Editing Heat transfer coefficient (section)

=== External flow, vertical plane ===
Recommendations by Churchill and Chu provide the following correlation for natural convection adjacent to a vertical plane, both for laminar and turbulent flow.<ref>{{cite journal |last1=Churchill |first1=Stuart W. |last2=Chu |first2=Humbert H.S.|title=Correlating equations for laminar and turbulent free convection from a vertical plate |journal=International Journal of Heat and Mass Transfer |date=November 1975 |volume=18 |issue=11 |pages=1323–1329 |doi=10.1016/0017-9310(75)90243-4|bibcode=1975IJHMT..18.1323C }}</ref><ref>{{cite book |last1=Sukhatme |first1=S. P. |title=A Textbook on Heat Transfer |date=2005 |publisher=Universities Press |isbn=978-8173715440 |pages=257–258 |edition=Fourth}}</ref> ''k'' is the [[thermal conductivity]] of the fluid, ''L'' is the [[characteristic length]] with respect to the direction of gravity, Ra''<sub>L</sub>'' is the [[Rayleigh number]] with respect to this length and Pr is the [[Prandtl number]] (the Rayleigh number can be written as the product of the Grashof number and the Prandtl number).

:<math>h \ = \frac{k}{L}\left({0.825 + \frac{0.387 \mathrm{Ra}_L^{1/6}}{\left(1 + (0.492/\mathrm{Pr})^{9/16} \right)^{8/27} }}\right)^2 \, \quad \mathrm{Ra}_L < 10^{12}</math>

For laminar flows, the following correlation is slightly more accurate. It is observed that a transition from a laminar to a turbulent boundary occurs when Ra''<sub>L</sub>'' exceeds around 10<sup>9</sup>.

:<math>h \ = \frac{k}{L} \left(0.68 + \frac{0.67 \mathrm{Ra}_L^{1/4}}{\left(1 + (0.492/\mathrm{Pr})^{9/16}\right)^{4/9}}\right) \, \quad \mathrm10^{-1} < \mathrm{Ra}_L < 10^9 </math>