Please explain in detail. I don't know why you have to insert electrode just to calculate the heat transfer rate. In COMSOL, there are some tutorial on electrochemical and DC/AC problems. It's easy to follow.

Dear Giho Jeong,

Actually, we want to test our nanofluids using this setup (constant heat flux)

Then you'd like to heat the system by Joule heating? Or you are thinking of heating by conduction/convection? You can do either by selecting the heat transfer and AC/DC module, as far as I know. And you might know how you can model the fluid sub-domain surrounded by steel pipe with patch-type electrodes on the wall. Actually, it's not easy imagine your system.

I guess you're going to test the heat transfer augmentation with fluid with nanoparticles.