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Coffee House => Coffee House Boards => CH / Science and Technology => Topic started by: JaXanim on March 03, 2004, 11:37:42 PM
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For several weeks now, our regional newspaper has published letters to the editor on the subject of aircraft vapour trails and how they are formed.
Many readers have submitted their observations about these fascinating phenomena and have put forward their hypotheses as to the physics involved. Many people have said that the trails are created by the jet engines. However, several writers have dismissed that idea. Instead, they assert with apparent authority, that the trails are caused by the precipitation of water vapour in low pressure vortices at the wingtips of planes.
Anyone care to comment on the physics of vapour trails?
Cheers,
JaX
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It's easy, not a matter of physics, but rather, chemistry.
Hydrocarbon + oxygen --> carbon dioxide + water
The water vapour comes from the combustion itself.
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i`ve a feeling that soot , or impurities in the exhaust, actually form the `seed` around which ice crystals can grow...
Due to the combination of vortexes and water produced by the engines (+ that in the air already)...
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JaXanim wrote:
For several weeks now, our regional newspaper has published letters to the editor on the subject of aircraft vapour trails and how they are formed.
Many readers have submitted their observations about these fascinating phenomena and have put forward their hypotheses as to the physics involved. Many people have said that the trails are created by the jet engines. However, several writers have dismissed that idea. Instead, they assert with apparent authority, that the trails are caused by the precipitation of water vapour in low pressure vortices at the wingtips of planes.
Anyone care to comment on the physics of vapour trails?
Cheers,
JaX
Actually it's both. The Vapour trails you see from large Jet airliners are from the exhaust water, heat, soot and NOX etc from the engines.
But at the wing tip you have a point where high pressure under the wing "leaks" up around the wing tip to the low pressure above the wing, commercial aeroplanes have Winglets (those little vertical fins on the wing tips) to top this, and as it leaks up it forms a vortex. These vortices heat and compress the air and it mixes with the cool air around it, water condenses out.. etc...
You will only really see a wintip vortex on a small military jet, usually when it in a high G turn and there is a lot of energy in the vortex.
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Let's have some physics. Water from combustion, high-g flight, etc don't tell us much about the mechanism of vapour trail formation. What conditions prevail when trails are formed? Some days they do, some days they don't. Why?
JaX
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JaXanim wrote:
Let's have some physics. Water from combustion, high-g flight, etc don't tell us much about the mechanism of vapour trail formation. What conditions prevail when trails are formed? Some days they do, some days they don't. Why?
JaX
I don't know. :-(
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JaXanim wrote:
Let's have some physics. Water from combustion, high-g flight, etc don't tell us much about the mechanism of vapour trail formation. What conditions prevail when trails are formed? Some days they do, some days they don't. Why?
JaX
When conditions are right they will form :-)
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THANKS FOR THAT, BLOODLINE. :-)
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Vapor trails are a concerted effort by the government to spread a mind controlling substance over the population.
Resistance is futile...
:pissed:
:-o
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OK, now we're gettin' somewhere. But what's the physics? Surely, someone knows a bit about water vapour?
JaX
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I think the simple fact is that in cooler air the water vapour becomes supercooled upon leaving the engine. As it hits the jetstream formed by the plane, it drops out of vapour and forms liquid droplets - giving the misty white haze that you see from jets, and occassionally, scraped from the air by wingtips.
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Hi,
Well, it takes a certain amount of humidity. A wing flies by creating a VERY low pressure above the wing. expanding air cools, water vapor condenses in this low pressure area and becomes visable. if cold enough this vapor will freaze and stay suspended for a while.
Chris
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Hi,
Well, it takes a certain amount of humidity. A wing flies by creating a VERY low pressure above the wing. expanding air cools, water vapor condenses in this low pressure area and becomes visable. if cold enough this vapor will freaze and stay suspended for a while.
Chris
Darn double post
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Watching films of high speed/high-g flight, it's clear that vapour precipitation from the upper side of the wing is quite transient. You can see a standing wave of fog at the lowest pressure zones. However, this re-evaporates towards the trailing edge, where the upper and lower wing pressures equilibrate. I can't see this being the mechanism for vapour trails. In any case, commercial flights do not create those conditions. It's usually seen in military aircraft. Concorde showed it when landing, but most civil flights do not.
Why do trails form on some days and not others? What's the physics?
JaX
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Hi,
Altitude and humidity are the big factors here.If cold enough in the low presure area, the vapor can freaze, the conditions must be just right. combination air temp, wet and dry bulb, which factors in humidity. also altitude and velocity have a role and also wing loading.
Chris
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Ah, the wet and dry bulb thermometer. That sounds more like the proper physics to me. Anyone care to elaborate on this? I think the truth is coming together here.
JaX
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Yea, i remember seeing a vapour trail coming off a rear wing on a formula one car once...
(no-where near the engine exhaust)
This was clearly the vortex creating a low pressure in which moisture condensed out of the air..
(a bubble chamber in effect)...
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@blobrana
Yes, that's the transient precipitation I talked about earlier. The car didn't leave a trail right around the track did it. If the right conditions were to prevail, it might happen, but that's highly unlikely.
So, what is it about flying that's different from the F1 scenario?
JaX
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JaXanim wrote:
@blobrana
Yes, that's the transient precipitation I talked about earlier. The car didn't leave a trail right around the track did it. If the right conditions were to prevail, it might happen, but that's highly unlikely.
So, what is it about flying that's different from the F1 scenario?
JaX
On a cold day, my car leaves a vapour trail, even when it's moving slowly. The vapour tends to drift upwards though.
Now, that's just the water from burning petrol at the rate of approx. 3 ltr/hr.
A 747 (according to this (http://www.airnewzealand.co.nz/aboutus/resourcecentre/aircraft_statistics.htm) has a mac fuel capacity of 173 tonnes, for a range of 13.000 km and a cruising speed 0of 930kph.
That's a burn rate of 12.4 tonnes of fuel per hour.
Aviation jet fuels are C8-C16 hydrocarbons. As a stab in the dark say that the average is round about C about 12 and H around 22.
That works out to be about 15 tonnes of water produced per hour. It doesn't seem unreasonable to suspect that if the upper atmosphere is cold and saturated, then that amount of condensate would be visible.
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Hi, Actually blobrana is right you don't need an engine. Living in southern California I have been privlaged to see the shuttle come in.when high up with the heavy wing loading it creates an extreame low pressure and if the humidity is not too high the vapor freazes and hangs for a while. Of course you all know when the shuttle returns it is a glider. Now if you were to watch it land at florida you will see the transient vapor trails that can't hang around to long. Humidity & temps too high.
Chris
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I think blobrana is getting nearer the hub (nub?) of it. I need to do some calculations myself. We need numbers like the volume of water vapour ejected at upper atmosphere conditions to get a better picture of trail generation.
Yes Chris, the Shuttle trails are the transient type caused by wing tip/lifting body precipitation. I'd suggest that you *could* see airliner trails on the same day lasting significantly longer than the Shuttle's, eh?
Another clue. Aircraft vapour trails don't usually form when the atmosphere is warmer than minus 50 Celcius (very approx).
Right, we need to use the gram molecular volume. Anyone got that to hand? I also need the air pressure at say 30-40,000 feet.
JaX
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QuikSanz wrote:
Hi, Actually blobrana is right you don't need an engine. Living in southern California I have been privlaged to see the shuttle come in.when high up with the heavy wing loading it creates an extreame low pressure and if the humidity is not too high the vapor freazes and hangs for a while.
Show me a commercial jet liner that does Mach 9.
Yes, if you look out of the window when you're going through cloud you can often see condensation in the air coming off the wingtip. But this is not the mechanism that causes contrails in high altitude commercial flight.
Get a pair of binoculars and take a look next time you see a jet making a contrail. The trail starts well back from the wing and not in line with the wing tip but with the engine.
The trails that form from shocked air cling to the edge that creates them generally. Far back from the wing they just don't spring into existance.
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@ JaXanim
qoute,
I also need the air pressure at say 30-40,000 feet.
I used to know this but it's been too long. Any pilot should have the answer to that one. Any Pilots out there.
Chris
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@fluffy...
That's right, contrails are caused by the engines. The wing precipitation is transient and appears within a standing wave in the low pressure zones. Once the pressure equilibrates (near the trailing edge) the fog re-evaporates.
There's a lot of useful stuff about this on the Net and we should be seeing a pretty definitive explanation of vapour trails quite soon.
BTW, the gram molecular volume is 22.4 litres, so a 747 produces quite prodigious volumes of water vapour at 35,000 feet. More later.
JaX
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@ Fluffy
By the time it gets to here it's barely over mach 1 cause it will land @ Edwards as soon as it clears the mountains. Aproxx 100 miles.
Chris
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@QuikSanz
I've managed to find this on the Net HERE (http://www.engineeringtoolbox.com/25_462.html). Taking 35,000 feet as an average height for commercial flights, the air pressure is 179mmHg or 24 kPa. This compares with 760mmHg/101kPa at ground level.
In other words, the pressure is less than a quarter of that at ground level.
I'm gonna do some calcs using the Combined Gas Law to determine the volume of vapour expelled by a 747 at that altitude. I'll take blobrana's data as a starter. Maybe this will refine as more data comes in on aviation fuel.
Cheers,
JaX
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JaXanim wrote:
@QuikSanz
Maybe this will refine as more data comes in on aviation fuel.
JaX
Maybe best to just buy some jet fuel and do an empirical test. :-o
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@ JaXanim,
AWEB won't work on that site, bummer. I'll either check it at work or my A1 when it shows up monday. As for engine related trails I belive the coditions are relativly simaler. We also get other neet shows in So.Cal. Once in a while they'll launch a military rocket from Vandenberg AF base. with the Oxygen/ Hydrogen fuel they use it makes for a great trail at least 100 mi. long. Now that stuff has to freeze.
Chris
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Here's a nice little shot of the Shuttle coming in for a landing, with a nice little trail.
(http://www-pao.ksc.nasa.gov/kscpao/images/medium/97pc1048-m.jpg)
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That's a nice picture. Note the fog precipitation in the low pressure standing wave atop the wings. This re-evaporates almost instantaneously and doesn't produce a significant trail. The tips create a short-lived trail.
@QuikSanz
Yes, that's got to be the ultimate contrail! From my reading so far, this vapour almost certainly does freeze into minute ice crystals.
@blobrana
Aviation kerosene is mainly a C10-C12 hydrocarbon mixture, but that doesn't have any significant effect on water volume. In fact I'm taking your estimates on this. The old story that a gallon of petrol creates a gallon of water isn't so far of, is it?
JaX
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JaXanim wrote:
@blobrana
Aviation kerosene is mainly a C10-C12 hydrocarbon mixture, but that doesn't have any significant effect on water volume.
Think those were my numbers your refering to. As to the hydrogen content, I was just guestimating what the average H would roughly be for such a mix of hydrocarbons.
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@Fluffy...
Yes, sorry. I'm using your figures for the water creation rate. Assuming kerosene is a saturated hydrocarbon fraction, its empirical formula will be C(n)H(2n+2). The carbon number(and therefore the hydrogen content) makes relatively little difference to the weight of water created on combustion. It's only important at low molecular weights. So a gallon of kerosene oxidises to create about a gallon of water (as liquid of course). Convert this to vapour at 35,000 feet and you'll have some idea of what vapour trails involve.
At this point, the numbers look astronomical. I'll put them up when I check them again. The vapour rate of a 747 (4 engines) is just mind boggling!
Cheers,
JaX
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JaXanim wrote:
At this point, the numbers look astronomical. I'll put them up when I check them again. The vapour rate of a 747 (4 engines) is just mind boggling!
Yes, they are. Some scientists are also starting to believe that jets can cause changes to weather patterns because of what they do up there.
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JaXanim wrote:
@Fluffy...
Assuming kerosene is a saturated hydrocarbon fraction, its empirical formula will be C(n)H(2n+2). The carbon number(and therefore the hydrogen content) makes relatively little difference to the weight of water created on combustion.
Cheers,
JaX
When I was looking up jet fuel all I found was the weights of fractions expressed in terms of C. IN addition to the saturated hydrocarbons, jet fuel also has such stuff as napthalene (C10H8), so I jogged the H down a bit.
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@KennyR
That's right. There is a lot of evidence that contrails affect weather. For example, when flights were grounded for three days following the 9/11 tragedy, US daytime air temperature was one degree celcius above average. For more data on this, do a Google on vapour trails and weather.
@Fluffy...
Yes, it can get a bit complex, but the volume of water generated is gonna be about right whatever the specific composition of the fuel. A gallon of water per gallon of fuel is applicable to almost any hydrocarbon fuel whether its simple hydrocarbons or mixtures with aromatics.
The numbers are collossal! I'll post the results sometime tomorrow. In the meanwhile, I'm interested in anyone's observations/comments on vapour/con trails.
Cheers,
jax
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stop being so patronising Jax:
That's right
:lol:
The Lockhead SR-71 Blackbird and the Stealth Bomber both have pipes which inject cfc's into the exhaust to eliminate vapour trails
/me waves bye bye to o-zone layer over afganisthan and iraq :-)
speaking of which... B-52 Bomber (http://aa.domaindlx.com/pkx89/b-52-981995b.jpg)
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@sir_inferno
Presumably, making the Ozone Hole even bigger!
Sorry....SIR!...?
JaX
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This is my take on the generation of vapour trails. It's rather long, so ignore it if it's not of interest to you. (Sorry also to Wayne, who has asked us to be brief.)
It's in two parts. The volume of vapour involved and the mechanism of trail formation.
Taking Fluffy..'s data on a 747 allows you to calculate the first part. I've assumed it's flying at 35,000 feet. The numbers vary a bit for other altitudes, but 35,000 seems about average for commercial jets.
A 747 produces 15 tonnes/hour of water in the exhaust gas of its four engines. The volume of water vapour this creates can be calculated using Avogadro's Law and the Combined Gas Law (Boyle's + Charles' Laws).
Avogadro says that the vapour created from one gram mole of any substance occupies 22.4 litres at N.T.P. (Normal Temperature and Pressure) which is 0C(273K) and 760mmHg.
A gram mole of water is 18 grams, so every hour the 747 creates:
15,000,000/18 g.moles
= 833,333 g.moles
The volume of this at NTP = 833,333 x 22.4 litres
= 18,670,000 litres
To convert this to 35,000 feet requires the Combined Gas Law: P1.V1/T1 = P2.V2/T2
At 35,000 feet the atmospheric pressure is 179 mmHg and the temperature is -65.6F(-54C/219K). These data are available
HERE (http://www.grc.nasa.gov/WWW/K-12/airplane/atmos.html) and HERE (http://www.engineeringtoolbox.com/25_462.html).
So: 760 x 18,670,000/273 = 179 x V/219
Therefore: V = 63,590,000 litres/hour
= 17,700 litres/second
Blimey, I hope somebody will confirm these figures!
The second part is what happens to this invisible vapour?
The atmosphere will support a certain water vapour content. Its capacity to do so is directly proportional to temperature and pressure. Hot air holds more than cold. High pressure more than low.
Under specific conditions, water vapour will condense into liquid droplets (fog). These conditions occur at the Dew Point (http://www.ar-tiste.com/ThermoOfFog2.html) of the air. If the Dew Point is below 0C, the fog may freeze into ice crystals, so it's called the Frost Point (http://www-atm.physics.ox.ac.uk/main/dept/dobson.html).
Ice formation usually requires a nucleator such as dust, soot, pollen, bacteria, etc. around which the crystals form. Without this, water drops can go down to -40C without freezing.
If the atmosphere is at or near the Dew Point, any excess water vapour added to it will condense into fog/ice crystals. This is when vapour trails are produced. Wing tip/vortex precipitation produces transient trails because no extra water is involved. This fog re-evaporates very quickly.
If the atmosphere is very dry, it will be well above its Dew Point and the extra vapour introduced remains in the gaseous phase. So no vapour trail is produced.
In 1998, NASA flew a jet in circles until its vapour trail created a Cirrus cloud covering 1,400 square miles! See HERE (http://en.wikipedia.org/wiki/Contrail).
Does anybody fancy checking the numbers? All the required data are available where indicated.
Cheers,
JaX
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On a cold day, my car leaves a vapour trail, even when it's moving slowly. The vapour tends to drift upwards though.
This is something so obvious to people living in northern climates, that we barely even think about it. In the middle of winter, at temperatures around -20 to -35 C (or colder :evilgrin: ) morning rush hour traffic produces dense ice fogs that often don't disapate until 10:30/ 11:00 AM.
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Seeing that this has happened, there really must definitely be life on mars. :-)
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@aardvark
A kind of ground level vapour trail from cars. All the principles involved with airplanes apply to cars as well. You just need the air to be at or near the Dew/Frost Point.
I guess car trails rise because they are slightly warmer, initially at least, than the surrounding air. I think aircraft trails probably rise somewhat, but 'cos the plane is moving so fast, the trail is always way behind it. You may notice that trails can be near or far away from the engines. It depends on how much the gas has to cool before it condenses.
When it's as cold as you say, I bet the car fog persists at ground level for quite a while eh?
Cheeers,
JaX
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Until the sun burns it off. The car vapour trails do not last long but disperse adding to the general fog levels. You do have a kind of condensation giving a fairy like frosting on trees, ushes and powerlines. (even the prostitues get 'hoarfrost') :-D
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@aardvark
An occupational hazard, eh?
JaX
[Edit: Hoare frost happens when the air is at/below the Frost Point. This makes the vapour condense and freeze into ice crystals. These stick to any solid surface, trees, bushes (and prostitutes) to form a crystal coating. Beautiful to see.]
Cheers,
jaX