# Uniform Heat Source vs Thermal Boundary Condition

Hello,

I would like to clarify the difference between two options in HEAT solver. What I want is to heat a wire so as to thermally tune a waveguide.
a) Use a thermal boundary condition with the following properties:
Geometry: solid
Solid : wire
Model: Power value in W

b) Use of a “Uniform Heat Source” with the dimensions of the wire and the same value in “Total Power” field

Hi,

I believe that both methods can be used, however the uniform heat source can only be rectangular, so if your wire has a curved shape which is not rectangular then you should apply the thermal boundary condition to the wire object.

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Hi. In some cases, both the uniform heat source and the thermal boundary condition will give identical results. However there is a fundamental difference between the two methods can become very important in a lot of cases. Generally speaking the uniform heat source is a better option if you want to introduce some heat input in your simulation. The reason is as follows:

When you use the thermal boundary condition on a solid to create a heat source in the simulation volume, the solver injects that amount of heat energy into the simulated structure from the surface of the solid. However, the solid itself does not get included in the simulation volume in this case. The following screenshot will clarify this.

On the left you can see the wire (heater) where the thermal boundary condition is applied. On the right is the temperature profile given by the solver. You can notice the vacant region in the plot. You can also see that the vacant region is preventing heat to go though the heater (solid) which is unphysical.

In this other screenshot we have now used a uniform heat source to apply the same amount of power to the solid heater. In the temperature plot on the right we can see that there is not empty region and heat from the top surface of the heater can go through the solid. This results in a higher temperature at the heater and this result would be the more accurate one.

Now in cases where the heater is at one edge of the simulation volume and removing the solid from the simulation volume does not matter, both option would give the same result. However in all other cases the uniform heat source is the better choice.

Following @nlui’s comment about the uniform heat source being restricted to rectangular shape; a solution to this limitation would be to use the import heat source instead. You can use the script environment to create a matrix for the heat input as a function of x,y,z and then load it into the import heat source to create a heat input of any arbitrary shape.

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Thanks for the answer. What I noticed is that if I want to use the uniform heat source in a 2D simulation I have to set the z span of the source equal the normalization length of the HEAT SOLVER to get correct results. Am I correct?

You are right. The input to the uniform heat source is net power in units of Watt. So the source needs to have a finite volume to calculate the power density in units of W/m^3. If you want the net power getting applied to your simulated device in a 2D simulation then the length of the source needs to equal the norm length.

There are actually two ways to do this,

1. One option is to use the 3D source type and set the span of the source in the third dimension equal to norm length.

2. Another option is to use one of the 2D source types and either define the span of the source in the third dimension using the “equivalent length” input or enable the “use solver norm length” option to make it equal to the norm length automatically.

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