Reynolds analogy
Many people confuse heat transfer coefficient (htc) based on Reynolds Analogy and htc based on standard wall functions for temperature.
Reynolds analogy: Reynold's analogy relates Nu and Cf  analogy bewteen Cf and Nu. The Reynolds analogy is: Cf/2 = St (1) It assumes Pr = 1. St = Nu/(Re*Pr) (2) Nu = htc*L/kf (3) Re = U*L*density/viscosity (4) Plugging 2 to 4 into one, we get: htc = Cf*U*kf*density/(2*viscosity) htc = Cf * density * U * Cp/2 where Cf is skin friction coef., U is reference velocity, and kf is thermal conductivity of fluid. Note that htc = q"/(Twall  Tref). Now what reference velocity should be used to find htc. Also, when applying this htc, what reference temperature should be used? So, it is not a very useful definition of htc. 

