Thermal effusivity
It is an intensive quantity defined as the square root of the product of the material's thermal conductivity (
to denote the thermal responsivity, although its usage along with an exponential becomes difficult.
are the temperature of the two bodies, then upon contact, the temperature of the contact interface (assumed to be a smooth surface) becomes[6] Specialty sensors have also been developed based on this relationship to measure effusivity.
Thermal effusivity and thermal diffusivity are related quantities; respectively a product versus a ratio of a material's intensive heat transport and storage properties.
[2] By contrast a body's effusivity (also sometimes called inertia, accumulation, responsiveness etc.)
is its ability to resist a temperature change when subjected to a time-periodic, or similarly perturbative, forcing function.
[5] This relationship can be demonstrated with a very simple "control volume" back-of-the-envelope calculation: Consider the following 1D heat conduction problem.
after being brought into contact, heat will have diffused across the boundary between the two materials.
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.
Even though the underlying heat equation is parabolic and not hyperbolic (i.e. it does not support waves), if we in some rough sense allow ourselves to think of a temperature jump as two materials are brought into contact as a "signal", then the transmission of the temperature signal from 1 to 2 is
Clearly, this analogy must be used with caution; among other caveats, it only applies in a transient sense, to media which are large enough (or time scales short enough) to be considered effectively infinite in extent.
An application of thermal effusivity is the quasi-qualitative measurement of coolness or warmth "feel" of materials, also known as thermoception.
It is a particularly important metric for textiles, fabrics, and building materials.
Rather than temperature, skin thermoreceptors are highly responsive to the inward or outward flow of heat.
Thus, despite having similar temperatures near room temperature, a high effusivity metal object is detected as cool while a low effusivity fabric is sensed as being warmer.
[2] For a diathermal wall having a stepped "constant heat" boundary condition imposed at
performs nearly the same role in limiting the initial dynamic thermal response (rigorously, during times less than the heat diffusion time to transit the wall) as the insulation U-factor
plays in defining the static temperature obtained by the side after a long time.
In remote sensing applications, thermal inertia represents a complex combination of particle size, rock abundance, bedrock outcropping and the degree of induration (i.e. thickness and hardness).
[4] The temperature of a material with low thermal effusivity changes significantly during the day, while the temperature of a material with high thermal effusivity does not change as drastically.
[12] Thermographic inspection encompasses a variety of nondestructive testing methods that utilize the wave-like characteristics of heat propagation through a transfer medium.
These methods include Pulse-echo thermography and thermal wave imaging.
Thermal effusivity and diffusivity of the materials being inspected can serve to simplify the mathematical modelling of, and thus interpretation of results from these techniques.
[13] When a material is measured from the surface with short test times by any transient method or instrument, the heat transfer mechanisms generally include thermal conduction, convection, radiation and phase changes.
The diffusive process of conduction may dominate the thermal behavior of solid bodies near and below room temperature.
A contact resistance (due to surface roughness, oxidation, impurities, etc.)
Evaluations with high heat dissipation (driven by large temperature differentials) can likewise be influenced by an interfacial thermal resistance.
All of these factors, along with the body's finite dimensions, must be considered during execution of measurements and interpretation of results.
This is a list of the thermal effusivity of some common substances, evaluated at room temperature unless otherwise indicated.
