### 1.2.5. Heat Transfer

A heat transfer model is used to predict the temperature throughout the flow. Heat transfer by conduction, convection, and (where appropriate) turbulent mixing and viscous work are included.

The following options for heat transfer available in CFX:

#### 1.2.5.1. None

Select this option if your simulation does not involve the modeling of heat transfer. It eliminates the heat transfer calculation from the governing equations. This option reduces the number of calculations performed, and subsequently the time required, by the CFX-Solver.

#### 1.2.5.2. Isothermal

This model requires you to enter a uniform temperature for the fluid in absolute temperature terms. This can be used for the purpose of evaluating fluid properties that are temperature-dependent; for example, the density of an ideal gas. For general fluids, a constant temperature can be used as the basis for a series of isothermal simulations using temperature-dependent fluid properties. You may also use this option to create an initial results file for a more complex model. Heat transfer is not modeled.

#### 1.2.5.3. Thermal Energy

This models the transport of enthalpy through the fluid domain. It differs from the `Total Energy` model in that the effects of mean flow kinetic energy are not included. It consequently reproduces the same results as the `Total Energy` model when kinetic energy effects vanish, and is therefore adequate for low speed flows where kinetic effects are negligible.

For cases where viscous heating and the effects of turbulence on it are important, it is recommended that you use the `Total Energy` model.

#### 1.2.5.4. Total Energy

This models the transport of enthalpy and includes kinetic energy effects. It should be used where kinetic energy effects become significant, for example gas flows where the Mach number exceeds 0.3. The selection of the `Total Energy` model has implications for whether the fluid is modeled as compressible or incompressible (see Compressible Flow) and for which buoyancy model (see Buoyancy) is employed by the CFX-Solver.

The mathematical model for heat transfer is available in Transport Equations in the CFX-Solver Theory Guide.

Note: The Incl. Viscous Work Term option should be selected for a case involving MFR (multiple frames of reference) and the Total Energy equation. For details, see The Total Energy Equation in the CFX-Solver Theory Guide.

#### 1.2.5.5. Turbulent Flux Closure

When the heat transfer model is set to either Thermal Energy or Total Energy, the Turbulent Flux Closure for Heat Transfer may be optionally selected in the Turbulence settings. When Turbulent Flux Closure for Heat Transfer is not selected, the default is the Eddy Diffusivity model with the turbulent Prandtl number of 0.9. The turbulent Prandtl number may be non-constant using by expressions (CEL), for example, as a function of physical space or solution variables.

By default the turbulent Prandtl number specified for heat transfer will also be applied as the turbulent Schmidt number for component mass fractions and combustion scalars.

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