Glossary of Terms

Thermal Conductivity

Thermal conductivity (often denoted by k, λ, or κ) refers to the intrinsic ability of a material to transfer or conduct heat.

Thermal conductivity is quantified using the International Systems of Unit (SI unit) of W/m•K (Watts per meter per degree Kelvin), and is the reciprocal of thermal resistivity, which measures an objects ability to resist heat transfer.

The rate at which heat is transferred by conduction through a unit cross-section area of a material when a temperature gradient exits perpendicular to the area.

Thermal Diffusivity

Thermal diffusivity (with a unit mm²/s) is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the rate of transfer of heat of a material from the hot end to the cold end.

Thermal Effusivity

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∙ρ∙C_ρ

A material’s thermal effusivity is a measure of its ability to exchange thermal energy with its surroundings. Although, the quantity can be expressed in bulk property terms, it is not measured in bulk property terms. If material is not a solid, then other heat transfer mechanisms exist, modifying the “k” expressed in thermal effusivity to thermal conductance.

See Thermal Effusance

Thermal Effusance

Thermal Effusance of a material is defined as the square root of the product of the material’s thermal condustance and its volumetric heat capacity.

e = √k∙ρ∙C_ρ

A material’s thermal effusance is a measure of its ability to exchange thermal energy with its surroundings, when the heat transfer mechanism is not thermal conductivity, typically found in measurement of a non-solid.

See Thermal Effusivity