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.

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 T_m 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.