MHD Turbines...

A thread of 2 messages, 7 July 1993 to 8 July 1993, in the order they were sent.

#5730 MHD Turbines...

nosnhoJ enaD · 7 July 1993

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.

This is dumb.  It means we have electricity to begin with, but we want to
push it through this other device before we use it...Also, we don't get
any thrust from this, as far as I can see....

Option 2:  We're burning Hydrogen as a reactant and using the heat/whatever
to power a turbine, which pushes the water from which electricity is
extracted with an MHD setup.

This is also dumb.  Why not just pull the power off the turbine directly?
Plus, the Hydrogen needs a reactant which we also need to lug along, or it
won't work in anything but atmospheres with a reactant in suitable quantities.

Option 3:  The Hydrogen Exhaust (Water, if we're using Oxygen as our
reactant) is accelerated (?) by an MHD (or has power pulled off it, which
means it's *deccelerated*) and then pushed through a turbine.

This is also stupid.  Why use both for generating electricity?  At TL 8,
the turbine is more effecient than the MHD, although we could probably
fudge this with "hi-tech" superconductors, or whatever :)

Option 4:  We've got a turbine precompressing the atmosphere before pushing
it through the MHD (a la modern turbo-jets) <assuming that we're using the
atmosphere as the reactant...if not, no precompressing is necessary>.

This is dumb, 'cause it won't fly in a vacuum.  And really, what's the use
of a grav vehicle that can't be used in a vacuum (or any non-compatible
atmosphere, really).

So, basically, I can see no rationale for something called an MHD turbine
as either a good powerplant or a source of thrust.  Am I wrong?

Dane

[email removed]
[email removed]
TNS Stringer ------------ Terra/Solomani Rim (1827 G867975-8)
"I'm not a real Scientist, but I play one in an RPG."

#5739 Re: MHD Turbines

George W Herbert · 8 July 1993 · reply to #5730

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