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Rayleigh number outside riser

Rayleigh number gas/duct

Symbol
$\mathrm{Ra}_{ext}$
Formulae
$\mathrm{Gr}_{ext} \mathrm{Pr}_{gas}$duct to ambient
$\frac{g \mathrm{Pr}_{gas} \beta_{gas} \left(\theta_{de}-\theta_{air}\right) {L_d}^3}{{\nu_{gas}}^2}$Chippendale, IEC
Related
$\beta_{gas}$
Volumetric thermal expansion coefficient for gas [1/K]
$g$
Standard acceleration of gravity [m/s$^2$]
$\mathrm{Gr}_{ext}$
Grashof number, duct in air
$L_d$
Length of duct [m]
$\nu_{gas}$
Kinematic viscosity for gas [m$^2$/s]
$\mathrm{Pr}_{gas}$
Prandtl number for gas
$\theta_{air}$
Ambient air temperature [°C]
$\theta_{de}$
Temperature of duct outer wall [°C]
Used in
$h_{conv,ext}$
Convection heat transfer coefficient, duct to air [W/(K.m$^2$)]