FLuent－换热器的相变模拟计算ppt课件

1、 UGM 2001New Initiatives at Fluent Inc.Phase Change in Heat ExchangersBrian Bell, Fluent Inc.UGM UGM 2001Motivation Demonstrate the use of Fluent to model phase change in heat exchangers Processes of interest Condensation Evaporation Boiling Illustrate how to model one such process through use of a

2、detailed example Shell-and-tube condenser Provide motivation for users to begin developing models of their own UGM 2001Outline Problem Description Shell-and-tube condenser Pure vapor condensation Non-condensable gases Modeling Approach Porous medium Heat and mass transfer modeling Model Implementati

3、on User-Defined Functions and User-Defined Memory Results Steam condenser with non-condensable gases Commercial chiller UGM 2001Description of Problem Shell-and-tube condenser UGM 2001Goals of CFD Modeling Condenser performance characterized by heat and mass transfer rate CFD allows evaluation of fa

4、ctors affecting heat and mass transfer in condenser Tube bundle configuration Tube arrangement Number of passes Location of inlet ports Baffles Pressure drop Velocity field Non-condensables Location and configuration of purge system Results allow identification of potential design UGM 2001Film Conde

5、nsation ProcessDriving potential for condensation is the temperature difference between vapor and cooling waterDriving potential variation caused by Pressure dropRise of cooling water temperatureNon-condensablesTPH2OPairCondensate layerTube wallCooling UGM 2001CFD Modeling Theory Porous medium appro

6、ach Tube bundle treated as porous medium Enables computationally efficient modeling of entire condenser Comparison with detailed modeling approach In 2-D, O(100)-O(1000) control volumes per tube versus more than one tube per control volume Heat and mass transfer models Condensation rate calculation

7、Condensation rate determined from local flow field and cooling water temperature Liquid film flow rate tracked in bundle from top to bottom Cooling water temperature tracked from inlet to UGM 2001Porous Medium Approach Representation of tube bundle as porous medium Porosity is only required paramete

8、r Porosity defined as ratio of fluid volume to total volume PduExample: staggered tube bundle with equilateral triangular layout2Pd321Porosity, b, expressed as: UGM 2001Transport Equations Generic transport equation for porous medium approachVAAdVRddAAVconvectiondiffusiondistributed resistanceEqn.co

9、ntinuity1x-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 Fluent Distributed resistance terms most significant in tube bundle UGM 2001Evaluation of Mod

10、eling Approach Advantages Computationally efficient Does an alternate, tractable approach exist? Approach demonstrated to give meaningful data by several authors Disadvantages Loss of some flow details due to averaging Can be overcome by detailed modeling of small regions of UGM 2001Heat Transfer Pr

11、ocess Film condensation on horizontal tubeCooling WaterTube WallCondensate FilmLiquid-vapor InterfaceRefrigerantVapor Latent heat released at liquid-vapor interface transferred to cooling UGM 2001Heat Transfer Model Heat transfer is modeled by coupling of thermal resistance network with CFD codeTcwT

12、t,iTt,oTiRcwRtubeRcondCooling WaterCFD code provides interface temperature, Ti Cooling water and tube thermal resistances are generally well-knownFilm heat transfer coefficient is required for R UGM 2001Film Heat Transfer Coefficient Critical component of heat transfer model Obtain from experiment02

13、 0 0 04 0 0 06 0 0 0C o n d e n s a te R e y n o ld s N u m b e r05 0 0 01 0 0 0 01 5 0 0 02 0 0 0 02 5 0 0 03 0 0 0 03 5 0 0 04 0 0 0 04 5 0 0 0Heat Transfer Coefficient W / (m2C)01 0 0 02 0 0 03 0 0 04 0 0 05 0 0 06 0 0 07 0 0 08 0 0 0Heat Transfer Coefficient BTU / (hrft2F)3 D T u b e , P u re R

14、-1 3 4 a3 5 C1 6 0 0 0 W / m27 5 0 0 W / m2, C Q1 6 0 0 0 W / m2, C Q3 1 0 0 0 W / m2, C Q4 7 0 0 0 W / m2, C Q6 3 0 0 0 W / m2, C Q Or obtain from literature Steam condensation on smooth tubesFigure courtesy of Kansas State University,Professor Steve Eckles, and Duane L. R UGM 2001Modeling Assumpti

15、ons Effect of liquid on flow field is neglected Approach can also be implemented in Eulerian-Eulerian multiphase framework Satisfactory model for liquid phase representation not currently available Published results of this type of model do not appear to show significant advantage Vapor is assumed t

16、o be saturated No superheating Vapor temperature determined from pressure field calculated by CFD UGM 2001Implementation of Model with UDFs UDFs are required for: Source terms required by porous medium approach Condensation rate Pressure drop in porous region Representation of tube bundle Porosity C

17、ondensate film flow rate accounting Cooling water temperature calculation with multiple tube UGM 2001Cooling water temperature calculation for each segmentEvery iteration, condensation rate is summed over each segmentInlet cooling water temperature = outlet temperature from previous segmentSegment o

18、utlet cooling water temperature calculated by energy balance.Log-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 cell zoneExample:

19、2 Pass bundleN = 2, M = 4inoutpcwTTcmQ mvcw,ivocw,vcw,ivocw,vlmTTTTTTlnTTTTT UGM 2001Tube Bundle Grid Structure Structured, cartesian grid used in tube bundle Each 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 c

20、reated with UDFsGrid generator, solver do NOT utilize structureUsed to track condensate film flow rate1k1,j1,icond3k1,j1,ifilm2k1,j1,ifilm1k1,j1, UGM 2001Source TermsAlgorithm for source term in continuity equationObtain pressure, velocity and species mass fraction (if necessary) from current soluti

21、on 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 termSi+1 = Si + a (So Si)Required for solution stability. Alpha typically

22、 0.01 0.10Value 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 UGM 2001Define_On_Demand Functions Define_On_Demand functions executed once per iteration Update co

23、ndensate film mass flow rate Update cooling water temperature Assume uniform temperature for each bundle segment New values stored in User-defined memory Automatic Define_On_Demand execution possible Example: UGM 2001Solution AlgorithmInitialize Solution: Assign porosity, tube bundle orientationUpda

24、te cooling water temperature and liquid condensate mass flow rateCalculate source termsSolve flow equationsYesNoSolution Converged?S UGM 2001Examples Steam condensation with non-condensable gasesMcAllister Condenserfrom: Bush et al., 1990, Proc. Int. Symp. On Condensers and C UGM 2001McAllister Cond

25、enser Geometry Boundary conditions and model inputs Shell 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: .

26、011 kg/sInletPressure: 27670 PaAir mass fraction: UGM 2001Condenser Grid 15,000 Control Volumes Simple geometry allows structured grid throughout domainGrid profile in x-z UGM 2001Results Condensation RateInlet mass flow rateCFD: 2.124 kg/sExp.: 2.032 kg/sError: 4.5%Cooling water temperature contour

27、sVolumetric condensation rate 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 kg/ UGM 2001Effect of Air on Condensation RateExperimentNon-condensableeffects includedNon-condensableeffects not includedInlet mass flow rate2.0322.1242.652Volumetric condensation rate contours without airV