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SXDFC
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Water Injection Question

Tue Jan 20, 2009 3:35 am

This is one term that I have seen quite often on here and in books as well, and I would like to know if someone can please explain this process to me?
 
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Starlionblue
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RE: Water Enjection Question

Tue Jan 20, 2009 4:07 am

Water injection (note spelling) was used to add power to piston engines and early jet engines, typically during the take-off phase.

Water was injected into the intake air.

In piston engines, this would cool the engine, allowing greater compression ratios without knocking, and thus higher power.

As I understand it, in jets it is a bit different, and has to do with making intake air denser and thus increasing power.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
lowrider
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RE: Water Injection Question

Tue Jan 20, 2009 4:36 am



Quoting Starlionblue (Reply 1):
As I understand it, in jets it is a bit different, and has to do with making intake air denser and thus increasing power

Right, it does this by cooling the air. Alcohol was used in some applications because it evaporates more readily.
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DiamondFlyer
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RE: Water Injection Question

Tue Jan 20, 2009 4:37 am

The way I understood water injection to work in turbine engines, was partially due to the cooling of the water on the part of the engine. However, I was under the impression that the majority of the benefits with water and a turbine came from the fact that, the water added a significant amount of mass to the equation. By throwing more mass out of the back of the engine, the whole system would have a greater force, due to F=ma, and physics and such.

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tdscanuck
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RE: Water Injection Question

Tue Jan 20, 2009 4:37 am



Quoting Starlionblue (Reply 1):
In piston engines, this would cool the engine, allowing greater compression ratios without knocking, and thus higher power.

As I understand it, in jets it is a bit different, and has to do with making intake air denser and thus increasing power.

The density effect also works in piston engines. The knock effect has no influence on jets because they can't know anyway.

Tom.
 
sabenapilot
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RE: Water Injection Question

Tue Jan 20, 2009 8:52 am



Quoting DiamondFlyer (Reply 4):
The way I understood water injection to work in turbine engines, was partially due to the cooling of the water on the part of the engine.

It is not the engine which was cooled, but the air it sucked in.

During the take-off phase from hot and high airports, water (or occasionally alcohol) was injected into the intake of the engine to cool the intake off and as such make it denser, which improved the performance of the jet engine.

This was especially important because the early jet engines were of the straight jet type and thus had no bypass ratio, meaning the amount of air taken in was limited and anything which could help win the engine more thrust was welcome.

Quoting DiamondFlyer (Reply 4):
However, I was under the impression that the majority of the benefits with water and a turbine came from the fact that, the water added a significant amount of mass to the equation. By throwing more mass out of the back of the engine, the whole system would have a greater force, due to F=ma, and physics and such.

Actually, its mass more of a hinderness if you think about it.

As we all know, the spontaneous ignition of the compressed fuel-air mixture inside the cumbustion chamber of a jet engine is responsible for a sudden expansion of this mixture which is then let to escape via the engine's conic outlet and creates the propulsion which is called thrust.

Now, if you inject water to cool off the intake air, you can make it denser indeed, but because the water molecules injected to cool off the intake air will take take up volume too, there will also be less usable air molecules left inside the combustion chamber to react with the fuel. As such the expansion will be less and the maximum thrust will be somewhat reduced again.

Now, since these 2 effects work in oposite ways, it is a matter of finding the right water quantity to inject: not enough and the cooling advantage will not be significant enough to note any increase in thrust, too much and the thrust reduction due to the presence of water inside the cumbustion chamber will be bigger than the gain drawn from lowering the intake air temperature...

A key problem is that the amount of water ideally has to be varied according to the ambinent temperatures, thus making the injection system very complicated really.
Some methods had variable amounts to get maximum efficiency (at the expense of maximum complexity) others didn't (and were fairly straight forward, but also relatively inefficient).

By using alcohols iso water, you'd improve on the water method described above because you're using a cooling fluid which evaporates more easily when injected (and thus cools more), is lighter than water (less dead weight to carry around) and takes up less volume inside the combustion chamber, but you'd make it far more difficult to use in day-to-day operations, because contrary to water, the alcohols used were not widely available in large quantities at the hot and high airports you'd be needing them... To my knowledge, alcohols were only really used by the military.

In the end the water injection method proved to offer only a very small increase in thrust, it was a highly complicated system to maintain and operate on a regular basis and it was soon made redundant by the use of (low) bypass engines.

Many early operators who had straight jet engines with a water injection system fitted to them had it turned off after a few years because it really wasn't worth it and only the military continued to use the method on a more or less regular basis.
 
FredT
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RE: Water Injection Question

Tue Jan 20, 2009 6:33 pm

Turbines:

Turbines work by heating air, thus increasing the volume of the air. The expansion expels the air out the exhaust nozzle, giving thrust. The more you can heat the air, the more it will expand and the more thrust the engine will generate.

The theoretical limit is to use all the air coming through the gas generator for combustion. This you cannot do, for a very practical reason: There's a limit to the temperature you can build engine components such as combustion chambers and turbine blades to withstand. In order to keep the engine components from melting, you have to limit the temperature. You do this by limiting the amount of fuel supplied to the engine. You do not inject enough fuel to use all the air in the combustion process, thus decreasing the total temperature gain but also decreasing the thrust generated.

Now, add water. The water will evaporate, turning into steam, adding volume and cooling the air flow through the engine. Suddenly you can inject more fuel without exceeding the maximum temperatures. This means more energy added and a larger expansion of the air inside the engine.

Mass flow is also a critical part of the thrust equation for a turbine engine, and the water injected becomes part of the total mass flow.
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FredT
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RE: Water Injection Question

Tue Jan 20, 2009 6:53 pm

Reciprocating engines:

The power generated by a piston engine is largely a function of the amount (mass) of air you can send through the engine. You increase the power at a given RPM by increasing the amount of air in the combustion chambers. The limiting factors are the physical capability of the engine to withstand the high pressures without blowing gaskets etc, the ability of the exhaust valves etc to withstand the temperatures generated and the threshold for detonation.

Normally, the fuel-air mixture is compressed and then ignited by the spark plug(s), creating one or several flame fronts starting at the spark plug(s). This burns the fuel-air mixture in a controlled manner, giving a controlled pressure rise in the combustion chamber.

If you sintroduce too much fuel-air mixture into the combustion chamber, the pressure rise as the mixture is compressed will increase the temperature of the fuel-air mixture to the point where it ignites. This ignition will be much more instantaneous than the controlled combustion initiated by the spark plugs, and quite likely happen sooner as well. This creates a sharp pressure increase in the combustion chamber, usually resisting the upgoing movement of the piston. This is what is known as detonation (as opposed to ignition), or sometimes knocking. The heat and pressure peaks from detonation will in short time destroy an engine.

So, we want maximum power out of a given engine but can't introduce more fuel-air mixture as this causes detonation. What to do? How about cooling the air before it goes into the combustion chamber? This will decrease the density of the air, enabling us to shove more air in there. It will also mean we can increase the temperature more before the engine starts knocking. One way to do this is to add an intercooler in the induction system.

Another way is to inject water. The water sucks up the heat from the air, giving you a larger margin to detonation. This means you can use higher manifold pressures, putting more air through the engine, and also enables you to heat the air more before reaching the detonation threshold. It effectively cools the engine. Coincidentally, you use rich mixture at high power setting for the very same reason, using excess fuel rather than water to cool the engine.

This also allows you to burn more fuel. This could be done by using a richer mixture combined with water injection. Quite a hassle to implement though. Cunning minds decided to instead add methanol to the water. The methanol serves dual purposes. It provides the extra energy the water enables you to add, meaning you do not have to muck about with the fuelling separately. It keeps the water from freezing at low temperatures. Alcohol is also more resistant to detonation than gasoline, adding even more to your detonation margin.


What it comes down to in the end, for turbines as well as recips, is adding water to the air flow in order to enable the air to absorb more energy without unwanted (and often disastrous) side effects.
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Northwest727
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RE: Water Injection Question

Tue Jan 20, 2009 7:04 pm

Taking my meteorology minor up on this, its the Latent Heat of Vaporization that causes the air to cool. Latent heat is defined as the heat needed to change phases of water, but it is not enough heat for one to feel (sensible heat). Avoiding confusing physics and numbers, in order to evaporate, water needs heat. Inject water upstream of the compressor assembly, it will evaporate thus taking heat out of the air. The air then becomes cooler, and denser. Therefore, the engine "experiences" more airflow into it, thus increasing thrust. The downside as mentioned above, is that water vapor (steam) doesn't burn, and is actually less dense than air itself.

I also read once that water injection had an effect on the life of engine parts, but I can't remember whether it was negative or positive.
 
windy95
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RE: Water Injection Question

Wed Jan 21, 2009 1:48 am

On the KC-135A models with the P&W J-57 we used the water injection system often. Fuel loads and temps dictated the use. If I remember right we carried 575 gallon's of de-mineralized water in a saddle tank located in the wheel well center-line. You required a minimum of 110 seconds of water on T/O roll with 120 being about norm. You had two electric driven pump's in the water tank. One to serve the Inboards and one to serve the outboards. There was then a pneumatic operated pump on each engine to pressurize the H2O on the engine. The cooling of the engine allowed more fuel to be dumped into the burner can's for a higher EPR. Max EPR with water on the A model I believe was 1.81. Water injection was not used once the OAT reached near freezing and was not needed due to the colder and denser air.

As far as maintenance on the system if you had a failure on one engine it was usually the butterfly valve or the engine pump. Being a pneumatic turbine fed with bleed air it had a tendency to blow especially on very hot days. Doing a trim was horrible if you where the man under the engine turning the screws. You had to set military power and water injection and the noise and vibration really tore you up.
 
ex52tech
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RE: Water Injection Question

Wed Jan 21, 2009 4:48 am



Quoting Windy95 (Reply 9):
If I remember right we carried 575 gallon's of de-mineralized water in a saddle tank located in the wheel well center-line.

670 gallons sticks in my head, but it has been years.

quote=SXDFC,reply=0]This is one term that I have seen quite often on here and in books as well, and I would like to know if someone can please explain this process to me?
[/quote]

Regarding the J-57 engine on the KC-135A, which is a good example of how water injection worked on the early turbojets. 1/3 of the water was injected into the inlet at the face of the N1 compressor, and 2/3 injected into the combustion chamber. You would actually see about a 2% drop in N2 compressor RPM and a significant increase in fuel flow ( there was a water pressure signal sent to the fuel control), and subsequently a jump in Engine Pressure Ratio (thrust).
The engine would not over-speed or over-temp. Now if you wanted to simulate water injection and you were to apply nitrogen to the water pressure bellows on the fuel control to simulate the engine was in water in the summer, you could easily over-speed and over-temp the engine..........the water worked that well to slow it down and cool it down, as Fred T stated above.

Just to clear up what might be confusing to some, alcohol was not used on any jet engines, maybe on some low compression ratio turboshaft engines, it is used on piston engines. Anything combustible would ignite inside the compressor.....not good.

Quoting Windy95 (Reply 9):
As far as maintenance on the system if you had a failure on one engine it was usually the butterfly valve or the engine pump. Being a pneumatic turbine fed with bleed air it had a tendency to blow especially on very hot days. Doing a trim was horrible if you where the man under the engine turning the screws. You had to set military power and water injection and the noise and vibration really tore you up.

Not sure what you mean by butterfly valve on the J-57, unless you mean the control valve for the air that operated the pump. I have seen a lot of engine pumps fail, simply because they turned so fast, and had sealed bearings, which was just asking for trouble, they should have been oiled by turbine oil.

The only butterfly type valve we had in the system, was the "Whitiker" valve, it was just down stream from the pump, on the RH side of the engine, and would not open unless the water pressure was above 200psi, or if it was full of metal from a previous pump failure, and nobody changed it with the pump.........saw that happen a lot.

YUP turning those screws on that fuel control was not for the faint of heart, kneeling down under that engine and turning that screw while that engine was in water would scare the hell out of you.......I don't care who you are.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
windy95
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RE: Water Injection Question

Wed Jan 21, 2009 12:52 pm



Quoting Ex52tech (Reply 10):
Not sure what you mean by butterfly valve on the J-57, unless you mean the control valve for the air that operated the pump. I have seen a lot of engine pumps fail, simply because they turned so fast, and had sealed bearings, which was just asking for trouble, they should have been oiled by turbine oil.

The only butterfly type valve we had in the system, was the "Whitiker" valve, it was just down stream from the pump, on the RH side of the engine, and would not open unless the water pressure was above 200psi, or if it was full of metal from a previous pump failure, and nobody changed it with the pump.........saw that happen a lot

For some reason we called the Whitaker vavle the Butterfly valve. Do not ask me why. And yes those water pumps needed some kind of lube. The 200 psi come back to me now and I believe yu are right with the 670 gallons of water. I really disliked when on deployment when we had to use the Demin kit. It could take hours to fill that tank

Quoting Ex52tech (Reply 10):
You would actually see about a 2% drop in N2 compressor RPM and a significant increase in fuel flow ( there was a water pressure signal sent to the fuel control), and subsequently a jump in Engine Pressure Ratio (thrust).

Fuel flow on T/O ran between 12 to 13 thousand lbs per hour per engine. You ate quite a bit of your fuel on Climbout. I remember taking off with 180,000lbs in a MITOT with the BUFF's and landing an hour later because we had burned off or transfered all of our fuel into the B-52's on Climbout. Fun stuff.
 
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Moose135
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RE: Water Injection Question

Wed Jan 21, 2009 8:42 pm



Quoting Windy95 (Reply 9):
Max EPR with water on the A model I believe was 1.81.

Actually, we set it at 2.83 at the start of takeoff roll, and it increased slightly as airspeed built up. Dry takeoff EPR varied between 2.2 and 2.4 (I think, it's been 20 years - but 2.83 is imprinted on my brain!)

Quoting Ex52tech (Reply 10):
670 gallons sticks in my head, but it has been years.

You are correct, it was 670 gallons. I flew out of KGUS, and used to feel bad for the crew chiefs on Alert during the winter. If it was between 20F and 40F, they had to run the tank heaters to keep the water from freezing. If it went below 20F, they had to dump the water, only to have to refill it when it went back above 20F.

The heaviest takeoff I remember in the A-model was out of Pease, at the start of a fighter drag to Mildenhall. Our gross weight was in the 285K range, so we had 175K+ of fuel on board. We rumbled down that 12,000 foot runway, making noise and black smoke, but not much acceleration. I think we rotated at the 2,000 foot remaining marker,not sure where we actually lifted off.
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windy95
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RE: Water Injection Question

Wed Jan 21, 2009 9:20 pm



Quoting Moose135 (Reply 12):
Actually, we set it at 2.83 at the start of takeoff roll, and it increased slightly as airspeed built up. Dry takeoff EPR varied between 2.2 and 2.4 (I think, it's been 20 years - but 2.83 is imprinted on my brain!)

You are correct. Seem's like a long time ago and some of the numbers are fuzzy. I have to go pull my old handbook out of the attic.

Quoting Moose135 (Reply 12):
You are correct, it was 670 gallons. I flew out of KGUS, and used to feel bad for the crew chiefs on Alert during the winter. If it was between 20F and 40F, they had to run the tank heaters to keep the water from freezing. If it went below 20F,

Spent many a night baby sitting the Hobart's as they heated the water. During the winter on Alert the best news was alway's "dump the water".

Quoting Moose135 (Reply 12):
The heaviest takeoff I remember in the A-model was out of Pease, at the start of a fighter drag to Mildenhall. Our gross weight was in the 285K range, so we had 175K+ of fuel on board

I was probably there. Spent six years there starting at the end of 1984. Most of the ETTF mission's would leave out of there. As far as fuel loads I remember once going out of Hickham with a 185K load and prayed to the water God's all the way until the water ran out. The only good thing with that type of weight is you did not have to carry any pax.
 
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Moose135
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RE: Water Injection Question

Wed Jan 21, 2009 11:39 pm

You probably were there, Windy. It was April 87. We were joining up with a KC-10 and a gaggle of F-4s out of Seymour Johnson. Took us almost a week to get out. We would check out of the off-base hotel at 4am, show up for mission brief and pre-flight, then wait on the weather. For 4 days, it was usually crappy at one of the divert fields along the route, so we'd scrub the mission, go back to the hotel, get our old rooms back, and hit the sack for a couple of hours. Finally on the last day, the weather gods smiled on us, and we were able to get out. It wasn't all bad, we got to a Red Sox game at Fenway one afternoon, and took a drive up the coast of Maine another day.
KC-135 - Passing gas and taking names!
 
SXDFC
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RE: Water Injection Question

Thu Jan 22, 2009 2:04 am

How is the water injected into the engines? Is another thing I wanted to know.

Thanks for the information on this, I really appreciate it!
 
sdf880
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RE: Water Injection Question

Thu Jan 22, 2009 4:38 am

I remember sitting in the jumpseat on a dart poewered aircraft and on takeoff roll at about 70 knots the first officer turned the water meth on. The water was supposed to be turned on taking the runway not half way into the take-off. I thought the engines were going to jump out of their mounts. I remember any/all the engine gauges going nuts, EGT, Torgue, RPM's went crazy. The captin continued the take-off and at about 400 feet he calmly looked over to the f/o and informed him he had just bent all the props forward about a foot. The scared young f/o was quiet for a second then tried to talk but really didn't know what to say. The captain let it go for a few more seconds then said "everythings OK, don't ever do that again"

SDF880
 
ex52tech
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RE: Water Injection Question

Thu Jan 22, 2009 3:28 pm



Quoting SXDFC (Reply 15):
How is the water injected into the engines? Is another thing I wanted to know.

On the J-57s on the KC-135A there is a round manifold just behind, and attached to, the Inlet Guide Vane case, that case also supports the #1 bearing in the engine. That spray ring distributed 1/3 of the water into the engine's N1 compressor. Let me back up a little.....after the water leaves the turbine pump it passes through the Whitiker valve (simple pressure check valve) or butterfly valve if you will. Then it passes through the forward water regulator, and that regulator sends 1/3 of the water to the inlet. From there the water go through the aft water pressure regulator then it travels down the top of the engine to the aft manifold which injected the water through the diffuser case ( the front of the combustion chamber) through ports in the diffuser case. The aft manifold looks like a bunch of little pig tails going around the engine, my guess is that they curled the lines up like that to take up the shock loads of 200+ p.s.i. water hitting the lines instantaneously. By the way 200p.s.i. was the minimum pressure for the system to operate......I have no idea what the max is, but I have witnessed a few blown lines, so it gets pretty high.

Now the water pump on the engine was controlled by a valve that let high pressure compressor air into the turbine on the water pump. This control valve was operated by a micro switch mounted on the fuel control linkage, that micro switch was powered by pressure switches in the water injection system. The micro switch would not allow you to reduce power below a certain power setting, you could flame out the engine by reducing the power to low. Had that happen to me on an engine run-up to trouble shoot a water injection problem......engine flamed out, water caused the turbine cases to shrink faster than the turbine wheels, and the engine seized......NOT GOOD.

There were two ways to put and engine into water injection.

#1 Walking into water: You would pressurize the aircraft water injection pumps, then advance the throttles until the micro switches were engaged, and presto you were in water.

#2 Banging water (my personal favorite) : You would take the engines up to a preset engine power setting where the micro switches were already made, and then pressurize the aircraft's water injection system. The engines would literally jump when they took water. This method was lots of fun, especially if you were standing next to the engine. This method is quite enjoyable if you have the new guy standing there next to you, the look on their faces is always a good "Kodak moment".

The turbine water pumps were always a problem. They turned at an unbelievable RPM, you could hear them spin up above the noise of the engine. They could and did occasionally come apart, you stayed away for them when they were operating.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
Tristarsteve
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RE: Water Injection Question

Thu Jan 22, 2009 4:48 pm



Quoting Starlionblue (Reply 1):
and early jet engines

Very interesting the use of water in the military stovepipes, but we used it in slightly more modern engines.
When I worked in BAH in 1977, GF had four BAC111-400 that had water injected Spey engines. Tank of pure (demineralised) water, an air driven pump, and injection only into the combustion chambers. The same engine was fitted to the Trident 2.
GF used this for nearly every take off in the summer. The increased power was essential at OAT of 38degC. an an engine flat rated to 24degC.
There were two types of Spey water injected engines. The W engines and the DW engines. I know one increased thrust, the other just brought the thrust back at higher OATs.
Even with these engines there were days when the fleet was grounded, the OAT was too high for an aircraft with an engine flat rated at so low a temp. Details are a bit hazy now!!!.
We also had a weekly visit from an Air India B747-100 with water injection. The refuellers at BAH only had a hand pump to fill the tanks, which was Ok for the 111, but a lot of pumping for a 747!
The system was quite reliable in regular use. But BEA with its Trident2 had a lot of problems, mainly because it was not used often, and the pumps failed. In the end they kept a few aircraft serviceable, and decommissioned the others. I still remember the red wooden pumps we fitted in the holes!

In 1978 GF was going to replace the BAC111, and the B737-200 arrived and all the problems disappeared, due to the much bigger engines.
 
prebennorholm
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RE: Water Injection Question

Fri Jan 23, 2009 1:41 am

Finally some very good explanations of water intection.

Especially FredT in reply #6 an #7 explains the function beautifully for tubine and reciprocating engines.

To sum it up, in turbine engines water intection serves (at least) four purposes:

1. Injected in front of the compressor, evaporation of the water cools the air, thereby increasing the inlet mass flow at a given crompression ratio.

2. Injected into the combustion chambres it cools the combustion, allowing a higher fuel flow without exceeding max EGT (exhaust gas temperature).

3. The higher fuel flow increases total gas expansion during combustion = more power.

4. The significant mass of the injected water adds significantly to the total gas masses which are accelerated through the tail pipe = more power.

I read somewhere many years ago that a J-57 powered B-52 would spend 5 tons of water during slightly less than two minutes at takeoff. That corresponds very well with the figures giving by Windy95 and others in previous posts covering KC-135A details.

For reciprocating engines a mixture of methanol and water was used, as explained by FredT in reply #7. It was usually called Methmix. For some engines a fifty-fifty mixture was specified, others used 45% methanol and 55% water.

Methanol (CH3OH - sometimes named methyl alcohol) is a fuel with some interesting properties. Since every methanol molecule contains an oxygen atom, then it can burn in a much, much richer mixture than gasoline, releasing a lot of expansion in a cooler combustion. Mixed with gasoline it also improves the anti-knock properties (increases octane number) which allows a higher inlet pressure and correspondingly increased total fuel injection.

The down side is that "milage" on methanol is hurrible compared to gasoline. That doesn't mean anything when used only to boost takeoff power.

Methanol also has roughly the same anti-freeze properties as ethanol (C2H5OH - the "power" stuff in whiskey). It mixes as easily as ethanol with water. Therefore it is the perfect stuff to make sure that nowhere in the system the water will freeze, never mind what pressure variations or temperatures are present.

Methanol also tastes much the same as ethanol. The downside is that is doesn't make you drunk, but makes you blind before it kills you. Misunderstanding that cost quite a few lives at airports in the old "Methmix days".
Always keep your number of landings equal to your number of take-offs
 
ex52tech
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RE: Water Injection Question

Fri Jan 23, 2009 5:28 am



Quoting Prebennorholm (Reply 19):
I read somewhere many years ago that a J-57 powered B-52 would spend 5 tons of water during slightly less than two minutes at takeoff. That corresponds very well with the figures giving by Windy95 and others in previous posts covering KC-135A details.

The B-52G held 1300 gallons of demineralized water, and would burn it in 120 seconds. Pretty much the same as the KC-135A.

The F and G models were the same due to the J-57-P-43WB engine being on both models. The water pump was mounted on the gearbox of the even engines in the same pad as the CSD, and Generator were on the odd engines.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
411A
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RE: Water Injection Question

Fri Jan 23, 2009 7:55 am



Quoting Sabenapilot (Reply 5):
Many early operators who had straight jet engines with a water injection system fitted to them had it turned off after a few years because it really wasn't worth it and only the military continued to use the method on a more or less regular basis.

Not with the airlines that I few for, they didn't.
Very necessary for heavy weight takeoffs, even near sea level.
 
sabenapilot
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RE: Water Injection Question

Fri Jan 23, 2009 9:47 am



Quoting Prebennorholm (Reply 19):
Finally some very good explanations of water intection.

Interesting topic indeed.
Personally I've never seen an engine with water injectin in operation: I wonder if there are any operators still using it today?

I'd like to point out one thing however, which I have repeatedly read:

Quoting Prebennorholm (Reply 19):
4. The mass of the injected water adds significantly to the total gas masses which are accelerated through the tail pipe = more power.

Although all said is accademically correct, unlike the 3 other purposes you've enumerated before, this 4th purpose is a phenomenon only valuable to straight jet engines.
Think about it for a second...

Quoting Moose135 (Reply 12):
On the KC-135A, it was 670 gallons



Quoting Windy95 (Reply 9):
You required a minimum of 110 seconds of water on T/O roll with 120 being about norm.



Quoting Ex52tech (Reply 20):
The B-52G held 1300 gallons of demineralized water, and would burn it in 120 seconds.

That's 163 gallons per engine in 2 minutes, so just over 5 liters (and thus kilograms) per second, since were talking water here. Compared to the air mass per second flowing through the engine, 5 kgs additional mass isn't really adding much, so that 4th point becomes mainly accademic in nature, unless for straight jet engines, which are limited in the volume (and thus mass flow) of air taken in.
Anybody who happens to know the saturated mass flow per second of the J-57 engine? Compare it to the mass flow through a HBPR engine of today....

On the other hand, the 3 other purposes of water injection you've enumerated would remain advantageous on by high by pass ratio engines too, were it not that the technological advances in the field of manufacturing have brought us far better compression ratio's and better alloys, able to generate more than enough thrust without complicated systems like water injection.

Quoting 411A (Reply 21):
Not with the airlines that I few for, they didn't.
Very necessary for heavy weight takeoffs, even near sea level.

Then you've probably flown soms pretty old planes for a fairly long period of time?

Pretty cool stuff to do I image, although I must say I'd definitely feel much more at ease in a brandnew A330 or 'underpowered' A340 (if you're to believe some here on A.net), than in any plane needing water injection to get airborne at sea level!

Long live the HBPR engines and their newer alloys!
 
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Starlionblue
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RE: Water Injection Question

Fri Jan 23, 2009 1:12 pm



Quoting Sabenapilot (Reply 22):
Pretty cool stuff to do I image, although I must say I'd definitely feel much more at ease in a brandnew A330 or 'underpowered' A340 (if you're to believe some here on A.net), than in any plane needing water injection to get airborne at sea level!

Heh. Well, it's not really "to get airborne" is it? More like "to meet engine out requirements".

But I see your point!
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
windy95
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RE: Water Injection Question

Fri Jan 23, 2009 1:16 pm

The loss of water was a serious thing. Losing water on all four engines was almost the equivalent of losing thrust from an entire engine. The Air Force lost a number of KC-135 aircraft due to water injection problems on takeoff.





Quoting Ex52tech (Reply 20):
Quoting Prebennorholm (Reply 19):
I read somewhere many years ago that a J-57 powered B-52 would spend 5 tons of water during slightly less than two minutes at takeoff. That corresponds very well with the figures giving by Windy95 and others in previous posts covering KC-135A details.

The B-52G held 1300 gallons of demineralized water, and would burn it in 120 seconds. Pretty much the same as the KC-135A.

Also add the 12 to 13k pph fuel burn and we would lose a lot of weight very quick. Which was the reason why the B-52 needed that tanker out fron to top it off on it's way up.
 
ex52tech
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RE: Water Injection Question

Fri Jan 23, 2009 4:37 pm



Quoting Sabenapilot (Reply 22):
Pretty cool stuff to do I image, although I must say I'd definitely feel much more at ease in a brandnew A330 or 'underpowered' A340 (if you're to believe some here on A.net), than in any plane needing water injection to get airborne at sea level!

When the CFM-56 engine came along for the R model 135 I think everyone was relieved. The A model 135 was a led sled on takeoff, a real nail biter at times. The Air Force kept the A model on as long as it did because they could afford to, and finally the argument for long term cost savings won the day, so they got re-engined.

As far as flying around in a brand new Airbus as compared to an old Boeing...........personally I would take the old high time Boeing. NW has already scrapped some of their first A320s, not even 20 years old, at the time they were still flying a few 30+ year old DC-9s with astronomical amount of cycles on them.

Back in the days when the KC-135A and the B-52G were still on the front line, we worked with what we had, nobody thought they were overpowered. But it was fun to watch and work on.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
411A
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RE: Water Injection Question

Sat Jan 24, 2009 12:37 am



Quoting Sabenapilot (Reply 22):
Then you've probably flown soms pretty old planes for a fairly long period of time?

Yup, quite a lot, in years gone by....turbojet powered.
Water injection provided decreased field length requirements and increased second segment climb ability.
When fan engines came along...much better, in both departments.
 
Tristarsteve
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RE: Water Injection Question

Sat Jan 24, 2009 8:55 am



Quoting 411A (Reply 26):
and increased second segment climb ability.

So how long was the water on?
On the BAC111 the water tank drain valve opened when airbourne. (to dump the unused water and stop it freezing)
 
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Francoflier
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RE: Water Injection Question

Sat Jan 24, 2009 11:51 am



Quoting Sabenapilot (Reply 22):
I wonder if there are any operators still using it today?

Probably not on jet engines, but I know of a few older Tprops that still grace the skies which use water or water/methanol injection.

F-27 (and Fairchild equivalent), Let 410 and some early Jetstream 31 come to mind.
I'll do my own airline. With Blackjack. And hookers. In fact, forget the airline.
 
windy95
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RE: Water Injection Question

Sat Jan 24, 2009 1:44 pm



Quoting TristarSteve (Reply 27):
So how long was the water on?
On the BAC111 the water tank drain valve opened when airbourne. (to dump the unused water and stop it freezing)

Same on the KC-135

Quoting Ex52tech (Reply 25):
When the CFM-56 engine came along for the R model 135 I think everyone was relieved.

And the T/O accidents stopped happening once the 135's started receiving first the TF33's on the E model and the CFM's on the R.
 
timz
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RE: Water Injection Question

Sat Jan 24, 2009 6:56 pm



Quoting Sabenapilot (Reply 5):
Many early operators who had straight jet engines with a water injection system fitted to them had it turned off after a few years

Aside from JT3C-powered 707s and DC-8s, whose turbojet airliners used water? (Oh yes-- besides the Comet IA.)
 
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CALTECH
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RE: Water Injection Question

Sat Jan 24, 2009 7:15 pm



Quoting Ex52tech (Reply 20):
The B-52G held 1300 gallons of demineralized water, and would burn it in 120 seconds. Pretty much the same as the KC-135A

I remember the stencil saying '1195 gallons' on the G model for approx a 120 second burn. Can't remember if that was Imperial gallons.
You are here.
 
411A
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RE: Water Injection Question

Sun Jan 25, 2009 1:34 am



Quoting TristarSteve (Reply 27):
So how long was the water on?

Until the tanks went dry...usually about 2.5 minutes.
 
ex52tech
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RE: Water Injection Question

Mon Jan 26, 2009 6:39 pm



Quoting CALTECH (Reply 31):
I remember the stencil saying '1195 gallons' on the G model for approx a 120 second burn

Probably correct, 1300 stuck in my head, but I was not absolutely sure.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
ex52tech
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RE: Water Injection Question

Mon Jan 26, 2009 6:49 pm



Quoting Timz (Reply 30):
Aside from JT3C-powered 707s and DC-8s, whose turbojet airliners used water? (Oh yes-- besides the Comet IA.)

The DC-10-40 had it also.

The water was injected into a port right on the fuel nozzles. The tank held 600 gallons, and was located in the fuselage wing fairing just behind the wing, and forward of the bulk cargo door. Three pumps in the tank, and the plumbing traveled through the LH wheel well, then along the front spar of each wing.

It was only used during certification. We never used it operationally. Most of the plumbing was removed in the mid 90's. The pylon and engine plumbing was removed very early on.

The trim gear did have a selection for water injection, and one of the trim heads was fitted with a water injection trim screw driver. I wish I could have used it just to see how much of a jump you would get.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
timz
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RE: Water Injection Question

Mon Jan 26, 2009 8:59 pm

Sure-- but when I said "turbojet" I meant non-turbofan.
 
ex52tech
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RE: Water Injection Question

Tue Jan 27, 2009 6:34 pm



Quoting Timz (Reply 35):
Sure-- but when I said "turbojet" I meant non-turbofan.

Sorry......missed that. You probably have it covered then.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
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CALTECH
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RE: Water Injection Question

Mon Feb 02, 2009 7:42 pm



Quoting Ex52tech (Reply 33):
Probably correct, 1300 stuck in my head, but I was not absolutely sure.

Where and did you work on B-52s ? Which model ? Worked at Minot AFB and at the training base of SAC at Castle AFB.
You are here.
 
access-air
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RE: Water Injection Question

Thu Feb 05, 2009 7:38 pm



Quoting SDF880 (Reply 16):
I remember sitting in the jumpseat on a dart poewered aircraft and on takeoff roll at about 70 knots the first officer turned the water meth on. The water was supposed to be turned on taking the runway not half way into the take-off. I thought the engines were going to jump out of their mounts. I remember any/all the engine gauges going nuts, EGT, Torgue, RPM's went crazy. The captin continued the take-off and at about 400 feet he calmly looked over to the f/o and informed him he had just bent all the props forward about a foot. The scared young f/o was quiet for a second then tried to talk but really didn't know what to say. The captain let it go for a few more seconds then said "everythings OK, don't ever do that again"

SDF880

Which Dart powered plane was it and where from?

Access-Air
Remember, Wherever you go, there you are!!!!
 
sdf880
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RE: Water Injection Question

Fri Feb 06, 2009 1:27 am



Quoting Access-Air (Reply 38):
Which Dart powered plane was it and where from?

Rather not say the airline involved as I worked for them too but the aircraft was a YS-11

SDF880
 
broke
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RE: Water Injection Question

Fri Feb 06, 2009 6:09 pm

You can add the Allison (now Rolls-Royce) 501D13H turboprop engine used on Frontier Airlines Convair 580's to the list of water injected powerplants. Frontier used it on high hot days in the Rocky Mountains.
 
sdf880
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RE: Water Injection Question

Fri Feb 06, 2009 9:08 pm



Quoting Broke (Reply 40):
You can add the Allison (now Rolls-Royce) 501D13H turboprop engine used on Frontier Airlines Convair 580's to the list of water injected powerplants. Frontier used it on high hot days in the Rocky Mountains.

I rode on several of those way back when. DEN-DRO, DRO-FMN, and FMN-ABQ. Nice rides and I loved the take-off from FMN as it seems we took off from a runway on a mesa.

SDF880
 
ex52tech
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RE: Water Injection Question

Sat Feb 07, 2009 7:05 pm

Quoting CALTECH (Reply 37):
Where and did you work on B-52s ? Which model ? Worked at Minot AFB and at the training base of SAC at Castle AFB.

Worked on H models, and then G models at El Forko Grande (GFAFB) Worked in the engine shop overhaul and on the line. Actually got to overhaul TF-33s, and both the J-57-59, and J-57-43. Line work was the best though.

Oh.......and I would rather do two tours in Grand Forks than one in Minot, nothing personal, but Minot is absolutely positively in the middle of nowhere. I would have drank myself to death there, not that Grand Forks was a "happening place" either, but it was closer to civilization.

[Edited 2009-02-07 11:11:01]
"Saddest thing I ever witnessed....an airplane being scrapped"
 
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DocLightning
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RE: Water Injection Question

Mon Feb 09, 2009 8:04 am



Quoting Sabenapilot (Reply 5):

Now, if you inject water to cool off the intake air, you can make it denser indeed, but because the water molecules injected to cool off the intake air will take take up volume too, there will also be less usable air molecules left inside the combustion chamber to react with the fuel. As such the expansion will be less and the maximum thrust will be somewhat reduced again.

So the mixture in a jet engine is very thin. The outrageous pressures in the combustor allow the mixture to burn. Were fuel and air mixed stoicheometrically, the can and everything downstream would melt! Air is by FAR not the limiting reagent. So I think that adding mass in this case might actually help.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
dakota123
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RE: Water Injection Question

Mon Feb 09, 2009 9:48 pm



Quoting Sabenapilot (Reply 5):
Now, if you inject water to cool off the intake air, you can make it denser indeed, but because the water molecules injected to cool off the intake air will take take up volume too, there will also be less usable air molecules left inside the combustion chamber to react with the fuel. As such the expansion will be less and the maximum thrust will be somewhat reduced again.

I don't think this is the case, within reason. Granted, the prime effect is without question the reduction of the heat of compression, but 2/3 of the air passing through the compressor is for cooling, not combustion. So unless the engine is well and truly flooded there's enough air to support combustion.* Further, the mass of steam (what is it, 1200:1 ratio of the volume of steam to water?) acts on the turbine, which spins faster, which makes the compressor spin faster, which increases the amount of air available for all purposes (combustion and cooling). So the whole process pretty much takes care of itself. Unless you eat into the surge margin and stall the compressor.

I (well, my vendor) did manage to pretty much destroy an industrial unit when tuning a de-NOx water injection system (injecting into the combustor, not the compressor). Something was wayyyy wrong in the watering schedule and about 10 times the safe water amount was injected all of the sudden. The engine shook like a wet dog and flamed out. Cracked a turbine disk (among other things). Good times.

* It certainly would be advantageous if the flow path were designed, variable guide vanes programmed etc. with water injection in mind, i.e to "expect" that the mass of water was there.
“And If I claim to be a wise man, well surely it means that I don’t know”
 
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CALTECH
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RE: Water Injection Question

Tue Feb 10, 2009 12:57 am



Quoting Ex52tech (Reply 42):
Worked on H models, and then G models at El Forko Grande (GFAFB) Worked in the engine shop overhaul and on the line. Actually got to overhaul TF-33s, and both the J-57-59, and J-57-43. Line work was the best though.

Oh.......and I would rather do two tours in Grand Forks than one in Minot, nothing personal, but Minot is absolutely positively in the middle of nowhere. I would have drank myself to death there, not that Grand Forks was a "happening place" either, but it was closer to civilization.

Worked a little line on Hs ended up on the Gs. Line work is much better. Didn't care for the hangar work. Hey, nothing personal taken, Minot AFB WAS in the middle of nowhere. If I remember right, the town was half the population of the base, and the town closed up at 6pm Saturday night. We did drink ourselves to death up there. Needed alot of anti-freeze. Castle AFB was much funner and more pleasant. It too was in the middle of nowhere, but an hour or two drive, and there were many things to do.

The water injection system was such a maintenance hog. Gate valve and bellows changes all the time. Nothing but afterburners, beat the sight of KC-135s and Buffs doing MITOs with H2O injection. The smoke, the roar, will stay with me forever,

http://www.youtube.com/watch?v=Cq6Hpxyrhyo&feature=related
You are here.
 
ex52tech
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RE: Water Injection Question

Tue Feb 10, 2009 2:25 pm



Quoting DocLightning (Reply 43):
The outrageous pressures in the combustor allow the mixture to burn.

As the air enters the combustion chamber it accelerates and the pressure drops considerably, otherwise you would burn up the burner can/cans, the nozzle guide vanes, and turbines.


Quoting Dakota123 (Reply 44):
Granted, the prime effect is without question the reduction of the heat of compression,

I have to disagree, the water injected into the compressor raises compression pressure/ratio, the water injected into the combustion chamber decreases temperature. There is always an increase in fuel flow when an engine is in water injection.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
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Moose135
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RE: Water Injection Question

Wed Feb 11, 2009 12:39 am



Quoting Ex52tech (Reply 46):
There is always an increase in fuel flow when an engine is in water injection.

Seeing the fuel flow gauges swing was a good indication that the engines "took water" on takeoff.
KC-135 - Passing gas and taking names!
 
ex52tech
Posts: 553
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RE: Water Injection Question

Wed Feb 11, 2009 4:10 am



Quoting Moose135 (Reply 47):
Seeing the fuel flow gauges swing was a good indication that the engines "took water" on takeoff.

I always got a kick out of watching the EPR, and fuel flow gage needles JUMP when the engine took water. Not to mention the slight kick in the pants you felt............and the noise.......and the smoke........  bigthumbsup   bigthumbsup  I sure do miss that.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
tdscanuck
Posts: 8573
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RE: Water Injection Question

Wed Feb 11, 2009 6:10 am



Quoting Ex52tech (Reply 46):
As the air enters the combustion chamber it accelerates and the pressure drops considerably, otherwise you would burn up the burner can/cans, the nozzle guide vanes, and turbines.

Air should be slowing down as it enters the combustion chamber...you want the fuel to be added at the lowest possible Mach number. The whole point of good burner design is to preserve total pressure. It shouldn't be accelerating until after the fuel has burned. and it's heading into the turbine nozzle.

Tom.

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