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)
- cp : 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'.
| 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 |
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.
References
- ^ 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).
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