Magnetohydrodynamics - Propelling Liquid Metal with Magnets!

[Music] hi magnetohydrodynamics it's a long word for a very old principle the same principle that drives basically all of our electric motors and operates our electric generators effectively when a charged particle such as an electron moves it's associated with an electromagnetic field and by convention we use what's called the right hand rule which states that if the electron is moving in the direction of my right hand thumb the field is considered to be moving in the direction of my right hand fingers for your left handers out there you're not being excluded because the field doesn't actually rotate or move it simply has a chirality associated with it a polarity associated with it that by convention we consider this to be the standard orientation of the field and as long as we stay with that convention that's that's fine it's consistent if we take a wire like this which is basically an electron tube and we place it near another wire similar to it and cross the gap with another conductor following the right-hand rule the field in this conductor is wrapping this way the field in this conductor is wrapping this way in the field and this return conductor is wrapping this way as a result these fields are all opposing each other and as a current were to move through here as the electrons move through here they all want to sort of move away from each other and so these rods these wires would want to expand away from each other and this armature would want to move away from the conductive loop that's basically the principle behind an electric motor if the armature is free to move and we place current in here the armature would want to move along the rails if the rails were round sort of like a bicycle tires and the armature was connected through an electrical feed the armature would move around within that bicycle wheel pattern and form rotational torque you produce a motor the field dough generated by this little loop is extremely weak and the force is the product of those very weak fields it's the field of this times the field in this and therefore the amount of current that you would have to flow through this loop to produce any meaningful force would have to be thousands of amps practically that's difficult to achieve and that's why when you look at an electric motor you will see many wraps of wire because if I effectively put say a thousand tiny little wires inside of this open tube and ran an amp through each one I would get the same electrical field outside of the tube as if I ran a thousand amps through the tube so an electric motor they're sort of recycling their electrons to produce the fields necessary to cause meaningful motion nevertheless it still would require a lot of current to generate a lot any significant force and so often these motors and generators are augmented by the use of a permanent magnet a magnet is formed by crystals that have a an extreme amount of polarity to them there's a de cemetery to the electron distribution as a consequence the little particles little crystals are extremely powerful magnets but before this is formed into a block magnet the powder that's going to be centred into this is random and so there's no net or macroscopic type of magnetic field outside of the the sample after they Center this into a block they expose it to an extremely strong electromagnetic pulse which Orient's the crystals all in one unidirectional orientation so that the bulk magnetic field of the magnet is not that dissimilar from the individual magnetic field of little particles and that will stay permanently unless you heat the magnet beyond its Curie temperature which is where the crystals become mobile enough that they can disorient or you expose it to a very strong contra magnetic field that can sort of flip the the orientation of the crystals but as long as you don't do those extreme things to the magnet it's basically forever if you place a magnet in the same location as the little wire loop that we produced it's still a magnetic field and as a result the force on this armature is the product of not a weak magnetic field times a weak magnetic field or two low numbers it's the product of a large number and a small number and so there's much more force produced by this armature if you have magnets located in the in the motor and that's why when you look at permanent magnet motors or generators they use these magnets not because the wires are not producing a field but to augment the field this process though can be inverted and if you imagine for a second that we've produced motion in this wire because we've the electrons within it want to move right angles to this field and the right angles to the current moving through it if instead what we do is we actually force the armature through a magnetic field what happens is the electrons within the armature because they are moving this way right hand rule they produce a magnetic field and that magnetic field interacts with the permanent magnetic field of the magnet therefore those electrons want to move in right angles to the magnetic field of the magnet and to the direction of motion and therefore those electrons will be induced to flow along the conductor therefore we generate a current of electrons and that's how we form a generator so you can flip it it's just two different points of view for the same process the important thing though is this doesn't have to be a wire and that's where magnetic hydrodynamics comes into play here I have a relatively modest strength n40 magnet and on either side I have two nickel electrodes what I'm going to do is I'm going to fill this container with water very pure distilled water is a very poor conductor and as a matter of fact that very purist waters are often characterized as to their electrical resistance their deionized and the higher the electrical resistance the purer the water as a matter of fact super pure water is actually corrosive because it is lacking in ions and we can get into that in another video but nevertheless we can enhance the conductivity if we add to the water salt regular sodium chloride table salt salts have a very low ionic bond strength and so in the positive sodium's and the negative chlorides are exposed to the very polar water molecules the water molecules will intersperse and cause the positive and the negative ions to separate from each other and flow around inside of the medium in which we placed them so if i put that in there and let me get a little stirrer these ions can do the same thing that the electrons do they're much more massive and there's far fewer of them and so an ionized water like salt water or sea water conducts electricity but not very well so we need to use a fairly high voltage to cause any significant current to move between these electrodes so as the salt is dissolving I'm going to attach two leads from this power supply to the electrodes now if you've read the book Hunt for Red October the Russians developed a submarine based on a propulsion system that used magneto hydrodynamics kind of interesting because the process does work that should be saline enough now once that stops flowing what I'm going to do is I'm going to place a little dye in between the electrodes just so that you can visualize what's going on a little better and there's a couple of YouTube videos out there that show this it's kind of a neat little phenomenon hopefully it's not flowing too fast right now maybe a little bit give a little bit of time okay not much now if I take the power supply and I apply a voltage what will happen is as the voltage goes up we start to generate a current which you'll see over here and the current will cause the dye to flow now if I take my little q-tip my little toothpick and Hance that a little bit you'll see it's streaming kinda neat huh now if I turn the voltage up a little bit higher you can see the flow is pretty impressive but if I was working for the the Soviets during the development of the submarine I think I'd probably end up in Siberia if I was proposing to drive a boat with this nevertheless the force that's being generated is proportional to the current which right now is about 0.3 amps and to the magnetic field so what happens if we get a little stronger magnet and a much stronger field okay so now what I've done is I replaced this apparatus with a new one and in this case instead of using an N 40 magnet I'm using an N 50 2 magnet the number at the end represents the strength of the magnet this is a much stronger magnet in addition you'll notice that the channel width is only half as great as a result the current flow is proportional to the contact area of the liquid across the electrodes but also proportional to the distance that the current has to flow making it narrower means that for a given voltage drop we're going to get more current across here and as a consequence we'll get much higher flows from this apparatus as I'll demonstrate so when I start to turn up the current here the voltage you'll notice that you get pretty darn significant flow so maybe I won't end up in Siberia quite yet we're up at three and a half amps when we were only generating about 0.3 amps before in addition you'll notice this foam that's forming here it's how they discovered or they were able to locate the Russian submarine and this is the hydrolysis of water that occurs when you get over a potential between the electrodes of 1.2 volts the electrical potential is actually disassociating the hydrogen and the oxygen and those bubbles are the foam of the hydrogen and oxygen being released now as I said before a generator and a motor are basically two viewpoints of the same process and so this process can be inverted and we can use this process to generate electricity let me show you that all right now what I've done is I've taken the same salt water I placed it in this little barrette what I'm going to do is I'm going to open up the valve and we're going to flow the liquid and you'll see the voltage here that occurs when we start a flow of the ionic liquid past the magnetic field now as you notice it's not a lot but nevertheless it does show that there is some potential and the reason that the voltage is so low is simply because we have very little velocity here and we don't have a very large surface area in a very weak magnet but 56 millivolts are being generated by this system in order to get real power out of this we would have to flow the fluid much faster and we would want the met the gap to be much smaller and the magnetic field if possible much stronger however the conductivity of ionic liquids is so low that it would be difficult to get a tremendous amount of power unless we could really improve the conductivity and a better alternative is to get away from the ionic liquids and go to something more like metal and specifically liquid metal so we've got a nice apparatus outside and I'm going to show you what happens when we do that okay what I'm going to do is repeat the demonstration now with a liquid metal and because I don't like to work with mercury when I can and certainly not inside I've decided to use a low melting point on metal alloy that will conduct electricity but is substantially safer this alloy is called sera low 136 and it is a sort of a member of a family of what are called eutectic alloys these are alloys of different low-temperature melting metals that when combined the individual metals interfere with the crystallization of their neighbors and so as a result that combination melts at a much lower temperature than the individual metals that are contained within it sera whoa 136 is a little unusual in that it's one of the few metals that does not contain any gallium and does not contain any cadmium so it tends to be substantially safer it's made out of a combination of lead bismuth tin and indium and what I've done is I've placed some of this in this channel and we're going to demonstrate how this conducts typically this material when it hardens looks like pewter and when it's liquid it looks like liquid solder and it's frequently used by mold makers to makes a liquid metal soldiers it can be used in a machining operation to fixture very delicate components that are otherwise very difficult to to hold and then when you're done with the machining operation you can simply warm the part a little bit and pop the the pop part out because it melts at 136 degrees Fahrenheit I'm outside simply to allow me to use this heater to bring this large warrant of water bath up to above 136 degrees but you can see on the temperature probe is right now at about a hundred and forty-two degrees and that's why this is why this is liquid so now the material in the channel is in contact with two electrodes like we had before it's on top of the magnet and I've got it hooked up to a power supply and when I apply the current if I'm not sure you're gonna be able to see all this at the same time you may want to go through the video a couple of times or we may repeat this depending on how we edit this but as I turn on the voltage the current will rise very quickly and you'll want to watch what happens to the liquid in the channel as the current goes up so I'm going to start rise raising the voltage now here we go see it moving up and turned it off and it goes back again don't turn it on again and you can see that the metal even though it's extremely dense is able to move up the same Hill that the ionic liquid was able to move up and the current that has been flowing through there is about 15 or 16 amps as a matter of fact at the end it was beginning to spark you the capability of moving metal with MHD types of pumps has been applied to cooling nuclear reactors cooling very difficult to cool high-temperature devices where you don't want moving parts you don't want pumps or pistons or anything that may be damaged by the the high temperatures and you don't want to include them in the fluid loop and so this is used around the world for cooling purposes and when we go inside I'm going to show you how we can use this for a kind of unusual application okay so up to this point in order to demonstrate some of the forces involved I've relied on the powerful magnetic field of a permanent magnet but because as I explained earlier there are forces that are generated by the current if you get enough current you can actually produce very significant forces and so what I've done is I've set up a little demonstration here using a power supply to charge a rather large electrolytic capacitor this is a 4700 micro farad 450 volt capacitor and this capacitor is going to be discharged or shorted through what's called a pancake thyristor or an scr these are rather unusual solid-state switches that can handle extremely high voltages thousands of volts and can handle tremendous currents tens of thousands of amps and the way they work is you effectively charge or allow current to flow through one side and then there is a small gate that is switch and the way the gate works is that there is a small bias voltage that I create between the ground of the thyristor and the upper voltage that's produced by the three volts from the battery so effectively by providing three volts above ground to the discharge end of the switch the switch will then open and allow all of the voltage and all of the current from the capacitor to discharge through this Loup I have formed between these two narrowly spaced conductive rods back down to ground what we'll do is I'll demonstrate this and show that you can get noticeable forces generated if you develop a sufficient amounts of current and we're going to start charging the capacitor this is the bolts DC across this capacitor as the power supply is charging it will go to about 50 volts or there abouts and then when I push this button I will close the switch and will allow the current to go through the two conductors watch the two parallel conductors one two three you see they moved apart the connector at the end was also feeling a force it wanted to move this way but it's it's tethered by the weight of the leads the current output from this capacitor is proportional to the voltage the current the energy is proportional to the square of the voltage but the current is proportional to the voltage the forces between these rods however is proportional to the square of the current the field in one interacts with a field and the other so if I double the field here and I double the field here I get four times the force so if I charge the capacitor up to say a hundred volts or twice as high I get four times the force between these two conductive rods once we give this up to about a hundred volts then we'll hit the switch and you'll see what the difference is one two three substantially different now these are the same forces that are used to operate a railgun and one of the interesting things is that the current that is flowing through here is actually pretty substantial it's on the order of thousands of amps and I'm going to demonstrate exactly what that is with an oscilloscope okay now I've set up the oscilloscope and what we're doing is we're measuring the voltage across the capacitor and when we triggered the circuit will show the decay of the voltage across the capacitor as it goes through the dead short one of the things about these capacitors is that they have a fairly high internal resistance so it's really the capacitor itself that limits the discharge speed not these wires this is significant as I'll explain it in just a second but what we'll do is bring this up to about a hundred volts and when I fire this on three you're gonna see a tracing of the voltage that goes across those two one two three now if you see on this scan the voltage has decayed to about 30% of its initial point in these are 50 micro micro second skill grids in proxy approximately a hundred to 150 microseconds now what that means is that all of almost all of the energy of the capacitor has been discharged within about one six thousandth of a second and that's significant because the two ways that we're going to be going with this information is we're building a very large yoke style magnet to produce a much higher field than the ones that you've seen in these little tests test devices to produce a much more powerful MHD pump as well as a much more powerful MHD generator that will operate on seawater in addition what we're going to be doing is using some of this technology to build a railgun the principles of a railgun are very easy to understand I mean we've just gone through that but there are two very very difficult problems that have to be solved with those devices the first one is that when a rail gun fires the pressures that you see here which are microscopic based on a few thousand amps are multiplied thousands of times and the forces that are pushing the rails apart when a rail gun fires are measured in thousands of kilograms the sliding armature that goes between the two rails has to make almost perfect contact but have almost no resistance and as you can imagine that would be extremely difficult to do because if you have just a little bit too much resistance before any load has been produced as the projectile or the armature that you're going to be sliding can lock up and when it does all the current will flow through it it will weld in position it will probably blow up and if you have enough clearance that it's able to get through what will happen is under the forces of the of the discharge the rails can move microscopically apart you'll get an air gap you'll develop arcs and plasma and waste a lot of power and potentially again blow up your elect your armature so one of the techniques that we're going to be using uses the same thing that I demonstrated with the liquid metal we're going to use a liquid metal SAPO to help provide good contact with relatively low machining tolerances the US military has released information that they were researching this early on Russian researchers have put out some papers and the comments are this looks very promising but it could be kind of expensive I don't think that that should be a limitation at least if if the process works and so we're going to try that the second aspect of this is that you'll see if you watch a few YouTube programs or youtubers that are trying to build rail guns they will frequently take a large number of these capacitors they'll wire them up in parallel and then they'll use the armature itself as the switch it's providing the disc as I demonstrated this forty-seven hundred microfarad capacitor which at about a hundred volts is storing about 25 joules will discharge almost all of its power within about a 6,000th of a second if you imagine a railgun that say about half a meter long if the initial velocity of the projectile is close to zero and the final terminal velocity is what's ever coming out of the gun the average speed of the projectile is half that so if you travel half a meter in one 6,000th of a second you're essentially traveling at a speed of roughly 3,000 meters per second on average and a peak speed of 6,000 meters per second that's orbital velocity far higher than any railgun fires so the problem with discharging these capacitors straight through a dead short is that you end up utilizing all of your power very early on generally welding your light with your projectile or blowing it up and not getting the power out when you need it what you need is to use what's called a pulse forming Network that essentially gets all the power out of the capacitors that you can but not any earlier then the projectile leaves the end of the gun and a pulse forming Network introduces inductances to slow down the pulse and tailor it to the length of the time of flight through the through the rails in addition almost all of the youtubers that have done this as I said dead short these electrolytic capacitors through these rails and through their projectile when they do so they create tremendous inductive fields as we've demonstrated here and those inductive fields as soon as the voltage drops in the capacitor will then refeed the capacitor in the opposite polarity and this sort of refeeding will continue until the capacitor reaches a certain voltage and then it will again feed back into the field and this sort of up and down voltage reversal across the capacitor is called ring down and that ringing in a polarized capacitor is lethal so it's surprising that any of them survive more than one pulse but they certainly wouldn't survive many and that's one of the reasons why you want to use a thyristor or a rectifier to protect the capacitors in the process of building the railgun so as we move on with this project we're going to be going in both of these directions but you can get an idea of some of the principles behind what we're doing and I'm looking forward to getting the magnet together and I'm looking forward to getting some real current through the leads so we'll show you that as soon as those things get a little bit more mature but I want to thank you very much for watching I would really appreciate it if you'd subscribe because it really does help us a lot in any case you have a wonderful evening good night [Music]

Channel: Tech Ingredients

Views: 790,596

Rating: 4.9342933 out of 5

Keywords: MHD, Magnetohydrodynamics, Magnet, Magnets, Eutectic alloys, Cerrolow 136, Electrolytes, Railgun, Liquid metal, Sabot, Electrolytic capacitor, Silicon controlled rectifier, SCR, Right Hand Rule, Electromagnetic Force, Neodymium magnets, The Hunt for Red October

Id: LS3GQk9ETRU

Length: 28min 35sec (1715 seconds)

Published: Thu Jul 26 2018

Reddit Comments

[u/nvaus]

I realize this is a bit of an unconventional post to qualify as 'artisan', but I think this guy's presentation and exhaustive knowledge pass the threshold. I can't get enough. Here's the next follow up in the series: https://youtu.be/B015P0XFl9g

[u/TheKidd]

I just rewatched 'The Hunt for Red October' last night, and the caterpillar drive on the Russian Sub was supposed to be Magnetothermodynamic. Great timing for this video to appear on my front page!

[u/fossil98]

This guy is what 4 year old me thinks of when you say 'engineer'.

[u/supratachophobia]

This guy is building a railgun. And now I understand how it works. Best Monday ever....

[u/Wrongallalong]

I loved watching both of these videos! Terribly informative and well explained. I Was however terrified at every moment he was Loading those magnets in the second video. Fun stuff And I’m excited for the next one.

[u/crazystu3]

A superb science pun went unrecognized at 12:28.

[u/Flaming_Pancakes]

This is great. Thanks for posting this. I would love to take a class from this guy.

[u/iDontTrustMyself]

Is that Bill Hader's dad?

[u/trevdak2]

Oof... Did anyone else get immensely seasick by the camera movement in this video? Normally I can handle any sort of camera shake but this took all of 15 seconds to make my guts churn