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Substitution coefficient u

Substitution coefficient for the calculation of the thermal resistance to ambient.

Symbol
$u$
Formulas
$\frac{2L_{cm}}{D_{o}}$Standard case
$\frac{2\left(L_{cm}-d_{psc}\right)}{D_{o}}$point source correction
$\frac{2\left(L_{cm}+d_{im}\right)}{D_{o}}$non-isothermal earth surface
$\frac{2\left(L_{cm}-d_{psc}+d_{im}\right)}{D_{o}}$point source correction & non-isothermal earth surface
Related
$d_{im}$
Non-isothermal earth surface imaginary layer of soil [m]
$D_{o}$
Outer diameter [m]
$d_{psc}$
Point source correction [m]
$L_{cm}$
Depth of laying [m]
Used in
$g_{u}$
Geometric constant of circle buried
$T_{4iii}$
Thermal resistance ambient [K.m/W]
$T_{e}$
External thermal resistance of tunnel [K.m/W]