# Variable Thermal Resistance

Variable resistance in thermal systems

• Library:
• 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

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

## 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.