Emissivity of a very high temperature object

In summary, the conversation discusses the emissivity of Platinum at high temperatures, particularly in the range of 1400-2000 K. It is noted that at room temperature, Platinum has a low emissivity between 0.1 and 0.2. However, at higher temperatures, the emissivity can approach one. The conversation also mentions that metals are generally shiny and reflective, making them poor emitters, but their emissivity improves at high temperatures. Data is provided from various sources to confirm this behavior in Platinum. Ultimately, it is concluded that Platinum remains a bad emitter at high temperatures, with an emissivity range of 0.1-0.2 up to 1800 K.
  • #1
onno
8
1
I am looking for data on the total hemispherical emissivitie from a Platinum surface at high temperature 1400 - 2000 K. In this temperature range the Platnium surface will color from red, orange to white.
I learned that, practically, the bove mentioned emissivity will approach to one in this temperature range.
Platinum has a low emissivity at room temperature: something between 0.1 and 0.2 (as most website tables how). Does the emissivity of this metal go to 1 when white hot?
Materialquality of Platinum is shiny.
 
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  • #2
I once built a small wood stove from a shiny stainless steel beer keg.
First test I set a fire inside and set it alight, waiting for the lovely radiant heat. Good fire, no heat. Fire raging, no heat. Waiting. Finally the stove starts to be visibly emissive (orange glow) and there is suddenly good heat coming from stove.
Solution for the stove was black stove paint...worked fine. Second test behaved like a wood stove should.
I believe this represents a general response of metals...they are shiny reflective (and hence very bad emitters) only to frequencies below their plasma frequencies. So when hot enough they emit well,. Here is some data for Au and Ag:
https://en.wikipedia.org/wiki/Reflectance
Pt should be similar
 
  • #3
Hello Hutchphd

You say: "So when hot enough they emit well"
We are designing an oven using a current driven, very thin Pt filament. I have data on filament temperature as function of the heating current. If I compare the measured temperature with some heat transfer simulations I come to the conclusion that above a 1000 C the emissivity is way above the quoted Pt emissivity of 0.2 at room temperature .
I Its going the values 0.5-0.8 above 1000 C is my first estimation. Looking for data though to confirm this.

Best regards
 
  • #4
From:
https://www.researchgate.net/publication/23557347_Temperature_dependence_of_the_emissivity_of_platinum_in_the_IR
the emissivity of platinum is measured in the near infrared as a function of temperature. In the wavelength range of study and the temperature range of 650-1100 K, we find the emissivity to be independent of temperature to within experimental error.

But this:
https://www.omega.co.uk/literature/transactions/volume1/emissivitya.html
Shows:
260°C e=0.05
538°C e=0.10

And this one that digs rather deeply into the subject shows it depends on the wavelength:
http://coimbra.ucsd.edu/publications/papers/2019c_Orosco_Coimbra.pdf(above found with:
https://www.google.com/search?q=emissivity+of+platinum+vs+temperature)

Cheers,
Tom
 
  • #5

What is emissivity and how does it relate to very high temperature objects?

Emissivity is a measure of an object's ability to emit thermal radiation. It is often represented by the Greek letter epsilon (ε), and its value ranges from 0 to 1. A value of 1 indicates that the object is a perfect emitter, while a value of 0 indicates that it is a perfect reflector. Very high temperature objects, such as those found in industrial processes or astrophysical environments, typically have high emissivity values due to their high energy and temperature levels.

How is the emissivity of a very high temperature object measured?

Emissivity can be measured using various techniques such as spectrophotometry, thermal imaging, or pyrometry. These methods involve measuring the amount and wavelength distribution of thermal radiation emitted by the object and comparing it to a known reference. The emissivity value is then calculated based on this comparison.

What factors can affect the emissivity of a very high temperature object?

The emissivity of a very high temperature object can be affected by several factors, including its material composition, surface roughness, and temperature. Objects with smooth and dark surfaces tend to have higher emissivity values, while those with rough and reflective surfaces have lower emissivity values. Additionally, the emissivity of an object can also change with temperature, with most materials having a higher emissivity at higher temperatures.

Why is the emissivity of a very high temperature object important to consider?

The emissivity of a very high temperature object is important to consider because it affects the amount of thermal radiation it emits, which can have practical implications in various fields. For example, in industrial processes, the emissivity of materials used in furnaces or reactors can impact their energy efficiency and performance. In astrophysics, the emissivity of stars and other celestial bodies is crucial in understanding their energy output and behavior.

Can the emissivity of a very high temperature object be altered or controlled?

Yes, the emissivity of a very high temperature object can be altered or controlled to some extent. For example, surface treatments and coatings can be applied to change the material's surface properties and thus its emissivity. Additionally, adjusting the object's temperature can also affect its emissivity. However, for most materials, the emissivity is an intrinsic property that cannot be significantly altered without changing the material itself.

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