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Thermal resistance 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
Formulas
$\frac{\rho_{t}}{2\pi} \ln\left(\frac{Do_{t}}{Di_{t}}\right)$circular tunnel
$\frac{2\rho_{t} t_{t}}{2U_{ti}+8t_{t}}$rectangular tunnel
Related
$Di_{t}$
Diameter (inner) tunnel [m]
$Do_{t}$
Diameter (outer) tunnel [m]
$\pi$
Archimedes' constant $\pi$
$\rho_{t}$
Thermal resistivity wall [K.m/W]
$t_{t}$
Thickness wall [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 wall (outer) [°C]
$\theta_{to,z}$
Temperature wall (outer) z [°C]