# Describing boundary conditions

## Introduction

The boundary conditions are described using region known as non-material:

• mainly face regions in 3D, possibly line regions
• mainly line regions in 2D, possibly point regions

For additional information on regions, see chapter Physics: principles.

## Non-material regions

Non-material regions enable the imposing of boundary conditions (BC) on faces or lines.

A region of type…

is a BC that enables one to impose…

Heat exchange surface

or *

heat source

the thermal exchanges by convection/radiation (local or infinite)

or

a heat source (uniform or space dependent)

Imposed temperature

a temperature (uniform or space dependent)

Thermal insulated region

a tangential thermal flux

… on the faces/lines that form the region
Note: * Note that neither has any physical meaning

## Interpretation

A graphic interpretation of boundary conditions is given in the figure below.

## Important

The boundary conditions (heat flux, heat transfer, etc.) must be chosen correctly so that the quantity of heat entering can be discharged. Otherwise, the temperature may become infinite or it may decrease even below absolute zero. (See Balance of thermal exchanges / § Interprétation des résultats)