Acoustic Energy & Surprising Ways To Harness It (Intro To Thermoacoustics)

hi everyone in this video i will be starting what will probably be a short series on the topic of thermo acoustics thermoacoustics is a field of study that explores the relationship between heat and acoustic resonance which can be applied in the real world to create things like engines that run on heat alone with only one or two moving parts or air conditioners that can pump heat from one place to another using only sound waves in this video we will explore a series of thermoacoustic experiments which will come together at the end in the assembly of a working thermoacoustic engine the first thing i'll demonstrate is an interesting experiment which will give obvious meaning to the term thermoacoustic we will use heat to make sound this can be demonstrated by applying heat to an object which has a natural acoustic resonance like a tube or a vial a familiar way to hear the resonance of objects like these is by blowing over the mouth in such a way that the air inside begins vibrating and you hear a tone many different instruments work by taking advantage of this resonant property usually initiated by the flow of air from your lungs but the same result can be achieved with a careful application of heat in this quartz tube i have a coil of nichrome wire suspended one quarter of the way from the bottom this is the same wire used in a toaster or hair dryer to generate heat when electricity is passed through it if i turn up the voltage the wire will begin to glow red which produces heat and we should hear a sound [Music] the simplest explanation for this sound is that the hot wire causes the air inside the pipe to become warmer which causes convection the hot air rises this pulls fresh cold air into the bottom of the tube as hot air exits the top and so we get air flow which we have already seen can cause a pipe to resonate if this flow of air is what's causing the sound then turning the pipe sideways should stop the hot air from rising and the sound along with it so let's see what happens [Music] and there it is the sound stops when we turn the pipe sideways so the rising hot air must be causing or at least contributing to it now if this convective airflow is the only thing causing the pipe to resonate we should be able to place this coil anywhere along the inside of the pipe and still get more or less the same sound as long as the air is moving it turns out that will not work the coil will only make the tube resonate if it's placed in this position or very close to it this is because the sound that a pipe produces when it resonates has very specific physical dimensions this is often illustrated by using a rotating helix as a representation of the shape of a sound wave one quarter of the way up from the bottom of an open-ended tube is a high-energy section of its resonant frequency which means the hot wire is adding energy to the wave right where it needs it most both the airflow and the heat contribute to the standing wave which we hear as sound when this was first discovered it was found that a singing tube could not only be created by adding heat to a high energy section of a resonant wave but the same result can be achieved by instead cooling the air at a section that has low energy when resonating with a cooled tube the only difference is that the air will be drawn in from the top as the cold air flows downward now this is a horizontal version of the upright singing tube which will produce a similar effect but in a slightly different way instead of nichrome wire i'm using a bit of steel wool this time one third of the way from one end which will be heated from the outside with an alcohol flame this is a test tube with one closed end so we can't rely on convection to move air from one side to the other to get some movement of air we will instead have to create a temperature gradient which will cause air to expand inside the tube in one section and contract in another to do that i will wrap part of the steel wool in a bit of damp paper towel now when the alcohol flame is placed under the exposed part of the wool the air inside the tube should begin oscillating between the hot and cold sides it will expand from the heat in this area move forward into the cold section where it will contract and again move backward once this starts the expansion and contraction cycle will quickly synchronize with the natural acoustic resonance of the tube and once again we should hear a sound notice that the tone from the large tube and the smaller test tube are very nearly the same despite the tubes being very different sizes in fact the test tube is almost exactly half the length of this one which is precisely why the sound is similar for this tube both ends are open so a sound wave travels straight through it the test tube has a closed end so a wave has to enter reflect off the closed end and travel the same distance back again to get out that makes the test tube and the larger upright tube effectively the same length as far as sound is concerned there is a very slight difference between the tone of these two tubes which does give me the opportunity to show you one more auditory phenomenon before we go turning this concept into an engine now if i get one of these tubes singing at a time it's a consistent flat tone but listen what happens when i turn on the other one at the same time which has a tone which is very slightly different now those pulses that you heard are called beats it's what happens when two sound waves start interfering with one another when they synchronize the result is a loud pulse as the pressure wave from each tube stacks on top of each other this is sort of like what happens when you're sitting in traffic with your blinker on every once in a while it will synchronize with the blinker of the car in front of you but only for a short amount of time before they break out of phase again now you don't need to know about beets to build a thermoacoustic engine i just think it's cool the next step will be to finally build a thermo acoustic engine but before that we'll take a quick pause to look at this video's sponsor the educational courses from my sponsor brilliant are an excellent resource for keeping your mind sharp and achieving your learning goals i've been a fan of brilliance approach to education for several years as they use interactive tools and challenging problems to help you intuitively understand what can seem like very abstract concepts in math science and computer science brilliance courses are very good at taking these seemingly complicated subjects and breaking them down to their simplest building blocks so you understand at a deep level how and why a concept works not just that it does i think everyone has had an experience in education where you have to learn a formula or principle without having any idea why it does what it does which for me caused all kinds of problems as i tried to go further in those subjects i had no foundation to stand on brilliance courses do not have this problem i especially appreciate their courses like the scientific thinking course which goes all the way to the beginning by developing the very mindset that these subjects should be approached with check out brilliant and i know you'll enjoy learning something new through their courses use my link brilliant.org forward slash nighthawk and you'll get 20 off a premium membership if you're one of the first 200 to sign up so now that we know how to use heat to produce sound which is really just a steady vibration of the air the only barrier remaining to turning this into an engine is to transfer that vibration to a physical object and get mechanical motion in return one way of doing this is with a drum skin this is a finished thermoacoustic engine and you can see i've stretched a piece of rubber over the mouth of the test tube with a weight in the center you can see also that the bit of steel wool on the inside is much further forward in the tube than for the version that was only meant to make sound i've done this because by adding a diaphragm over the mouth of the tube i've introduced a change to the overall resonance of the system the diaphragm itself has a natural resonance that it prefers to vibrate at depending on how tightly it's stretched the effect this has is basically the same as extending the length of the test tube artificially the rubber represents the same resonant properties as an invisible length of tube if it stuck out approximately this far you can imagine if the tube were actually this long then the heat would have to be applied further forward to once again be in the correct position to produce sound one third of the way from the back that explains the new position of the steel wool this distance is approximately one-third the total length of both the real test tube and the simulated tube that adding a diaphragm represents and with that we have enough information to understand how a thermoacoustic engine works which is by far the hardest part building one is comparatively easy if you only have the right measurements to put it together the goal is for the tube length and the application of heat to be ideally suited to resonate in unison with a physical object in this case a rubber diaphragm i was helped in my initial efforts to figure this out by another youtuber whose channel name is blade attila check out his channel in the video description below if you'd like to see more thermo acoustic engine designs after this one i ended up using a 20 by 180 millimeter test tube for the engine body this is held by using a stiff piece of copper wire bent into a loop at the top and mounted to a block of wood as a base close hanger wire would be a perfect option if you wanted to build something similar there are two features of this engine that are hard to put into exact numbers first is the tightness of the rubber diaphragm and second is the density of the steel wool cylinder the space that this steel will occupies inside the tube can be precisely measured it's exactly 25 millimeters long and 120 millimeters from the closed end but the density or how tightly the wool should be packed to form this cylinder is hard to put into numbers the best way to tell if the right amount of wool has been used is by sight it should be low enough density that you can easily see objects through the fibers as you look through the test tube to make this steel wool cylinder take a pad of steel wool and unroll it then cut a 25 by 25 millimeter square remove at least half the fibers from this square and then roll it into a small cylinder with the grain of the fibers facing parallel to the cylinder's length then push this into the test tube so that it comes to rest 120 millimeters from the closed end for this engine's diaphragm i use a circle of rubber cut from the side of a balloon and i add some extra weight to the center by snapping a tiny cylinder magnet to each side this will lower the frequency of the diaphragm and also give us the potential to extract electricity from the engine by surrounding the magnet with a coil of copper wire the diaphragm is placed over the mouth of the test tube and held tight with a rubber o-ring to keep pressure from building up in the tube when we apply heat a tiny hole should be punctured in the diaphragm with a needle this won't affect the resonance it'll just stop the diaphragm from inflating like a balloon in the same way that a temperature gradient was created earlier by wrapping a portion of the steel wool in a damp paper towel the same is done here and then we'll be ready to give the engine a test my alcohol flame is placed directly under the exposed section of the steel wool and this tube needs to get hot enough that the wool actually begins to discolor from the heat once that happens if everything's been done correctly the magnet on the front should begin to vibrate if it doesn't the first thing to adjust will be the tightness of the diaphragm it will most often need to be pulled tighter but with a few adjustments you should end up with a working engine hahaha there it is currently vibrating at about 120 hertz we've successfully converted heat into mechanical motion now in this configuration i can only run the engine for a few minutes at a time before the paper towel dries out and it catches fire so i created a little water cooled heat sink from a piece of copper pipe split down one side and this allows it to be slightly adjustable so i can bend it to tightly wrap around a test tube for good thermal contact on top of this i soldered a small copper end cap which i can fill with water from an eye dropper and in this way run the engine pretty much indefinitely the heat sink will keep the cold side of my steel wool at or below the boiling point of water for as long as the engine is running as long as i don't let it go dry now this is a very small model engine with limited applications but i did attempt to power a small light bulb by placing a coil of wire around the vibrating magnet the coil is perhaps not quite appropriately sized to make best use of the magnetic flux but you can tell it is drawing some power because the magnet slows down from electromagnetic resistance when the coil is placed over it i was not able to get the bulb to light up probably because my coil is not optimal for the task but it is definitely producing at least some electricity thermoacoustic engines are not known for high energy output but the real interest is in applications where simplicity and durability are key with next to no moving parts and what few parts there are being easy to build they are low cost highly wear resistant devices thermoacoustics also have the advantage of being able to operate on temperature gradients such as you might find created by a regular home furnace putting that temperature gradient to work without diminishing the overall heat that makes it into your home in a future video i hope to demonstrate the opposite of a thermoacoustic engine instead of turning heat into sound we will use sound to move heat from one place to another in a thermoacoustic refrigerator thermo-acoustic refrigeration is perhaps the most promising application for this technology and now that we understand how an engine works i think we're ready to dive deeper that will have to wait until a future video in the meantime leave me comments below support this channel on patreon if you'd like to help with the costs of equipment and time to produce these videos and thank you for watching i'll see you next time [Music] you

Channel: NightHawkInLight

Views: 460,061

Rating: 4.9678102 out of 5

Keywords: Thermo Acoustics, Resonance, Sound, Energy, Waves, Particles, NightHawkInLight

Id: abswNCqnMRQ

Length: 18min 2sec (1082 seconds)

Published: Fri Jan 22 2021