Where can I found the value of Nu for a 2-D channel.

I want to Nu for Re=40,000 based on double the channel height (2H) or 20,000 (based on charac lenght H)

You'll find it in any standard heat transfer textbook (e.g., Holman, Ozisik, etc.). For tubulent flow, it is usually given as correlations in the form:

Nu = c * Re^m * Pr^n

where Re and Pr are Reynolds and Prandtl numbers, and c, m, n are the correlation parameters.

but if it is a mixed convection case, add Gr in the corelation formula. you can find this in any heat transfer book.