temperatureNon-condensablesTPH2OPairairCondensate layerTube wallCooling water6Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001CFD Modeling TheoryPorous me
7、dium approachTube bundle treated as porous mediumEnables computationally efficient modeling of entire condenserComparison with detailed modeling approachuIn 2-D, O(100)-O(1000) control volumes per tube versus more than one tube per control volumeHeat and mass transfer modelsCondensation rate calcula
8、tionuCondensation rate determined from local flow field and cooling water temperatureuLiquid film flow rate tracked in bundle from top to bottomuCooling water temperature tracked from inlet to outlet7Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Porous Medium Approach
9、Representation of tube bundle as porous mediumPorosity is only required parameterPorosity defined as ratio of fluid volume to total volume PduExample: staggered tube bundle with equilateral triangular layoutPorosity, b, expressed as:8Company Confidential Copyright 2001 Fluent Inc. All rights reserve
10、d. UGM 2001Transport EquationsGeneric transport equation for porous medium approachconvectiondiffusiondistributed resistanceEqn.continuity1x-mom.uy-mom.vspeciesw w Distributed resistance takes form of source terms that model details of the flow that are not resolved by the gridPorosity in convectio
11、n and diffusion terms not modeled in FluentDistributed resistance terms most significant in tube bundle region9Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Evaluation of Modeling ApproachAdvantagesComputationally efficientuDoes an alternate, tractable approach exist?
12、Approach demonstrated to give meaningful data by several authorsDisadvantagesLoss of some flow details due to averagingCan be overcome by detailed modeling of small regions of condenser10Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Heat Transfer ProcessFilm condensat
13、ion on horizontal tubeCooling WaterTube WallCondensate FilmLiquid-vapor InterfaceRefrigerantVaporLatent heat released at liquid-vapor interface transferred to cooling water11Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Heat Transfer ModelHeat transfer is modeled by c
14、oupling of thermal resistance network with CFD codeTcwTt,iTt,oTiRcwRtubeRcondCooling WaterCFD code provides interface temperature, Ti Cooling water and tube thermal resistances are generally well-knownFilm heat transfer coefficient is required for Rcond12Company Confidential Copyright 2001 Fluent In
15、c. All rights reserved. UGM 2001Film Heat Transfer CoefficientCritical component of heat transfer modelObtain from experimentOr obtain from literatureSteam condensation on smooth tubesFigure courtesy of Kansas State University,Professor Steve Eckles, and Duane L. Randall13Company Confidential Copyr
16、ight 2001 Fluent Inc. All rights reserved. UGM 2001Modeling AssumptionsEffect of liquid on flow field is neglectedApproach can also be implemented in Eulerian-Eulerian multiphase frameworkuSatisfactory model for liquid phase representation not currently availableuPublished results of this type of m
17、odel do not appear to show significant advantageVapor is assumed to be saturatedNo superheatingVapor temperature determined from pressure field calculated by CFD code14Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Implementation of Model with UDFsUDFs are required for
18、:Source terms required by porous medium approachuCondensation rateuPressure drop in porous regionRepresentation of tube bundleuPorosityuCondensate film flow rate accountinguCooling water temperature calculation with multiple tube passes15Company Confidential Copyright 2001 Fluent Inc. All rights res
19、erved. UGM 2001Cooling water temperature calculation for each segmentuEvery iteration, condensation rate is summed over each segmentuInlet cooling water temperature = outlet temperature from previous segmentuSegment outlet cooling water temperature calculated by energy balance.uLog-mean temperature
20、 for each segment calculated based on vapor temperature and cooling water inlet and outlet temperaturesTube Bundle RepresentationBundle consists of N passes and M segmentsEach segment defined as unique cell zoneExample:2 Pass bundleN = 2, M = 416Company Confidential Copyright 2001 Fluent Inc. All ri
21、ghts reserved. UGM 2001Tube Bundle Grid StructureStructured, cartesian grid used in tube bundleEach control volume has unique i,j,k indexi=1j=1k=1i=1j=2k=1i=1j=3k=1i=1j=3k=2i=1j=2k=2i=1j=1k=2i =1j=1k=3i =1j=2k=3i=1j=3k=3Grid structure created with UDFsuGrid generator, solver do NOT utilize structur
22、eUsed to track condensate film flow rate17Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Source TermsAlgorithm for source term in continuity equationObtain pressure, velocity and species mass fraction (if necessary) from current solution valuesObtain film Reynolds numb
23、er and cooling water temperature from User-Defined MemoryCalculate heat flux based on current value of solution variables Translate heat flux into volumetric mass source termUnder-relax source termuSi+1 = Si + a (So Si)uRequired for solution stability. Alpha typically 0.01 0.10uValue of source term
24、from previous iteration, So, stored in User-Defined MemorySource term in momentum equations Calculated using empirical correlations with tube bundle porosity and current velocity18Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Define_On_Demand FunctionsDefine_On_Demand
25、 functions executed once per iterationUpdate condensate film mass flow rateUpdate cooling water temperatureuAssume uniform temperature for each bundle segmentNew values stored in User-defined memoryAutomatic Define_On_Demand execution possibleExample:19Company Confidential Copyright 2001 Fluent Inc.
26、 All rights reserved. UGM 2001Solution AlgorithmInitialize Solution: Assign porosity, tube bundle orientationUpdate cooling water temperature and liquid condensate mass flow rateCalculate source termsSolve flow equationsYesNoSolution Converged?Stop20Company Confidential Copyright 2001 Fluent Inc. A
27、ll rights reserved. UGM 2001ExamplesSteam condensation with non-condensable gasesMcAllister Condenserfrom: Bush et al., 1990, Proc. Int. Symp. On Condensers and Condensation21Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001McAllister Condenser GeometryBoundary conditio
28、ns and model inputsShell Dimensions 1.02 m X 1.22 m X 0.78 mCooling water flow directionInlet temperature: 17.8 CInlet velocity: 1.19 m/sTube BundleSingle pass, 4 segmentsOuter Diameter: .0254 mInner Diameter: .0242 mPitch: .0349 mPorosity: 0.52PurgeMass flow rate: .011 kg/sInletPressure: 27670 PaAi
29、r mass fraction: 0.0012222Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Condenser Grid15,000 Control VolumesSimple geometry allows structured grid throughout domainGrid profile in x-z plane23Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 200
30、1ResultsCondensation RateInlet mass flow rateCFD: 2.124 kg/sExp.: 2.032 kg/sError: 4.5%Cooling water temperature contoursVolumetric condensation rate contours24Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001McAllister Condenser Flow FieldVelocity MagnitudeMax: 34.4 m/s
31、Min: 0.02 m/sPressureMax: 27,663 PaMin: 27,530 PaAir Mass FractionMax: .534Min: .00122Condensation Rate *Max: 6.1 kg/smMin: 0.0 kg/sm* Minimum condensation rate in tube bundle is 0.18 kg/sm25Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Effect of Air on Condensation R
32、ateVolumetric condensation rate contours without airVolumetric condensation rate contours with airComparison of modeling results with and without non-condensable gases26Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Effect of Inundationon Condensation RateTracking cond
33、ensate film flow rate from upper tubes to lower tubes allows use of inundation correction factorContours of condensate film mass flow rateContours of volumetric condensation rate27Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001ConclusionsModeling phase change processes
34、 in heat exchangers is possible in Fluent 5 through the use of UDFs This approach is well-suited for falling film condensation or evaporation processesHeat and mass transfer models must be provided by the userCFD modeling of McAllister condenserDescription of model development process intended to serve as a reference for users who wish to develop similar modelsAccurate results demonstrate the potential of this approach28Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 结束结束
