Not that I've tried this, but have you defined a solid shape that the energy would come from disable it so it does not affect flow, and try setting it as a heat source?
Similar to creating a mock part to define a local mesh but define a local heat instead
yes, this feature is built into Flow Sim. I've used it and it works.
Here's an excerpt from the help file:
In Volume Source you can specify internal (volume) heat sources in the form of Temperature, Heat Generation Rate or Volumetric Heat Generation Rate (in both cases a positive value denotes heat generation, a negative value denotes heat absorption). You can specify Volume Source in a component (a part or subassembly in assemblies, as well as a body in multibody parts) that is treated as either solid or fluid. If the component is treated as a solid, heat conduction in solids has to be considered. If the component is to be treated as a fluid, you must disable the component in the Component Control dialog box.