Heat transfer to air is usually by natural convection and radiation. By combining these a single heat transfer coefficient can be obtained that is dependent on temperature.
If you have an air gap then you need to consider convection in the gap and radiation between the two surfaces. For small gaps radiation will be dominant and you could neglect the convection to air. Refer to McAdams Heat Transfer or other such books.

corus

thanks corus!

and by the way , do you know how can i simulate this in Abaqus? because i´ve try to simulate the air as a solid, and then made the interaction between the surfaces, but i only find the gap conductance property and not the film property.
 
Thank´s