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Posted by dow on September 21, 2010, 3:24 pm

No liquid is truly incompressible, *especially* water.

If you fill a vessel completely with water, seal it closed, then heat
it past 374C, then (assuming the vessel doesn't explode) the water
will imperceptibly change into the single fluid. If the vessel is
transparent, you won't see anything happen. But, of course, the water
molecules are still just as close together as they were in the liquid,
and the material will still be fairly incompressible. Even at much
higher temperatures, if the volume is held constant, the material will
still be fairly incompressible, not at all like an ideal gas. Only if
you let the volume increase until the molecules are far apart will the
material resemble an ideal gas.

It's possible to go from liquid water at room temperature to water-
vapour at the same temperature without passing through any change of
state. Heat the water, at constant volume, until it is above its
critical temperature. Then, keeping the temperature above critical,
allow the material to expand until its density is as low as the
density of water vapour at room temperature.. Then cool it to room
temperature. At no point in the process will two phases be present.


Posted by daestrom on September 21, 2010, 11:21 pm
On 9/21/2010 11:24 AM, dow wrote:

The container will burst or leak while you are heating it.  The thermal
expansion of water, versus the compressibility, would have the pressure
rising many hundreds of atmospheres while you heat the water.

Any moving parts that are trying to 'compress' water in the liquid phase
(deliberately or accidentally) will most likely break. (many a
piston-cylinder steam engine have been destroyed because of liquid in
the cylinder).

But, of course, the water

I don't think that is the path you could use to demonstrate this.  While
heating the water up to the critical point, you can't maintain a
constant volume.  As I said above, heating the water causes it to expand
(even without a phase change) and trying to maintain a constant volume
during this stage is virtually impossible. (or at least you would need
pressures far in excess of many *thousands* of atmospheres to maintain
the liquid volume constant).

Now, if you first pressurize it and keep the pressure above the critical
pressure while heating it, so the fluid stays 'liquid' until you've
exceeded the critical temperature but still allow it to expand, you only
need a pressure vessel of >3206 psia.  You can allow the water to expand
while heating but maintain it in the liquid phase.

And then allow the fluid to expand further while super-critical and then
coordinate the pressure-temperature reduction so as to stay on the vapor
side, you can move from 'liquid' phase to 'vapor' phase with no period
of having both phases present at the same time.

But what would be the purpose in that?

My point was simply that if you take super-critical water and continue
heating it, it begins to act more and more like an ideal gas (linear
PV=kT).  If Morris wants to stay in the 'non-linear' area of water
because he's trying to use it in a heat engine, then he's near the
saturation line and that means he could end up on the 'liquid' side at
some point and piston-cylinder equipment doesn't like suddenly finding
water in the way of where the piston wants to go (bang, snap, clunk,


Posted by Morris Dovey on September 21, 2010, 11:59 pm
 On 9/21/2010 6:21 PM, daestrom wrote:

I'm still struggling to wrap my head around this thing, but I think I
want to stay just below the critical temperature.

For the engine I'm working on, the piston shouldn't find water in the
way - because the piston /is/ water.

For the next phase, there will be a free (no mechanical linkage) piston
- but I have a lot to learn from the fluid piston engine before
venturing on to that.

Morris Dovey

Posted by dow on September 22, 2010, 2:24 pm
You are obviously imagining trying to keep super-critical water at
constant volume on a large scale - litres at least - which would
certainly be difficult. But on small scales, using the kinds of
laboratory equipment that are used to investigate high-pressure phases
of various materials, including water-ice, achieving a few thousand,
or even tens of thousands, of atmospheres is routine.

The pressure at the bottoms of the deepest places in the ocean is over
1000 atm., and the water is significantly compressed down there.


Posted by daestrom on September 22, 2010, 9:39 pm
 On 9/22/2010 10:24 AM, dow wrote:


Well, yeah.  That did seem to be the point of Morris' discussion.  I
suppose a laboratory experiment where the surface area is very small,
such pressures could be contained.


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