Regarding emissivity
 

What is the differences between the external emissivity and the internal emissivity? please explain it clearly.

This sounds like FLUENT terminology. If so - I recommend asking in the FLUENT forum. In what follows I'll ignore their interpretation (which is not fully clear to me) and give you mine.

Suppose you have a semitransparent domain with some of its boundary surfaces facing the ambient surroundings. This surface has two sides, which may have different emissivities. I would call the emissivity of the side towards the inner domain "internal emissivity", and the other, facing the surroundings, "external emissivity".