# Film temperature

In heat transfer and fluid dynamics, the film temperature is an approximation to the temperature of a fluid inside a convection boundary layer. It is calculated as the arithmetic mean of the temperature at the surface of the solid boundary wall and the free-stream temperature $T_{\infty}$.

The film temperature is often used as the temperature at which fluid properties are calculated when using Prandtl number, Nusselt number, Reynolds number or Grashof number to calculate a heat transfer coefficient, because it is a reasonable first approximation to the temperature within the convection boundary layer.

Somewhat confusing terminology may be encountered in relation to boilers and heat exchangers, where the same term is used to refer to the limit (hot) temperature of a fluid in contact with a hot surface.

Symbol
$\theta_{film}$
Unit
°C
Formulae
 $\frac{\theta_e}{2}+\frac{\theta_{di}}{2}$ Riser closed at both ends $\frac{\theta_e}{2}+\frac{\theta_{air}}{2}$ Riser open at both ends, object to gas $\frac{\theta_{di}}{2}+\frac{\theta_{air}}{2}$ Riser open at both ends, gas to duct $\frac{\theta_e+\theta_{di}+2\theta_{air}}{4}$ Riser open at top and closed at bottom, object to gas $\theta_{di}$ Riser open at top and closed at bottom, gas to duct $\frac{\theta_{de}}{2}+\frac{\theta_{air}}{2}$ Air around riser $\frac{\theta_e}{2}+\frac{\theta_a}{2}$ PAC/GIL in air $\frac{\theta_c}{2}+\frac{\theta_{gas}}{2}$ PAC/GIL conductor to gas $\frac{\theta_{gas}}{2}+\frac{\theta_{encl}}{2}$ PAC/GIL gas to enclosure
Related
$\theta_a$
$\theta_{air}$
$\theta_c$
$\theta_{de}$
$\theta_{di}$
$\theta_e$
$\theta_{encl}$
$\theta_{gas}$
Used in
$\mathrm{Gr}_{da}$
$\mathrm{Gr}_{prot}$
$T_{conv,sa}$
$T_{rad,sa}$
$T_{rad,sun}$
$W_{conv,sa}$
$W_{rad,sa}$