Re: Water injection ?

From: (Simon Craig)
Organization: OzEmail Pty Ltd - Australia
Date:         31 Oct 95 00:31:29 
References:   1
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> We were discussing about how water as material itself could increase
> the quantity of boost generated by the engines in first place, and thus
> increasing the overall power. Any possibility this theory could make
> it ?
> Thanks for listening!
> Gernot

Water was used in the past to basically increase the mass flow through the
engine.  In places of high ambient temperature (the tropics, etc) engine
power is significantly decreased due to temperature, and so water
injection was used to boost jet engine power back up to or above what it
would be in a colder climate.  The 707 and the 747 with earlier JT9s used
water injection quite frequently, but as engines became more powerful,
water injection was no longer necessary.

Water was sprayed (in the case of the JT9) though the fuel manifold (a
seperate pipe to the fuel though!) so as to increase the mass of flow
through the turbine, thus a pressure and temperature drop across the
turbine is reduced, increasing the jet pipe pressure, and thrust output.
Also, the water drops the turbine inlet temperature, meaning more fuel can
be scheduled without exceeding the metal specifications of the turbine
(melting...).  More fuel means more rpm, and the fast the engine spins,
again the more thrust is produced.  It worked quite well, however, the
tanks that held the demineralised water, and the pumps, which were very
high flow, added up to a significant amount of weight.  So, there were
disadvantages also.