Rate at which heat spreads throughout a material
In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It is a measure of the rate of heat transfer inside a material and has SI units of m2/s. It is an intensive property. Thermal diffusivity is usually denoted by lowercase alpha (α), but a, h, κ (kappa), K, ,D, formula_1 are also used.
The formula is:
formula_2
where
Together, ρcp can be considered the volumetric heat capacity (J/(m3·K)).
As seen in the heat equation,
formula_3
one way to view thermal diffusivity is as the ratio of the time derivative of temperature to its curvature, quantifying the rate at which temperature concavity is "smoothed out". Thermal diffusivity is a contrasting measure to thermal effusivity. In a substance with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its volumetric heat capacity or 'thermal bulk'.
Thermal diffusivity is often measured with the flash method. It involves heating a strip or cylindrical sample with a short energy pulse at one end and analyzing the temperature change (reduction in amplitude and phase shift of the pulse) a short distance away.
References.
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