1. Yes, it can be considered as internal forced convection

 2. This paper described detailedly the NU.

Look at this doc it may be helpful for your topic. Good luck

Dear Dr. Ma

I do appreciate your attention to my question and your answer.

In order to your suggested paper, I would like to know that what kind of heat sinks are considered in the realm of microchannel ?

Sincerely yours.

Dear Mr. Boulahia

Thank you for your answer and suggestion. 

when Hydraulic diameter Dh

This doc will be helpful

Dear Dr. Ma

Thanks a lot for your answer.