5. Thermal Diffusvity α

Thermal Diffusvity (\(\alpha\))

\[\boxed{\alpha = \frac{k}{\rho C_P}}\]

  • Thermal diffusivity is a measure of the transient thermal response of a material to a change in temperature.

  • The larger the value of \(\alpha\), the faster will the heat diffuse through the material and its temperature will change with time.

  • This will result either due to a high value of conductivity \(k\) or a low value of \(\rho, C_P\).

  • Thermal diffusivity is a convenient collection of physical properties for transient solutions of the heat equation.

  • Recollect about ‘kinematic viscosity’ (\(\nu=\mu/\rho\)), which is also called as ‘momentum diffusivity’; and \(D_{AB}\).

Material Thermal Diffusivity
(cm\(^2\)/s) at 300 K
Copper 1.15
Aluminum 0.97
Stainless Steel (304) 0.042
Silicon Dioxide (Polycrystalline) 0.0083
Water 0.0014
Polyvinyl Chloride (PVC) 0.0008
Alcohol 0.0007
Air 0.19

  • Metals and gases have relatively high value of thermal diffusivity and their response to temperature changes is quite rapid.

  • The non metallic solids and liquids respond slowly to temperature changes because of their relatively small value of thermal diffusivity.