Heat transfer in a horizontal rectangular duct
 

I need to analyse the flow in a horizontal rectangular duct and heat transfer is involved in the problem.

But I'm not sure if I should activate the gravity (buoyancy) then, because activating gravity I think will turn the scenario into a vertical duct flow.

I use a velocity inlet and pressure outlet (at atmospheric pressure) and turbulence is involved.

Could some one please clarify about this

You need gravity if any bouyancy effects occur, such as liquids at the bottom, gas at the top, or heavy particles sinking and light one rising.

You can define the gravity vector in any direction you like, so you can model a horizontal duct with bouyancy.