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Convection heat transfer coefficient, surface to air

When the velocity of surrounding fluid is less than approximately 0.5 m/s in air, natural convection will have the dominating influnece and a value of 200 W/(K.m$^2$) may be used.

If the flow is due to free convection, the heat transfer coefficient and the Nusselt number depend on the Grashof number. In this case, the approximate heat transfer coefficient can be determined using the following numerical equation. (Source:

Symbol
$h_{conv,sa}$
Unit
W/(K.m$^2$)
Formulae
$\frac{k_{gas} \mathrm{Nu}_{prot}}{D_o}$general equation for PAC/GIL
$12\sqrt{V_{air}}+2$approximate equation
Related
$D_o$
Outer diameter [m]
$k_{gas}$
Thermal conductivity for gas [W/(m.K)]
$\mathrm{Nu}_{prot}$
Nusselt number, surface to air
$V_{air}$
Air velocity [m/s]
Used in
$W_{conv,sa}$
Convection heat transfer, surface to air [W/m]