See "Heat Conduction in Solid Phase", Wikipedia. No radiation term is required.

Thanks for the answers. I should have been more clear, I'm interested to model radiation from the boundaries to the ambient free stream. I'll go with S2S after reading a bit more about it.

If you want your model to be simple and know only the emissivity of the material, you can include the effect of radiation on the increased heat flux by increasing the coefficient of heat exchange by convection with the coefficient of heat exchange by radiation. I.e. if you apply the boundary condition

q = αpv (Ts - Ta)

where q is a heat flux and Ts, Ta are surface and ambient temperatures, respectively, the coefficient αpv is a convection-radiation heat exchange coefficient, which can be expressed as:

αpv = αp + αr

where αp represents the heat exchange by convection and αr by radiation. For calculation of the effect of radiation in cooling concrete I apply the following formula (For Ta > 5oC):

αr = e [4.8 + 0.075 (Ta- 5)]

where e is the emissivity of concrete. However, I am not 100% sure if it is general and can be used for any material knowing its emissivity.

you must read Heat and mass transfer by Anthony F.. Mills Table 3.8

thank you for all your answers, I ended up going with a simple representation for now as Agnieszka described 

Hi,

just a comments and a question with tentative answers,

- When using S2S model in Fluent, you cannot exploit geometrical symmetries of your computational domain.

- If I well understood, you have to set the radiation of your solid body toward the environment. Here comes the question:  is your computational domain including the external environment?

The answer to this will discriminate on the approach to follow:

answer no: then  you have just to set a BCs that includes radiation and convection (as already mentioned above). This can be done in Fluent in the Wall boundary setting explicitly the convective heat exchange coefficient, the  body surface emissivity and the external environment temperatures.

Answer yes: then you need a model for radiative heat exchange and this can be S2S (the one based on view factors) provided your domain is an enclosure and does not exploit symmetries or the DOM (Discrete Ordinate Model based on a sort of  transport equation(s) of radiating energy intensity).  

I hope this helps.

Yours Sincerely,

Maurizio

Thank you Maurizio. Initially I did not include the environment in the computational domain, so I used the wall BC.

Now I am including air as the external fluid domain, so I will have to include a Radiation model. I have an axis of symmetry since the simulation is 2D axisymmetric. If S2S does not work, what other model you think would be easier to implement? I don't know any radiative properties other than the surface emissivity.