Thermal diffusivity

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

<math>\kappa = {k \over {\rho c_p}}</math>

where: <math>k\,</math> is thermal conductivity (SI units: watts per metre-kelvin, W∙m^-1∙K^-1) <math>\rho c_p\,</math> is the volumetric heat capacity (density kg∙m^-3 times specific heat capacity J∙kg^-1K^-1; SI units: joules per cubic metre-kelvin, J∙m^-3∙K^-1)

The SI units for thermal diffusivity are square metres per second or m²∙s^-1.

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, <math>\kappa\,</math> ~ 10^-6 m²∙s^-1.

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