Applications

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Knowledge of emissivity is important in many aspects of human activities. For instance, it is a key property for: 

•  non-contact temperature
measurement
•  remote sensing
•  solar energy applications
•  energy conservation

In particular, emissivity is essential for non-contact temperature measurement in many industrial sectors, e.g. steel, electrical, ceramics, engine technology, plastic, power generation, utilities, glass, environment and quality of life, semiconductors, medical, food and transportation.

Pyrometry in steel production. A pyrometer (ringed) monitors the temperature of the steel as it is coiled off the production line. (Photo courtesy of Land Infrared Ltd.)

For heat transfer calculations, knowledge of the thermal radiative properties of materials is needed to predict the thermal performance of components used in building construction, nuclear plant, aerospace products, power generation, automotive products, environmental applications, electrical and electronic devices, transportation, food-related products, detection systems for defence and solar plant among other applications.

Radiation is one of the three fundamental modes of heat transfer, the others being conduction and convection. Radiation differs from the other modes in two important respects - firstly, no medium is required for transport of energy by radiation, and secondly, the rate of heat dissipation by radiation varies approximately as the fourth power of the absolute temperature, while that by other modes varies approximately as the first power of temperature. For these reasons radiation is the dominant mode of heat transfer at high temperatures and in the absence of an atmosphere.

The properties of a material that control the rate of heat transfer by radiation between an object and its environment are known as its thermal radiative properties and are emissivity, absorptivity, reflectivity and transmissivity.  

  • emissivity is used in the Planck radiation equation to calculate the temperature of a surface when its thermal radiation is measured using a pyrometer· 
  • emissivity is used in equations to calculate heat transfer by thermal radiation

In many cases the emissivity value can be enhanced (e.g. through polishing, roughening, oxidation, shaping of a material surface) to improve heating or cooling by thermal radiation.

Below are two examples of applications of thermal radiative property measurements.


In collection of solar energy, use of an absorber provided with a selective coating (e.g., black chrome and black nickel) helps to reduce heat losses and increase solar gain efficiency. The proper adjustment of the parameters which govern the manufacturing process of the coatings to obtain high absorptance at the short wavelength (high solar absorptance) and low emissivity at long wavelengths (meaning less heat is radiated in the infrared) needs the radiative properties of the coating to be accurately characterised and measured.

 

In aerospace measurement of emissivity of critical components is essential for a correct prediction of thermal performance and for laboratory simulation. Characterisation in terms of emissivity of zirconia films used for coating turbine blades of high performance jet engines is an example.

 

     
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