# Variable Thermal Resistance

Variable resistance in thermal systems

Libraries:
Simscape / Foundation Library / Thermal / Thermal Elements

## Description

The Variable Thermal Resistance block lets you model a variable heat transfer process in generalized terms, independent of whether it is by conduction, convection, radiation, or a combination thereof. Thermal resistance is an abstract quantity that relates the thermal conductivity, the heat transfer coefficient, and the radiation coefficient. Thermal resistance of this block can vary with time, according to the input physical signal at port R.

The heat transfer equation for the Thermal Resistance block is:

`$R\cdot Q=\Delta T$`

where:

• R is the thermal resistance.

• Q is the heat flow rate.

• ΔT is the temperature difference between layers.

Thermal resistance is related to the other heat transfer quantities as follows:

`$R=\frac{D}{k\cdot A}=\frac{1}{h\cdot A}=\frac{1}{r\cdot A\left({T}_{A}^{2}+{T}_{B}^{2}\right)\left({T}_{A}+{T}_{B}\right)}$`

where:

• D is material thickness, that is, distance between layers.

• A is area normal to the heat flow direction.

• k is thermal conductivity of the material.

• h is convection heat transfer coefficient.

• TA and TB are temperatures at ports A and B, respectively.

### Variables

To set the priority and initial target values for the block variables prior to simulation, use the Initial Targets section in the block dialog box or Property Inspector. For more information, see Set Priority and Initial Target for Block Variables.

Nominal values provide a way to specify the expected magnitude of a variable in a model. Using system scaling based on nominal values increases the simulation robustness. Nominal values can come from different sources, one of which is the Nominal Values section in the block dialog box or Property Inspector. For more information, see Modify Nominal Values for a Block Variable.

## Ports

### Input

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Input physical signal that specifies the thermal resistance value.

### Conserving

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Thermal conserving port associated with layer A.

Thermal conserving port associated with layer B.

## Parameters

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If the input physical signal falls below this value, the resistance remains at this value.

## Version History

Introduced in R2017b