Thermal effusivity

In Thermodynamics, the thermal effusivity of a material is defined as the square root of the product of the material's thermal conductivity and its volumetric heat capacity.[1]

e = {(k\rho c_p)}^{1/2}
Thermal Effusivity Sensor
Thermal effusivity sensor typically used in the direct measurement of materials.

Here, k is the thermal conductivity, \rho is the density and c_p is the specific heat capacity. The product of \rho and c_p is known as the volumetric heat capacity.

A material's thermal effusivity is a measure of its ability to exchange thermal energy with its surroundings.

If two semi-infinite bodies initially at temperatures T1 and T2 are brought in perfect thermal contact, the temperature at the contact surface Tm will be given by their relative effusivities.[2]

T_m = T_1 + (T_2-T_1){e_2 \over (e_2+e_1)}

This expression is valid for all times for semi-infinite bodies in perfect thermal contact. It is also a good first guess for the initial contact temperature for finite bodies.

Direct measurement of thermal effusivity be performed via specialty sensors, as pictured to the right.

See also

References

  1. http://www.evitherm.org/default.asp?ID=277
  2. H. D. Baehr and K. Stephan, Wärme- und Stoffübertragung 4. Auflage, Springer, (2004), p 172, ISBN 3-540-40130-X

External links

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