CFX13_10_热传导

1、Chapter 10CFX热传导,CFX热传导设置,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 follows None: Energy Transport Equation not solved Isothermal:

2、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 specific heats

3、. 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 heating effe

4、cts 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 equation Heat tra

5、nsfer 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 panel Enable the Viscous Work term (Total Energy), or Viscous Dissipation term (Thermal Energy), if vi

6、scous 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 rates To account for ra

7、diation, Radiative Intensity Transport 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 (scatt

8、ering away fromthe direction) Local emission In-scattering (scattering into the direction),Radiation,Several radiation models are available which provide approximate solutions to the RTE Each radiation model has its assumptions, limitations, and benefits,Radiation Models,Choosing a Radiation Model,T

9、he optical thickness should be determined before choosing a radiation model Optically thin means that the fluid is transparent to the radiation at wavelengths where the heat transfer occurs The radiation only interacts with the boundaries of the domain Optically thick/dense means that the fluid abso

10、rbs 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 gases tend to absorb radiation The P1 models gives reasonable accuracy without too much computational effort,Choosing a Radiation Model,For opti

11、cally thin media the Monte Carlo or Discrete 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 reso

12、urces Surface to Surface Model Available 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

13、 Carlo model can be used There is no radiation in opaque solid domains,Inlet Static Temperature Total Temperature Total Enthalpy Outlet No details (except Radiation, see below) Opening Opening Temperature Opening Static Temperature Wall Adiabatic Fixed Temperature Heat Flux Heat Transfer Coefficient

14、 Radiation Quantities Local Temperature (Inlet/Outlet/Opening) External Blackbody Temperature (Inlet/Outlet/Opening) Opaque Specify Emissivity and Diffuse Fraction,Heat Transfer Boundary Conditions,Domain Interfaces,GGI connections are recommended for Fluid-Solid and Solid-Solid interfaces If radiat

15、ion is modelled in one domain and not the other, set Emissivity and Diffuse Fraction values on the side which includes radiation Set 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 mesh

16、ing 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 Fluid-Fluid Domain Interface On the Add

17、itional 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 interface types (Fluid-Solid etc) ca

18、n use the Thin Material option to represent coatings etc.,Thermal Contact Resistance,A Thermal Contact Resistance can be specified using the same approach as Thin Wall modeling Just select the Thermal Contact Resistance option instead of the Thin Material option,Natural Convection,Natural convection

19、 occurswhen temperature 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 = (r rref) g is added to the momentum equations Th

20、e density difference (r rref) is evaluated using either the Full Buoyancy model or the Boussinesq model Depending 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 d

21、omains 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 showing IMBALANCE levels for fluid and solid domains View the imbalance information printed at the end of th

22、e solver output file Use a Conservation Target when defining Solver Control in CFX-Pre,Heat Transfer Variables,The results file contains several variables related to heat transfer Variables starting with “Wall” are only defined on walls,Where Tref is the Wall Adjacent Temperature or the tbulk for ht

23、c temperature if specified,Twall,qw,Mesh,Control Volumes,Temperature 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

24、 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 temperature to use instead of the Wall Adjacent Temperature Wall Heat Flux, qw This is the total heat flux into the domain by all