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1、 UGM 2001FLuent换热器的相变模拟计算Stillwatersrundeep.流静水深流静水深,人静心深人静心深Wherethereislife,thereishope。有生命必有希望。有生命必有希望 UGM 2001MotivationDemonstrate the use of Fluent to model phase change in heat exchangersProcesses of interestuCondensationuEvaporationuBoiling Illustrate how to model one such process through us
2、e of a detailed exampleShell-and-tube condenserProvide motivation for users to begin developing models of their own devices 2 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001OutlineProblem DescriptionShell-and-tube condenseruPure vapor condensationuNon-condensable gases
3、Modeling ApproachPorous mediumHeat and mass transfer modelingModel ImplementationUser-Defined Functions and User-Defined MemoryResultsSteam condenser with non-condensable gasesCommercial chiller condenser 3 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Description of
4、ProblemShell-and-tube condenser 4 Company Confidential Copyright 2001 Fluent 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 configurat
5、iontTube arrangementtNumber of passestLocation of inlet portstBafflesuPressure dropuVelocity fielduNon-condensablestLocation and configuration of purge systemResults allow identification of potential design improvements 5 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001
6、Film Condensation ProcessDriving potential for condensation is the temperature difference between vapor and cooling waterDriving potential variation caused by Pressure dropRise of cooling water temperatureNon-condensablesTPH2OPairairCondensate layerTube wallCooling water 6 Company Confidential Copyr
7、ight 2001 Fluent Inc. All rights reserved. UGM 2001CFD Modeling TheoryPorous medium 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 o
8、ne tube per control volumeHeat and mass transfer modelsCondensation rate calculationuCondensation 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 outlet 7 Company Confidenti
9、al Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Porous Medium ApproachRepresentation 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
10、, expressed as: 8 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Transport EquationsGeneric transport equation for porous medium approachconvectiondiffusiondistributed resistanceEqn.continuity1x-mom.uy-mom.vspeciesw w Distributed resistance takes form of source terms t
11、hat model details of the flow that are not resolved by the gridPorosity in convection and diffusion terms not modeled in FluentDistributed resistance terms most significant in tube bundle region 9 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Evaluation of Modeling Ap
12、proachAdvantagesComputationally efficientuDoes an alternate, tractable approach exist?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 condenser 10 Company Confidential Copy
13、right 2001 Fluent Inc. All rights reserved. UGM 2001Heat Transfer ProcessFilm condensation on horizontal tubeCooling WaterTube WallCondensate FilmLiquid-vapor InterfaceRefrigerantVaporLatent heat released at liquid-vapor interface transferred to cooling water 11 Company Confidential Copyright 2001
14、Fluent Inc. All rights reserved. UGM 2001Heat Transfer ModelHeat transfer is modeled by coupling 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
15、 transfer coefficient is required for Rcond 12 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Film Heat Transfer CoefficientCritical component of heat transfer modelObtain from experimentOr obtain from literatureSteam condensation on smooth tubesFigure courtesy of Kans
16、as State University,Professor Steve Eckles, and Duane L. Randall 13 Company Confidential Copyright 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 mod
17、el for liquid phase representation not currently availableuPublished results of this type of model do not appear to show significant advantageVapor is assumed to be saturatedNo superheatingVapor temperature determined from pressure field calculated by CFD code 14 Company Confidential Copyright 2001
18、Fluent Inc. All rights reserved. UGM 2001Implementation of Model with UDFsUDFs are required for:Source terms required by porous medium approachuCondensation rateuPressure drop in porous regionRepresentation of tube bundleuPorosityuCondensate film flow rate accountinguCooling water temperature calcu
19、lation with multiple tube passes 15 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Cooling water temperature calculation for each segmentuEvery iteration, condensation rate is summed over each segmentuInlet cooling water temperature = outlet temperature from previous s
20、egmentuSegment outlet cooling water temperature calculated by energy balance.uLog-mean temperature 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
21、cell zoneExample:2 Pass bundleN = 2, M = 4 16 Company Confidential Copyright 2001 Fluent Inc. All rights 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
22、=3i =1j=2k=3i=1j=3k=3Grid structure created with UDFsuGrid generator, solver do NOT utilize structureUsed to track condensate film flow rate 17 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Source TermsAlgorithm for source term in continuity equationObtain pressure, v
23、elocity and species mass fraction (if necessary) from current solution valuesObtain film Reynolds number 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 termu
24、Si+1 = Si + a (So Si)uRequired for solution stability. Alpha typically 0.01 0.10uValue of source term from previous iteration, So, stored in User-Defined MemorySource term in momentum equations Calculated using empirical correlations with tube bundle porosity and current velocity 18 Company Confiden
25、tial Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Define_On_Demand FunctionsDefine_On_Demand functions executed once per iterationUpdate condensate film mass flow rateUpdate cooling water temperatureuAssume uniform temperature for each bundle segmentNew values stored in User-defined memo
26、ryAutomatic Define_On_Demand execution possibleExample: 19 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Solution AlgorithmInitialize Solution: Assign porosity, tube bundle orientationUpdate cooling water temperature and liquid condensate mass flow rateCalculate sourc
27、e termsSolve flow equationsYesNoSolution Converged?Stop 20 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001ExamplesSteam condensation with non-condensable gasesMcAllister Condenserfrom: Bush et al., 1990, Proc. Int. Symp. On Condensers and Condensation 21 Company Confid
28、ential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001McAllister Condenser GeometryBoundary conditions 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: .02
29、54 mInner Diameter: .0242 mPitch: .0349 mPorosity: 0.52PurgeMass flow rate: .011 kg/sInletPressure: 27670 PaAir mass fraction: 0.00122 22 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Condenser Grid15,000 Control VolumesSimple geometry allows structured grid throughou
30、t domainGrid profile in x-z plane 23 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001ResultsCondensation RateInlet mass flow rateCFD: 2.124 kg/sExp.: 2.032 kg/sError: 4.5%Cooling water temperature contoursVolumetric condensation rate contours 24 Company Confidential Cop
31、yright 2001 Fluent Inc. All rights reserved. UGM 2001McAllister Condenser Flow FieldVelocity MagnitudeMax: 34.4 m/sMin: 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
32、kg/sm 25 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Effect of Air on Condensation RateVolumetric condensation rate contours without airVolumetric condensation rate contours with airComparison of modeling results with and without non-condensable gases 26 Company Con
33、fidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001Effect of Inundationon Condensation RateTracking condensate film flow rate from upper tubes to lower tubes allows use of inundation correction factorContours of condensate film mass flow rateContours of volumetric condensation rate 2
34、7 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. UGM 2001ConclusionsModeling phase change processes 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 mode
35、ls 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 approach 28 Company Confidential Copyright 2001 Fluent Inc. All rights reserved.
