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.

$e={(k\rho c_{p})}^{1/2}$

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 T_m will be given by their relative effusivities.

$T_{m}=T_{1}+(T_{2}-T_{1}){e_{2} \over (e_{2}+e_{1})}$

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 may be performed via specialty sensors, as pictured to the right.

See also

• Heat capacity
• Heat equation
• Laser flash analysis
• Specific heat capacity
• Thermal conductivity
• Thermal diffusivity
• Thermal inertia

External links

• A reference defining various thermal properties
• Thermal heat transfer on HyperPhysics site