Fuel readout in Kilograms not litres
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Water injection in turbojet
Let's continue to be Nerds and Geeks, and talk about liters, weight and mass.
xxx
I am surprised nobody talked much about jet engines...
After all, what these machines do for power is mass acceleration.
So, let us talk about the past century and jet engines.
Jet fuel is say, of a specific gravity of about .81 (call it 81% of weight of distilled water).
Of course, as mentioned, varies with temperature. Hot fuel is "lighter".
Now, let's talk about the old jet engines with water injection
There were the P&W JT3C-6 with water (called J-57 in the USAF/NAVY).
Some 707-120 and DC8-10 had these "wet engines".
Even the 747s... there were JT9D engines with water...
Last letter W in their designation, as JT9D-7AW, or -7FW
These 747 got some 2,000 lbs of thrust extra per engine using "water".
xxx
Now, most "aviation experts" (many here) said "water... to keep engine cold".
When you go full power, put water ON, keeps EGT low...
Can I smile...?
What water did really, was to increase accelerated mass flow.
Extra mass of water (specific gravity 1) to jet fuel (SG of .81)...
Result was more power... without increase in EGT, as water does not burn.
xxx
Imagine an inventor finding way to have "Hg (mercury) injectors" SG of some 13.
Would be a bit of vitamins for our engines...!
Happy contrails
xxx
I am surprised nobody talked much about jet engines...
After all, what these machines do for power is mass acceleration.
So, let us talk about the past century and jet engines.
Jet fuel is say, of a specific gravity of about .81 (call it 81% of weight of distilled water).
Of course, as mentioned, varies with temperature. Hot fuel is "lighter".
Now, let's talk about the old jet engines with water injection
There were the P&W JT3C-6 with water (called J-57 in the USAF/NAVY).
Some 707-120 and DC8-10 had these "wet engines".
Even the 747s... there were JT9D engines with water...
Last letter W in their designation, as JT9D-7AW, or -7FW
These 747 got some 2,000 lbs of thrust extra per engine using "water".
xxx
Now, most "aviation experts" (many here) said "water... to keep engine cold".
When you go full power, put water ON, keeps EGT low...
Can I smile...?
What water did really, was to increase accelerated mass flow.
Extra mass of water (specific gravity 1) to jet fuel (SG of .81)...
Result was more power... without increase in EGT, as water does not burn.
xxx
Imagine an inventor finding way to have "Hg (mercury) injectors" SG of some 13.
Would be a bit of vitamins for our engines...!
Happy contrails
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I realized I wasn't fully clear about how water injection worked exactly, so I swiped this question/answer session from a discussion on airliners.net
While less than exact (e.g., pressure at the compressor outlet / burner inlet is way above atmospheric, so the water 'boils' at more than 100°C) the quotes above at least give some idea of how it works.
The SG is a red herring here; water injection would work pretty nearly just as well if water had the same SG as fuel.
Jet engine thrust is obtained by accelerating a large mass of air through the engine. The fuel provides the energy needed to accelerate the air, but the added mass flow of the burning fuel is negligeable relative to the mass flow of the air.
Unless I'm much mistaken, water injection works in two ways:
- the evaporating water cools down the air at the burner inlets, so more fuel can be injected without burning out the turbine, hence the mass flow of the air can be increased,
- the water also increases the total mass flow (in the example of the KC-135 above by over a ton/minute). But I'll have to get out my sliderule to figure out what percentage of total mass flow we're talking about.
CJ
Basic question if you please. Since I'm not a science type person, the injection of water to increase thrust has always seemed counter-intuitive to me since it would seem to have the effect of lowering the operating temperature of the engine (I've always understood the Law of Thermodynamics as indicating that to increase efficiency one must increase the temperature difference between the source & receiver. Do I have that right?). What causes the extra thrust? Superheated steam?
Without getting into a long mechanical engineering post here, the water is diffused (atomised) into the inlet and the combustion chamber (just forward of the burner cans).
As we all know, water does not burn. But what it does is drasticly increase the air flow density, and add additional oxygen as the water evaporates. This is basicly what gives all water injected engines the additional thrust, increasingly dense air flow and added oxygen.
Since the water is mixed with the air flow, and not contained in a pipe or boiler, it can never become superheated steam. The atom sized droplets boil off and vaporize as soon as they reach 212 degrees F (100 degrees C).
As we all know, water does not burn. But what it does is drasticly increase the air flow density, and add additional oxygen as the water evaporates. This is basicly what gives all water injected engines the additional thrust, increasingly dense air flow and added oxygen.
Since the water is mixed with the air flow, and not contained in a pipe or boiler, it can never become superheated steam. The atom sized droplets boil off and vaporize as soon as they reach 212 degrees F (100 degrees C).
On airplanes like the KC-135A/Q and B-52F/G the J-57 produced around 11,600lbs of dry thrust for a fully cowled engine. The addition of water injection added about 800lbs of additional thrust for a total of about 12,400lbs.
In the KC-135A/Q we used 640 gallons of water during the 2 minutes water injection was used, the B-52 used 1280 gallons, as they had twice as many engines. In the KC-135 the water added 5,600lbs to the weight of the airplane. Water injection could be used, in the KC-135 only, down to 20 degrees F. This water was heated in the water tank, in the KC-135 through the use of 8 5KW heating elements. The water had to be heated to a minumum of 60 degrees F (15.6 degrees C) if the outside temp was 40 degrees F, or less to keep the water from turning into ice. The B-52 did not use water below 40 degrees F.
In the KC-135A/Q we used 640 gallons of water during the 2 minutes water injection was used, the B-52 used 1280 gallons, as they had twice as many engines. In the KC-135 the water added 5,600lbs to the weight of the airplane. Water injection could be used, in the KC-135 only, down to 20 degrees F. This water was heated in the water tank, in the KC-135 through the use of 8 5KW heating elements. The water had to be heated to a minumum of 60 degrees F (15.6 degrees C) if the outside temp was 40 degrees F, or less to keep the water from turning into ice. The B-52 did not use water below 40 degrees F.
Originally Posted by BelArgUSA
What water did really, was to increase accelerated mass flow.
Extra mass of water (specific gravity 1) to jet fuel (SG of .81)...
Result was more power... without increase in EGT, as water does not burn.
Extra mass of water (specific gravity 1) to jet fuel (SG of .81)...
Result was more power... without increase in EGT, as water does not burn.
Jet engine thrust is obtained by accelerating a large mass of air through the engine. The fuel provides the energy needed to accelerate the air, but the added mass flow of the burning fuel is negligeable relative to the mass flow of the air.
Unless I'm much mistaken, water injection works in two ways:
- the evaporating water cools down the air at the burner inlets, so more fuel can be injected without burning out the turbine, hence the mass flow of the air can be increased,
- the water also increases the total mass flow (in the example of the KC-135 above by over a ton/minute). But I'll have to get out my sliderule to figure out what percentage of total mass flow we're talking about.
CJ
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Goeienavond Christiaan -
From an old jet engine book...
xxx
The increase in thrust is due to an increase in mass air flow, due to the inlet air temperature being reduced, plus the addition of the weight (mass) of the water-methanol itself
xxx
Again, I just repeat the "mis-information" of my past instructors...
Sorry, I am as educated as they were...
Happy contrails
From an old jet engine book...
xxx
The increase in thrust is due to an increase in mass air flow, due to the inlet air temperature being reduced, plus the addition of the weight (mass) of the water-methanol itself
xxx
Again, I just repeat the "mis-information" of my past instructors...
Sorry, I am as educated as they were...
Happy contrails
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For info Christiaan - Some numbers from 747 engines...
Sorry , these are not metric units.
The dry JT9D-7F is a 48,000 lbs thrust engine, the -7FW (wet) is 50,000 lbs.
I do not have the mass airflow for the above engine.
xxx
For the JT9D-7Q, a 52,500 lbs thrust engine, it is 1,668 lbs/second.
TSFC is indicated as .378...
Fan thrust is 41,000 lbs, core thrust is 11,500 lbs.
Bypass ratio is 4.9 to 1...
And compression ratio is 24.4...
xxx
Hope it helps for your slide rule
Happy contrails
Sorry , these are not metric units.
The dry JT9D-7F is a 48,000 lbs thrust engine, the -7FW (wet) is 50,000 lbs.
I do not have the mass airflow for the above engine.
xxx
For the JT9D-7Q, a 52,500 lbs thrust engine, it is 1,668 lbs/second.
TSFC is indicated as .378...
Fan thrust is 41,000 lbs, core thrust is 11,500 lbs.
Bypass ratio is 4.9 to 1...
And compression ratio is 24.4...
xxx
Hope it helps for your slide rule
Happy contrails
Dont forget the BAC 1-11 spey , and the Dart on the Andover [military avro 748] for those to young to remember. Cant remember all the theory but didnt the methanol have an effect as well as the water ? Have to dig the old rolls royce book out !!!
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This seems to turn into a "Water Injection" thread, but since we seem to have settled the original question.... why not !
BelArgUSA,
Goeienavond ook van hier.
Your book seems to match what I thought.
Thanks a lot for the figures, that will save me a lot of scratching around.
bvcu,
I think most of the ones you quote didn't smoke quite as spectacularly as the KC-135 or B-52.....
The methanol may have been there more as an anti-freeze? My guess, but I'd be pleased to find out more.
CJ
BelArgUSA,
Goeienavond ook van hier.
Your book seems to match what I thought.
Thanks a lot for the figures, that will save me a lot of scratching around.
bvcu,
I think most of the ones you quote didn't smoke quite as spectacularly as the KC-135 or B-52.....
The methanol may have been there more as an anti-freeze? My guess, but I'd be pleased to find out more.
CJ
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Slightly off topic but similar in a way. Previous life on oil tankers and always wondered why not much was made of loading fuel without taking account of temperature for payment. Individually it probably doesn't make much odds but over a fleet per year must mount up. Oil business very careful about it as when loading 30 000t of gasoil in Venezuela at 30 deg.C and delivering to Boston in winter makes a huge change in volume. Ship calibrated in cubic metres and so dealt with metric tonnes with density at 15 deg. C but of course USA in barrels at 60F (not the same) with an API figure. Remarkably the figures worked out pretty closely.
