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Thermal Effusivity vs Thermal Effusance

September 17, 2019

There is a great deal of discussion and use of thermal effusivity for the measurement of coolness or warmth feel of materials on textiles or fabrics. 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 = √k∙ρ∙Cρ

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

When a textile or fabric is measured from a 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.

e = √k∙ρ∙Cρ

e is thermal effusance
k is thermal conductance
p is density
Cp</em is heat capacity

A few further thoughts:

1. When measuring non-solids, such as fabrics and textiles for thermal effusance with short test times, the results are most useful for comparison purposes. It allows user to qualitative measure the feeling of coolness or warmth or determine which material is more effusive (conductive) then another.
2. When measuring any materials for thermal effusance with short test times, the thermal conductivity which is often incorrectly expressed in terms of a bulk property. In these cases, the k (thermal conductivity) should not be considered valid and used for only comparison purposes.

What is Thermal Effusivity
What is: Thermal Effusance

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