where Pr = μCp ⁄k, and
ReD = ρUextD ⁄μ.
D (the tube diameter) and the velocity
U are correlation inputs. All material data are evaluated at
(T + Text) ⁄2 except
μs, which is evaluated at the wall temperature,
T. The laminar-turbulent transition at
ReD = 2500 is handled by the use of a smoothed Heaviside function with a transition of size
250, which corresponds to 10% of the threshold value.