humid is better in turbines

[Roger Heflin]

KasaRyan at aol.com wrote:
> 
> Whoo Whee! opened up a can o worms on this one!
> 
> Ok, one of the reasons I put in the explaination I did was that I knew that
> some early jet aircraft, the 707 among them, would dump hundreds of gallons
> of water into the combustion chambers of the engines on takeoff.  This
> probably lowered the temp slightly, but more importantly dramatically
> increased the volume of what was coming out the back of the engine, and also
> pushing on the turbine to compress more air.  The more force you chuck out
> the back, the more the plane goes forward!  Sir Issac had that one down, as
> far as Harry ricardo goes, I dont have his book on combustion chamber design,
> but I do believe he plugged away at it for a while.
> 
> Roger, you seem to have the thermo book handy, so I would like this
> calculated.  If you have one gram(or cc) of water at 100C, and you add enough
> energy to vaporize it into gas, how much energy does this take, and at 1 ATM,
> how much volume does it take up once vaporization is complete?  If you take 1
> cc of air at 100C, and you add the same amount of energy, is the resulting
> volume increase more or less than for the water which goes thru the state
> change.  (IS this PV=nRT? or something of that sort)
> 
> I believe I was wrong on the water being in a liquid form on a humid day, but
> what if you use injection, and have liquid water in the combustion chamber.
> 
> If this goes on too long, i will probably have to dust off the books and look
> it up myself.
> 
You might need to, I won't be able to run the numbers until tonight.

I believe you have 2 test cases, all closed systems, I don't believe
humidity enterns in to it as water vapor acts as an ideal gas.

	case #1: 1 mole an ideal gas at 50C with xx joules then 
		added to it what is the ending pressure.
	case #2: 1 mole (minus a small amount for the water) of an ideal
		gas and 1 mole of water (18ml) at 50C with xx joules added
		to it, what is the ending pressure?

I suspect strongly case #2 (same physical volume starting) will produce
a higher ending pressure, this requires that you get the water into the
combustion chamer as a liquid, and then add enough energy to convert it 
to an ideal gas.    If it goes inth the combustion chamber as a gas, it 
will act pretty much like an ideal gas, and no gain is made, and also it
would have displaced some oxygen coming in so reduced the amount of energy
that you could add to things.   I am pretty sure the Harrier jets do
this trick also to increase thrust, though only for short periods of time,
I suspect that the water is weight inefficient so you don't want to use
it for long times (only use to hover (harrier) and takeoff and other
emergencies).

				Roger
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