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RE: 'Glow' discharge
> >One experiment I want to do is related to MHD power generation.
> I'd like to
> >be able to ionise the air moving along a non-conductive pipe, whilst also
> >causing a hefty current to flow across it (through the gas). The
> electrodes
> >need to be longitudinal with respect to the pipe, and maybe 1 - 2" apart
> and
> >parallel. If the theory works, it should be possible to blow gas through
> the
> >pipe and extract power from a coil wound around the pipe.
>
> Usualy a magnetic field (from magnets, >10Kgauss) is used and voltage is
> generated at the electrodes; you probably know this though. I don't quite
> get your plan.
The physics would seem to indicate that you can either go with a magnetic
field and pull the power out of the electrodes, or go the opposite way
around and pull the power out through the coil. Maybe I'm just being naive,
but as you'd need to ionise the gas anyway, it would make more sense to pump
lots of voltage into the gas which would both deal with the requirement for
ionisation and also the requirement to pass a heavy current through the gas.
Again, I may be crazy, but this approach should reduce losses, because the
kinetic energy of the expansion in the gas caused by the heating associated
with the ionisation will (to an extent) be picked up by the coil, just as
the kinetic energy of the gas flow itself will be.
> Low resistivity, or highly conductive plasma for low loss, like you will
> want, will take high currents. At atmospheric pressures the discharge will
> kink into filamentary arcs. So you will have to operate in short <100uS
> pulses or continous RF.
What about high temperature gas (such as what comes out of a propane burner,
say, at maybe 500 - 1000C and probably already ionised to a certain extent)
moving at high velocity? What if the electrodes are arranged as a 180 degree
out of phase double helix (fairly open, so the 'wavelength' is longer than
the diameter of the helix), so that the gas doesn't just blow along the axis
of the electrodes? It would probably be necessary to use tungsten for the
electrodes and maybe some kind of ceramic or glass for the tube (maybe
Teflon?) to cope with those temperatures. But if it works, it would be
possible to build a very compact, efficient electrical generator with no
moving parts.
Sarah