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I am having trouble understanding a steam turbine

  1. Aug 10, 2010 #1
    I know the basic theory to how a steam turbine works. but I have a gap in my understanding of how the thermal dynamics work.

    I'll outline a scenario and detail the bit I don't understand.

    simple steam turbine

    1/ heat source
    2/ boiler
    3/ steam jet
    4/ impeller

    The heat source heats the water in the boiler.
    Pressure builds in the boiler
    The heated water exits the jet, as the pressure drops the exiting heated water turns to steam.
    The steam strikes the impeller which spins.
    the spinning impeller is used to do work.

    My problem is where does the energy to do the work come from?

    The steam strikes the impeller at speed thus as the steam strikes the impeller some of the velocity is transferred to the impeller.

    My question is what happens to the heat energy?

    When the water exits the jet and changes to steam does the steam lose thermal energy which is converted to kinetic energy?

    When the steam strikes the impeller does it lose further heat energy or is only the kinetic energy transferred?

    How does one calculate the different thermal energies at different points of the process?

    That last one I know would be rather complex, I would just like a general idea it doesn't have to be accurate.

    Please ignore thermal losses from thermal conductivity and radiation assume everything is made from a super insulator of some sort.

  2. jcsd
  3. Aug 10, 2010 #2
    Adding heat to water (or pretty much anything) makes it's molecules move around faster -- heat increases their kinetic energy (in fact heat _is_ kinetic energy in a very basic sense) . If they go fast enough they "boil" off of the liquid and become a gas -- this is called a phase-change. The gas takes up a lot more space than the same amount of liquid, I think I remember about 1700x as much space, and so the pressure above the boiling liquid increases dramatically. If you allow this pressure to vent, in your jet for instance, it can push something -- the active bouncing molecules of water hit your impeller blade and transfer their kinetic energy to it. In doing so they lose their new-found energy and condense back to good old liquid water. And the cycle can repeat.

    So the work comes from the energy of the burning fuel you put under the boiler. I just answered (or made an attempt thereof) a question about fire, so scroll around this list to get an idea of what I think explains where the energy actually comes from...

    For calculating the energies, I'd first try the wiki article on Steam Engines and then jump over to the Thermal Efficiency link. Hopefully that will lead you to more specific information.
  4. Aug 17, 2010 #3
    Sorry for the delayed response.

    Thanks for that.

    I was under the misunderstanding that heat was a vectorless vibration, rather than a vector force. And thus the force derived from steam was from the expansion from the fluid state into the gas state (phase change). I had assumed the water molecules that made up steam still contained the same energy level and therefore vibrated at the same level as when heated and in the fluid state.

    From what you say the molecules actually have a vector, thus the vibration described in other texts is actually the apparently random movement of molecules as they interact with other molecules, in the gas state this would be them literally bouncing off each other.

  5. Aug 18, 2010 #4
    I think heat, as a macroscopic average, is vectorless. But the kinetic energy of each molecule (that is averaged) is a vector quantity. They bounce off each other, the walls of the container, the plunger or whatever of the pressure sensor, and the blades of your impeller.

    You can go with the macroscopic description, "expansion from the fluid state into the gas state", or poke deeper to expand on what "expansion" means. The molecules in steam have to have a certain minimum kinetic energy to escape the liquid state, but there is a large range of actual energies. For a fun thought experiment read up on "Maxwell's Demon"...
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