Re: MHD Turbines

From: [email removed] (nosnhoJ enaD)
Subject: MHD Turbines...
>Okay, somebody more in the know than I needs to explain this to me:
>
>An MHD Turbine, used in most of the Grav vehicles for power AND thrust,
>seems to me to be the following device:  a torus of water, accelerated by
>a magnetic field, which is then pushed through a turbine to generate
>electricity.

Err, no, that's a watercraft MHD drive.  Let me back off a bit and
explain from the theoretical side first...

MHD is merely the study of interactions between conductive fluids
(gases, plasma also) and electrical and magnetic fields.
The primary rule of MHD is that when a conductive fluid has
an electrical field passing through it in one direction,
and a magentic field perpendicular to that, it is accellerated
perpindicularly to _both_ of those fields... or, if a moving
conductive fluid is subjected to a magnetic or electrical
field it will establish the other field perpendicularly to
the existing one... 8-)

That's the basic stuff.  It gets more complex (I know physics
grad students who work in the field exclusively) and more painful
to work with (the typical reaction to someone describing
advanced MHD effects in a physics conference is that everyone
"smiles and nods" without understanding it at all ;-).

A MHD engine is one that takes a working fluid (gas, plasma...)
and subjects it to an accellerating field of perpendicular electric
and magnetic fields.  For a water vehicle, you use seawater (which
is highly conductive).  For space vehicles, you have to bring along
fuel of some sort and ionize it (the usual process is to have a
low powerion engine, say using xenon, feeding the MHD engine...)
and then runs it through the paired fields to accellerate it further.

An MHD _turbine_ does the opposite; instead of taking electrical
power and accellerating something, it takes something that's moving
and generates electrical power.  For example, the exhaust of a jet
engine is a hot, partially ionized, fast moving gas.  To generate
electricity from it, you can run the exhaust through a magnetic
field and put electrodes at the sides perpendicular to the magnetic
field... you get a current.

MHD turbines are exceptionally effecient ways of generating power,
but have the problem that they erode electrodes very quickly.
Work today is slowly proceeding on making them more effecient that
way, but it's slow going.  But you can make them today, if you're
willing to swap electrodes every few hours ;-)

- -george william herbert