Learning Forum

[scorsett]

Hello,

I am currently running a lumerical heat simulation for a thermo optic modulator and was attempting to use the transient simulation to get an idea for the response time. Upon setting the simulation to transient, I realized I had to switch my heat source from the uniform heat to a boundary condition. I switched to the power boundary condition and set the value of the power equal to that of the heat source power, and selected the same solid for the boundary, however the temperature I recorded with my monitor as well as the power flux on a separate debugging monitor were different for the uniform source and the power boundary condition. I am just interested in knowing what the difference is between the source and the boundary, and also why there is this discrepancy in temperature?

Thank you!

[kghaffari]

Hi Sabrina,

Great question here. Before I clarify the difference between uniform heat source vs power boundary condition (and the potential reason for the mismatch), please note that you can use both heat sources in a transient simulation:

When using uniform heat source you will have to use a shutter to specify when the source will be turned on. This option is available in HEAT solver under transient tab, and in newer releases also availble in the source itself. On the other hand, with power boundary condition you will have access to it’s time-related properties under it’s own settings. Below are two transient HEAT examples, one using each of the two approaches:

• Metamaterial Microbolometer – Transient with heat source: https://optics.ansys.com/hc/en-us/articles/360041621694-Metamaterial-microbolometer
• Transient heat flow in a graphene-coated glass – Transient with (temperature power) boundary condition: https://optics.ansys.com/hc/en-us/articles/360042273534-Transient-heat-flow-in-a-graphene-coated-glass

There is also a difference between the analysis done with uniform heat source vs power boundary condition; for power boundary condition the heat is emitted from the surface of an object; its boundaries. Therefore, the area of the object is not included in the calculation. You will also notice that the results of HEAT with power boundary doesn’t include the area inside.This is not the case when performing simulations (transient or steady-state) with uniform heat source. This difference could be the reason for the mismatch you see in the results. 

Best regards,

Khash

[kghaffari]

There is also another info that could be helpful to you here or in your future HEAT analysis; When the simulation is complete you will have access to a set of results named "boundaries". Here you will see the area of your HEAT boundary conditions and the power dissipated through them. Since the input and output power should match, by checking the results here (Pout) you can double check, how much power is introduced in the simulation and whether it matches your expectations.

Best