### 1.2.3. Conjugate Heat Transfer

CFX enables you to create solid regions in which the equations for heat transfer are solved. This is known as conjugate heat transfer, and the solid regions are known as solid domains.

Within solid domains, the conservation of energy equation can account for heat transport due to solid motion, conduction, and volumetric heat sources:

| (1–126)

where , , and are the enthalpy, density, and thermal conductivity of the solid, respectively, and is an optional volumetric heat source.

The reference frame for a solid domain is determined by the Domain Motion setting in CFX-Pre, and can be either `Stationary` or `Rotating`. In equation Equation 1–126, none of the terms depend on the reference frame. As a consequence, the Domain Motion setting has no effect on the solution field in a solid domain. However, this setting is still required in some situations, as shown in the examples presented below.

The term that has the solid velocity is an optional term that accounts for motion of the solid with respect to the reference frame, and corresponds to the Solid Motion setting in CFX-Pre. For details, see Solid Motion in the CFX-Pre User's Guide. This is an advection term applied to the temperature field. The solid domain mesh does not actually move, but because the temperature field is moved, this setting can be used to model solid motion that is tangential to the solid boundaries (you cannot advect the temperature field through a solid boundary and outside the solid domain). Examples of when solid motion can be used include:

- A continuous sheet of material moving along a conveyor belt

- A material being continuously extruded

- A solid that rotates about its symmetry axis

In each of these examples, the solid domain must have the form of a simple extruded or revolved volume to satisfy the tangential motion condition.

A rotating solid that is adjacent to a fluid domain can be modeled in different ways:

- If the rotating solid is connected to a rotating fluid domain, and both have the same rotational speed (such as for a turbine blade case), then the Domain Motion setting for both domains should be set to `Rotating`. Solid motion should not be used in this case. In addition, if a periodic sector is used for the fluid and solid domain, and the periodic boundaries do not line up circumferentially, a Frozen Rotor interface should be used.

  Note that, in this case, you can also set the solid Domain Motion setting to `Stationary` because Equation 1–126 continues to apply even when the solid Domain Motion setting is changed. Although this is a valid setup, it can be confusing, and CFX-Pre will issue a warning. You can ignore the warning message for this specific case, but for clarity it is recommended that you set the solid Domain Motion setting to `Rotating` and specify the same rotational velocity as for the adjacent fluid domain.

- If the rotating solid is a body of revolution (that is, the fluid solid interface is a surface of revolution), such as a solid disk spinning in a fluid, and the fluid domain is stationary, then there are two valid approaches:

  - For the solid domain, you can set the Domain Motion setting to `Rotating`. As was stated above, this setting does not change the equation solved in the solid domain, but it makes it clear that a frame change model is required to account for the reference frame change. If the Frozen Rotor model is used, then the solution will be the same as for a stationary solid. If there is non-axisymmetric flow in the fluid domain, such as a hot jet impinging on the disk surface (not at the axis of rotation) then a transient simulation with a transient rotator stator interface should be used, which is equivalent to rotating the solid domain mesh as the solution proceeds. The hot jet will effectively “see” a different part of the solid disk as the simulation progresses.

  - For the solid domain, you can set the Domain Motion setting to `Stationary`, then select the Solid Motion option and specify a velocity to account for the rotation of the solid. This can be done with a steady-state solution because it will activate the term involving as mentioned above. In this case, the hot jet will always “see” the same part of the solid disk, but the temperature field will be advected around the solid disk to account for the solid motion. When a rotating solid has a stationary heat source applied, such as a brake disk where the brake pad is modeled as a heat source, it is best to use this approach because it is computationally less expensive than a Transient Rotor Stator approach. A hot jet impinging on the disk can also be considered a stationary heat source.

Note that applying a Domain Motion setting of `Rotating` to a solid domain, or adding solid motion, will not have any effect on the fluid momentum equations, and therefore it is necessary to specify a rotational wall velocity on the fluid side of a domain interface whenever a solid is rotating in a stationary fluid domain (that is, whenever there is a frame change across the fluid-solid interface).

Additional information on plotting variables at a solid-fluid interface is available in Solid-Fluid Interface Variable Values in the CFX Reference Guide.

Release 18.2 - © ANSYS, Inc. All rights reserved.