Effect of water's expansion on combustion pressure

[Dust]

It's been said that water, when it phase shifts, expands to 1800? times it's previous size. I have seen some mention that this increases combustion pressure. Last week, I was told by a member of another board that the psi increase is negligible. Not being an ME, I don't know what it would take to work out the formula. But lets say at 15% of fuel flow, what would water actually do to the overall pressure inside a combustion chamber?

[Richard L]

When liquid water changes to vapor, the volume increase is over a thousand fold. Once it is in vapor form, it obeys the Boyles Law like any other gas. As temperature continues to increase so does the volume.

pV=mRT

p= observed pressure
V= total volume
m= mass
R= gas constant (gas specific)

If phase change of water makes little pressure difference, the steam engine will never work. The volume of the combustion chamber varies with crank angle, the pressure increase will vary with temperature, volume and position of the piston.

It is thermodynamic topic, a separate study of of Mechanical Engineering. Once you have familiar with the first and second law of thermodynamics, you can then apply it to the engine dynamics (Mechanical engineering).

Here is a good link to make the above clearer.
http://www.ohio.edu/mechanical/thermo/Intro/Chapt.1_6/Chapter2a.html

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[Richard L]

Earlier, I posted the following chart on another thread, represents the total internal energy of water. At 370 C, water comes a gas (single molecule).

I will try to explain this in more details: Below 370C, water exists as vapor. Vapor is of just cluster of molecules stack against each other. The gaps between the molecules increase with temperature, causing overall volume increase.

The study of gradual phase change phenomenon is called enthalpy. Simple put, water molecules are held together by an internal energy called Van der waal force. This force weakens when external energy is applied, the gag between each molecule increases and eventually reach total separation, the single molecule become gas. At this time, the Boyles law applies.


You can look this up on the steam table or enthalpy property of water in any reference book.

http://www.aquamist.co.uk/forum/WPT.gif


Since pv = constant. Keeping pressure constant means change of volume is proportional to temperature change. You can invert the above chart and you get your answer there.