Blackbird From , joined Dec 1969, posts, RR: Posted (5 years 11 months 2 weeks 3 days 21 hours ago) and read 1945 times:
I remember when I was asking questions about the GE-4 and the PW JTF-17A, and found out that they both used low afterburner in cruise someone said that some afterburner actually improves specific range.
First of all, what's specific range? Second of all, how does it improve it?
Short answer: because it makes speed go up more than fuel consumption.
Longer answer: it would be aircraft-dependent, and probably even mission-dependent, since it has to do with the L/D you can get with and without afterburner, the speed you can get with and without afterburner, and the drag as a function of speed with and without afterburner. My guess would be that any aircraft whose specific range improves with afterburner use is underpowered (at least for the mission in question) and would in fact have an even higher specific range with slightly more powerful engines and no afterburner use.
The main point of lighting the afterburner is to increase the exhaust velocity. By heating the exhaust stream, its pressure increases, which translates into higher velocity. There is a minor mass increase due to the added fuel, but that's pretty negligible.
Given that the mass does not change (negligible contribution from the A/B and the front of the engine doesn't start pumping through more air), the only way thrust can increase is if the exhaust velocity increases.
FredT From United Kingdom, joined Feb 2002, 2184 posts, RR: 26 Reply 7, posted (5 years 11 months 2 weeks 2 days 4 hours ago) and read 1794 times:
I intended to quote Oly720man (reply #4) and not you. Just to make it absolutely clear (and less confusing to those reading the thread) you were right in your initial post, which is what I intended to reinforce by my reply to post #4. Messing up the quote didn't help a bit, my sincere apologies.
I thought I was doing good trying to avoid those airport hotels... and look at me now.
3201 From , joined Dec 1969, posts, RR: Reply 8, posted (5 years 11 months 2 weeks 2 days 4 hours ago) and read 1787 times:
Quoting FredT (Reply 7): I intended to quote Oly720man (reply #4) and not you. Just to make it absolutely clear (and less confusing to those reading the thread) you were right in your initial post, which is what I intended to reinforce by my reply to post #4. Messing up the quote didn't help a bit, my sincere apologies.
No worries, that's what I guessed (and I should have quoted his and not yours as well!), and thanks!
Rwessel From United States of America, joined Jan 2007, 1990 posts, RR: 2 Reply 10, posted (5 years 11 months 1 week 3 days 22 hours ago) and read 1681 times:
Quoting Lehpron (Reply 9): I thought combustion decreases pressure but burning fuel increases temperature which would translate to a higher velocity flow?
Assuming volume remains constant, pressure is linearly dependant on the number of molecules of gas and the temperature. That's a slight oversimplification, but close enough for most purposes. It breaks down at very high and very low pressures, and in the presences of gasses who's molecules tend to "stick" to each other for various reasons. Also, the simple rule breaks down if some of the reactants or products are other than gases.
The temperature thing is simple. Double the temperature (relative to absolute zero), and you double the pressure. For example, if you heat a gas from 0C to 100C (273K to 373K) the pressure will increase about 37%.
As for the number of molecules, that depends on the exact reaction. For example, if you combine hydrogen gas and oxygen, you get two water molecules out of every three input molecules (2H2 + O2 --> 2H2O), which will result in a one-third reduction in pressure. That reaction releases a fair bit of heat too, of course, so the net is often a very significant pressure increase).
For hydrocarbons heavier than methane, all reactions increase the molecule count. For example, jet fuel is a mix of hydrocarbons, with an average length in the ballpark of 12. So the basic reaction is approximately 2C12H26 + 27O2 --> 24CO2 + 26H2O. So pressure increases by a ratio of about 50/29, plus what’s caused by the temperature increase.
Non-gas reactants and products are mostly a non-issue for jet engines. The unburned fuel is pretty well vaporized by the sprayers and by mixing it with the quite warm compressed air (although that does cool the incoming air a bit), and the exhaust stream doesn't cool enough to condense the water vapor until it's well out of the tailpipe.
The only point of burning fuel with air in an internal combustion engine is to increase the pressure. In a jet engine, that compressed gas is then expanded, which increases its velocity, and then that fast moving gas is let out the big hole in the back, which is what causes thrust (mass times velocity). Ideally, you want to gas to be released at the ambient pressure, because if you release it still compressed, you're wasting energy (the compressed gas will just uselessly expand sideways once it's out of the nozzle). You can see that quite clearly in rocket engines designed to work in space vs. ones designed to work well within the atmosphere - the expansion bells are proportionately much bigger on the former.
Rwessel From United States of America, joined Jan 2007, 1990 posts, RR: 2 Reply 12, posted (5 years 11 months 1 week 3 days 20 hours ago) and read 1664 times:
Quoting Rwessel (Reply 10): For hydrocarbons heavier than methane, all reactions increase the molecule count.
I should point out that the increase (or decrease) in the number of gas molecules due to the chemical reaction applies only to those molecules actually participating in the reaction. If you're burning a hydrocarbon fuel in air on earth, at most the 21% of the atmospheric gas that's oxygen participates (the 78% nitrogen and 1% argon are mostly non-reactive in this scenario, and while some of the trace stuff will react, there's not enough of it to matter).
3201 From , joined Dec 1969, posts, RR: Reply 13, posted (5 years 11 months 1 week 2 days 23 hours ago) and read 1604 times:
Quoting Blackbird (Reply 11): Would a turbofan with an outer-annular combustion chamber in lieu of an afterburner provide the increased specific range while avoiding excessive emissions problems?
My guess is still that the increased specific range is a side-effect of under-sized engines for the mission and aircraft in question. A turbofan that is sized correctly, allowing both airframe and engine to operate in their optimal design regimes, will provide the best specific range.
Just doesn't make sense that afterburner, which has a low efficiency, would increase overall efficiency. Given that you've heard it did increase specific range, which is a metric of overall efficiency, I created a scenario that could make that true given my understanding of engine efficiency. It's all conjecture, I have no information whatsoever on this.
3201 From , joined Dec 1969, posts, RR: Reply 17, posted (5 years 11 months 4 days 22 hours ago) and read 1524 times:
Quoting Blackbird (Reply 16): My only guess other than the jet engine being relatively underpowered for the altitude/speed/mach-number, is the exhaust velocity is WAY higher on an afterburner than on a non afteburning jet-engine.
Yes, but that's not an efficient way to translate chemical energy into thrust. If it were, afterburners would be used a lot.
B2707SST From United States of America, joined Apr 2003, 1350 posts, RR: 60 Reply 19, posted (5 years 11 months 4 days 18 hours ago) and read 1499 times:
Quoting Blackbird (Thread starter): I remember when I was asking questions about the GE-4 and the PW JTF-17A, and found out that they both used low afterburner in cruise someone said that some afterburner actually improves specific range.
First of all, what's specific range? Second of all, how does it improve it?
Correct - the GE4 would have used some afterburning at cruise, and the JTF-17A would have used duct heating (afterburning in the bypass stream, not the core stream) at cruise.
As mentioned above, specific range is an instantaneous measure of range, often quoted as as nm/pound of fuel. The chart below shows the B2707-100's specific range at various altitudes and gross weights for maximum dry thrust, minimum augmented (afterburning) thrust, and maximum augmented thrust conditions.
Note that the maximum specific range for any gross weight occurs a relatively small amount of augmentation. This surprised me as well when I first saw the chart. I have no explanation other than that the increased fuel consumption resulting from some degree of afterburning was more than offset by the drag reduction resulting from achieving a higher flight level than would be possible with dry thrust alone.
If a specific range curve is presented on the Y axis with aircraft weight on the X axis (assuming optimum cruise altitude for each gross weight), taking the definite integral of the curve between the beginning and ending aircraft weights gives the cruise range achievable for a given initial gross weight and a given amount of fuel burn. For the 2707-100 "reference mission," specific range approximated 0.15 nm/lb at initial cruise (61,000 feet, gross weight approximately 570,000 lbs.) and reached about 0.21 nm/lb at the top of descent (68,000 feet, gross weight approximately 390,000 lbs.), burning about 180,000 pounds of fuel to cover 3,227 nm during cruise.
Note that by 1971, the afterburner had been deleted from the GE4 design, so cruise obviously would have occurred without augmented thrust. I don't have comparable charts for the 2707-300, and even if they were available, it would be difficult to isolate the effects of the new "dry" GE4 configuration from the switch to a less efficient fixed delta wing.
Xv408 From United Kingdom, joined Nov 2006, 52 posts, RR: 0 Reply 20, posted (5 years 11 months 4 days 14 hours ago) and read 1483 times:
Burning fuel in the afterburner increases the gas temperature, but this is principally used to increase gas velocity, not pressure. Note that when an afterburner is lit, the exhaust nozzle opens up significantly. This is to keep the pressure drop across the turbine largely unchanged. Some afterburning engines use convergent/divergent nozzles to expand the hot gas more in the engine, but the trade is between extra weight and complexity vs. small increase in thrust.
Note also that burning fuel in the bypass duct is generally not efficient because the pressure is low. In some high-supersonic engines, the pressure recovery in the intake is sufficient to counter this, e.g. the J58 in the Blackbird, which at Mach 3 is effectively a ramjet.