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External heat transfer coefficient

The external heat transfer coefficient (or outside film coefficient) is a convection coefficient, calculated from the Nusselt number of the ambient fluid.

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:

When the velocity of surrounding fluid is less than approximately 0.05 m/s in water, natural convection will have the dominating influnece and a value of 200 W/(K.m$^2$) may be used.

Symbol
$h_{ext}$
Unit
W/(K.m$^2$)

Default
200.0

Formulae
$\frac{\mathrm{Nu}_w k_w}{D_{ext}}$pipe in water
$2100\sqrt{V_w}+350$approximation for free convection
Related
$D_{ext}$
Overall outer diameter of pipe [m]
$k_w$
Thermal conductivity of water [W/(m.K)]
$\mathrm{Nu}_w$
Nusselt number, surface to water
$V_w$
Velocity of water [cm/s]
Used in
$e_{limit}$
Limit of thickness of soil layer [m]
$h_{amb}$
Pseudo film coefficient of ambient fluid at ground level [W/(K.m$^2$)]
$U_{exposed}$
OHTC of part of pipe in contact with water [W/(K.m$^2$)]