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Grashof number, duct in air

Symbol
$\mathrm{Gr}_{ext}$
Formulae
$\frac{g \beta_{gas} {L_d}^3}{{\nu_{gas}}^2} \left(\theta_{de}-\theta_{air}\right)$
Related
$\beta_{gas}$
Volumetric thermal expansion coefficient for gas [1/K]
$g$
Standard acceleration of gravity [m/s$^2$]
$L_d$
Length of duct [m]
$\nu_{gas}$
Kinematic viscosity for gas [m$^2$/s]
$\theta_{air}$
Ambient air temperature [°C]
$\theta_{de}$
Temperature of duct outer wall [°C]
Used in
$\mathrm{Nu}_{ext}$
Nusselt number, duct to air
$\mathrm{Ra}_{ext}$
Rayleigh number outside riser