hI LINa,

It is always nice to share my own understanding with you! I studied to solve my own problem as You might have noticed that I haven't received any detailed solution from any experts in this group. Experts are running out of time!!!! 

No worry, I'll try to answer your question!

In your numerical solution, where you need to solve the Navier stroke equation, especially when you focus on the energy equation, you have to consider the heat source. Now, such a heat source can be added either as constant heat flux or any solar radiation model available in Fluent. Both options allow you to solve your numerical solution but the desired accuracy in heat transfer can't be obtained always that comes with a trade-off. 

In your case, you can add the radiative heat transfer coefficient value as a boundary condition (on the thermal tab in Fluent), which is only available for the radiation model. There are many resources of formula to calculate this value that you should cover already. Remember that this coefficient is a heat loss coefficient with surrounding (for your case). You also can add convection heat transfer coefficient value as it conjugates with radiative heat transfer but will depend on your problem.