Wall function heat transfer coefficient
recently i was doing a project of a flow in pipe under uniform heat flux and used the wall function K-Epsilon (Non-Equilibrium Wall Function) with recommended Y+>30. I tried to validate my results with experimental results using h=q/(Twall-Tf) and the "wall function based HTC" provided by ANSYS and the Wall func. HTC showed the best agreement. I found the picture below randomly across the web.
I tried to search all the way in the Fluent theory guide and the fluent manual about wall function heat transfer coefficient to get an idea about how it is evaluated but couldn't manage to find it.
Can someone help me explain where I can find how is it calculated?