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1、 UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 1 New Initiatives at Fluent Inc.Phase Change in Heat ExchangersBrian Bell, Fluent Inc.UGM UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 2 Motivation Demonstrate the use of Fluent to model pha
2、se change in heat exchangersProcesses of interestuCondensationuEvaporationuBoiling Illustrate how to model one such process through use of a detailed exampleShell-and-tube condenser Provide motivation for users to begin developing models of their own UGM 2001Company Confidential Copyright 2001 Fluen
3、t Inc. All rights reserved. 3 Outline Problem DescriptionShell-and-tube condenseruPure vapor condensationuNon-condensable gases Modeling ApproachPorous mediumHeat and mass transfer modeling Model ImplementationUser-Defined Functions and User-Defined Memory ResultsSteam condenser with non-condensable
4、 gasesCommercial chiller UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 4 Description of Problem Shell-and-tube condenser UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 5 Goals of CFD Modeling Condenser performance characterized by heat and
5、 mass transfer rate CFD allows evaluation of factors affecting heat and mass transfer in condenseruTube bundle configurationtTube arrangementtNumber of passestLocation of inlet portstBafflesuPressure dropuVelocity fielduNon-condensablestLocation and configuration of purge system Results allow identi
6、fication of potential design UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 6 Film Condensation ProcessDriving potential for condensation is the temperature difference between vapor and cooling waterDriving potential variation caused by Pressure dropRise of cooling wate
7、r temperatureNon-condensablesTPH2OPCondensate layerTube wallCooling UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 7 CFD Modeling Theory Porous medium approachTube bundle treated as porous mediumEnables computationally efficient modeling of entire condenserComparison wi
8、th detailed modeling approachuIn 2-D, O(100)-O(1000) control volumes per tube versus more than one tube per control volume Heat and mass transfer modelsCondensation rate calculationuCondensation rate determined from local flow field and cooling water temperatureuLiquid film flow rate tracked in bund
9、le from top to bottomuCooling water temperature tracked from inlet to UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 8 Porous Medium Approach Representation of tube bundle as porous mediumPorosity is only required parameterPorosity defined as ratio of fluid volume to to
10、tal volume PduExample: staggered tube bundle with equilateral triangular layout2Pd321Porosity, b, expressed as: UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 9 Transport Equations Generic transport equation for porous medium approachVAAdVRddbbAAVconvectiondiffusiondist
11、ributed 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 grid Porosity in convection and diffusion terms not modeled in FluentDistributed resistance terms most significant in tube bundle UG
12、M 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 10 Evaluation of Modeling Approach AdvantagesComputationally efficientuDoes an alternate, tractable approach exist?Approach demonstrated to give meaningful data by several authors DisadvantagesLoss of some flow details due to
13、 averagingCan be overcome by detailed modeling of small regions of UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 11 Heat Transfer Process Film condensation on horizontal tubeCooling WaterTube WallCondensate FilmLiquid-vapor InterfaceRefrigerantVapor Latent heat release
14、d at liquid-vapor interface transferred to cooling UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 12 Heat Transfer Model Heat transfer is modeled by coupling of thermal resistance network with CFD codeTcwTt,iTt,oTiRcwRtubeRcondCooling WaterCFD code provides interface te
15、mperature, Ti Cooling water and tube thermal resistances are generally well-knownFilm heat transfer coefficient is required for R UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 13 Film Heat Transfer Coefficient Critical component of heat transfer model Obtain from exper
16、iment0200040006000C ondensate R eynolds N um ber050001000015000200002500030000350004000045000Heat Transfer Coefficient W / (m2C)010002000300040005000600070008000Heat Transfer Coefficient BTU / (hrft2F)3D Tube, Pure R -134a35 C16000 W / m27500 W / m2, C Q16000 W / m2, C Q31000 W / m2, C Q47000 W / m2
17、, C Q63000 W / m2, C Q Or obtain from literatureSteam condensation on smooth tubesFigure courtesy of Kansas State University,Professor Steve Eckles, and Duane L. R UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 14 Modeling Assumptions Effect of liquid on flow field is n
18、eglectedApproach can also be implemented in Eulerian-Eulerian multiphase frameworkuSatisfactory model for liquid phase representation not currently availableuPublished results of this type of model do not appear to show significant advantage Vapor is assumed to be saturatedNo superheatingVapor tempe
19、rature determined from pressure field calculated by CFD UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 15 Implementation of Model with UDFs UDFs are required for:Source terms required by porous medium approachuCondensation rateuPressure drop in porous regionRepresentati
20、on of tube bundleuPorosityuCondensate film flow rate accountinguCooling water temperature calculation with multiple tube UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 16 Cooling water temperature calculation for each segmentuEvery iteration, condensation rate is summed
21、 over each segmentuInlet cooling water temperature = outlet temperature from previous segmentuSegment 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
22、RepresentationBundle consists of N passes and M segmentsEach segment defined as unique cell zoneExample:2 Pass bundleN = 2, M = 4inoutpcwTTcmQ mvcw,ivocw,vcw,ivocw,vlmTTTTTTlnTTTTT UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 17 Tube Bundle Grid Structure Structured,
23、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 structureUsed to track condensate film flow rate1k1,j1,icond3k1,j1,ifi
24、lm2k1,j1,ifilm1k1,j1, UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 18 Source TermsAlgorithm for source term in continuity equationObtain pressure, velocity and species mass fraction (if necessary) from current solution valuesObtain film Reynolds number and cooling wat
25、er 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 from previous iter
26、ation, So, stored in User-Defined MemorySource term in momentum equations Calculated using empirical correlations with tube bundle porosity and current UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 19 Define_On_Demand Functions Define_On_Demand functions executed once
27、per iterationUpdate condensate film mass flow rateUpdate cooling water temperatureuAssume uniform temperature for each bundle segmentNew values stored in User-defined memory Automatic Define_On_Demand execution possibleExample: UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reser
28、ved. 20 Solution AlgorithmInitialize Solution: Assign porosity, tube bundle orientationUpdate cooling water temperature and liquid condensate mass flow rateCalculate source termsSolve flow equationsYesNoSolution Converged?S UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved.
29、 21 Examples Steam condensation with non-condensable gasesMcAllister Condenserfrom: Bush et al., 1990, Proc. Int. Symp. On Condensers and C UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 22 McAllister Condenser Geometry Boundary conditions and model inputsShell Dimensio
30、ns 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 PaAir mass fraction: UGM 2001Company
31、Confidential Copyright 2001 Fluent Inc. All rights reserved. 23 Condenser Grid 15,000 Control Volumes Simple geometry allows structured grid throughout domainGrid profile in x-z UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 24 Results Condensation RateInlet mass flow r
32、ateCFD: 2.124 kg/sExp.: 2.032 kg/sError: 4.5%Cooling water temperature contoursVolumetric condensation rate UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 25 McAllister Condenser Flow FieldVelocity MagnitudeMax: 34.4 m/sMin: 0.02 m/sPressureMax: 27,663 PaMin: 27,530 PaA
33、ir Mass FractionMax: .534Min: .00122Condensation Rate *Max: 6.1 kg/smMin: 0.0 kg/sm* Minimum condensation rate in tube bundle is 0.18 kg/ UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 26 Effect of Air on Condensation RateExperimentNon-condensableeffects includedNon-con
34、densableeffects not includedInlet mass flow rate2.0322.1242.652Volumetric condensation rate contours without airVolumetric condensation rate contours with airComparison of modeling results with and without non-condensable UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 2
35、7 Effect of Inundationon Condensation Rate Tracking condensate film flow rate from upper tubes to lower tubes allows use of inundation correction factorExperimentInundation effectsnot modeledInundation effectsmodeledInlet mass flow rate2.0322.1242.064Contours of condensate film mass flow rateContour
36、s of volumetric condensation UGM 2001Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 28 Conclusions Modeling 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 models must be provided by the user CFD 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
