Is water injection a redundant technology?
Modern fan engines (High Bypass) just don't need it. Water injection (or water/meth) was required to boost and cool older turbojets. As a subject for MODERN jets, it has no relevance.
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Water Injection
The LET 410UVP uses the Walter M601E engine which has water injection. Link to website here:
http://www.walter.cz/htmlen/ramen.htm
Mel
http://www.walter.cz/htmlen/ramen.htm
Mel
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As an indication of the effect water/meth had on the engines of that era, the Avons [200 series IIRC] fitted to the tanker version of the Valiant, gained an extra 1000lbs of thrust, moving them up from 10,000lbs to 11,000lbs. This sometimes moved Stop and Go back to the right side of each other, not always the case in those early days.
The Tyne turboprops in the Belfast uses water meth in temperatures above ISA to maintain the rated SHP. It works up to 31Deg C and then the power falls off quite dramatically! We used it almost all the time for hot/high fields and always if we used 10deg flap for max perfomance.
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Water Injection
There's a degree of confusion crept in on the thread. Water injection can be with or without methanol added.
First of all, there is no value using water to cool a jet engine. You can't carry enough of it and it would be counter-productive anyway to have super-heated steam chasing around the airframe rapidly corroding everything in sight.
The value comes in cooling the inflow air, thereby increasing its density, which means a greater burn can be sustained, which generates more power.
The difference in temperature between in and out can be used to measure thermal efficiency; the greater the temperature difference front and rear, the higher the efficiency. So at sea level - and relatively high inlet temperatures therefore - water cooling the air means it's at a temperature of several thousand feet higher, but with the density advantage of the lower level. Result = bigger bang (or in fact, higher volume of jet efflux available for generating thrust).
Methanol adds an anti-freeze effect to the water, so enabling even lower inlet temperatures to be achieved and with obvious advantageous consequences over straight water. However, it's also highly combustible, so adds to the fuel and burning properties as well.
It doesn't matter whether it's a high or low by-pass jet, turbojet or turboprop; the principles are the same. However the water/meth is usually (but not always) sprayed into the compressor stages. Clearly on a high bypass, that means NOT the big fan at the front, but the teeny-weeny stages that actually form the front-end of the hot core that drive the fan.
I am not aware of any physical, engineering or other reason why water-meth shouldn't be used on any jet or piston engine, provided it's not used where it would cause the engine to burn out or over-power. It's a perfectly valid way of overcoming the Weight-Altitude-Temperature (WAT) limit.
First of all, there is no value using water to cool a jet engine. You can't carry enough of it and it would be counter-productive anyway to have super-heated steam chasing around the airframe rapidly corroding everything in sight.
The value comes in cooling the inflow air, thereby increasing its density, which means a greater burn can be sustained, which generates more power.
The difference in temperature between in and out can be used to measure thermal efficiency; the greater the temperature difference front and rear, the higher the efficiency. So at sea level - and relatively high inlet temperatures therefore - water cooling the air means it's at a temperature of several thousand feet higher, but with the density advantage of the lower level. Result = bigger bang (or in fact, higher volume of jet efflux available for generating thrust).
Methanol adds an anti-freeze effect to the water, so enabling even lower inlet temperatures to be achieved and with obvious advantageous consequences over straight water. However, it's also highly combustible, so adds to the fuel and burning properties as well.
It doesn't matter whether it's a high or low by-pass jet, turbojet or turboprop; the principles are the same. However the water/meth is usually (but not always) sprayed into the compressor stages. Clearly on a high bypass, that means NOT the big fan at the front, but the teeny-weeny stages that actually form the front-end of the hot core that drive the fan.
I am not aware of any physical, engineering or other reason why water-meth shouldn't be used on any jet or piston engine, provided it's not used where it would cause the engine to burn out or over-power. It's a perfectly valid way of overcoming the Weight-Altitude-Temperature (WAT) limit.
Splitbrain
As you have observed, in days of old engine manufacturers utilised Water/Water Methanol Injection to increase mass air flow through the engine when higher ambient temperatures resulted in reduced ITT/EGT margins.
Although these systems overcame the problem to some degree, they came with additional penalties such as weight (fluids & system components), additional component maintenance -not to mention the risks associated with the carriage of methanol.
As a result of substantial improvements in hot section material technology and cooling efficiencies, modern gas turbine engines are capable of producing the same amount of SHP/Thrust but at greatly increased ambient temperatures without the need for weighty and sometimes dangerous boost systems and therefore provide increased efficiencies and safety for the operators and crew/passengers alike.
Hope this helps?
As you have observed, in days of old engine manufacturers utilised Water/Water Methanol Injection to increase mass air flow through the engine when higher ambient temperatures resulted in reduced ITT/EGT margins.
Although these systems overcame the problem to some degree, they came with additional penalties such as weight (fluids & system components), additional component maintenance -not to mention the risks associated with the carriage of methanol.
As a result of substantial improvements in hot section material technology and cooling efficiencies, modern gas turbine engines are capable of producing the same amount of SHP/Thrust but at greatly increased ambient temperatures without the need for weighty and sometimes dangerous boost systems and therefore provide increased efficiencies and safety for the operators and crew/passengers alike.
Hope this helps?
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Hilife, thanks for your input. You summarise what I'd more or less concluded myself, that material, design and technology advances have more or less made water injection redundant on the latest engine variants.
Thank you all.
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Yes, I'd agree with Hilife too. Thought it would add an unnecessary complication to my post, so thank you for bringing it out. All that luggage and maintenance is a pain if you don't really need it. There might still be a case for the hot/hi though, especially military jets operating at edges of envelope.
And you're right, I did interpreted the previous post as for some form of jacket or external spraying.
Re the injection into the combustion chamber, IIRC that's the preferred method for a more conventional axial flow jet, because it gets a more even distribution through the mixture and because it's possible to pump a lot more in, faster and more effectively. I went for the soft option to illustrate. I should have known better than to over-simplify on PPRuNe!!
Nevertheless, I think you may still be referring to the cooling effect on the inlet air to the combustion chamber (laws of thermodynamics and physics will do it anyway), though, rather than other cooling effects. I can see a case for cooling the turbine though (as opposed to inlet).
The only other possibility I can see - but I think it unlikely- is to use the water to improve the boundary layer around the combustion chamber, so that higher temps can be maintained in the burn.
I'm a bit sceptical because it would probably have been easier to design the larger boundary layer with inflow air from the start rather than augment it in some way. But if there's a RR designer watching, by all means pronounce me wrong (but put us all out of our misery with the answer too please!).
And you're right, I did interpreted the previous post as for some form of jacket or external spraying.
Re the injection into the combustion chamber, IIRC that's the preferred method for a more conventional axial flow jet, because it gets a more even distribution through the mixture and because it's possible to pump a lot more in, faster and more effectively. I went for the soft option to illustrate. I should have known better than to over-simplify on PPRuNe!!
Nevertheless, I think you may still be referring to the cooling effect on the inlet air to the combustion chamber (laws of thermodynamics and physics will do it anyway), though, rather than other cooling effects. I can see a case for cooling the turbine though (as opposed to inlet).
The only other possibility I can see - but I think it unlikely- is to use the water to improve the boundary layer around the combustion chamber, so that higher temps can be maintained in the burn.
I'm a bit sceptical because it would probably have been easier to design the larger boundary layer with inflow air from the start rather than augment it in some way. But if there's a RR designer watching, by all means pronounce me wrong (but put us all out of our misery with the answer too please!).
Last edited by GlosMikeP; 10th Dec 2006 at 17:18. Reason: Turbine addition.
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Going back 40 years puts a strain on the old memory cells but I am fairly certain that the effect on the Avon of water/meth injection was to increase the max RPM at the limiting JPT. The throttles were fully advanced and the RPM in dry power was automatically limited to the max JPT. On selecting water/meth the RPM increased by some 2 to 3 % with the JPT remaining the same, a noticeable increase in thrust and significantly for stream take-offs, a much smokier exhaust. Certainly in that application the cooling of the turbine would seem to have been the raisin d'etre.
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Going back 40 years puts a strain on the old memory cells but I am fairly certain that the effect on the Avon of water/meth injection was to increase the max RPM at the limiting JPT. The throttles were fully advanced and the RPM in dry power was automatically limited to the max JPT. On selecting water/meth the RPM increased by some 2 to 3 % with the JPT remaining the same, a noticeable increase in thrust and significantly for stream take-offs, a much smokier exhaust. Certainly in that application the cooling of the turbine would seem to have been the raisin d'etre.
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actually, water injection only slightly increased the RPM's -- almost negligibly. I was a crew chief on KC-135A's at Castle AFB back in the early 80's. On a turbojet engine, water injection cooled the inlet charge, made the air denser, & therefor had more potential for expansion. was similar in practice to hitting afterburner. depended on the ambient air temperature more than anything else as to it's effectiveness. if it was 110 degrees out, & you had a heavy fuel load, you probably wouldn't have enough runway to get off the ground without water. the new CFM56 engines made it all a moot point though, quieter, more efficient, & about twice the thrust of the old J57's. The BUFF's still use the J57's btw.
Military Argosy (RR Dart engines) approach to El Adem one afternoon - couple of little misunderstandings led to being above water-meth cut-in RPM when Flt Eng switched WM pumps on in landing checklist.. .Quite dramatic moment as VP props tried to cope with sudden but initially unstable injection of additional push-juice! 
This question used to come up a couple of times a year in the office, and so I was given "custodiamship" of the following "standard answer" which came from a friend who was (and still is) a gas-turbine designer at RR. I've posted it before, and I know some people disagree with it, but I go with Gav's version - YMMV:
The following is a standard answer that was prepared by one of the gas turbine design specialists at RR:
Are you sitting comfortably? Good, then we shall begin ....
First of all, gas turbine water injection is a thrust augmentation device. The concept of injecting water into a gas turbine has got sod all to do with cooling the engine as one of the advantages of a gas turbine is that they are self cooling – ponder the extensive use of such beasts in stationary applications, such as on oil rigs, pipelines and for powering ships.
Now we’ve got that one nailed – just how can you increase an engine’s thrust by injecting water? At first glance, it seems an absurd thing to do. Well, it’s simple really, and there are three different ways of doing it:
1) Add the water at the front of the compressor
2) Add the water directly to the combustion chamber
3) Add the water immediately before the turbine section or just before the propelling nozzle.
Taking each one in turn:
1) Injecting water at the compressor face has the effect of lowering the temperature of the inlet air, (assuming the water is at a lower temp than the ambient temp, of course, but seeing as you will generally be using water injection on hot days, that's taken as read). Remember the old maxim of 'It's fookin' difficult to compress cold air and vladimir impossible to compress hot air' and you soon realise that lowering the inlet air temp allows you to get either:
a) the same level of pressure rise as before but from less power offtake or
b) more pressure increase for the same shaft power requirement.
Both of these effects give you greater thrust (via less power offtake or through higher pressure ratio respectively) but option 1b) is usually the one used. In a nutshell, you are fooling the engine into thinking the ambient temperature has suddenly gone down and gas turbines work best at low temperatures. Because you have lowered the inlet air temp then obviously you are lowering the compressor outlet temp as well. This allows you to add more fuel and gives you a greater delta t across the combustor. You are also putting more mass flow through your engine (because you've added the water and water is more dense than air), giving you greater thrust because thrust is directly related to mass flow. Additionally, you can utilise a water/ethanol mix if you so desire, with the ethanol being burnt in the combustor giving you even more bang.
From point 1b) you can see the problems that occur with gas turbines at high ambient air temperatures: Higher air temp = lower compressor efficiency = lower pressure ratio = less efficient combustion = lower resultant thrust because the turbine is using up all the available power to run the compressor = you ain't going nowhere.
2) Adding water directly to the combustion chamber is one for the theoretical physicists. What you are trying to do is induce blockage and temporarily reduce the volume of the chamber, thus increasing the pressure inside the combustion chamber as the efficiency of the combustion process is increased at higher pressure. It also has some other peculiar effects such as increasing the air flow speed which is not detrimental. This type isn't used much as it's difficult to model and understand and can lead to combustion instability, which is a bit of a bad thing (tm)
3) Adding water at the turbine face or just before the nozzle simply works by adding mass flow to the engine's exhaust thus giving you more stuff out the back = greater thrust.
Option 3 is the simplest and most straightforward whereas option 1b will most probably give you the greatest thrust increase. Sometimes you will get a water injection installation that gives you both compressor and nozzle injection to get even more increase at the expense of plumbing complexity.
If anybody ever says that water injection is for 'cooling the engine', just ask them exactly what a couple of gallons of water is supposed to do to a raging inferno at 1,200 degrees centigrade travelling at 200 meters per second. You should get a few blank faces in return...
€0.07 supplied,
PDR
The following is a standard answer that was prepared by one of the gas turbine design specialists at RR:
Are you sitting comfortably? Good, then we shall begin ....
First of all, gas turbine water injection is a thrust augmentation device. The concept of injecting water into a gas turbine has got sod all to do with cooling the engine as one of the advantages of a gas turbine is that they are self cooling – ponder the extensive use of such beasts in stationary applications, such as on oil rigs, pipelines and for powering ships.
Now we’ve got that one nailed – just how can you increase an engine’s thrust by injecting water? At first glance, it seems an absurd thing to do. Well, it’s simple really, and there are three different ways of doing it:
1) Add the water at the front of the compressor
2) Add the water directly to the combustion chamber
3) Add the water immediately before the turbine section or just before the propelling nozzle.
Taking each one in turn:
1) Injecting water at the compressor face has the effect of lowering the temperature of the inlet air, (assuming the water is at a lower temp than the ambient temp, of course, but seeing as you will generally be using water injection on hot days, that's taken as read). Remember the old maxim of 'It's fookin' difficult to compress cold air and vladimir impossible to compress hot air' and you soon realise that lowering the inlet air temp allows you to get either:
a) the same level of pressure rise as before but from less power offtake or
b) more pressure increase for the same shaft power requirement.
Both of these effects give you greater thrust (via less power offtake or through higher pressure ratio respectively) but option 1b) is usually the one used. In a nutshell, you are fooling the engine into thinking the ambient temperature has suddenly gone down and gas turbines work best at low temperatures. Because you have lowered the inlet air temp then obviously you are lowering the compressor outlet temp as well. This allows you to add more fuel and gives you a greater delta t across the combustor. You are also putting more mass flow through your engine (because you've added the water and water is more dense than air), giving you greater thrust because thrust is directly related to mass flow. Additionally, you can utilise a water/ethanol mix if you so desire, with the ethanol being burnt in the combustor giving you even more bang.
From point 1b) you can see the problems that occur with gas turbines at high ambient air temperatures: Higher air temp = lower compressor efficiency = lower pressure ratio = less efficient combustion = lower resultant thrust because the turbine is using up all the available power to run the compressor = you ain't going nowhere.
2) Adding water directly to the combustion chamber is one for the theoretical physicists. What you are trying to do is induce blockage and temporarily reduce the volume of the chamber, thus increasing the pressure inside the combustion chamber as the efficiency of the combustion process is increased at higher pressure. It also has some other peculiar effects such as increasing the air flow speed which is not detrimental. This type isn't used much as it's difficult to model and understand and can lead to combustion instability, which is a bit of a bad thing (tm)
3) Adding water at the turbine face or just before the nozzle simply works by adding mass flow to the engine's exhaust thus giving you more stuff out the back = greater thrust.
Option 3 is the simplest and most straightforward whereas option 1b will most probably give you the greatest thrust increase. Sometimes you will get a water injection installation that gives you both compressor and nozzle injection to get even more increase at the expense of plumbing complexity.
If anybody ever says that water injection is for 'cooling the engine', just ask them exactly what a couple of gallons of water is supposed to do to a raging inferno at 1,200 degrees centigrade travelling at 200 meters per second. You should get a few blank faces in return...
€0.07 supplied,
PDR
Following my stint in the U.S. Navy flying the F-8 Crusader, I went to work for Trans World Airlines as a pilot, starting out in the flight engineer seat on the B-707. TWA had all sorts of 707 models, but one was powered by the same turbojet engine used in the F-8, the famous J-57, called a JT3C in civilian life. It's been a long long time, but if I remember correctly the "straight pipe" (non fan) versions injected demineralized water (no other additives) into the combustion chamber for about 2 minutes on take off to increase the mass flow through the engine.
Anyone who's flown a non-fan jet knows about the poor static thrust produced by pure turbojets engines, which is why they invented fan engines in the first place. But in the mean time, water injection allowed heavy take offs to be made with conventional turbo jet powered airliners. The dry thrust was just over 11,000 pounds and "wet" it made about 13,000 pounds.
Unfortunately what's left of my brain can't remember how much water we held nor do I recall how fast we pumped it, but I do remember being impressed when they taught those two facts in ground school.
Any water not used prior to thrust reduction following take off was immediately dumped.
I also seem to recall that the pumps were powered by two electrical busses, one for engines 1 & 2 and another for 2 & 4. The FE's job was to protect this part of the electrical system at all cost during take off because losing one meant a huge reduction of thrust on one side. Why Boeing, who was and still is famous for their good engineering practices, wired it that way I'll never know. Anyhow it was an interesting system and a hot weather, heavy take off in an old water wagon was always a thrill.
Once in cruise, these aircraft were fast primarily because the non-fan engines were so slim.
Anyone who's flown a non-fan jet knows about the poor static thrust produced by pure turbojets engines, which is why they invented fan engines in the first place. But in the mean time, water injection allowed heavy take offs to be made with conventional turbo jet powered airliners. The dry thrust was just over 11,000 pounds and "wet" it made about 13,000 pounds.
Unfortunately what's left of my brain can't remember how much water we held nor do I recall how fast we pumped it, but I do remember being impressed when they taught those two facts in ground school.
Any water not used prior to thrust reduction following take off was immediately dumped.
I also seem to recall that the pumps were powered by two electrical busses, one for engines 1 & 2 and another for 2 & 4. The FE's job was to protect this part of the electrical system at all cost during take off because losing one meant a huge reduction of thrust on one side. Why Boeing, who was and still is famous for their good engineering practices, wired it that way I'll never know. Anyhow it was an interesting system and a hot weather, heavy take off in an old water wagon was always a thrill.
Once in cruise, these aircraft were fast primarily because the non-fan engines were so slim.
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Since this topic is in "military aviation", I'm assuming at least one military jet used water injection to combat high inlet temperatures for high speed flying.
I doubt it. I can't be certain, but where used in jets it would almopst always be for thrust at low speeds rather than high-speeds.
Some *piston* engines used water or alcohol/water injection for high-speed flying, but they were using it to allow higher boost pressures without knocking (which is not a feature in jets).
PDR
Some *piston* engines used water or alcohol/water injection for high-speed flying, but they were using it to allow higher boost pressures without knocking (which is not a feature in jets).
PDR