What is Thermal Effusivity?

In thermodynamics, thermal effusivity is the ability of how readily the material can draw heat from its surroundings or release it. Thermal effusivity combines the properties of thermal conduction (how rapidly heat can propagate through a material) and thermal capacity (how much heat can be stored in a given volume of material). In most materials, if one of these properties is high, the other tends to be low.

Thermal Effusivity Equation

In where, ${\displaystyle \lambda }$ is the thermal conductivity, ${\displaystyle \rho }$ is the density and ${\displaystyle c_{p}}$ is the specific heat capacity. The product of ${\displaystyle \rho }$ and ${\displaystyle c_{p}}$ is known as the volumetric heat capacity.

Application of Thermal Effusivity

Understanding the feeling of warmth and coolness of a material makes up the feeling of comfort. This is an important field of determining the thermal effusivity measurement of fabrics and textiles. The material responds to the body’s heat and wetness to help keep us at a comfortable temperature at all times. For example, when you’re feeling hot and sweaty, the fabric lets infrared radiation pass through. But when you’re colder and drier, it traps the heat in.

One of the common fabric applies to this feelings is the mattress. Waking up too warm or too cold can impact your sleep quality. It's generally agreed upon that innerspring mattresses sleep the coolest. The reason for this is because of the lack of dense, heat-retaining foam and instead, they have springs that allow for a lot more breathability.

A test method published by ASTM International that measures the rate of thermal transport between a heating element and a fabric specimen. Some of the comfort properties of a garment relate to initial thermal sensations (that is, cold or warm feeling upon initial contact), where lower thermal effusivity values indicate sensations of warmth and higher values indicate sensations of coolness. The thermal effusivity of different fabrics and their initial perceived surface temperature are important to assist product developers with fabric selection.

Using this method, a portable meter was developed to test the thermal effusivity of fabrics and textile. This thermal effusivity meter is called TPS-EFF by Thermtest. The TPS-EFF (Transient Plane Source-EFFusivity) is a portable meter for measuring the warm feel and cool touch of fabrics, textiles and other low effusivity materials. It uses a single-sided 30 mm diameter TPS sensor and short test times, to ensure one-dimensional heat flow.

Fig 1. Thermtest TPS-EFFusivity Meter, in accordance to ASTM D7984-16

Reference:

https://en.wikipedia.org/wiki/Thermal_effusivity
https://www.astm.org/Standards/D7984.htm
https://thermtest.com/tps-effusivity-meter
https://www.technologyreview.com/s/612909/this-new-fabric-will-automatically-cool-you-down-when-you-get-hot-and-sweaty/