# Thermal effusivity

It has been suggested that Volumetric heat capacity#Thermal inertia be merged into this article. (Discuss) Proposed since June 2020. |

In thermodynamics, the **thermal effusivity**, **thermal inertia** or **thermal responsivity** of a material is defined as the square root of the product of the material's thermal conductivity and its volumetric heat capacity.^{[1]}^{[2]}

Here, 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^{[i]} bodies initially at temperatures and are brought in perfect thermal contact, the temperature at the contact surface will be given by their relative effusivities.^{[3]}

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 using specialty sensors, as pictured.

## Thermal Effusivity vs Thermal Effusance ^{[4]}Edit

A materials thermal 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

ρ = is density

Cρ = is heat capacity

## ApplicationsEdit

One application of thermal effusivity is the quasi-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.

## See alsoEdit

## ReferencesEdit

**^**i.e. their thermal capacity is sufficiently large that their temperatures will not change measurably owing to this heat transfer

**^**A reference defining various thermal properties**^**Williams, F. A. (2009). "Simplified theory for ignition times of hypergolic gelled propellants".*J. Propulsion and Power*.**25**(6): 1354–1357. doi:10.2514/1.46531.**^**Baehr, H.D.; Stephan, K. (2004).*Wärme- und Stoffübertragung 4. Auflage*. Springer. p. 172. doi:10.1007/978-3-662-10833-8. ISBN 978-3-662-10834-5.**^**"Thermal Effusivity vs Thermal Effusance".*Thermal Effusivity*. 2019-10-11.

## External linksEdit

- "Thermal heat transfer".
*HyperPhysics*.