# Grey Diffusive Radiation

Source: https://help.sim-flow.com/boundary-conditions/grey-diffusive-radiation
Last updated: 2026-10-01

## Description

Grey Diffusive Radiation establishes a grey-diffuse scenario for radiation intensity, \(I\), to be used with the finite-volume discrete-ordinates model (fvDOM). The radiation temperature is derived from the boundary condition of the temperature field.

"Grey" means radiation properties are independent of wavelength, and "diffuse" indicates uniform distribution across all angles. This condition handles the interplay of radiative and convective heat transfer at surfaces, crucial for high-temperature applications where both modes are significant.

## Understanding Grey Diffusive Radiation

The Grey Diffusive Radiation boundary condition assumes grey-body radiation (wavelength-independent) and diffuse emission/reflection. It integrates radiative intensity with convective heat transfer, deriving radiation temperature from the temperature field boundary condition.

This allows for accurate modeling of heat transfer in environments where both radiation and convection are important, such as high-temperature furnaces or electronic cooling.

## Application & Physical Interpretation

The Grey Diffusive Radiation is used in simulations requiring detailed radiative transfer with the fvDOM model, where grey and diffuse assumptions are valid.

### Grey Diffusive Radiation in Industrial Furnace Design

**Example applications**: furnaces, combustion chambers

This problem can be solved using the [chtMultiRegionFoam (solver)](https://help.sim-flow.com/solvers/cht-multi-region-foam).

The Grey Diffusive Radiation models heat exchange between hot gases and walls via radiation and convection.

Example Boundary Conditions set for Industrial Furnace Design

| Physics | Temperature | Radiative Intensity |
| --- | --- | --- |
| Furnace Wall | Fixed Value | **Grey Diffusive Radiation** |
| Inlet | Fixed Value | Fixed Value |
| Outlet | Zero Gradient | Zero Gradient |

### Grey Diffusive Radiation in Thermal Management of Electronic Devices

**Example applications**: electronics cooling, solar panels

This problem can be solved using the [chtMultiRegionSimpleFoam (solver)](https://help.sim-flow.com/solvers/cht-multi-region-simple-foam).

The Grey Diffusive Radiation accounts for radiative emission from hot components and convective cooling.

Example Boundary Conditions set for Thermal Management

| Physics | Temperature | Radiative Intensity |
| --- | --- | --- |
| Electronic Component | Heat Source | **Grey Diffusive Radiation** |
| Air Inlet | Fixed Value | Fixed Value |
| Outlet | Zero Gradient | Zero Gradient |

Example usage of Grey Diffusive Radiation for Thermal Management in SimFlow

| Tutorial | Description |
| --- | --- |
| **[Electronics Cooling](https://help.sim-flow.com/tutorials/electronics-cooling)** | Analysis of heat dissipation in electronic devices with radiation and convection. |

## Grey Diffusive Radiation in SimFlow

To define the Grey Diffusive Radiation boundary condition in SimFlow, radiation must be enabled and the Discrete Ordinates (fvDOM) model selected. Navigate to the Boundary Conditions panel, select the appropriate boundary, and choose Grey Diffusive Radiation from the drop-down menu.

Define the surface properties: emissivity, absorptivity, and transmissivity.

![Grey Diffusive Radiation definition in SimFlow](https://help.sim-flow.com/boundary-conditions/images/grey-diffusive-radiation/grey-diffusive-radiation-bc01.png)

Figure 1. Grey Diffusive Radiation definition in SimFlow

## Grey Diffusive Radiation - Alternatives

In this section, we propose boundary conditions that are alternative to Grey Diffusive Radiation. While they may fulfill similar purposes, they might be better suited for a specific application and provide a better approximation of physical world conditions.

Grey Diffusive Radiation - Alternatives

| Boundary Condition | Description |
| --- | --- |
| [Marshak](https://help.sim-flow.com/boundary-conditions/marshak) | Used with the P1 radiation model for participating media. |
| [Marshak Fixed Temperature](https://help.sim-flow.com/boundary-conditions/marshak-fixed-temperature) | Marshak with fixed boundary temperature. |