I have a vertical heated pipe. I want to model a small suction tap in the side of the pipe, where i specify the flow rate at this tap. Then this fluid will be cooled down and injected back in the pipe a little above the suction tap. What boundary condition would i use for this suction tap, since i want to specify the flowrate for this internal section. (I need to inject the same fluid, because it is a diffusion problem, so I need the composition sucked out the same as put back in, but just lower temperature). Thanks in advance

For compressible flow, you can use a mass flow inlet for both places. Or for the upper tap you could use a pressure outlet and give it a target mass flow rate.

For incompressible flow, you could calculate the average velocity at the two taps and apply velocity inlet BCs. You can also use the pressure outlet with a target mass flow in incompressible flow.

Hope this helps, and good luck, Jason