Thermal diffusivity 

In heat transfer analysis, thermal diffusivity (symbol: \alpha\,, but note that the symbols κ, D, and k are all commonly used) is the ratio of thermal conductivity to volumetric heat capacity. It has the SI unit of m²/s.

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

where:

The denominator of the thermal diffusivity expression above, \rho c_p\,, can be identified as the volumetric heat capacity with the SI unit of J/(m³·K).

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

Some thermal diffusivities of common materials1
Material Thermal diffusivity
m²/s
Aluminium 8.418 × 10-5
Carbon steel (1%) 1.172 × 10-5
Copper 1.1234 × 10-4
Pure silver (99.9%) 1.6563 × 10-4
Aluminium oxide (polycrystalline) 1.20 × 10-5
Common brick 5.2 × 10-7
Pyrolytic graphite, parallel to layers 1.22 × 10-3
Pyrolytic graphite, normal to layers 3.6 × 10-6
Window glass 3.4 × 10-7
Nylon 9 × 10-8
Sandstone 1.12 × 10-6 to 1.19 × 10-6
Wood (Yellow Pine) 8.2 × 10-8
Air (1 atm, 300 K) 2.2160 × 10-5
Water vapour (1 atm, 400 K) 2.338 × 10-5
Engine oil (saturated liquid, 100 °C) 7.38 × 10-8

See also

References

  1. ^ J.P. Holman, Heat Transfer, 9th Ed., McGraw-Hill 2002 (which in turn cites Brown and Marco, Introduction to Heat Transfer, 3rd Ed, McGraw-Hill, 1958 and Eckert & Drake, Heat and Mass Transfer, McGraw-Hill, 1959).