1、Chapter 10Heat Transfer,Introduction to CFX,Governing Equations,Continuity,Momentum,Energy,where,Conservation Equations,Heat transfer in a fluid domain is governed by the Energy Transport Equation:The Heat Transfer Model relates to the above equation as followsNone: Energy Transport Equation not sol
2、vedIsothermal: The Energy Transport Equation is not solved but a temperature is required to evaluated fluid properties (e.g. when using 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
3、specific heats. An optional viscous dissipation term can be included if viscous heating is significant.Total Energy: This models the 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
4、 heating effects arise in the boundary layer, where kinetic energy effects become significant.,Sources,Viscous work,Convection,Transient,Conduction,Governing Equations,Governing Equations,For multicomponent flows, reacting flows and radiation modeling additional terms are included in the energy equa
5、tionHeat transfer in a solid domain is modeled using the following conduction equation,Source,Transient,Conduction,Selecting a Heat Transfer Model,The Heat Transfer model is selected on the Domain Fluid Models panelEnable the Viscous Work term (Total Energy), or Viscous Dissipation term (Thermal Ene
6、rgy), 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 significant,Radiation effects should be accounted for when is significant compared to convective and conductive heat transfer ratesTo accou
7、nt for radiation, Radiative Intensity Transport Equations (RTEs) are solvedLocal absorption by fluid and at boundaries couples these RTEs with the energy equationRadiation intensity is directionally and spatially dependentTransport mechanisms for radiation intensity:Local absorptionOut-scattering (s
8、cattering away fromthe direction)Local emissionIn-scattering (scattering into the direction),Radiation,Several radiation models are available which provide approximate solutions to the RTEEach radiation model has its assumptions, limitations, and benefits,Radiation Models,Choosing a Radiation Model,
9、The optical thickness should be determined before choosing a radiation modelOptically thin means that the fluid is transparent to the radiation at wavelengths where the heat transfer occursThe radiation only interacts with the boundaries of the domainOptically thick/dense means that the fluid absorb
10、s and re-emits the radiationFor optically thick media the P1 model is a good choiceMany combustion simulations fall into this category since combustion gases tend to absorb radiationThe P1 models gives reasonable accuracy without too much computational effort,Choosing a Radiation Model,For optically
11、 thin media the Monte Carlo or Discrete Transfer models may be usedDTM can be less accurate in models with long/thin geometriesMonte Carlo uses the most computational resources, followed by DTMBoth models can be used in optically thick media, but the P1 model uses far less computational resourcesSur
12、face to Surface ModelAvailable for Monte Carlo and DTMNeglects the influence of the fluid on the radiation field (only boundaries participate)Can significantly reduce the solution timeRadiation in Solid DomainsIn transparent or semi-transparent solid domains (e.g. glass) only the Monte Carlo model c
13、an be usedThere is no radiation in opaque solid domains,InletStatic Temperature Total Temperature Total EnthalpyOutletNo details (except Radiation, see below)OpeningOpening Temperature Opening Static Temperature WallAdiabatic Fixed Temperature Heat Flux Heat Transfer CoefficientRadiation QuantitiesL
14、ocal Temperature (Inlet/Outlet/Opening)External Blackbody Temperature (Inlet/Outlet/Opening)OpaqueSpecify Emissivity and Diffuse Fraction,Heat Transfer Boundary Conditions,Domain Interfaces,GGI connections are recommended for Fluid-Solid and Solid-Solid interfacesIf radiation is modelled in one doma
15、in and not the other, set Emissivity and Diffuse Fraction values on the side which includes radiationSet these on the boundary condition associated with the domain interface,Thin Wall Modeling,Using solid domains to model heat transfer through thin solids can present meshing problemsThe thickness of
16、 the material must be resolved by the meshDomain interfaces can be used to model a thin materialNormal conduction only; neglects any in-plane conduction,Example: to model a baffle with heat transfer through the thicknessCreate a Fluid-Fluid Domain InterfaceOn the Additional Interface Models tab set
17、Mass and Momentum to No Slip Wall This makes the interface a wall rather than an interface that fluid can pass throughEnable the Heat Transfer toggle and pick the Thin Material optionSpecify a Material and ThicknessOther domain interface types (Fluid-Solid etc) can use the Thin Material option to re
18、present coatings etc.,Thermal Contact Resistance,A Thermal Contact Resistance can be specified using the same approach as Thin Wall modelingJust select the Thermal Contact Resistance option instead of the Thin Material option,Natural Convection,Natural convection occurswhen temperature differences i
19、n the fluid result in density variationsThis is one-type of buoyancy driven flowFlow is induced by the force of gravity acting on the density variations,As discussed in the Domains lecture, a source termSM,buoy = (r rref) g is added to the momentum equationsThe density difference (r rref) is evaluat
20、ed using either the Full Buoyancy model or the Boussinesq modelDepending on the physics the model is automatically chosen,Solution Notes,When solving heat transfer problems, make sure that you have allowed sufficient solution time for heat imbalances in all domains to become very small, particularly
21、 when Solid domains are includedSometimes residuals reach the convergence criteria before global imbalances trend towards zeroCreate Solver Monitors showing IMBALANCE levels for fluid and solid domainsView the imbalance information printed at the end of the solver output fileUse a Conservation Targe
22、t when defining Solver Control in CFX-Pre,Heat Transfer Variables,The results file contains several variables related to heat transferVariables starting with “Wall” are only defined on walls,Where Tref is the Wall Adjacent Temperature or the tbulk for htc temperature if specified,Twall,qw,Mesh,Contr
23、ol Volumes,TemperatureThis is the local fluid temperatureWhen plotted on a wall it is the temperature on the wall, TwallWall Adjacent TemperatureThis is the average temperature in the control volume next to the wallWall Heat Transfer Coefficient, hcBy default this is based on Twall and the Wall Adja
24、cent Temperature, not the far-field fluid temperatureSet the expert parameter “tbulk for htc” to define a far-field fluid temperature to use instead of the Wall Adjacent TemperatureWall Heat Flux, qwThis is the total heat flux into the domain by all modes convective and radiative (when modeled),Heat
25、 Transfer Variables,Heat FluxThis is the total convective heat flux into the domainDoes not include radiative heat transfer when a radiation model is usedConvective heat flux contains heat transfer due to both advection and diffusionIt can be plotted on all boundaries (inlets, outlets, walls etc)At
26、an inlet it would represent the energy carried with the incoming fluid relative to the fluid Reference Temperature (which is a material property, usually 25 C)Wall Radiative Heat FluxThe net radiative energy leaving the boundary (emission minus incoming)Heat Flux + Wall Radiative Heat Flux = Wall Heat FluxOnly applicable when radiation is modeledWall Irradiation FluxThe incoming radiative fluxOnly applicable when radiation is modeled,
