Thermal diffusivity

In heat transfer analysis, thermal diffusivity (symbol: $$\alpha\,$$, but note that the symbols $$\kappa$$, $$D$$, and $$k$$ are all commonly used) is the ratio of thermal conductivity to volumetric heat capacity.


 * $$\alpha = {k \over {\rho c_p}}$$

where:
 * $$k\,$$ : thermal conductivity (SI units : W/(m K) )
 * $$\rho c_p\,$$ : volumetric heat capacity (SI units : J/(m3K) )
 * $$\rho\,$$ : density (SI units : kg/(m3) )
 * $$c_p\,$$ : specific heat capacity (SI units : J/(kg K) )

The SI units for thermal diffusivity is m2/s.

Substances with high thermal diffusivity rapidly adjust their temperature to that of their surroundings, because they conduct heat quickly in comparison to their thermal 'bulk'.

For common rock material, $$\alpha\,$$ ~ 10-6 m2/s.

Thermal diffusivity of air at 300 K is 0.000024 m2/s.

For a plot of thermal diffusivity of air as a function of absolute temperature see James Ierardi's Fire Protection Engineering Site