We need a "volume" for the Fluent to be able to use the heat generation which is in [W/m³]. Your length, the one meter into the screen and then the last dimension for the volume is the thickness. The effects of this thickness on the flow field is not really considered as we are not resolving the thickness of the wall in the mesh.

One thing to keep in mind as you are comparing this to experiment, is that we are apply this in a planar model. If your experiment is on a circular pipe where there is water on either side of the pipe (into and out of the screen), then would we expect our model to be a little to hot or too cold?