Reference value and Nusselt number
 

Hello guys

I have simulated a 2D smooth channel with the following boundary conditions and geometry:

-Overall length: 450mm
-Entrance length: 300mm (to be sure about fully-developed region)
-Channel height: 10mm
-Channel width: 180mm
-from 300 to 410mm , the wall is at the temperature of 450K (Test Section)
-the wallls out of the test section are insulated
-inlet Temp=300K (air)

After running the program, I had to change the "reference value length" to get logical Nusselt number!! I found the length about 13mm!!( I don't know why?!)
1. Does anybody exactly know about the adjustment in Ref. Value panel?
2. When I changed the working fluid to "water" I saw a huge decrease in Nusselts number with the same boundary conditions!! Do you know why?

Help please! Thanks in advance.