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Hello,
The busbar tutorial in the Introduction to COMSOL Multiphysics manual, version 5.2, shows how to do this. All manuals can be reached through File>Help>Documentation.
Best regards,
Jeff

My problem is not the same. In the busbar example the heat transfer to the ambient with constant T.ext is modeled, but I have a fluid with known flow rate and inlet temperature giving heat to the solid, and therefore, its temperature changes.

I know the heat transfer coefficient from experimental data that I trust and do not want to go for conjugate HT, lam. flow since my model is computationally heavy and I want to make less important parts as simple as I can.

Sorry I misunderstood, but your second post leaves me still a bit unclear as to what approach you want to follow. As I read it, your first paragraph suggests that you want to compute the flow and its temperature and couple that to the heat transfer in the solid (i.e. set up a full-fledged conjugate heat transfer model). On the other hand, your second paragraph suggests you're trying to stay away from that.
Are you saying that you can assume an expression for the fluid's temperature at the interface with the solid, T_ext? If so, you can type that expression in the GUI.
Jeff

Sorry if my explanations were unclear.
There is a flow of liquid with known flow rate and known inlet temperature, but I do not know how the temperature of the fluid will change as it passes through a channel in a solid.

I want to find a way to give the inlet temperature of the liquid, its flow rate, and the convective heat transfer coefficient for the liquid-solid interface (which is a constant value) to COMSOL, and COMSOL calculate the outlet temperature of the liquid and the temperature field in the solid and the other parts which are in thermal contact with the solid.

The solid gives the heat taken from the liquid to other parts and ambient air.

The differential equation that I derive for the liquid is
m_dot*cp*dT_f/dx=-h*perimeter*(T_f - T_s)
If T_s (temperature of the solid at the interface) was constant it was easy to solve my problem in COMSOL by solving the equation analytically and writing the resulting equation in COMSOL, but now that the solid temperature is not constant I do not know how to do it.

Intuitively, I don't see how you could avoid solving the full problem (i.e. Navier-Stokes + convection and conduction in the fluid, conduction in the solid) without introducing hard-to-justify simplifications since the temperature of the fluid and solid will not be homogeneous and will be influenced by the flow pattern.
I understand you would like to stay away from that approach, but if you want to give it a shot one way to set it up is covered on pages 110-124 of the Introduction to COMSOL Multiphysics manual, version 5.2.
Best,
Jef