Variable resistance in thermal systems

**Library:**Simscape / Foundation Library / Thermal / Thermal Elements

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.*r*is radiation coefficient.*T*_{A}and*T*_{B}are temperatures at ports A and B, respectively.

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.

Conductive Heat Transfer | Convective Heat Transfer | Radiative Heat Transfer | Thermal Resistance

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