Heat absorber
Heat absorber
(OP)
Hi,
I am trying to design an exhaust system that would make it very hard for a heat-seeking missile to identify.
I have read that the F-117 stealth fighter uses "heat conducting materials" in its exhaust ducts to take some heat away from the gasses leaving the engine.
I am thinking about surrounding the exhaust with the foam that the space shuttle uses to protect itself from the high temperatures experienced during re-entry to the earth's atmosphere. This material can hold a MASSIVE amount of heat energy.
However, my thermodynamics is a little rusty, since I haven't studied it for a couple of years.
I know that the zeroth law of thermodynamics says if the temperature of two bodies in contact is the same, there is no flow of energy between them.
I know that if the exhaust gas was held in a container, over time it would reduce its temperature until it was equal to the temperature of the ambient atmosphere.
The problem is that the exhaust gas is constantly being renewed, so the temperature of the gas at the point where it leaves the turbine is always the same (about 720 decgrees C). If the space shuttle foam was next to the gas flow, I think it would just heat up until it is also at 720 degrees C, and the exhaust gas would not be able to cool down.
Am I right or wrong?
I would think that two bodies would be in equilibrium when they both have the same internal energy, so if the absorbing material had a low specific heat capacity then it could maintain a higher temperature than the exhaust gas, allowing some reduction in exhaust temperature for a sustained period.
I am trying to design an exhaust system that would make it very hard for a heat-seeking missile to identify.
I have read that the F-117 stealth fighter uses "heat conducting materials" in its exhaust ducts to take some heat away from the gasses leaving the engine.
I am thinking about surrounding the exhaust with the foam that the space shuttle uses to protect itself from the high temperatures experienced during re-entry to the earth's atmosphere. This material can hold a MASSIVE amount of heat energy.
However, my thermodynamics is a little rusty, since I haven't studied it for a couple of years.
I know that the zeroth law of thermodynamics says if the temperature of two bodies in contact is the same, there is no flow of energy between them.
I know that if the exhaust gas was held in a container, over time it would reduce its temperature until it was equal to the temperature of the ambient atmosphere.
The problem is that the exhaust gas is constantly being renewed, so the temperature of the gas at the point where it leaves the turbine is always the same (about 720 decgrees C). If the space shuttle foam was next to the gas flow, I think it would just heat up until it is also at 720 degrees C, and the exhaust gas would not be able to cool down.
Am I right or wrong?
I would think that two bodies would be in equilibrium when they both have the same internal energy, so if the absorbing material had a low specific heat capacity then it could maintain a higher temperature than the exhaust gas, allowing some reduction in exhaust temperature for a sustained period.





RE: Heat absorber
Should be lots of info available on them.
To answer your question (sort of):
If you insulate the exhaust ducting then the exhaust gas stream will not cool down as it passes through the duct. Therefore the only thing that can happen is that it will be hotter when it leaves the duct than it would be if the duct were uninsulated.
RE: Heat absorber
RE: Heat absorber
B2 and F117 and F22 use mixing to cool the exhausts and hid them on the tops of the wings.
That's the only gig in town; mix and spread.
One thing that's rather funny is that the RAM coating on the fuselages are VERY good IR absorbers and emitters.
TTFN
RE: Heat absorber
I am aware that mixing with cool air is a good way to cool down the exhauust gas, And I guess if I used a thin but wide rectangular exhaust I can get a lot of mixing.
Its just that on wikipedia it says that the F-117 uses heat absorbing materials in its ducts to cool down the exhaust gases and I was trying to think of a way that this could be possible.
I suppose that contacting a mass with a large specific heat capacity would allow a large amount of energy to be absorbed, but this conduction would be likely to be pretty slow, too slow to cool the exhaust gas enough to have a good effect.
Thinking Fourier's Law of conduction (dQ/dt=kA.dT) to find the heat energy flow, I would need a material with a high value of thermal conductivity.
If the specific heat capacity was very large, the increase in temperature would be low compared to the amount of heat energy being transfered to the heat sink (Q=mc.dT). When the temperature of the heat sink is less than the gas, the temperature of the gas will decrease.
Does anybody know of a material with high thermal conductivity and specific heat capacity?
RE: Heat absorber
http://en.wikipedia.org/wiki/Stealth_technology
http://www.aeronautics.ru/f117a.htm
http://en.wikipedia.org/wiki/F-117
I suggest that you do the math on the amount of heat generated by a jet turbine and compare that to the specific heat of available materials.
TTFN
RE: Heat absorber
Just for the record, the zeroth law says that if systems A and B are each in thermodynamic equilibrium with C, then A and B are in thermodynamic equilibrium with each other.
RE: Heat absorber
RE: Heat absorber
Its just that on wikipedia it says that the F-117 uses heat absorbing materials in its ducts to cool down the exhaust gases and I was trying to think of a way that this could be possible.
snip
Wikipedia is unvetted information and, as such, inaccurate.
Words will hold still for anyone.........
RE: Heat absorber
Thermal equilibrium is based on temperature, not internal energy.
The mix and spread description above is valid and is potentially augmented by conducting heat out of the gas to other places that then reject it to the surroundings.
I agree with IRStuff's suggestion that you calculate the heat absorption capacity of the proposed solids. Don't forget that you are taking a significant fraction of the energy content of the burned fuel and proposing applying it to solids in the plane. Sort of like a flying foundry.
Jack
Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering
www.KleinfeldTechnical.com