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Very Basic Thermodynamics, H20, and Jet Engines

by bkamdg
Tags: engines, thermodynamics, water
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bkamdg
#1
Apr9-14, 05:02 PM
P: 2
Dear Physics People,

I am a religion teacher with a background in philosophy and theology. I haven't taken a physics or calculus in some years, so there is a lot of information that I'm missing in my head.

So here's the deal. Airlines spend millions of dollars on fuel a day (around $32 million). The airlines also spends millions to save very little on fuel each year. Winglets save about 3-5% and cost millions, switching from 30 pound flight kits to 3 pound iPads, and Delta even bought an oil refinery in order to save some money on the East Coast. Airlines will do a lot to save some money.

So here is where I need some help, well a lot of help, if the inside of a jet engine is around 3000 F and water breaks down around the same temp. Can I put water in the engine and have it break apart and use that to power the engine. Now I understand that I might have to cool the hydrogen but there is a lot of cool air around the engine so I could do that.

I know that hydrogen is very volatile so I want to break it down and then burn it right away. That way I'm not storing hydrogen in the plane. Now I don't need the hydrogen to run the airplane the whole time just maybe 5-10% of the fuel. There is a lot of math that I am missing, and I don't understand much of the math. So I need a lot of help figuring this out.

Thanks so much for the teaching me.
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boneh3ad
#2
Apr9-14, 05:56 PM
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When you take hydrogen and burn it, it needs to be in the presence of an oxidizer as with burning any other compound. The end result is water. If you burn hydrogen you get water.

Now, back to your scenario, you are suggesting dumping water into the chamber to try and split it, then burn the byproducts to produce energy. Unfortunately, splitting the water requires energy, which would be removed from the flow through the engine. Then burning it would produce energy. However, the second law of thermodynamics dictates that you aren't going to get as much energy out of such a process as what it would have taken to split the water in the first place, so the process results in a net loss of energy.

Also, this assumes that all of the water molecules split at that temperature, but they don't. At 3000F, perhaps only 1% or 2% of water molecules will split into their constituents on their own, so you will take a whole lot of energy out of the flow to heat the water and only get 1% or 2% of the total hydrogen back out. Even at 5500F only somewhere around half of the molecules would split.

Really, if you dumped water into your jet engine, all you would manage to do in all likelihood is either produce a lot of low-thrust steam or else just quench the engine.
Drakkith
#3
Apr9-14, 08:25 PM
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It takes exactly as much energy to split the hydrogen from the oxygen as you would get from combining them back together during combustion. The end result, ignoring the many, many losses of energy in a real combustion process, would be to have zero gain or loss of energy. Conservation of energy!

etudiant
#4
Apr10-14, 05:25 AM
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Very Basic Thermodynamics, H20, and Jet Engines

Water injection was used in early jet engines (1950s), to cool the incoming air as it goes through the compressor.
This improves the mass flow without the compressor exit temperatures getting too hot for the materials available.
The added plumbing was a serious maintenance headache, so much effort was spent to improve materials enough to eliminate it in newer designs.
bkamdg
#5
Apr10-14, 08:02 AM
P: 2
Thanks for the replies. I knew that there would be many problems with this idea but I didn't know how to figure it out. Thanks again for the help.


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