This video is taken from the HEAT 100 course on Lumerical University.

## Transcript

In this video we are going to learn about different boundary conditions available in

the HEAT solver.

The Boundary condition objects allow the user to define various thermal boundary conditions

at the simulation boundaries and interfaces.

All the available boundary conditions can be found in the “boundary conditions”

section under the “HEAT” tab.

Once added, they appear inside the “boundary conditions” group under the HEAT solver

in the objects tree.

In the property editor window of every boundary condition object, the Geometry tab defines

the location where the boundary condition object gets applied by using reference geometries.

This can be at the interface between two materials, at the surface of a geometric solid, at a

simulation boundary, or at the intersection of a solid and the simulation boundary.

To get familiar with the concept and different types of reference geometries, please visit

the “Reference Geometries” unit of this course.

The rest of the settings for different boundary condition types can differ due to the difference

in their definition and behavior.

A Temperature boundary condition can be used to define a constant temperature at a simulation

boundary and is most commonly used to set a surface or boundary such as the bottom of

a device or substrate to room temperature.

The BC mode can be steady-state or transient keeping in mind that a transient definition

is only valid in the case of a transient simulation.

In the steady-state mode, the temperature can remain constant by choosing the sweep

type to be single or can be swept linearly over a range of values by selecting the “range”

option.

In addition, it is possible to define a table of custom temperature values to sweep over

when the sweep type is set to “value”.

The thermal impedance option, when enabled, will allow the user to add some additional

thermal impedance at the boundary to define how resistive the boundary is to the conductive

heat flow.

A higher value means the boundary is closer to a thermally insulating boundary.

When a transient simulation is being performed, choosing the transient bc mode allows the

user to define a table of temperature versus time which can be used to define a time-varying

temperature at the corresponding boundary.

The Power boundary condition can be used to define a heat power flow at the boundary.

The flow could be inward (for positive values) or outward (for negative values) to model

heating or cooling effects with a constant rate at a boundary.

Just like temperature boundary condition, power boundary condition can be defined in

steady-state or transient modes and can be swept over a linear range or table of values.

As an alternative to power boundary condition, the Heat flux boundary condition can be used

to define a cooling or heating boundary condition at a surface by setting the amount of power

per unit area passing through that surface.

The convection boundary condition can be used to model heat loss to a fluid (e.g. air) in

contact with the simulated structure by means of convection heat transfer.

It can be defined at any external simulation boundary or at any specific interface (usually

between a solid and a fluid).

The amount of heat transfer depends on the temperature of the fluid which is defined

by the ambient temperature parameter.

A variety of models are available to define the convective heat transfer coefficient but

the constant model which defines the heat transfer through a constant coefficient (h)

is most commonly used.

A higher convective heat transfer coefficient means higher heat loss at this boundary.

For more information about the rest of available analytic models, please see the related links

below.

The Radiation boundary condition enables the user to model heat loss at a particular interface

due to thermal radiation according to the Stefan-Boltzmann law.

The temperature of the object’s surrounding medium and its emissivity, which is a measure

of its ability to emit energy as thermal radiation, need to be specified in order to define this

boundary condition.

An insulating boundary condition can model a thermally insulating boundary which does

not allow any heat transfer at that interface.

If no boundary condition is assigned to an external simulation boundary, it will be considered

insulating.

No settings other than geometry definition are required for this particular boundary

condition.

The last boundary condition in this list, the voltage boundary condition, is only used

in thermal and conductive simulations to apply a bias voltage to the electrical contacts

of the simulated structure.

Refer to the thermal-conductive and transient simulation section of this course to learn

more about the usage of the voltage boundary conditions.