# Heat Generation vs Heat Flow

I am running a steady state thermal simulation where a body at the center of my geometry is releasing heat (40W to be exact) to the bodies it’s attached to. Originally I had set the problem up such that I had 40W of heat leaving the surface of that body. As a test, I changed the boundary condition for that body to be a heat generation boundary condition with the W/m^3 set such that that body was generating 40W of heat. I received very different values for the temperature distribution and heat fluxes. I am confused as to why they are different. I would imagine that if the body was generating 40W of heat, it would also be releasing 40W of heat to the other bodies as well. Which boundary condition would be correct to use for this problem?

## Comments

Think about how the boundary is defined:

40W flux means we add 40W uniformly over the surface(s). If the flow (free stream temperature or HTC) isn't suitable to move the 40W we increase the temperature so that flux bc is met.

40W from the volume means that more heat can be removed from surfaces facing cold or fast moving flow (HTC or dT is higher) so we have a non-uniform flux and potentially a lower peak temperature.

Which is the correct bc is another question. Given the above which best matches your "real" system? Why?