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Thermal resistance of tunnel wall

The equation for rectangular tunnel is given in the book 'Kabel und Leitungen für Starkstrom' by L. Heinhold, 5th edition 1999.

Symbol
$T_{tw}$
Unit
K.m/W
Formulae
$\frac{\rho_t}{2\pi} \ln\left(\frac{Do_t}{Di_t}\right)$circular tunnel
$\frac{2\rho_t t_t}{2U_{ti}+8t_t}$rectangular tunnel
Related
$Di_t$
Tunnel inner diameter [m]
$Do_t$
Tunnel/trough outer diameter [m]
$\pi$
Archimedes' constant $\pi$
$\rho_t$
Thermal resistivity of wall [K.m/W]
$t_t$
Wall thickness [m]
$U_{ti}$
Circumference of inner rectangular tunnel wall [m]
Used in
$T_e$
External thermal resistance of tunnel [K.m/W]
$\theta_{to}$
Temperature of outer tunnel wall [°C]
$\theta_{to,z}$
Temperature of outer tunnel wall at z [°C]