Effervescent fuel
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Effervescent fuel
Found this to be interesting. Some aircraft have a max altitude without boost pump capability, some don't. Pehaps due to effervescence.
Jet fuel contains dissolved air and the amount depends on the temperature of the fuel and the altitude. As the airplane climbs to cruising altitude, and the ambient pressure decreases, air is liberated from the fuel as bubbles that rise to the surface. As the flight progresses the rate of effervescence decreases as the fuel comes into equilibrium with the atmospheric and fuel temperature conditions. This condition is called fuel weathering.
Jet fuel contains dissolved air and the amount depends on the temperature of the fuel and the altitude. As the airplane climbs to cruising altitude, and the ambient pressure decreases, air is liberated from the fuel as bubbles that rise to the surface. As the flight progresses the rate of effervescence decreases as the fuel comes into equilibrium with the atmospheric and fuel temperature conditions. This condition is called fuel weathering.
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Wow! Flown 16,000hrs in over 30 years and 15 turbine types - and never heard of it!
Air dissolved in kerosene? Really??
Where did that quote come from?
Max alt sans fuel pump is conventionally understood to be due to cavitation, ie vapourisation of the fuel.
Edit.
A brief internet search shows weathering to be a process in which the more volatile elements (benzene, naptha, toluenes etc) in an oil based fuel are stripped from it over time by evaporation of driven off by heat ie in a fire leaving the heavier fractions behind. I found no reference to Air though, and this process seems to take weeks or months. It appears to be a somehat niche science to do with analysing oil spill residue, forensic analysis of accelerants in arson fires and degradation of long-term oil fuel stocks. I also found no reference to it occurring due to lower atmospheric pressure at altitude and although it may perhaps be seen as somewhat similar to cavitiaion nb. cavitation is caused by mechanical action (eg of a fuel pump) and not just evaporation.
It certainly isn't in the mainstream of aviation knowledge.
Air dissolved in kerosene? Really??
Where did that quote come from?
Max alt sans fuel pump is conventionally understood to be due to cavitation, ie vapourisation of the fuel.
Edit.
A brief internet search shows weathering to be a process in which the more volatile elements (benzene, naptha, toluenes etc) in an oil based fuel are stripped from it over time by evaporation of driven off by heat ie in a fire leaving the heavier fractions behind. I found no reference to Air though, and this process seems to take weeks or months. It appears to be a somehat niche science to do with analysing oil spill residue, forensic analysis of accelerants in arson fires and degradation of long-term oil fuel stocks. I also found no reference to it occurring due to lower atmospheric pressure at altitude and although it may perhaps be seen as somewhat similar to cavitiaion nb. cavitation is caused by mechanical action (eg of a fuel pump) and not just evaporation.
It certainly isn't in the mainstream of aviation knowledge.
Last edited by noflynomore; 10th Jun 2017 at 12:05.
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Indeed the A320 on gravity feed has either FL360 as a limit with deaerated fuel or FL260 with non-deaerated fuel.
The decider as to whether the fuel is aerated or not is whether you have been above FL300 for more or less than 30 mins.
The decider as to whether the fuel is aerated or not is whether you have been above FL300 for more or less than 30 mins.
De-aerated fuel
JammedStab
...Jet fuel contains dissolved air and the amount depends on the temperature of the fuel and the altitude. As the airplane climbs to cruising altitude, and the ambient pressure decreases, air is liberated from the fuel as bubbles that rise to the surface..
On one past passenger aircraft with a high climb rate, those tanks where the fuel remained relatively static during the climb required the fuel to be de-aerated to ensure the air in solution in the fuel did not become a hazard as the ambient air pressure in the fuel tanks decreased during the climb.
There were two principal areas of concern. Firstly, that as the air in solution in the fuel expanded, the likelihood of fuel pump cavitation increased, and secondly, the possibility that transient spike increases in fuel tank pressure might lead to fuel transfer and loss via the vent system.
De-air pumps were used, which circulated fuel through spray nozzles within the tanks, and, at higher altitudes, the tanks were partly pressurised to counteract these concerns and also to counteract the effects of kinetic heating on the fuel and so prevent the lighter elements of the fuel boiling off.
noflymore
... the more volatile elements (benzene, naptha, toluenes etc) in an oil based fuel are stripped from it ... by heat ... leaving the heavier fractions behind...
Our ground engineers had another word for the “heavier fractions” you mention that could be left behind in our very thin tip tanks, due to kinetic heating and low ambient air pressure boiling off the lighter elements, if the poor F/E forgot to use that fuel as soon as practicable!
...Jet fuel contains dissolved air and the amount depends on the temperature of the fuel and the altitude. As the airplane climbs to cruising altitude, and the ambient pressure decreases, air is liberated from the fuel as bubbles that rise to the surface..
On one past passenger aircraft with a high climb rate, those tanks where the fuel remained relatively static during the climb required the fuel to be de-aerated to ensure the air in solution in the fuel did not become a hazard as the ambient air pressure in the fuel tanks decreased during the climb.
There were two principal areas of concern. Firstly, that as the air in solution in the fuel expanded, the likelihood of fuel pump cavitation increased, and secondly, the possibility that transient spike increases in fuel tank pressure might lead to fuel transfer and loss via the vent system.
De-air pumps were used, which circulated fuel through spray nozzles within the tanks, and, at higher altitudes, the tanks were partly pressurised to counteract these concerns and also to counteract the effects of kinetic heating on the fuel and so prevent the lighter elements of the fuel boiling off.
noflymore
... the more volatile elements (benzene, naptha, toluenes etc) in an oil based fuel are stripped from it ... by heat ... leaving the heavier fractions behind...
Our ground engineers had another word for the “heavier fractions” you mention that could be left behind in our very thin tip tanks, due to kinetic heating and low ambient air pressure boiling off the lighter elements, if the poor F/E forgot to use that fuel as soon as practicable!
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There are two different concepts here:
1. Fuel degassing (deaeration). This is the release of dissolved gasses, including oxygen, from the fuel.
2. Fuel weathering. This is the release of lighter / more volatile hydrocarbons from the fuel -- changing the composition of the fuel itself.
The author quoted in the first post seems to have mixed the two.
1. Fuel degassing (deaeration). This is the release of dissolved gasses, including oxygen, from the fuel.
2. Fuel weathering. This is the release of lighter / more volatile hydrocarbons from the fuel -- changing the composition of the fuel itself.
The author quoted in the first post seems to have mixed the two.
Probably something a bit long, pointy and rather quick...
When trying to identify a fuel gauging problem on another aircraft (loss of fuel quantity at the top of the climb), I did a lot of tests with a fuel gauge in a vertical perspex cylinder of Jet A1. Having it connected to an air data test set, I was able to simulate various rates of climb with varying fuel levels.
Fresh fuel would start to significantly de-gas at about 16,000ft, and over about 20 minutes would trail off to nothing. At around 20,000ft, bubbles were 1/2" diameter as they got to the top of the cylinder. Imagine opening a two litre bottle of sparkling water on a warm day and you will get the idea.
I used the fuel test facility built for the four-engined aeroplane mentioned, to do some fuel tank inerting research much later on. At the time, we thought that removing the dissolved oxygen in the fuel would be important, and tried all sorts of aquarium diffusers to put fine streams of nitrogen enriched air (AKA not quite pure nitrogen) through the fuel, although this proved to be unnecessary in the long run, as the dissolved oxygen is quite small in comparison to the ullage (space above the fuel) on anything other than a tank which is completely full.
Fuel degassing can be a problem in fuel lines which have a valve at each end, such as a tail to wing tank. If the valve are only opened at altitude, the fuel in the pipe will then de-gas, making a gravity transfer more difficult.
Piston-engined aircraft burning petrol type fuels are a different matter. We we told that the B-29 lost about 15% of its potential range due to fuel evaporation, although I can't find a reference at the moment to back that up.
When trying to identify a fuel gauging problem on another aircraft (loss of fuel quantity at the top of the climb), I did a lot of tests with a fuel gauge in a vertical perspex cylinder of Jet A1. Having it connected to an air data test set, I was able to simulate various rates of climb with varying fuel levels.
Fresh fuel would start to significantly de-gas at about 16,000ft, and over about 20 minutes would trail off to nothing. At around 20,000ft, bubbles were 1/2" diameter as they got to the top of the cylinder. Imagine opening a two litre bottle of sparkling water on a warm day and you will get the idea.
I used the fuel test facility built for the four-engined aeroplane mentioned, to do some fuel tank inerting research much later on. At the time, we thought that removing the dissolved oxygen in the fuel would be important, and tried all sorts of aquarium diffusers to put fine streams of nitrogen enriched air (AKA not quite pure nitrogen) through the fuel, although this proved to be unnecessary in the long run, as the dissolved oxygen is quite small in comparison to the ullage (space above the fuel) on anything other than a tank which is completely full.
Fuel degassing can be a problem in fuel lines which have a valve at each end, such as a tail to wing tank. If the valve are only opened at altitude, the fuel in the pipe will then de-gas, making a gravity transfer more difficult.
Piston-engined aircraft burning petrol type fuels are a different matter. We we told that the B-29 lost about 15% of its potential range due to fuel evaporation, although I can't find a reference at the moment to back that up.
Last edited by Mechta; 11th Jun 2017 at 12:07.
There are two different concepts here:
1. Fuel degassing (deaeration). This is the release of dissolved gasses, including oxygen, from the fuel.
2. Fuel weathering. This is the release of lighter / more volatile hydrocarbons from the fuel -- changing the composition of the fuel itself.
The author quoted in the first post seems to have mixed the two.
1. Fuel degassing (deaeration). This is the release of dissolved gasses, including oxygen, from the fuel.
2. Fuel weathering. This is the release of lighter / more volatile hydrocarbons from the fuel -- changing the composition of the fuel itself.
The author quoted in the first post seems to have mixed the two.
In other words, while your differentiations may be technically correct, within the industry it's generally all referred to as 'weathered fuel'.
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In other words, while your differentiations may be technically correct, within the industry it's generally all referred to as 'weathered fuel'.
Jet fuel will weather over a period of time as noflynomore describes. If returned to low altitude (generally back on the ground) will reabsorb air from the atmosphere over a period of days or weeks after having been taken to altitude and thus degassed.
From my rather hazy recollections, air in fuel exists in three states. The size of the groups of molecules and their degree of entrapment by the fuel determining the boundaries between the types.
From my rather hazy recollections, air in fuel exists in three states. The size of the groups of molecules and their degree of entrapment by the fuel determining the boundaries between the types.
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It would be interesting to actually see this fuel while it is weathering during the climb/initial cruise flight. I would be curious if the jet fuel resembles a glass of champagne/beer/7up or has more/less/larger/smaller bubbles.
Maybe TD racer knows.
