In heat transfer analysis, thermal diffusivity (symbol: , 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.

where:

• k : thermal conductivity (SI units: W/(m·K))
• ρ : density (kg/m³)
• c_p : specific heat capacity (J/(kg·K))

The denominator of the thermal diffusivity expression above, , 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 materials^[1]

Material	Thermal diffusivity (m²/s)
Pyrolytic graphite, parallel to layers	1.22 × 10−3
Pure silver (99.9%)	1.6563 × 10−4
Copper	1.1234 × 10−4
Aluminium	8.418 × 10−5
Water vapour (1 atm, 400 K)	2.338 × 10−5
Air (1 atm, 300 K)	2.2160 × 10−5
Aluminium oxide (polycrystalline)	1.20 × 10−5
Carbon steel (1%)	1.172 × 10−5
Pyrolytic graphite, normal to layers	3.6 × 10−6
Sandstone	1.12–1.19 × 10−6
Common brick	5.2 × 10−7
Window glass	3.4 × 10−7
Rubber	1.3 × 10−7
Nylon	9 × 10−8
Wood (Yellow Pine)	8.2 × 10−8
Engine oil (saturated liquid, 100 °C)	7.38 × 10−8

[edit] See also

• Thermodiffusion
• Heat capacity
• Specific heat capacity
• Heat equation
• Thermal conductivity
• Thermal effusivity
• Thermal time constant
• A plot of thermal diffusivity versus temperature for air (James Ierardi's Fire Protection Engineering Site).
• A Manufacturer measurement of thermal diffusivity versus temperature.

[edit] 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).