### 10.8.3. Boundary Details

The boundary condition options that appear will depend on the type of model you are using. Choices will come from the following:

#### 10.8.3.1. External Blackbody Temperature

This setting represents the effective blackbody temperature of any bodies beyond that boundary. For example, in the instance that a body has a wall temperature of and an emissivity of , in the absence of any attenuation, the effective blackbody temperature is given by . This temperature is not necessarily the same as the local temperature.

#### 10.8.3.2. Local Temperature

The local fluid temperature must be used to account for the incoming radiation energy. It is extremely useful for outlets or openings when the external blackbody temperature is either unknown or much lower than the expected outlet temperature.

#### 10.8.3.3. Radiation Intensity

Specify the mean radiation intensity only, and it is only supported by the P1 model.

#### 10.8.3.4. Radiative Heat Flux

You can specify the radiative heat flux directly, and it is only supported by the P1 model.

#### 10.8.3.5. Emissivity

At opaque boundaries, the emissivity must be supplied. It can be set as a function of spectral variables using CEL expressions when using the Multiband spectral modeling option.

#### 10.8.3.6. Diffuse Fraction

Diffuse fraction represents the ratio between the diffuse reflected energy and the total reflected energy at an opaque boundary. If the boundary is black (that is, unitary emissivity), this value has no meaning because no energy is reflected.

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