Dissipating heat in outer space... how??

[dxkj]

http://www.cnn.com/2003/TECH/space/12/09/jupiter.icymoons.ap/index.html


They say the heat from the nuclear reactor would be dissipated by a lot of fins.... how would that work in space? I know there is a very low density of atoms?


Maybe Im just stupid

 

[OrganizedChaos]

probably however the electronics in a regular satelite disapate heat. although i have no idea how that works. i don't see where that thermal energy is gunna go in a vacuum.

 

[Matthias99]

You can't cool by convection or conduction in space, as obviously there's no matter there to dump heat into. However, it's certainly possible to cool by radiation -- that is, a hot heatsink emitting infrared radiation out into "empty" space.

Honestly -- how do you think the Sun heats the Earth through a few million miles of vacuum? Same thing.

 

[rjain]

Yep, it's not easy and is an important consideration when designing a space vehicle.

 

[grant2]

If you read any heinlein, he discusses it a little bit. His solution was space suits that vent gas after it has been breathed by the occupant.

 

[RossGr]

The 3 heat transfer mechanisms are Conduction, Convection and Radiation.

Your Fan/heat sink rely on conduction to move the heat from the processor to the Heat Sink and convection to move the heat from the Heat sink to the air in your case.

Radiation is the method the earth gains and loses energy, the day time side adsorbs heat energy from the sun while the night time side radiates heat to deep space. Radiation depends on exposed surface area and the Temperature difference. Also involved is the Stephan-Boltzmann constant (generally lower case sigma ) = .1713 exp -8 Btu/(ft^2*hr*R^4)

Radiation heat loss per unit area goes like

e=(sigma)*T^4

Here the temperature must be in absolute units (K or R)

to get the total energy you need to look at the temperature difference between the body and the other "visible" surface. For a body in space the "visible" surface would be deep space at 3K.

According to theory you can freeze water on a warm summer night by placing it in a open mouth thermos bottle and pointing the open end at the dark sky. The water sees only the 3K temp of deep space and radiates its heat content to the nearly infinite sink of deep space.

 

[LurchFrinky]

I'm disappointed in you, Ross. You completely forgot about epsilon, the emissivity of the object .

 

[RossGr]

Originally posted by: LurchFrinky
I'm disappointed in you, Ross. You completely forgot about epsilon, the emissivity of the object .

Yep, sure did. I sort of neglected it on the KISS principle, that and I was a bit short on time.

Note the ugly British units on sigma, that was what was given in the book I had on hand. I really should have searched out a metric value.

I'll leave that to the reader!

 

[f95toli]

A neat illustration of this principle is glas-cryostats with liquid helium ( at 4.2K) . The cryostats is built in layers just like a thermos bottle: helium container-vacuum-liquid nitrogen-vacuum.. You need the liquid nitrogen because it is at 77 K and it "cools" the radiation from the surrounding, since the radiative heating is proportional to T^4 there is a huge difference between 77 and 300K. The nitrogen is totally transparant to visible light.

These cryostats are fun to play with because you can see the liquid helium, and if you reduce the preassure you can cool it further and make it superfluid which is really interesting.

 

[Mday]

Originally posted by: RossGr

Originally posted by: LurchFrinky
I'm disappointed in you, Ross. You completely forgot about epsilon, the emissivity of the object .

Yep, sure did. I sort of neglected it on the KISS principle, that and I was a bit short on time.

Note the ugly British units on sigma, that was what was given in the book I had on hand. I really should have searched out a metric value.

I'll leave that to the reader!

lmao

 

[Pulsar]

There are other methods of dissipating heat in space. The obvious way is to change heat into another form of energy that can be easily dissipated. Electrical generation being the most obvious.

But in space, the only true way to dissipate heat aside from change the form of energy, is by radiation, which is a VERY slow process.

 

[rjain]

This heat is usually from the power plant in the first place, actually

 

[LurchFrinky]

So, how would you dissipate electical energy in outer space anyway? Use it for propulsion to go around in circles?

 

[Matthias99]

The only way to "dissipate" electrical energy is to use it to power something (unless you suggest running a ground wire to the satellite!). This just generates waste heat again, putting you back where you started.

Radiation is "slow" here on Earth because the ambient temperatures around us don't produce enough of a differential for it to be significant in most applications (few environments would give you a "cold" temperature of less than 300K), and convection and conduction dominate heat transfer. In orbit, where you have, essentially, an infinite-capacity radiation heatsink held at 5-10K (ie, outer space), radiative cooling is much more effective.

 

[rjain]

And there's nothing to conduct to and no medium to convect heat.

 

[everman]

Radiate the heat in a way to aid propulsion?

 

[Def]

Off the top of my head the metric number for the Stephan-Boltzmann constant is 5.67*10^-8(just remember 5,6,7,8).

Forgot what the units are... maybe that's why Heat Transfer gave me fits from time to time.

 

[Def]

Guys... listen to what you're saying...

You would not go through the trouble to produce electrical energy only to have the problem of trying to dump it into space.


You use the electricity produced by the nuclear decay of radioactive material to do "useful" things, then dump the byproduct of this electricity production, waste heat energy, out into space. Regular powerplants operate in much the same way, although this is grossly oversimplified from the real process.

 

[RossGr]

Metric Stephan Boltzmann units would be

W*m^-2 * C^-4

 

[Pulsar]

 

[Pulsar]

Sorry about the double post.

Couldn't any excess energy be converted to light? Or is there no light-generating device in space that is capable of operating with only convection to drag the excess heat that isn't given off as light away from the device?

I'm probably missing the basic theory - that generating the light creates more waste energy in the form of heat than you get rid of in the form of light?

Just curious.

 

[f95toli]

When you radiate heat you ARE radiating light, it is just in the infrared part of the spectrum which we can not see without special equipment.

 

[ZeroNine8]

in fact, converting heat (infrared) radiation to visible radiation, even if done perfectly, would be less efficient than simple infrared radiation is higher energy than visible light due to wavelength (if I remember my physics correctly). Heat transfer via radiation in space is quite effective, as it has been stated several ways already. Does anyone know if the radiation could be harnessed and directed as an additional form of propulsion for the satellite? In theory, it should work, though the magnitude of the force may be negligible.

 

[ZeroNine8]

edit: I'm so important, I had to say it twice.

 

[Matthias99]

Does anyone know if the radiation could be harnessed and directed as an additional form of propulsion for the satellite? In theory, it should work, though the magnitude of the force may be negligible.

Yes -- in fact, there's no way for it not to affect you. Since you tend to emit radiation equally in all directions, though, this effect is really, really small. It would be like a lightsail with a surface area of a few square meters. A system built specifically to do this might be somewhat more effective, but IANANE (I Am Not A NASA Engineer).