Anyone played around with a magnetohydrodynamic drive?

Discussion in 'Propulsion' started by jim lee, Aug 27, 2012.

  1. jim lee
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    jim lee Senior Member

  2. PAR
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    PAR Yacht Designer/Builder

    Oh yeah, play with it all the time . . .

    Once you look at the energy requirements for this type of drive, you quickly realize why it's consider impractical.
     
  3. jehardiman
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    jehardiman Senior Member

    Yep, it's not the MHD that's hard, it's the flux capacitor rated to 1.21 gigawatts of electricity.
     
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  4. gonzo
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    gonzo Senior Member

    I think they were designed with a nuclear reactor in mind. That is about the only way to produce enough power.
     
  5. jim lee
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    jim lee Senior Member

    I wonder if they are horribly inefficient at low power? Or just inefficient all around so you need power to burn?

    -jim lee
     
  6. jehardiman
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    jehardiman Senior Member

    It's a field issue. They are fairly efficient at low field strengths, but then you need a huge field area...Rasing the field strength lowers efficiency due internal losses (including supercooling). In some respects they are just like any momentum disc...best efficiency is infinitely small acceleration over an infinitely large disc. IIRC, the Japanese tried to double the field strength but never got the platform over 10 knots with SCC windings.
     
  7. jim lee
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    jim lee Senior Member

    Where did you come up with this? And, what is a "momentum disc." I'm obviously missing some background info. here.

    -jim lee
     
  8. jehardiman
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    jehardiman Senior Member

    Do you understand how "normal" props, oars, paddlewheels work?
     
  9. jim lee
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    jim lee Senior Member

    I get the rotating wing idea for a prop. Paddles just seemed like pushing against water resistance.

    -jim lee
     
  10. Tim B
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    Tim B Senior Member

    Jim,

    A spinning propeller can be thought of in fluid dynamic terms as a volume within which a change in the fluid's momentum happens. More simply, think of an aircraft propeller. The blade tips describe a circle (edge of the disc) every rotation, and behind this "disc", the airflow is faster than it is in front. Since momentum = mass * velocity, we consider the propeller to cause a change in momentum, which also (rather conveniently) includes pressure terms, density terms and everything else required for the continuity equations. In "simple" theory (still degree-level) the disc is thought of as a thin entity. In CFD, you think of it as a volume, because it needs to have cells associated with it.

    Since we know that F=MA, this leads us to the conclusion that if dMom=Mass*dV then to change momentum both requires and produces a force (Newton's 3rd Law). The useful upshot of which is thrust.

    Consequently, all that is needed to produce thrust is a change in the fluid's momentum. And there are many ways this can be done. The problem with MHD, though, is the amount of power required.

    I think any development of this technology for ships will either be as a result of ion-drive type technology for spacecraft, or from commercial shipping. I don't see any military putting much resource into this due to it's basically anti-stealth properties.

    Hope this helps,

    Tim B.
     
  11. jim lee
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    jim lee Senior Member

    So a "momentum disk" is a mathematical construct for modeling thrust calculations?

    "The problem with MHD, though, is the amount of power required."

    This it a the bit I keep wondering about. What is it that makes this so inefficient?

    -jim lee
     
  12. Tim B
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    Tim B Senior Member

    Yep. It's just a way of reducing the problem to something that can be solved more easily. It's applicability depends on what you're trying to do, of course.

    The root of the problem with MHD (and this is not an area I know much about) is probably the required field strength, and the size (and weight) of the field generator, given that the field becomes weaker with distance and sea-water is only partially conductive (it's the impurities which conduct), while these can be accelerated, they are a relatively small fraction of the water in terms of density, and any viscous coupling to the surrounding fluid will be quite limited.

    Tim B.
     
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  13. jim lee
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    jim lee Senior Member

    Well, what I really like about them is that, at least in theory, they're so bloody simple. You can just about rip the shaft out of a car starter motor and have something to test with. As soon as I get some more free time, I think I'll try some experiments and see why no one uses them.

    -jim lee
     
  14. jehardiman
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    jehardiman Senior Member

    Yep, the field strength decreases with the distance squared from the center of the generator coil. It becomes a diminishing return if you try to increase field strength by increasing the coil size, i.e. you can't pack a coil that tight with present technology even using super cooling. So because you have small increases in momentum, you need a large momentum disc (i.e. field effected area) , but the large effector area requires a huge field, which in turn requires huge coils, which means large losses.
     

  15. jim lee
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    jim lee Senior Member

    I guess that's why the Japanese one used multiple small tubes for its drive.

    -jim lee
     
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