Diluting jet fuel with water to increase mass flow.

"toby" wrote in message
...
I looked on the web curious about thrust augmentation concepts for jet
engines and found a 1952? NACA report that detailed an experiment where
water was injected into the combustion chamber to increase mass flow and so
thrust.

However, the water and fuel were injected separately and the researchers
limited themselves to only adding 20% (I think) water because they thought
it would start interfering with combustion stability or something.

Has anyone heard of any research into pre-mixing the water and fuel together
to avoid this problem? Or any other similar research like running jets on
water-alcohol fuels, or water-oil emulsions to achieve the same effect.

Toby.

Water injection is today used on some airliners, particularly where they go to
hot & high airfields such as Johannesburg, South Africa as an aid to takeoff.
An emulsion probably would not ignite in the combustion chamber.
Mike.



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"Henry Spencer" wrote in message
...
In article et,
Carey Sublette wrote:
Not a jet, but the V-2 burned an alcohol-water fuel mixture (IIRC,
one-third water) to improve its thrust.

Not quite right. The alcohol-water mix, yes, but the motive wasn't higher
thrust. The presence of some water was partly to improve the fuel's
coolant properties, and partly to reduce flame temperature. Later
LOX/alcohol rockets, with better chamber cooling, eliminated the water.


Quoting Willy Ley in "Rockets, Missiles, and Space Travel" (1959), p. 451:

"The fuel of the V-2 was ordinary ethyl alcohol - in this case made from
potatoes - to which enough water had been added to bring its strength down
to 75 per cent by volume. The reason for the addition was the following: the
exhaust velocity of a rocket motor is determined, in the main, by combustion
pressure and temperature. But the natue of the exhaust is important too: the
exhause velocity will be higher, if the other factors remain unchanged, for
an exhaust consisting of lighter gas molecules. The combustion productions
of burning ethyl alcohol are CO2 and H2O, and of course the CO2 molecule is
by far the heavier. By adding water to the alcohol the proportion of water
molecules in the exhaust is increased and the average molecular weight
depressed. This addition also decreases combustion temperature, but at t
alesser rate. The mixture ratio which was actually used in the optimal
mixture for this purpose."

Carey Sublette

In article ,
toby wrote:
To answer your question, water injection (either straight de-ionised water
or a DI water/alcohol mix (50/50, IIRC) ) is still used occasionally on some
freight/commercial aircraft to increase thrust on take-off. It is stored in
a separate tank on the aircraft and is injected directly into the combustion
chamber as required.

Any idea whether this combustion chamber injection is fuel rich or is a
lean mixture.

Turbojet/turbofan engines invariably run lean, to keep turbine temperatures
within the bounds of the turbine-blade materials. That's why afterburners
are useful, because there's unused oxygen in the exhaust.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

"Henry Spencer" wrote in message
...
In article ,
toby wrote:
To answer your question, water injection (either straight de-ionised
water
or a DI water/alcohol mix (50/50, IIRC) ) is still used occasionally on
some
freight/commercial aircraft to increase thrust on take-off. It is
stored in
a separate tank on the aircraft and is injected directly into the
combustion
chamber as required.

Any idea whether this combustion chamber injection is fuel rich or is a
lean mixture.

Turbojet/turbofan engines invariably run lean, to keep turbine
temperatures
within the bounds of the turbine-blade materials. That's why afterburners
are useful, because there's unused oxygen in the exhaust.
--

MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |


Sure, this is how I understood it. I just wondered if the added water in ,
what
I now know to be called power boost fluids (thanks Cameron), would allow
one to reach stoichiometric combustion without hitting these temperatures.
As it seems that it is this temperature constraint which prevents
the engine from generating even more thrust.

If we are aiming at maximum thrust regardless of efficiency then we need to
add as much energy as possible. Unfortunately, with conventional fuels this
results in these intolerably high temperatures. The whole point of this
question is
to find out whether it would be possible to utilise all the available oxygen
by
absorbing this excess heat in the creation of super-heated steam from the
water
dilutent. Or, alternatively, whether it would be more effective to absorb
this excess
heat by running very FUEL RICH, with the water dilutent absorbing the extra
heat by
'steam reforming' the unburnt hydrocarbons. The second method might be
better
as it would produce an exhaust with a lower average molecular mass. And
produce
a cool fiery tail!

The paper I found online detailed experiments in injecting water and fuel
into the
combustion chamber separately, but they weren't happy using more than 20%
water,
as they felt it would mess up the combustion process.This is why I asked
whether
anyone had tried to get around this problem by using fuels with very high
water
contents, and that were effectively pre-mixed. The answer was, yes:
power-boost
fluids. I want to know if this had been taken to the extreme, and whether
mixtures which
allowed complete use of all the oxygen had been tried.

We know from watching brandy burning on a christmas pudding that even
fairly dilute
alcohol will combust in air without too much problem, so it seems that there
is considerable
scope to use mixtures like power-boost-fluids, or maybe ones even more
dilute, as
fuels in their own right. The point being that if you can double or triple
the T/W of a
jet engine it might be worth taking the efficiency losses for space apps
(Glen Olsons
Pogo and other vertical takeoff concepts in particular) because the
reduction in burnout
mass might pay for the lower isp.

Toby

In article et,
Carey Sublette wrote:
...The presence of some water was partly to improve the fuel's
coolant properties, and partly to reduce flame temperature...

Quoting Willy Ley in "Rockets, Missiles, and Space Travel" (1959), p. 451:
"The fuel of the V-2 was ordinary ethyl alcohol - in this case made from
potatoes - to which enough water had been added to bring its strength down
to 75 per cent by volume. ... The combustion productions
of burning ethyl alcohol are CO2 and H2O, and of course the CO2 molecule is
by far the heavier. By adding water to the alcohol the proportion of water
molecules in the exhaust is increased and the average molecular weight
depressed..."

Unfortunately, adding water to alcohol *reduces* performance, although not
quite as much as you might think. Molecular weight is actually a complete
red herring, the result of misunderstanding a performance equation(*).

(* People see an equation with "temperature/molecularweight" and fail to
realize that those two values are not independent in a chemical rocket.
With minor simplifying assumptions, temperature/molecularweight is easily
shown to be simply combustion energy release per unit mass. For example,
Hill&Peterson's "Mechanics and Thermodynamics of Propulsion", 2nd ed,
shows this. But energy/mass *cannot* be increased by adding non-reacting
mass. The more subtle issue, which for example makes it better to run
most fuel combinations fuel-rich, is nozzle efficiency as affected by gas
properties. People *think* molecular weight matters because both the
energy/mass and the gas properties are generally better for lighter and
simpler molecules, which also tend to have lower molecular weight. See
the "Performance" chapter of Clark's "Ignition!".)

Earlier in the same book, Ley's account of how the VfR first came up with
the alcohol-water mix mentions a vague (and fallacious) notion of higher
performance due to reduced LOX requirements (Ley's idea), but also the
ability to add cooling water to the fuel (Riedel's idea). The VfR engines
tended to burn out except when they had running water for cooling, and a
fuel that could have water mixed into it was a lot simpler than a separate
water supply. Riedel ran some experiments and determined that about 60%
alcohol seemed best. ("Rockets, Missiles, and Men in Space" -- R,M&ST was
an earlier edition of the same book -- pages 181-2.)

The Army project that followed on from the VfR's efforts used the same
general fuel (with a bit less water) simply because it had proven
workable, at a time when making a liquid rocket engine work at all was
quite a challenge. And the V-2 inherited that decision.

Clark's "Ignition!" comments (pages 8-9):

"Late in 1931 Klaus Riedel of the VfR designed a motor for a new
combination, and it was fired early in 1932. It used liquid oxygen, as
usual, but the fuel, conceived by Riedel and Willy Ley, was a 60-40
mixture of ethyl alcohol and water. The performance was somewhat below
that of gasoline, but the flame temperature was much lower, cooling was
simpler, and the hardware lasted longer. This was the VfR's major
contribution to propellant technology, leading in a straight line to the
A-4 (or V-2)..."

(Interestingly, Clark notes that Reaction Motors in the US later got
successful operation using LOX/gasoline/water, with the water mixed into
the gasoline during injection.)
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

"toby" :

Sure, this is how I understood it. I just wondered if the added water in ,
what
I now know to be called power boost fluids (thanks Cameron), would allow
one to reach stoichiometric combustion without hitting these temperatures.
As it seems that it is this temperature constraint which prevents
the engine from generating even more thrust.

If we are aiming at maximum thrust regardless of efficiency then we need to
add as much energy as possible. Unfortunately, with conventional fuels this
results in these intolerably high temperatures. The whole point of this
question is
to find out whether it would be possible to utilise all the available
oxygen
by
absorbing this excess heat in the creation of super-heated steam from the
water
dilutent. Or, alternatively, whether it would be more effective to absorb
this excess
heat by running very FUEL RICH, with the water dilutent absorbing the
extra
heat by
'steam reforming' the unburnt hydrocarbons. The second method might be
better
as it would produce an exhaust with a lower average molecular mass. And
produce
a cool fiery tail!

Now that I get the idea about what you are looking for may I suggest that you
also read the logs for the last year's work on Armadillo Aerospace's website.
There water has been added to a monopropellant (H2O2 + Alcohol) to:

a) Lower the burn temperature to enable to use of COTS material to built the
engines with.

b) To remove the explosive character of an high energy monopropellant.

c) To make manual handling easyier/safer.

I don't know if your idea of steam reforming from the extra water matters in
a H2O2 design but it does help in understanding why H2O2 design are as good
as they are.

Earl Colby Pottinger

PS. http://www.armadilloaerospace.com/

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