Thermal effusivity

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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}

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.

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References[edit]

  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

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