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Heat transfer coefficient for radiation

Heat transfer coefficient for radiation in channel acc. Heinhold eq. 18.21

Symbol
$\alpha_{st}$
Unit
W/(K.m$^2$)
Formulae
$\frac{K_t \sigma \left(\left(\theta_{de}+\theta_{abs}\right)^4-\left(\theta_t+\theta_{abs}\right)^4\right)}{\Delta \theta_s}$
Related
$\Delta \theta_s$
Temperature difference surface to ambient [K]
$K_t$
Effective emissivity of surface
$\sigma$
Stefan Boltzmann constant [W/m$^2$K$^4$]
$\theta_{abs}$
Absolute temperature [K]
$\theta_{de}$
Temperature of duct outer wall [°C]
$\theta_t$
Temperature of inner tunnel wall [°C]
Used in
$T_{st}$
Radiation thermal resistance, surface to tunnel [K.m/W]