A device subject to internal natural convection has a cavity in which air (or some other fluid) can move around internal components that give off heat. As the components heat up, the air within the device heats and moves due to buoyancy.
Examples of internal natural convection situations include an electronics device that has vents which allow flow in an out of the device and a module that is fully sealed. Each of these scenarios requires a slightly different set-up.
Either construct the flow volume in the CAD model or construct caps that fully enclose the flow volume. (This can be done using the Void Fill Geometry tool.)
Because the device is sealed, the internal flow volume should be created automatically when opened into Autodesk Simulation CFD. If not, ensure that there are no leaks or construct the flow volume in the CAD model.