Hi, Ricardo! Thanks again for your reply.
In my simulation, I wanted to demonstrate how much heat water loses due to radiation, without taking into account the effects of convection with air (for this, I changed the CP of air to 1). Based on your comments, I think that perhaps another radiative behavior might work better for water, since I am using a surface heat flux at the bottom of the duct. By doing so, I expect the water to heat up by convection until it reaches an equilibrium temperature, at which point the heat input from the bottom should be equal to the heat exchanged by radiation, since this is the only heat transfer mechanism available at the outlet of the water domain (convection with air is neglected, as mentioned above).
Finally, the Simscale documentation states that in conjugate heat transfer simulations, the DOM is used to calculate the behavior of radiation, while in convective heat transfer simulations, the face-to-face method is used. That is why I am considering that my simulation, being a conjugate heat transfer, uses the DOM. However, I would be interested in using the face-to-face method to validate my simulation by measuring the radiative heat on each wall of the environment and verifying that the sum of all these flows is equal to the heat lost at the interface.
BR