View source for Magnetohydrodynamic converter

{{Short description|Electromagnetic machine with no moving parts}}
__NOTOC__
A '''magnetohydrodynamic converter''' ('''MHD converter''') is an [[Electromagnetism|electromagnetic]] [[machine]] with no [[moving parts]] involving [[magnetohydrodynamics]], the study of the [[kinetics (physics)|kinetics]] of [[Electrical resistivity and conductivity|electrically conductive]] [[fluid]]s ([[liquid]] or [[Plasma (physics)|ionized gas)]] in the presence of [[electromagnetic field]]s. Such converters act on the fluid using the [[Lorentz force]] to operate in two possible ways: either as an [[electric generator]] called an [[magnetohydrodynamic generator|MHD generator]], extracting energy from a fluid in motion; or as an [[electric motor]] called an MHD accelerator or [[magnetohydrodynamic drive]], putting a fluid in motion by injecting energy. MHD converters are indeed reversible, like many electromagnetic devices.<ref name="The Silence Barrier">{{cite book 
 |last1=Petit |first1=Jean-Pierre
 |title=The Silence Barrier
 |series=The Adventures of Archibald Higgins
 |date=1983
 |publisher=Savoir Sans Frontières
 |url=http://www.savoir-sans-frontieres.com/download/eng/mursilence.htm
 |format=PDF
}}</ref>

[[Michael Faraday]] first attempted to test a MHD converter in 1832. MHD converters involving plasmas were highly studied in the 1960s and 1970s, with many [[Subsidy|government funding]] and dedicated [[Academic conference|international conferences]]. One major conceptual application was the use of MHD converters on the hot exhaust gas in a [[Fossil fuel power station#Coal|coal fired power plant]], where it could extract some of the energy with very high efficiency, and then pass it into a conventional [[steam turbine]]. The research almost stopped after it was considered the [[electrothermal instability]] would severely limit the [[efficiency]] of such converters when intense magnetic fields are used,<ref name="Velikhov 1965">
{{cite conference
 | last1=Velikhov |first1=E. P.
 | last2=Dykhne |first2=A. M.
 | last3=Shipuk |first3=I. Ya
 | year = 1965
 | title = Ionization instability of a plasma with hot electrons
 | conference = 7th International Conference on Ionization Phenomena in Gases
 | location = Belgrade, Yugoslavia
 | url = https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670016557.pdf
 }}
</ref> although solutions may exist.<ref>
{{cite journal 
 | last1 = Shapiro |first1 = G. I.
 | last2 = Nelson |first2 = A. H.
 | date = 12 April 1978
 | title = Stabilization of ionization instability in a variable electric field
 | journal=Pis'ma V Zhurnal Tekhnischeskoi Fiziki
 | volume = 4
 | issue = 12
 | pages = 393–396
|bibcode = 1978PZhTF...4..393S
 }}
</ref><ref>
{{cite journal 
 | last1 = Murakami |first1 = T.
 | last2 = Okuno |first2 = Y.
 | last3 = Yamasaki | first3 = H.
 | date = December 2005
 | title = Suppression of ionization instability in a magnetohydrodynamic plasma by coupling with a radio-frequency electromagnetic field
 | journal=Applied Physics Letters
 | volume = 86
 | issue = 19
 | pages = 191502–191502.3
 | url = http://ayuba.fr/pdf/murakami2005.pdf
 | doi = 10.1063/1.1926410
| bibcode = 2005ApPhL..86s1502M }}
</ref><ref name="Petit 2009">
{{cite journal
 | last1= Petit | first1= J.-P.
 | last2= Geffray | first2= J.
 | date=June 2009
 | title = Non equilibrium plasma instabilities
 | journal= Acta Physica Polonica A
 | volume = 115
 | issue = 6
 | pages = 1170–1173
 | doi = 10.12693/aphyspola.115.1170
| bibcode= 2009AcPPA.115.1170P
 | doi-access= free
 }}</ref><ref name="Petit 2013">{{Cite journal
 |last1=Petit |first1=J.-P.
 |last2=Doré |first2=J.-C.
 |title=Velikhov electrothermal instability cancellation by a modification of electrical conductivity value in a streamer by magnetic confinement
 |year=2013
 |journal=Acta Polytechnica
 |volume=53
 |issue=2
 |pages=219–222
 |doi=10.14311/1765
 |url=https://ojs.cvut.cz/ojs/index.php/ap/article/view/1765/1597
|doi-access=free
 |hdl=10467/67041
 |hdl-access=free
 }}</ref>

[[File:MHD converters (generator and accelerator).svg|Crossed-field magnetohydrodynamic converters (linear Faraday type with segmented electrodes). A: MHD generator. B: MHD accelerator.|center]]
<br>
<div style="font-size:88%; line-height: 1em; text-align: center;">Crossed-field magnetohydrodynamic converters<br>
(linear Faraday type with segmented electrodes)<br>
A: MHD generator. B: MHD accelerator.</div>

== MHD power generation ==
{{main|Magnetohydrodynamic generator}}
A [[magnetohydrodynamic generator]] is an MHD converter that transforms the [[kinetic energy]] of an electrically conductive fluid, in motion with respect to a steady magnetic field, into [[electricity]]. MHD power generation has been tested extensively in the 1960s with [[liquid metal]]s and plasmas as working fluids.<ref name="Haines 1974">{{cite journal
 |last1=Haines |first1=M. G.
 |last2=McNab |first2=I. R.
 |title=Magnetohydrodynamic power dynamics
 |date=1974
 |journal=Physics in Technology
 |volume=5
 |issue=4
 |pages=278–300
 |url=http://ayuba.fr/pdf/haines1974.pdf
 |doi=10.1088/0305-4624/5/4/I03
|bibcode=1974PhTec...5..278H
 }}</ref>

Basically, a plasma is hurtling down within a channel whose walls are fitted with electrodes. Electromagnets create a uniform transverse magnetic field within the cavity of the channel. The Lorentz force then acts upon the trajectory of the incoming electrons and positive ions, separating the opposite [[charge carrier]]s according to their sign. As negative and positive charges are spatially separated within the chamber, an [[voltage|electric potential difference]] can be retrieved across the electrodes. While work is extracted from the kinetic energy of the incoming high-velocity plasma, the fluid slows down during the process.

== MHD propulsion ==
{{main|Magnetohydrodynamic drive}}
A [[Magnetohydrodynamic drive|magnetohydrodynamic accelerator]] is an MHD converter that imparts motion to an electrically conductive fluid initially at rest, using cross electric current and magnetic field both applied within the fluid. MHD propulsion has been mostly tested with models of ships and submarines in [[seawater]].<ref name="PopMech 1990">{{cite magazine
 |last=Dane |first=Abe
 |title=100 mph Jet Ships
 |date=August 1990
 |magazine=Popular Mechanics
 |pages=60–62
 |url=http://ayuba.fr/pdf/popmech_aug1990.pdf
 |access-date=2018-04-04
}}</ref><ref name="PopSci 1992">{{cite magazine
 |last=Normile |first=Dennis
 |title=Superconductivity goes to sea
 |date=November 1992
 |magazine=Popular Science
 |publisher=Bonnier Corporation
 |pages=80–85
 |url=http://ayuba.fr/pdf/popsci_nov1992.pdf
 |access-date=2018-04-04
}}</ref> Studies are also ongoing since the early 1960s about [[aerospace]] applications of MHD to [[aircraft]] propulsion and [[flow control (fluid)|flow control]] to enable [[hypersonic flight]]: action on the boundary layer to prevent laminar flow from becoming turbulent, shock wave mitigation or cancellation for thermal control and reduction of the wave drag and form drag, inlet flow control and airflow velocity reduction with an MHD generator section ahead of a scramjet or turbojet to extend their regimes at higher Mach numbers, combined to an MHD accelerator in the exhaust nozzle fed by the MHD generator through a bypass system. Research on various designs are also conducted on [[Electrically powered spacecraft propulsion#Electromagnetic|electromagnetic plasma propulsion]] for [[space exploration]].<ref name="Sherman 1963">{{cite report
 |last1=Sherman |first1=A.
 |chapter=Magnetohydrodynamic Propulsion
 |date=January 1963
 |title=R63SD5
 |publisher=Air Force Office of Scientific Research
 |chapter-url=http://www.dtic.mil/dtic/tr/fulltext/u2/295721.pdf
 }}</ref><ref name="NASA 1971">{{cite report
 |last1=Carter |first1=A. F.
 |last2=Weaver|first2=W. R.
 |last3=Mcfarland |first3=D. R.
 |last4=Wood |first4=G. P.
 |chapter=Development and initial operating characteristics of the 20 megawatt linear plasma accelerator facility
 |date=December 1971
 |title=NASA-TN-D-6547
 |publisher=NASA
 |location=Langley Research Center 
 |hdl=2060/19720005094
 |chapter-url=http://hdl.handle.net/2060/19720005094
 |chapter-format=PDF
}}</ref><ref name="Litchford 2008">{{cite conference
 |last1=Litchford |first1=Ron J.
 |last2=Lineberry |first2=John T.
 |title=Magnetohydrodynamic Augmented Propulsion Experiment
 |date=May 2008
 |conference=Annual Technical Meeting
 |location=Yamanakako, Japan
 |publisher=Japan MHD Society
 |hdl=2060/20080033025
 |hdl-access=free
 }}</ref><ref name="Roy 2011">{{cite report
 |last1=Roy |first1=Subrata
 |last2=Arnold |first2=David
 |last3=Lin |first3=Jenshan
 |last4=Schmidt |first4=Tony
 |last5=Lind |first5=Rick
 |last6=Durscher |first6=Ryan
 |last7=Riherd |first7=Mark
 |last8=Houba |first8=Tomas
 |last9=Anderson |first9=Richard
 |last10=Zito |first10=Justin
 |last11=Casanova |first11=Joaquin
 |last12=Thomson |first12=Carlton
 |last13=Blood |first13=Daniel
 |last14=Tran |first14=Dong
 |display-authors=5
 |date=20 December 2011
 |title=Demonstration of a Wingless Electromagnetic Air Vehicle
 |id=AFRL-OSR-VA-TR-2012-0922
 |publisher=Defense Technical Information Center
 |editor1=Air Force Office of Scientific Research
 |editor2=University of Florida
 |asin=B01IKW9SES
 |url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a564120.pdf
 |archive-url=https://web.archive.org/web/20130517063830/http://www.dtic.mil/dtic/tr/fulltext/u2/a564120.pdf
 |url-status=live
 |archive-date=May 17, 2013
 }}</ref>

In an MHD accelerator, the Lorentz force accelerates all charge carriers in the same direction whatever their sign, as well as neutral atoms and molecules of the fluid through collisions. The fluid is ejected toward the rear and as a reaction, the vehicle accelerates forward.

== See also ==
* [[Plasma (physics)]]
* [[Lorentz force]]
* [[Electrothermal instability]]
* [[Wingless Electromagnetic Air Vehicle]]

==References==
{{reflist}}
{{refbegin}}
{{refend}}

==Further reading==
* {{cite book |last1=Sutton |first1=George W. |last2=Sherman |first2=Arthur |title=Engineering Magnetohydrodynamics |publisher=Dover Publications |series=Dover Civil and Mechanical Engineering |date=July 2006 |isbn=978-0486450322}}
* {{cite book |editor1-last=Molokov |editor1-first=Sergei S. |editor2-last=Moreau |editor2-first=R. |editor3-last= Moffatt |editor3-first=H. Keith |title=Magnetohydrodynamics: Historical Evolution and Trends |last1=Weier |first1=Tom |last2=Shatrov |first2=Victor |last3=Gerbeth |first3=Gunter |chapter=Flow Control and Propulsion in Poor Conductors |pages=295–312|publisher=Springer Science+Business Media |date=2007 |isbn=978-1-4020-4832-6 |doi=10.1007/978-1-4020-4833-3}}

[[Category:Electromagnetism]]
[[Category:Fluid dynamics]]
[[Category:Plasma technology and applications]]
[[Category:Energy conversion]]
[[Category:Propulsion]]