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A thermodynamic/aerodynamic doubt.

  1. Oct 2, 2007 #1
    A thermodynamic/aerodynamic doubt.........

    Hi peeps...........
    Physics forums rock..........

    k now. Before asking my doubt I need to ask small small doubts before the big one........
    So I guess this question will lead to a hot discussion eventually............

    Doubt number 1
    Is it legal to say that 1 litre of h2o in liquid state has a particular number of loosely arranged/bonded molecules and the same quantity when frozen to solid state has the same number of molecules but compactly arranged/bonded ?

    This doubt arose because,
    I read from my book that hot air is less dense and cold air is more denser, and so water methanol spraying technique is used to augment the thrust produced by a jet engine............

    My next doubt depends on your answers.........
    So keep em coming and I'll just wait..................
  2. jcsd
  3. Oct 2, 2007 #2

    D H

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    Most things expand as their temperature increases. Most things expand when transitioning from solid to liquid or liquid to gas.

    Water violates these general observations. Ice floats. Think about that for a second.

    Liquid water is densest at 4oC. Liquid water's density decreases below 4oC. This leads to interesting behaviors in lakes in the Northern Hemisphere at this time of year. There are multiple forms of solid water. The one we are most familiar with is less dense than liquid water.

    Now to answer your particular question: It is tautologically true. We measure quantity by mass or mole number. One mole of water contains 6.022e23 H2O molecules, regardless of the water's state of matter.
  4. Oct 2, 2007 #3


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    Not sure why this is in the ME/AE area, but anyway.

    It turns out the crystalline structure of frozen water is larger than free-floating liquid water molecules. Hence, 1 mol of frozen water displaces more volume than 1 mol of liquid water; subsequently it floats.

  5. Oct 2, 2007 #4


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    Thrust augmentation due to water ingestion is a technique that works pretty well but requires a lot of water that has to be deionized and as pure as possible. The idea behind it is get a lower compressor inlet temperature due to the evaporative cooling. Temperature drops, density goes up, mass flow goes up and thrust increases. Of course, there are always drawbacks and tradeoffs.
  6. Oct 2, 2007 #5


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    I missed the water/methanol injection part...

    While I have far less experience than fred in the Aerospace field, water/methanol injection is also used in the automotive industry for high performance aftermarket applications.

    My take on the advantages of W/MI in the automotive field-

    1) Injecting the W/M mixture helps cool an intake charge before compression in the cylinder (specifically after a compressor such as a supercharger or turbocharger). It acts similarly to an intercooler.

    2) While the water does not add anything to the combustion process, the methanol burns at a much lower temperature than the fuel, helping lower exhaust gas tempertures while adding power.

    3) Beacuse the water does not "react" in the combustion process, it is available to absorb heat during the process, further lowering EGT's. This primarily has to do with the fact that water's specific heat capacity is much higher than air, so it takes more energy to raise water's temperature by a certain amount than it would air. Also, if water droplets go through a phase change to steam, this will also absorb a significant amount of energy and aid in cooling the exhaust.
  7. Oct 3, 2007 #6
    ok assuming that you guys agree that it is legal to say so, here comes my next doubt..........

    If I say that h2o in solid state (ice in this case) has dead molecules, i.e molecules without any energy, is it a right statement or am I fantasizing?

    I know that it is the property of h2o (any liquid) to freeze at a particular temperature......
    But I needed a physical explanation so I came up with this hypothesis......
    I keep thinking that ice has 0 molecular activity, and so every molecule sits tightly one over the other without any motion, unlike water in a temperature say 50 degrees Celsius has molecules which have energies proportional to the temperature, and hence are in continuous random motion.......

    Your answers will eventually lead to my space shuttle doubt, all part of my mission to understand an aerodynamic principle...........
  8. Oct 3, 2007 #7


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    Now that is not true. All atoms have energy, unless they are at absolute zero (which is impossible, you can only get close). While they have less energy than a molecule in liquid water, they still have energy nonetheless.

    There's really no need to come up with a hypothesis of why this happens. You should instead read a book that explains it.

    The molecules in ice still have molecular energy that is proportional to it's temperature. As a point of fact, the molecules in the ice are theoretically moving, just not very much.

    Just quit beating around the bush, what's your REAL question???
  9. Oct 3, 2007 #8
    Dude I am just trying to learn my way, and I don't think I am hurting anyone out here
    I am just trying to be creative in thinking so I came up with that so called hypothesis.....
    And can you suggest me any book which explains this? cause I have none.......
    So the molecules in ice are theoretically moving, I love practicals more than theory, so can you please tell me what happens practically in ice......
    I'll definitely ask my question dude. You think I am wasting my time sitting and posting?
    I need to get somethings straight before asking my doubt......
  10. Oct 3, 2007 #9


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    Unfortunately, the answer is far less simple than you might hope. The molecules do have energy and vibrate, but once they form a hydrogen bond with a fixed crystal they don't move per say... All I'm saying is that we don't have to re-invent the wheel here, and the fact that you're so interested in crystalline ice structures with respect to the aerodynamics of the Space Shuttle makes me think you're going in an entirely wrong direction.

    Wikipedia article on Ice

    Wikipedia article on Hydrogen bonds

    Wikipedia article on Crystalline Structures

    Wikipedia article on Hexagonal Crystal Structures
    Last edited: Oct 3, 2007
  11. Oct 3, 2007 #10

    Woah when did I say that I am interested in that?

    And thanks a tonne for those websites.........

    So now my next set of doubt are..........

    What is compressibility?
    I know that compressibility is the change in density with respect to change in speed......
    When you say change in density, is it simply a change or a positive change or a negative change?

    I imagine that as speed of airflow increases (as in a space shuttle re-entry) the molecular activity (temperature) increases and so the density decreases..... Am I right?
    Is this the reason why supersonic/hypersonic flows over airfoils are called compressible flows?
  12. Oct 3, 2007 #11


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    Actually, you sort of did:

    Whatever, it doesn't matter... My inkling seems to have been right though, since you are essentially trying to tie fluid properties of water to supersonic air flow around the Space Shuttle. All of the talk we have had thus far about how ice forms and water/methanol injection is essentially useless :rolleyes:

    You know, you could have just as easily found the same pages by doing a quick search on Google or Wikipedia...

    Now you're really asking for more than you think... all you ever wanted to know by typing in "space shuttle" in Wikipedia.


    Summary of the Space Shuttle:

    Basics of Atmospheric Reentry (pay special attention to the Shock Layer Gas Physics section)

    Shock Waves:

    Supersonic Aerodynamics:
    Last edited: Oct 3, 2007
  13. Oct 4, 2007 #12
    Just an FYI.... water does add a lot more than you're thinking.

    When water turns to steam during the combustion process it's expansion rate is far greater than that of the fuel. Not only will this equate to more power being made but also cleaner emissions due to not needing as much fuel per power stroke ( for power to be made).

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