asr0dzjq
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(Hypothetical) Water injection on high-bypass engine

Mon Feb 17, 2020 12:59 am

If a water-injection system were to be installed on an aircraft high-bypass turbofan, where would the injectors be placed?
My guess is probably in the core, just after the low-pressure turbine, or in the bypass duct. Probably not injecting it into the combustion chamber, as cooling the flame would cause emissions to skyrocket.
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AirKevin
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Re: (Hypothetical) Water injection on high-bypass engine

Mon Feb 17, 2020 1:39 am

Some older JT9D engines for the 747 actually had water injection capability, not sure how they were set up.
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Re: (Hypothetical) Water injection on high-bypass engine

Mon Feb 17, 2020 2:36 am

asr0dzjq wrote:
If a water-injection system were to be installed on an aircraft high-bypass turbofan, where would the injectors be placed?
My guess is probably in the core, just after the low-pressure turbine, or in the bypass duct. Probably not injecting it into the combustion chamber, as cooling the flame would cause emissions to skyrocket.


Water injection was typically done prior to compressor inlet or into the compressor stages directly. The evaporation of the water cooled the flow through the compressor, improving pressure ratio (as the water evaporated and cooled the air, it caused subsequent stages to essentially operate at a higher corrected speed) and increasing the physical mass flow the compressor could pass (reducing the corrected flow). As noted, however, the water reduced flame temperatures causing incomplete combustion and the high amounts of particulate matter (smoke) seen in old photos of 707s at takeoff.

I
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Re: (Hypothetical) Water injection on high-bypass engine

Mon Feb 17, 2020 6:49 am

AirKevin wrote:
Some older JT9D engines for the 747 actually had water injection capability, not sure how they were set up.


On the JT9D the water is injected via the twenty (20) fuel nozzles into the combustion chamber.
From the longitudial section drawing of the JT9D-7(W) engine the following is stated :
"Each nozzle is connected to primary and secondary fuel manifolds and to the watermanifold".

See also : viewtopic.php?t=764067
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Re: (Hypothetical) Water injection on high-bypass engine

Tue Feb 18, 2020 7:43 pm

akiss20 wrote:
asr0dzjq wrote:
If a water-injection system were to be installed on an aircraft high-bypass turbofan, where would the injectors be placed?
My guess is probably in the core, just after the low-pressure turbine, or in the bypass duct. Probably not injecting it into the combustion chamber, as cooling the flame would cause emissions to skyrocket.


Water injection was typically done prior to compressor inlet or into the compressor stages directly. The evaporation of the water cooled the flow through the compressor, improving pressure ratio (as the water evaporated and cooled the air, it caused subsequent stages to essentially operate at a higher corrected speed) and increasing the physical mass flow the compressor could pass (reducing the corrected flow). As noted, however, the water reduced flame temperatures causing incomplete combustion and the high amounts of particulate matter (smoke) seen in old photos of 707s at takeoff.

I

So if you implement engine water injection, there's no way to avoid the smoke problem?
 
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Re: (Hypothetical) Water injection on high-bypass engine

Tue Feb 18, 2020 8:20 pm

CowAnon wrote:
akiss20 wrote:
asr0dzjq wrote:
If a water-injection system were to be installed on an aircraft high-bypass turbofan, where would the injectors be placed?
My guess is probably in the core, just after the low-pressure turbine, or in the bypass duct. Probably not injecting it into the combustion chamber, as cooling the flame would cause emissions to skyrocket.


Water injection was typically done prior to compressor inlet or into the compressor stages directly. The evaporation of the water cooled the flow through the compressor, improving pressure ratio (as the water evaporated and cooled the air, it caused subsequent stages to essentially operate at a higher corrected speed) and increasing the physical mass flow the compressor could pass (reducing the corrected flow). As noted, however, the water reduced flame temperatures causing incomplete combustion and the high amounts of particulate matter (smoke) seen in old photos of 707s at takeoff.

I

So if you implement engine water injection, there's no way to avoid the smoke problem?

No, the ones done in the past were just less sophisticated, it is possible to do a water injection system with little or no smoke and overall performance improvements. Check this out: https://mdao.grc.nasa.gov/publications/ ... _20_04.pdf
 
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Re: (Hypothetical) Water injection on high-bypass engine

Wed Feb 19, 2020 1:14 am

What if the water was injected right after the LPT and into the bypass duct, instead of before the LPC or in the combustion chamber?
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Re: (Hypothetical) Water injection on high-bypass engine

Wed Feb 19, 2020 8:49 am

All water injection systems on the early jets, incl. the JT9D's at the early 747-100/200 aircraft were to reduce the turbine inlet temperatures, because at that point in time the materials used in the first stages of the HPT were not able to withstand the temperatures belonging to the higher T/O thrust setting needed for the increased MTOW's
After the introduction of directional solidified , single cristal and (in the future) ceramic turbine blades, no water injection was (and will be) used, because of the very complicated, failure prown, additional systems to be added to the engine.
Today no airline will accept an engine with a water injection system installed.
Personally i made numereous T/O's with P&W JT9D-7W powered 747-206B's during the periode 1970-1990.
Before switching off the water injection system after 2,5 minutes wet T/O thrust or upon iminent water run out, we retarded the P/L's to avoid exceeding the red line EGT, because one of the engine fuel controls stayed at the "rich-wet"setting with no water available. Exceeding the red line for several seconds required at least a boroscoop HPT inspection or in the worst case : engine change.
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Re: (Hypothetical) Water injection on high-bypass engine

Thu Feb 20, 2020 1:26 am

asr0dzjq wrote:
If a water-injection system were to be installed on an aircraft high-bypass turbofan, where would the injectors be placed?
My guess is probably in the core, just after the low-pressure turbine, or in the bypass duct. Probably not injecting it into the combustion chamber, as cooling the flame would cause emissions to skyrocket.

Surely you mean after the HP compressor, increasing the density of the volume of air entering the combustion chamber.
 
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Re: (Hypothetical) Water injection on high-bypass engine

Thu Feb 20, 2020 1:46 am

Yikes! wrote:
asr0dzjq wrote:
If a water-injection system were to be installed on an aircraft high-bypass turbofan, where would the injectors be placed?
My guess is probably in the core, just after the low-pressure turbine, or in the bypass duct. Probably not injecting it into the combustion chamber, as cooling the flame would cause emissions to skyrocket.

Surely you mean after the HP compressor, increasing the density of the volume of air entering the combustion chamber.

No, I meant the low-pressure turbine.
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Re: (Hypothetical) Water injection on high-bypass engine

Thu Feb 20, 2020 1:47 am

LH707330 wrote:
No, the ones done in the past were just less sophisticated, it is possible to do a water injection system with little or no smoke and overall performance improvements. Check this out: https://mdao.grc.nasa.gov/publications/ ... _20_04.pdf

Thanks, that was an informative article. However, I don't understand why the SFC improves only by about 3-4 percent for LPC injection (and negative for combustor injection) when the water-fuel mix is in the vicinity of 1:2 or 1:1. You'd think that with a 33-50 percent reduction in fuel, the percentage drop in specific fuel consumption would at least be in the double digits.
 
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Re: (Hypothetical) Water injection on high-bypass engine

Fri Feb 21, 2020 12:57 am

AirKevin wrote:
Some older JT9D engines for the 747 actually had water injection capability, not sure how they were set up.

the water injection was behind the fan into the low pressure core. The problem was? The added power only served to offset the weight of the injection fluid. So they got rid of the Water Wagon concept.
 
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Re: (Hypothetical) Water injection on high-bypass engine

Fri Feb 21, 2020 10:49 am

When I worked for Gulf Air in Bahrain we had water injection on our fleet of 4 BAC111-400. It was widely used in the summer.
But even with water, the aircraft was limited by the OAT. I remember one hot day (around 39degC) when all 4 BAC111 were waiting for the temp to drop so they could depart from BAH.
Once a week an Air India B747 arrived and used water injection for its departure. The fuel service had a small water tank they towed behind the bowser with a hand pump.
Whereas the BAC111 used a few gallons of water, the B747 needed hundreds of gallons. But it only arrived once a week, so there was no incentive to get a better truck.
Then GF changed to B737-200 and the AI stopped coming, and we used the water tank to do compressor wash on the JT8D. (we fitted an electric pump!)
 
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Re: (Hypothetical) Water injection on high-bypass engine

Sat Feb 22, 2020 12:17 am

asr0dzjq wrote:
Yikes! wrote:
asr0dzjq wrote:
If a water-injection system were to be installed on an aircraft high-bypass turbofan, where would the injectors be placed?
My guess is probably in the core, just after the low-pressure turbine, or in the bypass duct. Probably not injecting it into the combustion chamber, as cooling the flame would cause emissions to skyrocket.

Surely you mean after the HP compressor, increasing the density of the volume of air entering the combustion chamber.

No, I meant the low-pressure turbine.


The purpose of water injection is to absorb heat from the incoming airstream, which allows more energy to be added in the combustion chamber without exceeding the turbine inlet temperature limit. Turbine inlet temperature is generally the critical "redline" constraining engine performance.

Water injection in the compressor absorbs some of the adiabatic temperature increase during the compression stages. This reduces thermal loads on the high-pressure compressor (which start to become significant) and all downstream stages.

The bypass flow plays no part in the combustion process, so injecting water into the bypass duct serves no purpose. Likewise, injecting water after the low-pressure or even after the high-pressure turbine doesn't help much, since you really need the temperature drop in the combustion chamber and the high-pressure turbine.
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Re: (Hypothetical) Water injection on high-bypass engine

Sun Feb 23, 2020 7:22 am

B2707SST wrote:
The purpose of water injection is to absorb heat from the incoming airstream, which allows more energy to be added in the combustion chamber without exceeding the turbine inlet temperature limit. Turbine inlet temperature is generally the critical "redline" constraining engine performance.

Water injection in the compressor absorbs some of the adiabatic temperature increase during the compression stages. This reduces thermal loads on the high-pressure compressor (which start to become significant) and all downstream stages.

The bypass flow plays no part in the combustion process, so injecting water into the bypass duct serves no purpose. Likewise, injecting water after the low-pressure or even after the high-pressure turbine doesn't help much, since you really need the temperature drop in the combustion chamber and the high-pressure turbine.


The more mass of the gas, the more thrust. Just assuming, but wouldn't water injected in the bypass lead to lower bypass exhaust speeds with increased mass? I would therefore expect this to be a good idea for takeoff.

"Water injection ...when used in a turbine engine...water is normally injected either at the compressor inlet or in the diffuser just before the combustion chambers. Adding water increases the mass being accelerated out of the engine, increasing thrust, but it also serves to cool the turbines. Since temperature is normally the limiting factor in turbine engine performance at low altitudes, the cooling effect lets the engine run at higher RPM with more fuel injected and more thrust created without overheating.[3] The drawback of the system is that injecting water quenches the flame in the combustion chambers somewhat, as there is no way to cool the engine parts without also cooling the flame. This leads to unburned fuel out the exhaust and a characteristic trail of black smoke."

Image

source: https://en.wikipedia.org/wiki/Water_injection_(engine)
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Re: (Hypothetical) Water injection on high-bypass engine

Mon Feb 24, 2020 6:11 am

Sokes wrote:
The more mass of the gas, the more thrust. Just assuming, but wouldn't water injected in the bypass lead to lower bypass exhaust speeds with increased mass? I would therefore expect this to be a good idea for takeoff.


Mass flow is directly proportional to thrust, but the fan also has to accelerate that additional mass, so it's doing more work. This is fine when mass flow increases due to colder denser air, since the core can also provide more work to power the fan. But injecting extra mass into the bypass duct means taking more energy from the core stage - in which case you might as well just build a bigger fan. Or use something like compressed air, which the core is already generating in much greater quantities than any feasible water tankage. The never-built Concorde B model was to use an improved Olympus 593 sometimes described as a "leaky turbojet" - bypass pipes were to run from the compressor to the exhaust nozzle in place of a deleted afterburner. This was predicted to reduce noise and fuel consumption while maintaining or increasing thrust by increasing mass flow at subsonic and transonic speeds.

But in any case, the rate of water injection is not significant in comparison with the airflow already passing through the engine. From a prior thread on this subject, the 747-200 would tank about 2,400 kg of water for injection over 2.5 minutes during takeoff. This works out to about 4 kg/sec per engine. The mass flow rate of the JT9D-7 at takeoff thrust is 698 kg/sec, so the water injection represents about 0.5%. This certainly doesn't hurt, but it's not as good as air since water vapor obviously doesn't support combustion. What really helps is the enormous amount of heat that water absorbs when it vaporizes (among the highest of any liquid). This lets you run the core hotter and more efficiently than you otherwise could. However, the weight and complexity of water injection systems are apparently greater than the performance gains on modern engines.
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Re: (Hypothetical) Water injection on high-bypass engine

Mon Feb 24, 2020 11:43 pm

B2707SST wrote:
asr0dzjq wrote:
Yikes! wrote:
Surely you mean after the HP compressor, increasing the density of the volume of air entering the combustion chamber.

No, I meant the low-pressure turbine.


The purpose of water injection is to absorb heat from the incoming airstream, which allows more energy to be added in the combustion chamber without exceeding the turbine inlet temperature limit. Turbine inlet temperature is generally the critical "redline" constraining engine performance.

Water injection in the compressor absorbs some of the adiabatic temperature increase during the compression stages. This reduces thermal loads on the high-pressure compressor (which start to become significant) and all downstream stages.

The bypass flow plays no part in the combustion process, so injecting water into the bypass duct serves no purpose. Likewise, injecting water after the low-pressure or even after the high-pressure turbine doesn't help much, since you really need the temperature drop in the combustion chamber and the high-pressure turbine.


There is some of that (absorbing heat) but flashing to steam is where the real action is at (~1,700x expansion). Absolutely brutal to combustors.
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Re: (Hypothetical) Water injection on high-bypass engine

Tue Feb 25, 2020 2:05 am

What would a reasonable water flow rate be if these things were to be installed on, say, a 767's CF6 engines before the LPC?
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Re: (Hypothetical) Water injection on high-bypass engine

Tue Feb 25, 2020 4:34 am

Before the LPC? CF680C2? Based on the electric power generation version (LM6000 PC) introducing approximately 18 GPM before the LPC will yield a ~6% to 8% increase in power relative to expected power for a given ambient. (Bleed air is used for atomization.) Injecting 45 gpm (1:1 ratio with fuel) into combustor via fuel nozzles will yield a ~15% power increase pretty much independent of ambient. Water must be of very high purity or things can get expensive in a hurry.
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Re: (Hypothetical) Water injection on high-bypass engine

Wed Feb 26, 2020 7:46 pm

CowAnon wrote:
LH707330 wrote:
No, the ones done in the past were just less sophisticated, it is possible to do a water injection system with little or no smoke and overall performance improvements. Check this out: https://mdao.grc.nasa.gov/publications/ ... _20_04.pdf

Thanks, that was an informative article. However, I don't understand why the SFC improves only by about 3-4 percent for LPC injection (and negative for combustor injection) when the water-fuel mix is in the vicinity of 1:2 or 1:1. You'd think that with a 33-50 percent reduction in fuel, the percentage drop in specific fuel consumption would at least be in the double digits.

If you read the article closely, it assumes that this would be a retrofit on a T800, hence the SFC drop. I bet that you'd get better numbers if you designed it that way from the start. If you increase the thrust briefly at takeoff, then you can make a smaller engine overall, so less weight and better SFC later in the mission. My guess as to why it's not done today is likely maintenance costs and logistics hassles outweigh the fuel savings.
 
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Re: (Hypothetical) Water injection on high-bypass engine

Tue Mar 03, 2020 1:33 pm

B2707SST wrote:
Sokes wrote:
The more mass of the gas, the more thrust. Just assuming, but wouldn't water injected in the bypass lead to lower bypass exhaust speeds with increased mass? I would therefore expect this to be a good idea for takeoff.


the 747-200 would tank about 2,400 kg of water for injection over 2.5 minutes during takeoff. This works out to about 4 kg/sec per engine. The mass flow rate of the JT9D-7 at takeoff thrust is 698 kg/sec, so the water injection represents about 0.5%


That explains why water isn't injected in the bypass. Thanks.
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Re: (Hypothetical) Water injection on high-bypass engine

Tue Mar 03, 2020 8:40 pm

LH707330 wrote:
CowAnon wrote:
LH707330 wrote:
No, the ones done in the past were just less sophisticated, it is possible to do a water injection system with little or no smoke and overall performance improvements. Check this out: https://mdao.grc.nasa.gov/publications/ ... _20_04.pdf

Thanks, that was an informative article. However, I don't understand why the SFC improves only by about 3-4 percent for LPC injection (and negative for combustor injection) when the water-fuel mix is in the vicinity of 1:2 or 1:1. You'd think that with a 33-50 percent reduction in fuel, the percentage drop in specific fuel consumption would at least be in the double digits.

If you read the article closely, it assumes that this would be a retrofit on a T800, hence the SFC drop. I bet that you'd get better numbers if you designed it that way from the start. If you increase the thrust briefly at takeoff, then you can make a smaller engine overall, so less weight and better SFC later in the mission. My guess as to why it's not done today is likely maintenance costs and logistics hassles outweigh the fuel savings.

Thanks again. The responses in this thread for and against water injection have been interesting. The discussion has been about NOx, smoke, and SFC at takeoff, but it seems to me that going from all-fuel to half fuel/half water for all phases of flight would result in large reductions in CO2 emissions. The typical tube+wings structure would have trouble accommodating the water and keeping it from freezing, but a flying wing structure would have the fuel and water in the same temperature-controllable hull as the passengers. The EKIP thick flying wing, which was proposed by the Soviet Union/Russia in the 1990s/2000s, also had the engines inside the hull, so it didn't even need insulated pipes between the engines and the fuel/water tanks. The designers considered using water injection with the water condensate from the exhaust:

https://www.youtube.com/watch?v=cThIbDONerI
http://www.ekip-aviation-concern.com/eng-b/1.shtml



Here's a little more about the water/fuel mixture mentioned in the video (aquazine):
http://www.oilru.com/or/36/716/
 
Sokes
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Re: (Hypothetical) Water injection on high-bypass engine

Wed Mar 04, 2020 6:13 am

CowAnon wrote:
...The designers considered using water injection with the water condensate from the exhaust:

https://www.youtube.com/watch?v=cThIbDONerI
http://www.ekip-aviation-concern.com/eng-b/1.shtml
...
Here's a little more about the water/fuel mixture mentioned in the video (aquazine):
http://www.oilru.com/or/36/716/


From your last link, apparently from Lukoil:
"According to the available data, just motor vehicles used on the territory of Russia burn approximately 110-115 million metric tons (MMT) of fuel and 12-15 MMT of lubricating oil every year. In the process, about 30 MMT of harmful substances are emitted into the atmosphere, including 15 MMT of carbon dioxide, 12 MMT of nitrogen oxides and 1 MMT of soot. The environmental situation would improve markedly is water-fuel emulsions were used as fuel."

115 weight units fuel require 12 weight units of lubrication oil? Are there no 4-stroke engines in Russia yet?
115 weight units fuel make only 15 weight units of CO2?
For 15 weight units of CO2 there is 1 weight unit of soot? That's really too much soot. It's about time that water gets mixed in the fuel.

Concerning your video:
I guess the jury is still out if the B737Max or Russia's flying saucer is the better concept. Boeing already missed out on the C-Series. I believe it's better for Boeing to err on the side of safety. They better hurry.

Do you have any more information on the 28 billion $ that Americans invested to build an experimental saucer, which they successfully crashed (see at 2:57)?
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Re: (Hypothetical) Water injection on high-bypass engine

Wed Mar 04, 2020 8:36 am

Sokes wrote:
CowAnon wrote:
...The designers considered using water injection with the water condensate from the exhaust:

https://www.youtube.com/watch?v=cThIbDONerI
http://www.ekip-aviation-concern.com/eng-b/1.shtml
...
Here's a little more about the water/fuel mixture mentioned in the video (aquazine):
http://www.oilru.com/or/36/716/


From your last link, apparently from Lukoil:
"According to the available data, just motor vehicles used on the territory of Russia burn approximately 110-115 million metric tons (MMT) of fuel and 12-15 MMT of lubricating oil every year. In the process, about 30 MMT of harmful substances are emitted into the atmosphere, including 15 MMT of carbon dioxide, 12 MMT of nitrogen oxides and 1 MMT of soot. The environmental situation would improve markedly is water-fuel emulsions were used as fuel."

115 weight units fuel require 12 weight units of lubrication oil? Are there no 4-stroke engines in Russia yet?
115 weight units fuel make only 15 weight units of CO2?
For 15 weight units of CO2 there is 1 weight unit of soot? That's really too much soot. It's about time that water gets mixed in the fuel.

Yep, the amount of CO2 is suspiciously low compared to the fuel weight.

You only need a particulate filter should keep the soot out of the air. Letting a water-fuel emulsion sit in a car's fuel tank for a week is more than enough time for nasty bacterial films to grow, though. I don't know how full or empty a typical airline keeps its working plane's fuel tanks when they're sitting on the ground, but I'd imagine the same problem would occur.

Concerning your video:
I guess the jury is still out if the B737Max or Russia's flying saucer is the better concept. Boeing already missed out on the C-Series. I believe it's better for Boeing to err on the side of safety. They better hurry.

Do you have any more information on the 28 billion $ that Americans invested to build an experimental saucer, which they successfully crashed (see at 2:57)?

No idea where the $28 billion comes from. My guess is it's a currency conversion error. The theory about an American saucer crash is explained in this Russian article (use an online translator if necessary): https://www.mk.ru/editions/daily/articl ... ssiya.html.
 
LH707330
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Re: (Hypothetical) Water injection on high-bypass engine

Sat Mar 07, 2020 10:16 pm

CowAnon wrote:
LH707330 wrote:
CowAnon wrote:
Thanks, that was an informative article. However, I don't understand why the SFC improves only by about 3-4 percent for LPC injection (and negative for combustor injection) when the water-fuel mix is in the vicinity of 1:2 or 1:1. You'd think that with a 33-50 percent reduction in fuel, the percentage drop in specific fuel consumption would at least be in the double digits.

If you read the article closely, it assumes that this would be a retrofit on a T800, hence the SFC drop. I bet that you'd get better numbers if you designed it that way from the start. If you increase the thrust briefly at takeoff, then you can make a smaller engine overall, so less weight and better SFC later in the mission. My guess as to why it's not done today is likely maintenance costs and logistics hassles outweigh the fuel savings.

Thanks again. The responses in this thread for and against water injection have been interesting. The discussion has been about NOx, smoke, and SFC at takeoff, but it seems to me that going from all-fuel to half fuel/half water for all phases of flight would result in large reductions in CO2 emissions. The typical tube+wings structure would have trouble accommodating the water and keeping it from freezing, but a flying wing structure would have the fuel and water in the same temperature-controllable hull as the passengers. The EKIP thick flying wing, which was proposed by the Soviet Union/Russia in the 1990s/2000s, also had the engines inside the hull, so it didn't even need insulated pipes between the engines and the fuel/water tanks. The designers considered using water injection with the water condensate from the exhaust:

https://www.youtube.com/watch?v=cThIbDONerI
http://www.ekip-aviation-concern.com/eng-b/1.shtml



Here's a little more about the water/fuel mixture mentioned in the video (aquazine):
http://www.oilru.com/or/36/716/

The problem with having water for the whole flight is the weight penalty. If you have roughly equal amounts of fuel and water to carry, your plane becomes heavier and you burn more fuel as a consequence. For a short burst on takeoff it might make sense, but for the whole flight, the water doesn't pay the way. For turbines that live on the ground and do not have to worry about weight, they use water injection all day long and save a bunch of fuel as a result.
 
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Re: (Hypothetical) Water injection on high-bypass engine

Thu Mar 12, 2020 3:46 am

And after doing a detailed analysis on multiple aircraft, I noted that water injection, whether in the core or in the bypass, only gives enough of an improvement at high payloads as that is when the thrust increase offsets the increase in TOW. For lightly-loaded/empty planes, it actually detracts from the performance.
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LH707330
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Re: (Hypothetical) Water injection on high-bypass engine

Thu Mar 12, 2020 4:09 am

asr0dzjq wrote:
And after doing a detailed analysis on multiple aircraft, I noted that water injection, whether in the core or in the bypass, only gives enough of an improvement at high payloads as that is when the thrust increase offsets the increase in TOW. For lightly-loaded/empty planes, it actually detracts from the performance.

How did you calculate that?
 
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Re: (Hypothetical) Water injection on high-bypass engine

Thu Mar 12, 2020 11:04 pm

LH707330 wrote:
asr0dzjq wrote:
And after doing a detailed analysis on multiple aircraft, I noted that water injection, whether in the core or in the bypass, only gives enough of an improvement at high payloads as that is when the thrust increase offsets the increase in TOW. For lightly-loaded/empty planes, it actually detracts from the performance.

How did you calculate that?

As an example (correct me if I'm wrong):
Airbus A346
MTOW 380000 kg
Rolls-Royce Trent 560 mass flow is 880 kg/s of air per engine
300 CFM of water injected into bypass during takeoff
1600 cu. ft. of water = c. 45300 kg
MTOW minus water weight = 334700 kg

Increase in weight = 380000/334700 = approx. 13.5%
Increase in thrust = (water density * 300 CFM)/(880 kg/s) = approx. 16.1%

Not much of a benefit. Modern engines move so much air that the water doesn't add as much to the mass as planes with low-bypass engines.
R.I.P.

Pan Am (B 19-10-1927, D 4-12-1991)
TWA (B 1-10-1930, D 1-12-2001)
Douglas (B 22-7-1921, D 23-5-2006)
 
LH707330
Posts: 2284
Joined: Fri Jun 15, 2012 11:27 pm

Re: (Hypothetical) Water injection on high-bypass engine

Fri Mar 13, 2020 3:00 am

asr0dzjq wrote:
LH707330 wrote:
asr0dzjq wrote:
And after doing a detailed analysis on multiple aircraft, I noted that water injection, whether in the core or in the bypass, only gives enough of an improvement at high payloads as that is when the thrust increase offsets the increase in TOW. For lightly-loaded/empty planes, it actually detracts from the performance.

How did you calculate that?

As an example (correct me if I'm wrong):
Airbus A346
MTOW 380000 kg
Rolls-Royce Trent 560 mass flow is 880 kg/s of air per engine
300 CFM of water injected into bypass during takeoff
1600 cu. ft. of water = c. 45300 kg
MTOW minus water weight = 334700 kg

Increase in weight = 380000/334700 = approx. 13.5%
Increase in thrust = (water density * 300 CFM)/(880 kg/s) = approx. 16.1%

Not much of a benefit. Modern engines move so much air that the water doesn't add as much to the mass as planes with low-bypass engines.

Ahh, I see what's going on here. There are two things to take into consideration:

1. The water should be injected into the core, which has a faster output, so your mass flow at that higher rate will give you more thrust
2. The water improves the thermodynamic efficiency of the core due to the latent heat of vaporization, which allows you to add more fuel to spin the fan faster while staying below critical turbine temps, which in turn gives you even more mass flow and thrust.
 
asr0dzjq
Topic Author
Posts: 171
Joined: Tue Oct 29, 2019 2:36 am

Re: (Hypothetical) Water injection on high-bypass engine

Fri Mar 13, 2020 4:02 am

LH707330 wrote:
Ahh, I see what's going on here. There are two things to take into consideration:

1. The water should be injected into the core, which has a faster output, so your mass flow at that higher rate will give you more thrust
2. The water improves the thermodynamic efficiency of the core due to the latent heat of vaporization, which allows you to add more fuel to spin the fan faster while staying below critical turbine temps, which in turn gives you even more mass flow and thrust.

Now let's say you're putting (on takeoff) 5 CFM before the LPC, an additional 5 CFM into the combustion chamber, 145 CFM after the LPT inside the core, and 145 CFM into the bypass. Could 10 CFM of water in the combustion chamber cause the engine to flame out?
R.I.P.

Pan Am (B 19-10-1927, D 4-12-1991)
TWA (B 1-10-1930, D 1-12-2001)
Douglas (B 22-7-1921, D 23-5-2006)
 
LH707330
Posts: 2284
Joined: Fri Jun 15, 2012 11:27 pm

Re: (Hypothetical) Water injection on high-bypass engine

Fri Mar 13, 2020 4:18 pm

asr0dzjq wrote:
LH707330 wrote:
Ahh, I see what's going on here. There are two things to take into consideration:

1. The water should be injected into the core, which has a faster output, so your mass flow at that higher rate will give you more thrust
2. The water improves the thermodynamic efficiency of the core due to the latent heat of vaporization, which allows you to add more fuel to spin the fan faster while staying below critical turbine temps, which in turn gives you even more mass flow and thrust.

Now let's say you're putting (on takeoff) 5 CFM before the LPC, an additional 5 CFM into the combustion chamber, 145 CFM after the LPT inside the core, and 145 CFM into the bypass. Could 10 CFM of water in the combustion chamber cause the engine to flame out?


10 CFM is 17t/hr if my numbers below are correct, are you proposing putting that in per engine or total? If an A346 burns ~8t/h in cruise, then it'd probably be closer to 20 at takeoff setting, so you'd have near 1:1 ratios at that setting. This is probably near the limit. I don't know why you'd want to put so much into the bypass duct, it doesn't help there. If mass flow is your concern, you're better off spinning the fan faster to get freely available mass (air) instead of carrying mass around to dump in the duct (water). If you're going to put the water in the engine, you should send it through the core for the reasons I mentioned above. More core flow means lower turbine temps and thus lower NOx and turbine wear, at the expense of maintenance for the water system. It could also potentially make sense to try to cool turbine blades with water from a thermodynamic standpoint (higher delta-T) and then use the resulting steam in the LPT, but I wager nobody's tried this because the risks of steam pressure containment, cavitation, etc. outweigh the benefits.

CFM 10
CC/m 283726.25
Liters/m 283.72625
Liters/hr 17023.575
T/hr 17.023575
 
asr0dzjq
Topic Author
Posts: 171
Joined: Tue Oct 29, 2019 2:36 am

Re: (Hypothetical) Water injection on high-bypass engine

Fri Mar 13, 2020 4:43 pm

LH707330 wrote:
asr0dzjq wrote:
LH707330 wrote:
Ahh, I see what's going on here. There are two things to take into consideration:

1. The water should be injected into the core, which has a faster output, so your mass flow at that higher rate will give you more thrust
2. The water improves the thermodynamic efficiency of the core due to the latent heat of vaporization, which allows you to add more fuel to spin the fan faster while staying below critical turbine temps, which in turn gives you even more mass flow and thrust.

Now let's say you're putting (on takeoff) 5 CFM before the LPC, an additional 5 CFM into the combustion chamber, 145 CFM after the LPT inside the core, and 145 CFM into the bypass. Could 10 CFM of water in the combustion chamber cause the engine to flame out?


10 CFM is 17t/hr if my numbers below are correct, are you proposing putting that in per engine or total? If an A346 burns ~8t/h in cruise, then it'd probably be closer to 20 at takeoff setting, so you'd have near 1:1 ratios at that setting. This is probably near the limit. I don't know why you'd want to put so much into the bypass duct, it doesn't help there. If mass flow is your concern, you're better off spinning the fan faster to get freely available mass (air) instead of carrying mass around to dump in the duct (water). If you're going to put the water in the engine, you should send it through the core for the reasons I mentioned above. More core flow means lower turbine temps and thus lower NOx and turbine wear, at the expense of maintenance for the water system. It could also potentially make sense to try to cool turbine blades with water from a thermodynamic standpoint (higher delta-T) and then use the resulting steam in the LPT, but I wager nobody's tried this because the risks of steam pressure containment, cavitation, etc. outweigh the benefits.

CFM 10
CC/m 283726.25
Liters/m 283.72625
Liters/hr 17023.575
T/hr 17.023575

So forget bypass injection. Now would changing 145 CFM after the LPT + 145 CFM in the bypass to 290 CFM after the LPT and nothing in the bypass work?
Also, I noticed that normally the Trent 500's get run at 2900-3200 RPM on takeoff. So now we have our configuration:

3500 Fan RPM at takeoff on all four engines
5 CFM water injection before the LPC
5 CFM injection in the combustion chamber
290 CFM injection after the LPT
R.I.P.

Pan Am (B 19-10-1927, D 4-12-1991)
TWA (B 1-10-1930, D 1-12-2001)
Douglas (B 22-7-1921, D 23-5-2006)
 
LH707330
Posts: 2284
Joined: Fri Jun 15, 2012 11:27 pm

Re: (Hypothetical) Water injection on high-bypass engine

Sat Mar 14, 2020 9:32 pm

asr0dzjq wrote:
LH707330 wrote:
asr0dzjq wrote:
Now let's say you're putting (on takeoff) 5 CFM before the LPC, an additional 5 CFM into the combustion chamber, 145 CFM after the LPT inside the core, and 145 CFM into the bypass. Could 10 CFM of water in the combustion chamber cause the engine to flame out?


10 CFM is 17t/hr if my numbers below are correct, are you proposing putting that in per engine or total? If an A346 burns ~8t/h in cruise, then it'd probably be closer to 20 at takeoff setting, so you'd have near 1:1 ratios at that setting. This is probably near the limit. I don't know why you'd want to put so much into the bypass duct, it doesn't help there. If mass flow is your concern, you're better off spinning the fan faster to get freely available mass (air) instead of carrying mass around to dump in the duct (water). If you're going to put the water in the engine, you should send it through the core for the reasons I mentioned above. More core flow means lower turbine temps and thus lower NOx and turbine wear, at the expense of maintenance for the water system. It could also potentially make sense to try to cool turbine blades with water from a thermodynamic standpoint (higher delta-T) and then use the resulting steam in the LPT, but I wager nobody's tried this because the risks of steam pressure containment, cavitation, etc. outweigh the benefits.

CFM 10
CC/m 283726.25
Liters/m 283.72625
Liters/hr 17023.575
T/hr 17.023575

So forget bypass injection. Now would changing 145 CFM after the LPT + 145 CFM in the bypass to 290 CFM after the LPT and nothing in the bypass work?
Also, I noticed that normally the Trent 500's get run at 2900-3200 RPM on takeoff. So now we have our configuration:

3500 Fan RPM at takeoff on all four engines
5 CFM water injection before the LPC
5 CFM injection in the combustion chamber
290 CFM injection after the LPT

I assume you mean 145 further aft in the core, was that a typo? I'm not sure how much additional thrust you'd get from adding water as an "afterburner" in the core, it may cool the flow and reduce the density, but I'd have to run numbers to be sure. Even if you get some effect, the core flow is such a small fraction of the overall that I don't think the mass flow of the added water will help you much in terms of overall thrust. You're better off chasing an RPM boost from a better-performing core.

Regarding the 5 and 5 in the core, the combustion chamber injection would likely increase particulates due to the flame quenching, so I doubt you'd want to do that. I'd try to determine how much you can spray in before the LPC and HPC, essentially to act as an intercooler and reduce the work that the compressors need to do (lower temp->lower pressure for same density). If you get this intercooling to permit a fan RPM increase from 3200 to 3500, then your thrust will go from 60k to about 72k. Alternately, you scale the engine down to make 60k with water, and then enjoy the weight savings. Again, the fact that nobody's doing this now suggests that it doesn't add up for some reason....
 
asr0dzjq
Topic Author
Posts: 171
Joined: Tue Oct 29, 2019 2:36 am

Re: (Hypothetical) Water injection on high-bypass engine

Sat Mar 14, 2020 11:38 pm

LH707330 wrote:
I assume you mean 145 further aft in the core, was that a typo? I'm not sure how much additional thrust you'd get from adding water as an "afterburner" in the core, it may cool the flow and reduce the density, but I'd have to run numbers to be sure. Even if you get some effect, the core flow is such a small fraction of the overall that I don't think the mass flow of the added water will help you much in terms of overall thrust. You're better off chasing an RPM boost from a better-performing core.

Regarding the 5 and 5 in the core, the combustion chamber injection would likely increase particulates due to the flame quenching, so I doubt you'd want to do that. I'd try to determine how much you can spray in before the LPC and HPC, essentially to act as an intercooler and reduce the work that the compressors need to do (lower temp->lower pressure for same density). If you get this intercooling to permit a fan RPM increase from 3200 to 3500, then your thrust will go from 60k to about 72k. Alternately, you scale the engine down to make 60k with water, and then enjoy the weight savings. Again, the fact that nobody's doing this now suggests that it doesn't add up for some reason....

Yeah, I meant injection after the LPT. And after doing some more calculations I determined that even at 900 ft/s the thrust improvement was minimal. So now I'm considering injecting only before the LPC and before the HPC.
R.I.P.

Pan Am (B 19-10-1927, D 4-12-1991)
TWA (B 1-10-1930, D 1-12-2001)
Douglas (B 22-7-1921, D 23-5-2006)
 
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Balerit
Posts: 624
Joined: Sun Aug 23, 2015 9:14 am

Re: (Hypothetical) Water injection on high-bypass engine

Sun Mar 15, 2020 10:03 am

Water injection, for example on the JT9, is for cooling the first stage turbine, nothing else and is injected into the combustor chamber alongside the fuel nozzles. When water injection pressure increases, the FCU is commanded to add extra fuel which gives the extra thrust. Any water in the incoming airflow decreases the air density causing loss of power.
Licensed Aircraft Maintenance Engineer (retired).

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