# Marshak Fixed Temperature

Source: https://help.sim-flow.com/boundary-conditions/marshak-fixed-temperature
Last updated: 2026-03-11

## Description

Marshak Fixed Temperature is a variant of the Marshak boundary condition, designed for scenarios where the temperature at the boundary is fixed and known a priori. It is used in radiative heat transfer simulations where the boundary temperature does not change during the simulation. The radiative heat flux is calculated based on the fixed temperature and the radiative properties of the material, considering emission and absorption but not allowing temperature variation in response to radiative heat transfer.

This condition is suitable for situations where the boundary temperature is controlled or maintained constant, such as surfaces with constant temperature heating or cooling.

## Understanding Marshak Fixed Temperature

The Marshak Fixed Temperature boundary condition fixes the temperature at the boundary to a specified value, \(T_{rad}\). It then calculates the radiative heat flux using the Marshak approximation, incorporating the fixed temperature and surface radiative properties (emissivity, absorptivity, transmissivity).

This ensures that the boundary acts as a perfect emitter at the fixed temperature, while accounting for absorption and transmission of incident radiation.

## Application & Physical Interpretation

The Marshak Fixed Temperature is used in radiative heat transfer simulations where the boundary temperature is known and constant, such as in controlled heating or cooling scenarios.

### Marshak Fixed Temperature in Controlled Temperature Environments

**Example applications**: furnaces with fixed wall temperatures, radiative cooling systems

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

The Marshak Fixed Temperature is applied to boundaries where temperature is maintained constant.

Example Boundary Conditions set for Controlled Temperature Environments

| Physics | Temperature | Radiative Intensity |
| --- | --- | --- |
| Fixed Temperature Wall | Fixed Value (\(T_{rad}\)) | **Marshak Fixed Temperature** |
| Inlet | Fixed Value | Fixed Value |
| Outlet | Zero Gradient | Zero Gradient |

## Marshak Fixed Temperature in SimFlow

To define the Marshak Fixed Temperature boundary condition in SimFlow, radiation must be enabled and the P1 model selected. Navigate to the Boundary Conditions panel, select the appropriate boundary, and choose Marshak Fixed Temperature from the drop-down menu.

Specify the fixed radiation temperature \(T_{rad}\) and the surface properties: emissivity, absorptivity, and transmissivity.

![Marshak Fixed Temperature definition in SimFlow](https://help.sim-flow.com/boundary-conditions/images/marshak-fixed-temperature/marshak-fixed-temperature-bc-01.png)

Figure 1. Marshak Fixed Temperature definition in SimFlow

## Marshak Fixed Temperature - Alternatives

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

Marshak Fixed Temperature - Alternatives

| Boundary Condition | Description |
| --- | --- |
| [Marshak](https://help.sim-flow.com/boundary-conditions/marshak) | General Marshak condition without fixed temperature. |
| [Grey Diffusive Radiation](https://help.sim-flow.com/boundary-conditions/grey-diffusive-radiation) | Used for opaque surfaces with variable temperature. |