Talk:Magnetohydrodynamic generator

Are magnets really needed?

I have a question to those who are really proficient in MHD generation. There exist some claims that degree of interaction between flowing molecules (gases) and ions on one hand and flowing molecules (gases) and electrons on other hand is vastly different. Therefore movement of ions in a gas flow will occur much faster than movement of free electrons. If this is correct then why we need to use a strong magnets to separate ions and electrons in MHD generator? If speed of the ions and electrons movement in the same gas flow is vastly different then doesn't charge separation suppose to occur by itself just due to a gas flow? Shouldn't majority of electrons concentrate at the beginning of the duct while many more ions at the end? Then only thing we need to generate current is to put an electrode at the beginning of the duct and another one at the end and let electrons flow from the inside of the duct through the external load to the end of the duct and recombine with ions there? And no need for a magnets. — Preceding unsigned comment added by 192.222.146.79 (talk) 10:45, 2 March 2020 (UTC)

I'd expect the ions (positive charge) to drag free electrons with them. Gas flow is probably insufficient to separate the charges. Interactions of free electrons with the flow gas needs to be defined. If they are creating negative gas ions temporarily, then they are traveling just as fast with the flow as the positive ions. If they are just colliding with the gas, (more frequently because of higher velocity than the ions) the net collisions will still stay with the gas velocity on average due to the electron's tiny mass it will be pushed along with the flow. WikiSlothBlobThing (talk) 03:55, 26 May 2021 (UTC)

todo

• Done:wikilink elements and pivotal words in sentences
• Done:unify terminology of 'tube' 'pipe' 'channel' 'cylinder' (I used the terminology of the Messerle reference).
• have others help with grammar and wording ...
• Done: look for an 'efficiency' for throughput for coal power plants (from amount of energy in fuel to amount of electricity produced from same fuel) (USed the Messerle reference)
• efficency of nuclear power plants from amount of fuel consumed to amount of energy produced.
• Done, sort of...: do the math to verify efficency of MHD generator ... 10 to 20% came from the babelfish german version and needs to be verified. (I didn't do the math, I shamelessly imported Messerle's results, whcih were from actual experiments).
• (Oops; I'll try to add it) please check: power generation example lacks any mention of a magnetic field, isn't this required?

Comments by Ray Van De Walker 19:34, 19 March 2007 (UTC)

The piece I couldn't fit in anywhere that merged from MHD dynamo; "In some cases, anomalous magnetic field production in the Madison Symmetric Torus reversed field pinch has been shown to be caused by the MHD dynamo."

I've tried to fix this up as best I can. To the best of my knowlage there must be a magnetic field at all times for this to work. cc24.137.78.34 21:20, 4 February 2006 (UTC)

Current developments?

I can't find any current examples of MHD generators in use for production of electric power. It's my impression that this ship has sailed...that the steady development of, eg. combined-cycle gas turbines with steam bottoming cycles have gained all the efficiency advantages that were once supposed for MHD, without the heart-breaking difficulty of trying to make a gas conduct electricity. Can anyone point at an MHD experiment more recent than the early '70s, which is all my "Standard Handbook for Electrical Engineers" has ? --Wtshymanski 01:50, 21 May 2005 (UTC)

I have to agree with you that this hasn't really caught on, but I am not an authority on the topic. It seems to me that the there could be a revival however by adding some new technology. The bit about the alkaline salts and plasma generation seems a bit out of date however. With the recent developments in cold one atmospheric glow plasma these would seem to be unnecessary as they are supposed to be quite efficient, although I could be mistaken. I have heard of one being used backwards once on an industrial scale, but that was for a niche application pumping liquid metal in a refinery or something along those lines. Also, shouldn't the formula about how much energy can be extracted include a variable for the strength of the magnetic field? The MHD propulsor formula used in the real world is (thrust)= (current) X (magnetic field strength) X (the distance between the electrodes) cc24.137.78.34 21:35, 4 February 2006 (UTC)

Being an enthusiast, I had a more recent reference work, the Messerle reference. I wrote the more recent history from that. I hope it's helpful. I bought Messerle some time ago, and I was surprised to find that it is not available at either A Libris or Amazon. However, many research libraries have copies, because many subscribed to the UNESCO energy series. Ray Van De Walker 19:38, 19 March 2007 (UTC)

generator types?

I was surprised to find no mention of the different generator types, so I added that information. The Faraday generator has been obsolete for more than forty years. I believe the AVCO Mk. V (1965) was a Hall generator. Ray Van De Walker 19:41, 19 March 2007 (UTC)

Magnetic Field Strength (no citation)

I was looking into why the magnetic field strength was stated to increase with the 7th power of the distance, and while I didn't exactly find a citeable source, I did come across this tidbit:

Close to the magnet, that depends on the detailed shape of the magnet. Farther away (say at distances more than a few times the size of the magnet) the magnetic field will generally fall off as the cube of the distance from the magnet. If you have a small magnet, the field two feet away will only be 1/8 as strong as the field one foot away. There are special magnets for which the field falls off even faster, but in general the 1/distance cubed law holds at large distances.

- Mike W.

That's the magnetic field strength Mike's talking about. The force something feels in a magnetic field depends on how the magnetic field changes with distance (paradoxically, objects would feel no net force in a perfectly uniform magnetic field! This is one reason why the shapes of the pole tips of magnets is important). So in fact, the force another magnet will feel will fall off with the distance to the fourth power. If the object which feels the force isn't a magnet but needs to have its magnetization induced by the presence of the magnetic field, the force gets weaker still with increasing distance. The field falls off with 1/distance cubed, the rate of change falls off with 1/distance to the fourth power, and if the induced magnetization is proportional to the field strength, the force felt falls off with 1/distance to the seventh power.

This is one of the reasons why we are reluctant to answer questions about how strong the force is between two magnets or between magnets and other stuff. The detailed shapes of the magnets are really important, and tiny changes in distance make a big change in the forces you feel, as you can tell with real magnets.

- Tom

It would seem that since the electromagnetic field in the fluid is induced by the coils, the strength would indeed depend on the field strength produced by the coils, and would be subject to the same strength increase/decrease due to distance as normal magnets on top of that. So this would result in $(1/x^{3})*(1/x^{4})=1/x^{7}$.

-- AfroThundr3007730 (talk) 03:51, 28 May 2015 (UTC)

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