heat transfer by radiation
heat transfer by radiation
(OP)
I have searched this site, my texts, and the internet looking for an answer to my question which is based on the fundamentals of heat transfer by radation. I'm sure I'm overlooking something simple here, but I cannot figure out what. This is not a school question, I have recently applied to a job that requested a basic understanding of heat transfer by radiation and I feel I should be able to answer this anyway.
My question relating to heat transfer radiation has to deal with the distance between two bodies.... I've gone through all my old texts and looked at websites and I have yet to see a comment about effects of varying distances on the radiation transfer other than it is irrelevant.
Can someone explain to me then how when standing beside a fire , say 1 ft away, I can feel it intensely but if I were to step back 10 feet, I would feel warm, but nothing more. If I were to walk back 100 feet, I wouldn't feel it at all?
The only thing I can guess is that at 1ft, I am getting the radiation effects plus conduction (heat transfer through the air). At 10 ft, I am not longer experiencing conduction, so I only feel radiation after passing through cool air, which acts as an insulator. And at 100 ft, the insulating effect of the air has totally eliminated the radiation effect.
But, if tha'ts true, shouldn't it matter what the medium is as well?
Cheers
My question relating to heat transfer radiation has to deal with the distance between two bodies.... I've gone through all my old texts and looked at websites and I have yet to see a comment about effects of varying distances on the radiation transfer other than it is irrelevant.
Can someone explain to me then how when standing beside a fire , say 1 ft away, I can feel it intensely but if I were to step back 10 feet, I would feel warm, but nothing more. If I were to walk back 100 feet, I wouldn't feel it at all?
The only thing I can guess is that at 1ft, I am getting the radiation effects plus conduction (heat transfer through the air). At 10 ft, I am not longer experiencing conduction, so I only feel radiation after passing through cool air, which acts as an insulator. And at 100 ft, the insulating effect of the air has totally eliminated the radiation effect.
But, if tha'ts true, shouldn't it matter what the medium is as well?
Cheers





RE: heat transfer by radiation
http://en.wikipedia.org/wiki/Inverse-square_law
RE: heat transfer by radiation
To prove it mathematically is a little bit more difficult but assume constant radiation density radially emitted from the small sphere, the amount of radiation that you will receive will be a ratio of the solid angle subtended by the person's body to that of a sphere at those intervals of 1', 10' and 100'. Note radiation intensities will decrease as the inverse of the radius square,ie, at the 1',10' and 100'.
RE: heat transfer by radiation
I think I see where you were going with that example chicopee, thanks.
Mintjulep, I checked out that link and it was helpful too. Thanks.
RE: heat transfer by radiation
If you look at your heat transfer text, a free one is available here (http://web.mit.edu/lienhard/www/ahtt.html), you'll see that conduction becomes negligible beyond about 1/2 inch, so even at 1 ft, it's all radiation.
If you are ever at a Disney resort, you can verify this by the fact that the gas flame bursts shower you with heat way quicker than the speed of sound.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: heat transfer by radiation
View factors for typical orientations and bodies are tabulated in some texts, such as Siegel + Howell's "thremal radiation heat transfer" or Mike Modest's text.
RE: heat transfer by radiation
RE: heat transfer by radiation
> If you were to feel constant "heating" regardless of distance, you would need an infinite energy output, since the implication is that if you were a million miles away, you could still feel the heat. That's patently absurd, and there is only one type of source that can even come close to doing that, and if it did come our way, all life on Earth would instantly cease to exist.
> A radiative heat source is no different than a 60W light bulb, which allows you to do reading close up, but is pretty useless as a light from 100 ft away.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: heat transfer by radiation
Cheers.
RE: heat transfer by radiation
BOTH bodies in the problem are radiating "heat energy" = you (the colder one) are not radiating as much, but you ARE radiating heat away from yourself in all directions, not just absorbing it from the hot body. The "hot body" is receiving a portion of that radiating heat form you. The "walls" of the chamber you are in are receiving radiation form you, the hot body, and the rest of the walls. Scratch all of those problems IF and ONLY IF you are in a prefect vacuum of infinite size. Space, in other words. Otherwise, just mention it as a complication you have to make adjustments for in the real world.)
The amount of radiation being lost is proportional to the body's surface (metal) condition and roughness and color and reflectivity. A "theoretical" black body is impossible to get in real life, so dirt, shininess, shape and texture matter. A LOT. In a theoretical problem, they can be ignored - but again, only if you make that statement that you are (deliberately) ignoring the complications. (This implies, of course, that "you" know how to take care of the complications in your real life of real work and real materials .....)
Radiation is released/emitted according to the 4th power of the ABSOLUTE temperature (degrees K - NOT ever degrees F or degrees C.) Get familiar with the conversion, and KNOW IT. Know why you need to use degrees K. Know room temperature, freezing temperature, and boiling temperature in degrees K - and memorize them.
RE: heat transfer by radiation
Macmet, also try to understand the relationship between transmissivity, reflectivity and absorptivity. In addition, the wavelength dependence of all of these properties.
RE: heat transfer by radiation
racookpe1978 refers to the color (or colour in the UK) but what is the colour you see? If you heat a metal bar until it glows red then is it still black but only appears to be red because of the radiation you see? Just a thought.
corus
RE: heat transfer by radiation
From the Wien's displacement law:
T*λmax = b
where:
b= 2.8977685×10−3 mK Wien's displacement contant
T = Absolute temperature of the emitting source [degrees Kelvin]
λmax = peack wave length [m]
As temperature increases the peak emissivity shifts towards shorter wave length. We can deduce that as temperature varies the color (or colour in the UK) changes.