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.
Here, k is the thermal conductivity, is the density and is the specific heat capacity. The product of and 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.
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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