Thermal Effusivity

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What is Thermal Effusivity?

A materials effusivity is a measure of its ability to exchange thermal energy with its surroundings. Although the quantity of thermal effusivity can be expressed in bulk property terms of e = √k∙ρ∙Cρ when measured, it is not measured in terms of bulk properties.

e is thermal effusivity
k is thermal conductivity
p is density
Cp is heat capacity

One application of thermal effusivity is the qualitative measurement of coolness or warmth feel of materials on textiles and fabrics. When a textile or fabric is measured from the surface with short test times by any transient method or instrument, the measured effusivity includes various heat transfer mechanisms, including conductivity, convection and radiation, as well as contact resistance between the sensor and sample. As the dominant property in the thermal effusivity expression is thermal conductivity, which is valid for solids only, the term used to properly describe the measured value is thermal conductance. When considering this point in terms of thermal effusivity, the more appropriate term becomes thermal effusance. With this understanding, thermal conductivity should never be backed out of the thermal effusivity expression. This is an important fact when considering qualitative use of thermal effusivity for testing non-solids.

What is Thermal Effusance?

When measuring non-solids, the dominant property in the thermal effusivity expression is thermal conductivity, which is valid for solids only, the term used to properly describe the measured value is thermal conductance. When considering this point in terms of thermal effusivity, the more appropriate term becomes thermal effusance.

e = √k∙ρ∙Cρ

e is thermal effusance
k is thermal conductance
p is density
Cp is heat capacity

As the measured thermal effusivity of textiles and fabrics encompasses various modes of heat transport (conductivity, convection and radiation), the results are referred to as thermal effusance.