Just speaking theoreticly, could you realize some thrust benefit by
injecting a fine spray of water into the exhaust which would turn to steam
and increase thrust? Of course, you'd have to refill your huge water tank
just like a steam locamotive, so it wouldn't be practical. Just wondering.
Brian Trubee
I think so. If it works for a combined cycle gas turbine it should work
for jet exhaust. Both devices work on similar principles.
Paul Lamar
I definitly DO NOT believe so. The operating theory and cycle of a steam
bottoming cycle is worlds away from just dumping water into the exhaust.
Efficiency in any thermal cycle consists of bringing the working fluid up
to a higher pressure, then heating it (possibly through a phase change),
before expanding it for useful work. That is a long way from just
squirting it in by the turbine. In a gas turbine there is limited gain
from injecting water to act as sort of an intercooler in the compressor,
just as there is some gain in positive displacement engines (piston/rotor)
as anti detonation fluid. I am afraid that most of these schemes belong
in a J C Witney catalog along with the magnets on a fuel line.
USEFUL expansion is the magic bullet for improved efficiency.
Mechanically varying compression ratio as an inverse function of
load has been done - heavy, complicated, expensive.
Turbocompounding works well AT DESIGN POINT altitude, near full
power etc.
The most efficient thermal engine ever built is the Space Shuttle Main
Engine
actually in the 90 percent range at altitude E
XPANSION! - Vance.
I don't thing Brian was advocating squirting it into the turbine Vance. Did you get
my messages on the Merlin exhaust stack thrust? I think he was talking
about squirting it into the exhaust expanding it into steam and
adding to the mass flow for the jet effect. Hopefully it would not affect the
overall temp to significantly reduce the kinetic energy. No turbine involved.
I agree with you if you squirt plain water upstream of the turbine that will
cool the gas defeating the purpose. Making steam with the left over exhaust
heat and then squirting the steam back into the gas turbine does indeed work.
I have lots of papers on the subject from the ASME Journal of Engineering
For Gas Turbines and Power. Don't make me scan them in and upload them :)
BTW what struck me about NACA Report 786 was the scaling of the turbo
compound concept. The R3350 TC at 2800 HP got the same percentage
of HP recovery that an R1350 550 HP engine got. Namely 21% at high altitude
and 9% at sea level. Apparently they both had the same ratio of kinetic energy
to heat energy in the exhaust. Surely the scaling will hold true down to 200 HP.
In NACA Report 765 the rule of thumb for kinetic mass flow contained in the exhaust
appears to be .002 pounds per second per main engine HP. So the 200 HP rotary would have
.4 pound per second mass flow in the form of kinetic energy to work with.
That is 24 pounds per minute. That seems to be right in the ball park for a
Garrett GT35R turbine which is similar in size to a TO4 which I don't have
a turbine map for yet.
Paul Lamar
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