I realized I wasn't fully clear about how water injection worked exactly, so I swiped this question/answer session from a discussion on !!!!!!!!!!!!!!
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).
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.
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.
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.
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