Ansys_12.0_CFX_官方教程__10(Heat Transfer)

1、Heat Transfer10-1ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training Manual10-1ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Chapter 10Heat TransferIntroduction to CFX Heat Transfer10-2ANSYS, Inc. Propriet

2、ary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training ManualGoverning EquationsContinuityMomentumEnergywhereConservation EquationsHeat Transfer10-3ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training Manual Heat transfer in a

3、 fluid domain is governed by the Energy Transport Equation: The Heat Transfer Model relates to the above equation as follows None: Energy Transport Equation not solved Isothermal: The Energy Transport Equation is not solved but a temperature is required to evaluated fluid properties (e.g. when using

4、 an Ideal Gas) Thermal Energy: An Energy Transport Equation is solved which neglects variable density effects. It is suitable for low speed liquid flow with constant specific heats. An optional viscous dissipation term can be included if viscous heating is significant. Total Energy: This models the

5、transport of enthalpy and includes kinetic energy effects. It should be used for gas flows where the Mach number exceeds 0.2, and high speed liquid flows where viscous heating effects arise in the boundary layer, where kinetic energy effects become significant. SourcesViscous workConvectionTransient

6、ConductionGoverning EquationsEtottotSUThUtpth)()() ()(Heat Transfer10-4ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training ManualGoverning Equations For multicomponent flows, reacting flows and radiation modeling additional terms are included in the e

7、nergy equation Heat transfer in a solid domain is modeled using the following conduction equationSourceTransientConductionHeat Transfer10-5ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training ManualSelecting a Heat Transfer Model The Heat Transfer mode

8、l is selected on the Domain Fluid Models panel Enable the Viscous Work term (Total Energy), or Viscous Dissipation term (Thermal Energy), if viscous shear in the fluid is large (e.g. lubrication or high speed compressible flows) Enable radiation model / submodels if radiative heat transfer is signif

9、icantHeat Transfer10-6ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training Manual Radiation effects should be accounted for when is significant compared to convective and conductive heat transfer rates To account for radiation, Radiative Intensity Tran

10、sport Equations (RTEs) are solved Local absorption by fluid and at boundaries couples these RTEs with the energy equation Radiation intensity is directionally and spatially dependent Transport mechanisms for radiation intensity: Local absorption Out-scattering (scattering away fromthe direction) Loc

11、al emission In-scattering (scattering into the direction)Radiation)(4min4maxradTTQHeat Transfer10-7ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training Manual Several radiation models are available which provide approximate solutions to the RTE Each ra

12、diation model has its assumptions, limitations, and benefits1) Rosseland Model (Diffusion Approximation Model)2) P-1 Model (Gibbs Model/Spherical Harmonics Model)3) Discrete Transfer Model (DTM) (Shah Model)4) Monte Carlo Model (not available in the ANSYS CFD-Flo product)Radiation ModelsHeat Transfe

13、r10-8ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training ManualChoosing a Radiation Model The optical thickness should be determined before choosing a radiation model Optically thin means that the fluid is transparent to the radiation at wavelengths w

14、here the heat transfer occurs The radiation only interacts with the boundaries of the domain Optically thick/dense means that the fluid absorbs and re-emits the radiation For optically thick media the P1 model is a good choice Many combustion simulations fall into this category since combustion gase

15、s tend to absorb radiation The P1 models gives reasonable accuracy without too much computational effortHeat Transfer10-9ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training ManualChoosing a Radiation Model For optically thin media the Monte Carlo or D

16、iscrete Transfer models may be used DTM can be less accurate in models with long/thin geometries Monte Carlo uses the most computational resources, followed by DTM Both models can be used in optically thick media, but the P1 model uses far less computational resources Surface to Surface Model Availa

17、ble for Monte Carlo and DTM Neglects the influence of the fluid on the radiation field (only boundaries participate) Can significantly reduce the solution time Radiation in Solid Domains In transparent or semi-transparent solid domains (e.g. glass) only the Monte Carlo model can be used There is no

18、radiation in opaque solid domainsHeat Transfer10-10ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training Manual Inlet Static Temperature Total Temperature Total Enthalpy Outlet No details (except Radiation, see below) Opening Opening Temperature Opening

19、 Static Temperature Wall Adiabatic Fixed Temperature Heat Flux Heat Transfer Coefficient Radiation Quantities Local Temperature (Inlet/Outlet/Opening) External Blackbody Temperature (Inlet/Outlet/Opening) Opaque Specify Emissivity and Diffuse FractionHeat Transfer Boundary ConditionsHeat Transfer10-

20、11ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training ManualDomain Interfaces GGI connections are recommended for Fluid-Solid and Solid-Solid interfaces If radiation is modelled in one domain and not the other, set Emissivity and Diffuse Fraction valu

21、es on the side which includes radiation Set these on the boundary condition associated with the domain interfaceHeat Transfer10-12ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training ManualThin Wall Modeling Using solid domains to model heat transfer t

22、hrough thin solids can present meshing problems The thickness of the material must be resolved by the mesh Domain interfaces can be used to model a thin material Normal conduction only; neglects any in-plane conduction Example: to model a baffle with heat transfer through the thickness Create a Flui

23、d-Fluid Domain Interface On the Additional Interface Models tab set Mass and Momentum to No Slip Wall This makes the interface a wall rather than an interface that fluid can pass through Enable the Heat Transfer toggle and pick the Thin Material option Specify a Material and Thickness Other domain i

24、nterface types (Fluid-Solid etc) can use the Thin Material option to represent coatings etc.Heat Transfer10-13ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training ManualThermal Contact Resistance A Thermal Contact Resistance can be specified using the

25、same approach as Thin Wall modeling Just select the Thermal Contact Resistance option instead of the Thin Material optionHeat Transfer10-14ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training ManualNatural Convection Natural convection occurswhen tempe

26、rature differences in the fluid result in density variations This is one-type of buoyancy driven flow Flow is induced by the force of gravity acting on the density variations As discussed in the Domains lecture, a source termSM,buoy = ( ref) g is added to the momentum equations The density differenc

27、e ( ref) is evaluated using either the Full Buoyancy model or the Boussinesq model Depending on the physics the model is automatically chosenHeat Transfer10-15ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598Training ManualSolution Notes When solving heat t

28、ransfer problems, make sure that you have allowed sufficient solution time for heat imbalances in all domains to become very small, particularly when Solid domains are included Sometimes residuals reach the convergence criteria before global imbalances trend towards zero Create Solver Monitors showi

29、ng IMBALANCE levels for fluid and solid domains View the imbalance information printed at the end of the solver output file Use a Conservation Target when defining Solver Control in CFX-PreHeat Transfer10-16ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.April 28, 2009Inventory #002598T

30、raining ManualHeat Transfer Variables The results file contains several variables related to heat transfer Variables starting with “Wall” are only defined on walls)(refwallcwTThqWhere Tref is the Wall Adjacent Temperature or the tbulk for htc temperature if specifiedTwallqwMeshControl Volumes Temper

31、ature This is the local fluid temperature When plotted on a wall it is the temperature on the wall, Twall Wall Adjacent Temperature This is the average temperature in the control volume next to the wall Wall Heat Transfer Coefficient, hc By default this is based on Twall and the Wall Adjacent Temperature, not the far-field fluid temperature Set the expert parameter “tbulk for htc” to define a far-field fluid tempera