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Afterburning Jets And NOx Emissions  
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Posted (7 years 10 months 3 weeks 4 days 2 hours ago) and read 2623 times:

How much emissions do afterburners produce in comparison to a jet operating at dry power?

22 replies: All unread, jump to last
 
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6515 posts, RR: 54
Reply 1, posted (7 years 10 months 3 weeks 4 days 1 hour ago) and read 2604 times:

Afterburners produce practically no NOx since the combustion happens at low pressure and therefore at relatively low temperature.

Afterburners first of all produce a lot of CO2. CO2 production is basically liniar to the fuel consumption, which is extremely high.

Afterburners will also emit a lot more "unburned fuel" vapor (CxHx) than a modern high efficency turbine engine running "dry".

Also CO emission is a lot higher than running "dry". That is mainly because the afterburner combustion happens in a low oxygen environment. The afterburner only has the oxygen which did not burn in the engine core. Therefore the oxygen contents is somewhat lower than the normal 21% in the atmosphere.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 2, posted (7 years 10 months 3 weeks 4 days 1 hour ago) and read 2601 times:

Some afterburners do slow the airflow down a bit more by making the burner wider before initiating the combustion, particularly on high-speed designs (think J-93, and early J-58 design), also some afterburners are more efficient than others (the J-93 was designed for continuous use). With these two things said, how much would they overall add to additional Nitrogen Oxide Production?

I'm also wondering that if an afterburner was attached to a jet-engine using a lean-premixed prevaporized combustor (ultra-low NOx), if you'd still get the same very low NOx effect?

Would it be correct to say that afterburners have little effect on Ozone Layer?

Andrea K


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 3, posted (7 years 10 months 3 weeks 3 days 7 hours ago) and read 2552 times:

I forgot to mention, the J-58 (A-12/SR-71 Engine) would be an exception to this rule, since it uses high-pressure air off the compressor to feed into the burner, so the pressure is much higher in the afterburner prior to combustion.

Okay, that said...

-Andrea K


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6515 posts, RR: 54
Reply 4, posted (7 years 10 months 3 weeks 3 days 1 hour ago) and read 2521 times:

Quoting Blackbird (Reply 2):
Would it be correct to say that afterburners have little effect on Ozone Layer?

Yes. Especially because since afterburners don't work very well at very high altitude. When entering the stratosphere, then most afterburners will have flamed out due to oxygen starvation.

Exactly NOx emission is mostly a problem with modern, high efficiency tubine engines at ground level. The very high compression ratio combined with the thick air at sea level (and take-off power) makes it very tricky to avoid or reduce combination of the oxygen and nitrogen in the atmosphere. The same high efficiency engines at cruise at the low ambient pressure at altitude produce a lot less NOx.

Water vapor is a much greater threat to the ozon layer. Every gallon of fuel burned produces approximately 1.2 gallons of water. That helps create tiny ice crystals (stratopheric clouds) which help to speed up the ozone destruction. But that's only a problem when emitted in the stratosphere. In the troposphere it will always just fall as rain or snow.

The tropopause between the troposphere and stratosphere is mostly placed around FL500 - FL550. Unfortunately it is somewhat lower - FL350 to FL400 - at or near the poles.

NOx is mostly a health problem in towns near busy airports in calm weather. But then the diesel engines running in the town are generally much worse.

NOx polution is a "local" and "temporary" polution. NOx molecules are unstable and pretty fast they split into N2 and O2 again. NOx is therefore a problem only when we have a constant, local production which cannot be blown away from inhabited areas.

Odinary healthy people will hardly ever notice even very severe NOx polution. But people, who suffer from certain breathing illnesses, will suffer more when the NOx level is high. It is not a "poison gas" such as CO. But it slightly affects the way oxygen is taken up in the blood in the lungs.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 5, posted (7 years 10 months 3 weeks 2 days 8 hours ago) and read 2475 times:

I got a question

The Lean Pre-Mixed Prevaporized (LPP) combustion type used on the HSCT design pre-vaporized, and then premixed the fuel in a fairly uniform fasion in an antechamber ahead of the combustion chamber. Then the mix is taken itno the combustion chamber, and burned up very rapidly.

I know it worked in one way since the fuel/air mix was nowhere near stoichiometric... But did the pre-vaporization reduce or eliminate large amounts of water-vapor?

Andrea K


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6515 posts, RR: 54
Reply 6, posted (7 years 10 months 3 weeks 2 days 3 hours ago) and read 2450 times:

Quoting Blackbird (Reply 5):
But did the pre-vaporization reduce or eliminate large amounts of water-vapor?

Jet fuel consists of hydrocarbon molecules. When burned the hydrogen part combines with oxygen from the atmosphere and creates H2O.

The only way to avoid that is not to burn the fuel - which means: Stay on the ground.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 7, posted (7 years 10 months 3 weeks 2 days 2 hours ago) and read 2443 times:

How much destruction to the ozone layer would a fleet of 500 SST type airplanes cause just as a result to water-vapor? (providing NOx levels were curbed to reasonable amounts)

Andrea K


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6515 posts, RR: 54
Reply 8, posted (7 years 10 months 3 weeks 2 days 1 hour ago) and read 2438 times:

Quoting Blackbird (Reply 7):
How much destruction to the ozone layer would a fleet of 500 SST type airplanes cause just as a result to water-vapor? (providing NOx levels were curbed to reasonable amounts)

Huh, the answer to that question could set 500 scientist disagree for decades. There are so many variables, not only fuel consumption and operating altitude, but also a lot of more or less unknown variables.

Another problem is that the effect would be very long term. The effect on the ozone layer could still be increasing even decades after those 500 SST planes were beer-canned and replaced by subsonic planes. An a "repair" of the ozone layer could last hundreds of years.

The problem is that there is very little exchange of the air in the stratosphere and the troposphere. The temperature gradient reversal in the tropopause practically created a wall between the troposphere and the stratosphere.

But no scientists will ever worry about it. Simply because it will never happen. The reason is that the supersonic transport plane has been superceeded by video conferences, netmeetings, global file server access, internet and all such stuff the same way as sailships, oceanliners, airships and steam trains have been superceeded by electric trains and subsonic airliners.

The history books will to some extent put Titanic, Graf Hindenburg, and the Concorde into the same basket. Of course the Concorde was by far the most successful of that trio. But there are several similarities, the most important being that none of them will ever happen again. Because today we have much more efficient ways to do the same thing.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineHaveBlue From United States of America, joined Jan 2004, 2121 posts, RR: 1
Reply 9, posted (7 years 10 months 3 weeks 1 day 22 hours ago) and read 2430 times:

Quoting Prebennorholm (Reply 8):
The history books will to some extent put Titanic, Graf Hindenburg, and the Concorde into the same basket. Of course the Concorde was by far the most successful of that trio. But there are several similarities, the most important being that none of them will ever happen again. Because today we have much more efficient ways to do the same thing.

Very interesting point Preb, and very true.



Here Here for Severe Clear!
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 10, posted (7 years 10 months 3 weeks 1 day 9 hours ago) and read 2408 times:

Probably true, but for the sake of hypotheticals, let's say 500-SST Type planes with a mach 3 cruise speed, with a typical cruise of 71,500 ft to 76,500 ft (With extremes of 70,500 to 78,000 feet). How much damage would that do to the Ozone layer due to water-vapor productions?

Andrea K.


User currently offlineStarglider From Netherlands, joined Sep 2006, 679 posts, RR: 44
Reply 11, posted (7 years 10 months 3 weeks 1 day 2 hours ago) and read 2389 times:

Quoting Blackbird (Reply 10):
Probably true, but for the sake of hypotheticals, let's say 500-SST Type planes with a mach 3 cruise speed, with a typical cruise of 71,500 ft to 76,500 ft (With extremes of 70,500 to 78,000 feet). How much damage would that do to the Ozone layer due to water-vapor productions?

Lab tests have shown that water vapor was found to cause a small ozone reduction when injected by itself. However, when coupled with NOx emissions, water vapor reduced the ozone reduction caused by NOx by a factor between 0.85 and 0.97, depending on the altitude and magnitude of the emission.

Apart from ozone, the atmosphere in which a potential fleet of 500 SSTs would fly will not be today's atmosphere. Carbon dioxide will have increased, methane and nitrous oxide are likely to have increased, and CFCs are expected to decrease. There will likely be an increased subsonic fleet perturbing the troposphere. One of the more relevant aspects of this changing atmosphere will be a decrease in stratospheric temperatures due to increased CO2. This will impact the dynamics of the stratosphere and both gas-phase and heterogeneous chemistry. Water vapor plays an important role in stratospheric chemistry. A changing atmosphere may change tropospheric conditions, and in particular, may modify the temperature of the tropical tropopause, thus affecting the lower stratospheric water vapor distribution.

Starglider


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 12, posted (7 years 10 months 3 weeks 19 hours ago) and read 2370 times:

So, if NOx were ultra-low, the water vapor wouldn't do as much harm?

User currently offlineStarglider From Netherlands, joined Sep 2006, 679 posts, RR: 44
Reply 13, posted (7 years 10 months 3 weeks 5 hours ago) and read 2335 times:

Quoting Blackbird (Reply 12):
So, if NOx were ultra-low, the water vapor wouldn't do as much harm?

Yes, H2O vapor dampens the negative effect of NOx on ozone. The effect on ozone with the coupled NOx and H2O emissions case is a factor 0.9 less than the NOx emissions only case. This effect is due to the interactions between NOx and HOx chemistry (H2O affects the composition, growth, and reactivity of aerosol reactions and provides a source for HOx).

A study that has been evaluating a total of 15 proposed 2015 aircraft fleet scenarios is shown in the table below. For each of the 15 scenarios, the table shows the calculated global averaged change in column ozone relative to a 2015 ambient atmosphere with and without trace gas emissions from proposed subsonic aircraft (see the 2 most RH columns in the table, one for a combination of sub- and supersonic aircraft, the second, most RH, for supersonic emissions only). For the results listed in the table, all model calculations used the normal gas-phase (or homogeneous) chemistry reactions. In all cases, column ozone decreases when NOx is emitted by supersonic aircraft. There is an increased sensitivity to ozone column depletion as the cruise altitude increases. The emissions of NOx from the subsonic scenario increases column ozone by approximately 0.7% globally. This increase is due to the CH4-NOx-smog mechanism that occur in the troposphere. Here is the table:



Big version: Width: 800 Height: 499 File size: 65kb


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 14, posted (7 years 10 months 2 weeks 6 days 2 hours ago) and read 2286 times:

Thanks Starglider,

-Andrea K


User currently offlineStarglider From Netherlands, joined Sep 2006, 679 posts, RR: 44
Reply 15, posted (7 years 10 months 2 weeks 5 days 11 hours ago) and read 2261 times:

Quoting Blackbird (Reply 14):
Thanks Starglider,

-Andrea K

You are welcome.


User currently offlineTripleDelta From Croatia, joined Jul 2004, 1124 posts, RR: 7
Reply 16, posted (7 years 10 months 2 weeks 3 days 11 hours ago) and read 2212 times:
AIRLINERS.NET CREW
PHOTO SCREENER

I have a similar question possibly closely related to this:

- at an airshow last year, we had a MiG-29 in attendance. As seen in the db, at normal power settings, its engines produce A LOT of smoke:

Hungarian AF MiG-29UB - smoke

However, on afterburner settings, they produce no smoke at all:

Hungarian AF MiG-29UB - no smoke

Is this due to the same effects that have been discussed here?

[Edited 2007-01-15 16:58:43]


No plane, no gain.
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6515 posts, RR: 54
Reply 17, posted (7 years 10 months 2 weeks 3 days 4 hours ago) and read 2186 times:

The gasses, which we have been discussing so far, are invisible.

Black smoke is soot particles. Half burned carbon rich fuel particles. It is not gas which blends with the atmosphere, but tiny, solid particles. The less efficient the combustion, the more soot is generated.

All turbine engines generate soot. But modern engines have a combustion so efficient that the soot is seldom visible. First generation airliner engines usually left a sustantial soot trail behind. Today soot trails are the mark of some military planes, either very old planes or planes with poorly designed engines. The Boeing B-52H is probably the worst smoker today.

Soot from aircrafts has never been a major environmental problem. It comes down with the rain and probably works as a "free" fertilizer on the fields.

But fifty years ago, in most large cities all buildings were more or less black of soot from coal burning from hundreds of thousands of chimneys.

Some of us still remember railway stations during the time of the coal fired steam locomotives. OMG !!!

In 1957 my school class was on a quite long excursion by train. Weather was nice, so we were more or less hanging out of the window all the way. When we came back home we were all black in the face.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 18, posted (7 years 10 months 2 weeks 3 days 3 hours ago) and read 2174 times:

Yeah, I remember that when an RA-5C Vigi pilot was talking about doing recon runs and they'd always use some degree of AB, in addition to the speed increase so that the J-79's wouldn't produce the characteristic black smoke.

User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6515 posts, RR: 54
Reply 19, posted (7 years 10 months 2 weeks 3 days 2 hours ago) and read 2172 times:

Quoting Blackbird (Reply 18):
Yeah, I remember that when an RA-5C Vigi pilot was talking about doing recon runs and they'd always use some degree of AB, in addition to the speed increase so that the J-79's wouldn't produce the characteristic black smoke.

Yeah, the J-79 was a real smoker. The reason for that is mostly that (unlike its P&W sisters - J-57/J-75) it was a single shaft engine. Even if it had a generous 17 stage compressor with innovative, variable stator vanes, then the lack of a high rpm high pressure stage limited the compression ratio to a modest max just over 13. That's about half or one third of modern turbofan airliner engines.

The afterburner would burn much of the soot making a low level approach a lot less visible.

Later versions of the J-79 - those used on the F-4E and later - did get away with much of the smoke due to improvements in the combustion chambers. But early Phantoms and the RA-5C Vigis could really make day into night.

I remember one sunny afternoon around 1970. I was on the sun deck of a car ferry, and two Luftwaffe RF-4C Phantoms passed at high speed 200 feet overhead. Most people of course got a chock, but I had seen the growing black smoke ball on the horizon half a minute in advance.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineTripleDelta From Croatia, joined Jul 2004, 1124 posts, RR: 7
Reply 20, posted (7 years 10 months 2 weeks 2 days 18 hours ago) and read 2168 times:
AIRLINERS.NET CREW
PHOTO SCREENER

Quoting Prebennorholm (Reply 17):
Black smoke is soot particles. Half burned carbon rich fuel particles. It is not gas which blends with the atmosphere, but tiny, solid particles. The less efficient the combustion, the more soot is generated.

Ah, I see. Thanks, that clears it up.

Quoting Prebennorholm (Reply 19):
I remember one sunny afternoon around 1970. I was on the sun deck of a car ferry, and two Luftwaffe RF-4C Phantoms passed at high speed 200 feet overhead. Most people of course got a chock, but I had seen the growing black smoke ball on the horizon half a minute in advance.

Also an easy way to spot any early MiGs. At airshows we always look for a thin cloud of smoke on the horizon, indicative of either the MiG-21s our AF flies, or "guest" -29s.



No plane, no gain.
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 21, posted (7 years 10 months 2 weeks 2 days 8 hours ago) and read 2154 times:

Preben,

You sure about that. If I recall correctly, the J-79 had a higher pressure ratio than the J-57. From what I remember it's combustor wasn't as good.

Andrea K


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6515 posts, RR: 54
Reply 22, posted (7 years 10 months 2 weeks 2 days 1 hour ago) and read 2136 times:

Quoting Blackbird (Reply 21):
You sure about that. If I recall correctly, the J-79 had a higher pressure ratio than the J-57. From what I remember it's combustor wasn't as good.

You're right, Andrea. The P&W J57 had an overall pressure ratio of 11 - 12, at least when we talk about the earlier versions.

But the J57 was many things. It was first of all the very first "large" US jet engine. It had a long and successful career and came in many improved versions. At least if we count the later fanned versions (TF33), then the pressure ratio grew to still unimpressive 16 in the most powerful versions.

Even if it was a two spool engine, then the J57 was also a bad smoker. If you google it, then you will also stumble over various soot reduction programs to keep 707s and DC-8s flying a few more years on JT3D engines. Some JT3D engines were in their old days rebuilt with substantially revised combustor chambers based on JT8D experience to put them below new, tightened legal emission kimits.

Also present day B-52s smoke really bad.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
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