This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
• 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: 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. 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 effects arise in the boundary layer, where kinetic energy effects become significant.
• 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 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
• For optically 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 resources
• 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 Carlo model can be used– There is no radiation in opaque solid domains
• 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 values on the side which includes radiation– Set these on the boundary
Training ManualThin Wall Modeling• Using solid domains to model heat transfer through 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 Fluid-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 interface types (Fluid-Solid etc) can use the Thin Material option to represent coatings etc.
• 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 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 the solver output file
– Use a Conservation Target when defining Solver Control in CFX-Pre
• Heat Flux– This is the total convective heat flux into the domain
• Does not include radiative heat transfer when a radiation model is used• Convective heat flux contains heat transfer due to both advection and diffusion
– It can be plotted on all boundaries (inlets, outlets, walls etc)• At 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 Flux– The net radiative energy leaving the boundary (emission minus incoming)– Heat Flux + Wall Radiative Heat Flux = Wall Heat Flux– Only applicable when radiation is modeled
• Wall Irradiation Flux– The incoming radiative flux– Only applicable when radiation is modeled