Chemtrail mystery does air temperature matter

originally posted by: Zaphod58
a reply to: mrthumpy

And like I said, what do I know. It's demonstrated fact that high bypass turbofans leave contrails where low and medium don't, and that's the major difference. But hey, maybe it's just magic.

Not magic. Just a cooler exhaust temperature from the work done turning the fan. Cooler exhaust means saturation is more likely in a wider ambient temperature range

yezz air temp and latent heat of evaporation

and mainly condensation nuclei

Well because 'Chemtrails' don't actually exist & you're nuts to think those heavy trails are anything but entirely normal products of aircraft transport, the question is moot. And you're clearly nuts.

[/sarc]

Look forward to reading the OP!

(Which should be the tagline for most of the aggressive naysayers in this forum at the end of each derogatory post..!!)

a reply to: Zaphod58

It gets confusing.

And that's no accident...

a reply to: mrthumpy

And the bypass air cools the engine as it goes around it. That's one of the reasons for them to bypass it instead of passing it through the engine.

originally posted by: Zaphod58
a reply to: mrthumpy

And the bypass air cools the engine as it goes around it.

By how much?

That's one of the reasons for them to bypass it instead of passing it through the engine.

As I understand it the reason for bypassing is that it is more efficient to use power from the exhaust to derive thrust from the fan rather than using the exhaust for thrust

a reply to: mrthumpy

I said one of the reasons to bypass, not the only reason to bypass. As for how much, it depends on the engine.

The bypass air mixes with the hot exhaust, and cools it, as well as cooling the engine as it goes around it. The cooling of the exhaust contributes to the engine making contrails where older engines didn't. The exhaust is cooler, but that isn't the only factor as to why a high bypass turbofan has the highest incidence of contrail formation of any jet engine.

originally posted by: Zaphod58
a reply to: mrthumpy

I said one of the reasons to bypass, not the only reason to bypass. As for how much, it depends on the engine.

The bypass air mixes with the hot exhaust, and cools it, as well as cooling the engine as it goes around it. The cooling of the exhaust contributes to the engine making contrails where older engines didn't. The exhaust is cooler, but that isn't the only factor as to why a high bypass turbofan has the highest incidence of contrail formation of any jet engine.

What happens to the heat the bypass air picks up from compression and cooling the engine?

a reply to: mrthumpy

It blends with the exhaust air, and forms a contrail.

originally posted by: Zaphod58
a reply to: mrthumpy

It blends with the exhaust air, and forms a contrail.

So doesn't make any difference to the temperature

"I want to believe these are all just contrails. But I do not remember any lines in the sky before 2000."

It's not just jet turbines that make contrails. Reciprocating (piston-driven) engines do too, and can make just as much of a condensation trail. Look here:

www.youtube.com...

That's not exhaust; that's condensation. Flight planners back then had to consider weather conditions such as temperature at altitude and dewpoint to plan ingress and egress flight routes, because contrails make a very small aircraft in a very large sky quite easy to see from many miles away by the enemy.

Sometimes, however (as in the video linked here), contrails were unavoidable and the strategic or tactical objective's importance overrode the risk of early detection.

You know what else? We weren't spraying crazyjuice on France, Belgium, or Germany. Those visible trails are composed of teeny tiny ice crystals because H20 is a byproduct of fuel combustion in an air-breathing engine--whether it be a jet turbine or a piston-driven powerplant.

originally posted by: dfnj2015

I want to believe these are all just contrails. But I do not remember any lines in the sky before 2000.

Here is a meteorological paper published in 1972 that mentions long lasting/persistent contrails, and how spreading contrails create cirrus clouds:

Measurements of the Growth of the Ice Budget in a Persisting Contrail(Note: This links directly to a PDF file)

Excerpt:

“It is often observed that contrails spread considerably…Under favorable conditions, a lateral spread of kilometers is observed…If sufficient air traffic exists, an entire overcast of contrail cirrus may develop and persist for hours with rapid growth in the ice budget of individual contrails.”

B-17's used chemtrails over Germany to try and Pacify the residents. I think that's were it started. in the attached photo you can see some of the chemical laden planes out front and the normal bomber to the rear

originally posted by: Zaboomafood
B-17's used chemtrails over Germany to try and Pacify the residents. I think that's were it started. in the attached photo you can see some of the chemical laden planes out front and the normal bomber to the rear

Sorry, but the bombers that appear to be "out front" are actually much higher. Higher means colder. And normal contrails are very temperature dependent. Even 5 degrees F can mean the difference between trail and no trail. And the normal adiabatic lapse rate is 3.5 degrees F (2C) per thousand feet of altitude. The difference in altitudes could very well have resulted from different models being used. The F model B-17 had a service ceiling of 36,500 feet with the Dash 97 engines. Earlier models were limited to 30,000 feet. The 4 engine B-24 was even more limited, having a service ceiling of 28,000 feet. To make a good solid contrail the air needs to be super-cooled so that the water vapor in the exhaust undergoes deposition, phase changing directly from vapor to ice, without going through condensation first. For that reason they should be called deptrails and not contrails since condensation is not involved. It is the opposite of sublimation where ice "evaporates" going from solid (ice) to vapor.

a reply to: EvidenceNibbler

The actual reason why high bypass engines leave more contrails is simply that they are more efficient. Contrail factor depends on the propulsion efficiency.

A more efficient engine extracts more energy from the fuel, so there is less energy left in the exhaust. In other words the exhaust is colder, and therefore it will condense faster (because it has to lose less energy before it condenses).

And if the exhaust condenses faster, then it can do so before it gets mixed out below the critical dilution for a contrail to form. Hence, contrails at a wider range of temperatures.

To answer the OP question though, it is down to humidity. Often cold winter days are associated with Arctic air. Arctic air sources tend to have low relative humidity, which is why the sky on those cold crisp winter days is so clear. Sometimes the relative humidity at 300mb on those days can be as low as 2 or 3%, so contrails cannot form.

originally posted by: Rob48
a reply to: EvidenceNibbler

The actual reason why high bypass engines leave more contrails is simply that they are more efficient. Contrail factor depends on the propulsion efficiency.

A more efficient engine extracts more energy from the fuel, so there is less energy left in the exhaust. In other words the exhaust is colder, and therefore it will condense faster (because it has to lose less energy before it condenses).

And if the exhaust condenses faster, then it can do so before it gets mixed out below the critical dilution for a contrail to form. Hence, contrails at a wider range of temperatures.

To answer the OP question though, it is down to humidity. Often cold winter days are associated with Arctic air. Arctic air sources tend to have low relative humidity, which is why the sky on those cold crisp winter days is so clear. Sometimes the relative humidity at 300mb on those days can be as low as 2 or 3%, so contrails cannot form.

I can't argue that fanjets are more efficient than turbojets, but the reason you state is not even close. You get the same thrust by moving a small amount of air really fast as you do moving a large mass of air more slowly. A turbojet or pure jet does the former. All the air goes through the core of the engine, being compressed in the compressor section, then combusted with fuel in the hot section, and then passing through the turbine section, spinning it and, by means of the gear box and shaft, the compressor section. In a fanjet, as much as 90% of the air bypasses the core and is accellerated by the fan blades, which are propelled by the turbine. Because the fan blades are inside the inlet duct and are small and numerous, they can be spun much faster than a turboprop propeller. And fluid dynamics tells us that propulsive efficiency is increased if the exit velocity of the propulsive fluid (air, in our case) is not much greater than aircraft speed. Thermal efficiency improves when the Turbine Inlet Temperature goes up, or the overall pressure ratio goes up, or both. This can be used on both turbojets and turbofans to improve efficiencies. So, the fanjet is more efficient because most of the propulsive force comes from the fan at slower speed. And with new alloys and materials higher TITs (turbine inlet temperatures) can be used, increasing thermal efficiency. The bottom line is, a turbo fan energises a greater mass flow with a given power (amount of fuel per second) than a turbo jet. It really doesn't have anything to do with "getting more energy out of the fuel." Jet fuel has 42.8 megaJoules of energy per kilogram.Thhat's all you can get out of it. And unless you dilute it, burning it in an oxygen rich environment wont give you less than 42.8 MJ/kg. A kilogram of jet fuel gives you the same energy whether it is combusted in a GE CJ-610 turbojet or a CF-6 fanjet. The exhaust temperature of a high bypass fan is a little lower than that of a turbojet because of mixing of the core exhaust and the ambient bypass air, and usually runs about 550C.

And your statement about humidity needs a little tweaking. As air cools, relative humidity, as opposed to absolute humidity, increases, since it takes less water vapor to saturate colder air. In any event, most of the water vapor causing contrails is the chemical byproduct of burning long chain hydrocarbons in an oxygen rich environment.

originally posted by: F4guy

originally posted by: Rob48
a reply to: EvidenceNibbler

It really doesn't have anything to do with "getting more energy out of the fuel."

Would you prefer "wasting less energy from the fuel" ?

originally posted by: mrthumpy

originally posted by: F4guy

originally posted by: Rob48
a reply to: EvidenceNibbler

It really doesn't have anything to do with "getting more energy out of the fuel."

Would you prefer "wasting less energy from the fuel" ?

It's not really wasted. "Energy" is defined as the capacity to do work. All of the energy in the fuel is being used to do work. It's just that in a high bypass (9:1) fan, the work gives a better result. It's the same as if you have a really good breakfast (say, 2 Egg McMuffins, fries and a chocolate shake) and decide to see how far you can run. You can run farther if you pace yourself than if you try a full-out sprint. In both cases, all the energy from your breakfast is used. One choice just gives a better result. In any case, insofar as contrails are concerned, you have pumped the same amount of hydrogen into the atmosphere to combine with the oxygen that is present, so you make the same amount of water vapor, and the same amount of carbon is emitted, providing nucleating particles around which ambient water vapor can deposit. I have made contrails with pure jets (Learjets with the CJ-610 turbojet), low bypass fans (F-4 with the J-79), and high bypass fans (747-400 with the CF-6) and haven't noticed any difference in the contrails, except that with the 4 engines on the 74, the trail is wider.
Now, if you want real gains in efficiency, slow down. Since parasitic drags increases by the square of the speed, you can choose to cruise at .84 rather than its MMO of .92 (for the -400). In fact some airlines pay bonuses based on fuel usage and have adjusted block times to reflect a lower cruise speed. The max certified mach number for the new 747-8 has been lowered to .90. On oceanic tracks, we are usually assigned a speed of .82-.84.
Of course, you get a huge boost in efficiency by chopping off 2 engines, hence the 777, 787 and A330. Thats why the 747 and A340 are disappearing in passenger service. The A380 may last a little longer since it can carry so many passengers and per seat mile efficiency is what really counts.

originally posted by: F4guy

originally posted by: mrthumpy

originally posted by: F4guy

originally posted by: Rob48
a reply to: EvidenceNibbler

It really doesn't have anything to do with "getting more energy out of the fuel."

Would you prefer "wasting less energy from the fuel" ?

It's not really wasted. "Energy" is defined as the capacity to do work. All of the energy in the fuel is being used to do work. It's just that in a high bypass (9:1) fan, the work gives a better result. It's the same as if you have a really good breakfast (say, 2 Egg McMuffins, fries and a chocolate shake) and decide to see how far you can run. You can run farther if you pace yourself than if you try a full-out sprint. In both cases, all the energy from your breakfast is used. One choice just gives a better result. In any case, insofar as contrails are concerned, you have pumped the same amount of hydrogen into the atmosphere to combine with the oxygen that is present, so you make the same amount of water vapor, and the same amount of carbon is emitted, providing nucleating particles around which ambient water vapor can deposit. I have made contrails with pure jets (Learjets with the CJ-610 turbojet), low bypass fans (F-4 with the J-79), and high bypass fans (747-400 with the CF-6) and haven't noticed any difference in the contrails, except that with the 4 engines on the 74, the trail is wider.
Now, if you want real gains in efficiency, slow down. Since parasitic drags increases by the square of the speed, you can choose to cruise at .84 rather than its MMO of .92 (for the -400). In fact some airlines pay bonuses based on fuel usage and have adjusted block times to reflect a lower cruise speed. The max certified mach number for the new 747-8 has been lowered to .90. On oceanic tracks, we are usually assigned a speed of .82-.84.
Of course, you get a huge boost in efficiency by chopping off 2 engines, hence the 777, 787 and A330. Thats why the 747 and A340 are disappearing in passenger service. The A380 may last a little longer since it can carry so many passengers and per seat mile efficiency is what really counts.

Well it is wasted if it's just going out of the back of the engine as heat instead of being converted to kinetic energy

originally posted by: mrthumpy

originally posted by: F4guy

originally posted by: mrthumpy

originally posted by: F4guy

originally posted by: Rob48
a reply to: EvidenceNibbler

It really doesn't have anything to do with "getting more energy out of the fuel."

Would you prefer "wasting less energy from the fuel" ?

It's not really wasted. "Energy" is defined as the capacity to do work. All of the energy in the fuel is being used to do work. It's just that in a high bypass (9:1) fan, the work gives a better result. It's the same as if you have a really good breakfast (say, 2 Egg McMuffins, fries and a chocolate shake) and decide to see how far you can run. You can run farther if you pace yourself than if you try a full-out sprint. In both cases, all the energy from your breakfast is used. One choice just gives a better result. In any case, insofar as contrails are concerned, you have pumped the same amount of hydrogen into the atmosphere to combine with the oxygen that is present, so you make the same amount of water vapor, and the same amount of carbon is emitted, providing nucleating particles around which ambient water vapor can deposit. I have made contrails with pure jets (Learjets with the CJ-610 turbojet), low bypass fans (F-4 with the J-79), and high bypass fans (747-400 with the CF-6) and haven't noticed any difference in the contrails, except that with the 4 engines on the 74, the trail is wider.
Now, if you want real gains in efficiency, slow down. Since parasitic drags increases by the square of the speed, you can choose to cruise at .84 rather than its MMO of .92 (for the -400). In fact some airlines pay bonuses based on fuel usage and have adjusted block times to reflect a lower cruise speed. The max certified mach number for the new 747-8 has been lowered to .90. On oceanic tracks, we are usually assigned a speed of .82-.84.
Of course, you get a huge boost in efficiency by chopping off 2 engines, hence the 777, 787 and A330. Thats why the 747 and A340 are disappearing in passenger service. The A380 may last a little longer since it can carry so many passengers and per seat mile efficiency is what really counts.

Well it is wasted if it's just going out of the back of the engine as heat instead of being converted to kinetic energy

If it is being accelerated out the back of the engine it has both mass and velocity and that is the definition of kinetic energy. And if it being burned, it is being converted from chemical potential energy into heat energy. Fuel is only wasted if it is unburned or incompletely burned, which was somewhat of a problem with the early F-4s and is why they left a big black trail behind them. The same problem occurred with early model B-52s with the low bypass J-57 engines.