Gnielinski's correlation for turbulent flow in tubes $Nu=\frac{(f/8)(Re_D-1000)Pr}{1+12.7(f/8)^{1/2}(Pr^{2/3}-1))}$ valid for, $0.5 \le Pr \le 2000$ $3000 \le Re_D \le 5\times 10^6$ where f is [[Darcy friction factor]] that can be obtained from a [[Moody chart]] or for smooth tubes using Petukhov correlation $f = (0.79\ ln(Re)-1.64)^{-2}$