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Anonymous Poster #1

Air Evolution from Jet A1 Fuel

10/27/2013 10:09 AM

Can anybody give data which tells the amount of air dissolved in Jet A1 fuel with respect to altitude variation?In Fuel properties book Ostwald coefficient is available for air but it gives the air solubility w.r.t. temperature of fuel. but how to arrive altitude variation effect?Can Henry's law be used which relates dissolved gas mole fraction and its partial pressure?

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#1

Re: air evolution from Jet A1 fuel

10/27/2013 11:05 AM

Higher altitude will yield less disolved gasses.

Jet A seems to work just fine on the ground, so what's the worry?

Put some in a vacuum chamber with a view port and watch it off gas as you lower the pressure.

Anyway, I don't know and this sounds like homework.

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#2

Re: air evolution from Jet A1 fuel

10/27/2013 12:43 PM

If air actually does dissolve in in Jet A1 fuel, I suspect that it will never be a constant value as pressure, temperature, and agitation change that amount.

I am very curious why anyone needs to know how much air is in any liquid fuel. Clearly the amount of air dissolved is insufficient for combustion. With the probable exception of microbial life, jet fuel will not support any life regardless of the amount of air dissolved.

So why do you wish to know how much air can be dissolved in jet fuel?

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Anonymous Poster #1
#3
In reply to #2

Re: air evolution from Jet A1 fuel

10/27/2013 1:07 PM

Thank you for the response and i agree that the dissolved air amount is depends on pressure, temperature and agitation. But in my case last two are negligibly changes.

I'm working on Aircraft fuel tank vent system sizing.

One of the requirement is, the vent system should take care that the rate of air evolution will not rise the tank pressure during steep climb to higher altitude. Ostwald coefficient says about 0.2 cubic m air will be dissolved in 1 cubic m of fuel at a particular temperature (20%).

The air evolution can be contained by pressurizing the fuel tank. But could not find a rational way to decide this value also for which the above data required.

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#4
In reply to #3

Re: air evolution from Jet A1 fuel

10/27/2013 4:06 PM

Obviously you are more versed in aviation jet fuel concerns than I am, but I have another question for you. Since pressurizing the fuel tank is a viable solution to this apparent problem of releasing air from jet fuel, why should anyone worry about how much air comes out of solution at all? In a worst case scenario of rapid ascent transitioning to a level high altitude flight won't this just extend the time it takes to equalize with outside pressure conditions. The pressure will never exceed the initial one atmosphere of pressure when the plane was on the ground. A plane with a steep angle of attack climb could conceivably have the outside pressure dropping faster than your vent can handle but so what. If this fuel tank is supplying fuel for the engine the volume of fuel will be getting smaller at the same time.

I'll grant you that this is an intriguing collection of differential equations to solve or at the very least approach. If the answer to this must be known, this seems more suited to a simple empirical test and measurement with a vacuum chamber than something that should be theoretically calculated.

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#5

Re: Air Evolution from Jet A1 Fuel

10/27/2013 5:27 PM

I think you could use the ideal gas law to account for the change in density due to pressure and temperature of the air. I think the Ostwald coefficient gives the actual volume ratio, which remains constant with pressure. But since the density changes with pressure, the mass of dissolved air decreases as the pressure decreases, almost proportionately.

Googling 'ostwald coefficient jet fuel' gave some interesting results, including FAA.gov. The FAA would be a good source, I think.

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Anonymous Poster #1
#8
In reply to #5

Re: Air Evolution from Jet A1 Fuel

10/28/2013 2:29 PM

Thank you very much sir for the reply and i got some material from FAA.org on testing of O2 release from fuel.

In your reply could you explain more on the "volume ratio remains constant with pressure". Here the pressure mentioned is above fuel surface. As the fuel is incompressible and so the dissolved gas also. So how pressure variation will affect mass of dissolved air?

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#9
In reply to #8

Re: Air Evolution from Jet A1 Fuel

10/28/2013 7:36 PM

I think the compressibility of the fuel can be ignored at the pressures you are dealing with, but that of the dissolved gas (air) surely cannot. You are dealing with, what, 2:1 (absolute) pressure ratio or more?

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Anonymous Poster #1
#10
In reply to #9

Re: Air Evolution from Jet A1 Fuel

10/28/2013 10:16 PM

Sir, sorry i could not get your point. 2:1 pressure ratio which point you are mentioning? here pressure reduction is atmospheric pressure variation, where at SL 101.3 kPa and at 15 km 12.5 kPa.

Could you please explain more on you point "volume ratio remains constant with pressure".

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#11
In reply to #10

Re: Air Evolution from Jet A1 Fuel

10/28/2013 11:15 PM

OK, so your actual pressure ratio is 101.3 / 12.5 = 8.1. If the temperature were constant (which it probably isn't), the density of the air at altitude would be about 1/8 of the density at sea level (SL). Since specific volume is 1/density, the air contained in the fuel at sea level would expand to about 8 times that volume at altitude. Since the fuel can only contain the same (initial) volume, 7/8 of that volume would evolve (assuming equilibrium is reached) and 1/8 would stay in solution.

You would still have to make the actual calculations, based on real conditions. That's as clear as I can make it. I have already broken my cardinal rule: Do not respond to anonymous posters. Thanks for reminding me why. Now I'm probably the laughing stock of the forum.

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#15
In reply to #8

Re: Air Evolution from Jet A1 Fuel

10/30/2013 11:02 AM

Wrong assumption. Whilst the liquid is non-compressible, what makes you think the gas (even dissolved in the liquid) is non-compressible. The standard volume of gas dissolved at lower pressure is less than that at standard pressure due to expansion of the gas. The volume ratio refers to the volume of gas dissolved at a standard condition. That is why pulling vacuum will de-gas this fuel every time.

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#6

Re: Air Evolution from Jet A1 Fuel

10/27/2013 9:25 PM

Did you ever figure out how to determine the cg of a rotating liquid mass?

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#12
In reply to #6

Re: Air Evolution from Jet A1 Fuel

10/28/2013 11:16 PM

How did you recognize him/her?

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#13
In reply to #12

Re: Air Evolution from Jet A1 Fuel

10/28/2013 11:45 PM

First, I am the laughing stock of the forum.

Well, any aerobatic pilot worth their salt would be interested in not only the CG of the fuel during a roll, but also the density of the fuel in the tank, right?

What do I know?

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#14
In reply to #13

Re: Air Evolution from Jet A1 Fuel

10/29/2013 9:02 AM

I'm not a pilot of any level but I suspect that the aerobatic pilot constantly tests and demonstrates the CG of the whole plane, not just the fuel. The only time the CG of just the fuel tank would be relevant is when designing the support of the tank in the aircraft. Certainly baffles in the fuel tank will be required in a light weight aircraft so that the fuel motion minimizes the relative speed of this fluid sloshing.

In an aerobatic plane I suspect the tank will not be vented to prevent fuel leakage.

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#7

Re: Air Evolution from Jet A1 Fuel

10/28/2013 10:39 AM

Depends a great deal more on temperature for the major constituents of air with air solubility increasing with temperature up to a certain limit. Carbon dioxide is much more soluble than nitrogen and oxygen (in aviation fuels), but this solubility decreases with temperature, and so tank heaters will help with this, but not with the other gases.

http://www.dtic.mil/dtic/tr/fulltext/u2/a132106.pdf

this link is fairly informative.

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