Acoustic Cooling & How To Manipulate Heat With Sound (Thermoacoustics Part 2)

hi everyone this video is part two of my series on thermo acoustics in part one we covered the basics of how heat can be used to produce sound and then how sound can be harnessed and converted into mechanical motion in the form of a thermoacoustic engine in this video i will reverse the process instead of using heat to make sound we will use sound to manipulate heat one practical application for this knowledge which we will explore is thermoacoustic refrigeration a process by which a finely tuned acoustic wave can extract energy from one place and concentrate it somewhere else it will help us to understand how this concept of manipulating heat with sound is possible if we first take a closer look at a sound wave i have this tube connected to a speaker at one end with a handful of styrofoam beads on the inside using a tone generator i can feed the speaker a signal and sweep through the frequencies until i find one that matches the tube's natural acoustic resonance watch what happens to the styrofoam beads when i do the lowest resonant frequency i encounter is called the fundamental frequency of the tube when we've hit this note the styrofoam beads begin to gather in a lump in the center if we were to illustrate the shape of this sound wave with two-dimensional lines it would look something like this a peak in the center just like we see reflected in the shape of the styrofoam and a node at each end a node is the place where the sound wave hits something that causes it to reflect back towards its source on one side it hits the end of the tube and on the other it hits the speaker which if the speaker is vibrating at just the right frequency it will give the wave a push back down the tube as soon as it's returned that's what makes this the fundamental frequency the speaker vibration exactly matches the amount of time it takes for the sound wave to travel down the tube and back again which will be a longer or shorter period of time depending on the tube's length but that doesn't exactly explain what's going on with this demonstration why is it that the styrofoam beads are so kind as to conveniently gather in this shape giving us a picture of the invisible sound wave to understand why the foam beads gather in this way we have to think about pressure as a sound wave approaches a node it becomes compressed kind of like a rubber ball hitting the ground when it springs backward the compression is released as it moves freely through space this makes the volume of air near a node experience large spikes in pressure while the area between nodes stays at a relatively constant low pressure because of the high pressure at the nodes the styrofoam beads are pushed to the center giving us this great visual experiment to work with this fact about air pressure spiking near the nodes of a sound wave will be very important for thermo acoustic refrigeration so remember that for later before we move on i couldn't help but push this demonstration a little further by doubling the fundamental frequency which gives us what is called the first harmonic note of this tube now we get an extra node in the center which is caused by opposing sound waves crashing into each other effectively this makes the sound behave as if the tube were half as long other harmonic frequencies divide the tube into even more sections each one separated by a node for thermoacoustics the fundamental frequency is typically the most useful to experiment with but these are fun to look at you might notice that each lump of foam beads is itself divided into smaller peaks this is actually a bit of a mystery there has been some research into why this happens but no conclusive answers some studies suggest that the air movement of a sound wave is subdivided into smaller pockets of movement each represented by these peaks i tend to think it has something to do with the phenomenon of beats which we looked at last video no matter what the true answer it's really interesting to think about now that we have a basic understanding of how a sound wave behaves inside of a tube we can begin to consider how this might be used to manipulate heat in addition to what we already know there's one more property of a gas-like air which is important to understand for this purpose when you compress a gas the energy that gas contains is concentrated which results in an increase in temperature we get a great example of this in a little fire starting device called a fire piston this is an airtight tube with a volume of air inside which becomes compressed when i push the piston downward this causes the air to increase in temperature so much that it can actually ignite a small piece of cotton some of you might know that a diesel engine also works in the same way the fuel in the engine is not ignited by a spark but instead spontaneously ignites from the increase in temperature as the piston compresses the air in the cylinder the amount of compression in a fire piston or diesel engine is extreme which is necessary for the energy of a gas to be concentrated so much that it can ignite solid or liquid fuel but the principle remains the same for any level of compression including the compression that occurs near the node of a sound wave now the pieces are starting to come together if we can generate heat by compression and compression with sound these are the building blocks we need for thermoacoustic devices of many kinds including perhaps unintuitively devices that can make things cold as well as hot thermoacoustic refrigerators which we are now prepared to learn about but first a quick pause for this video sponsor mel science supplies excellent chemistry sets physics experiments and other educational projects which allow you to explore science hands-on in your own home mel chemistry sets come first with a free starter kit that includes all the basics and then each experiment box is on a particular topic with two or three different reactions to demonstrate that topic these are really well thought out to give a beautiful look at the chemistry and there are both physical and digital resources to guide you through the experiment in-depth explanations of the science behind each reaction are accessible through the mel chemistry mobile app and website and vr chemistry lessons which can be viewed using a smartphone and the vr headset which comes in the starter kit overall it's a great way to have fun learning chemistry and inspire an interest in science use my link in the video description and the promo code nighthawk for 25 off the first month of any mel science subscription now let's take a look at an actual thermo acoustic refrigerator as an example of how we can put the concepts we've learned so far to practical use this one uses a glass resonator tube so we can easily see inside and an 8 inch subwoofer at the base which ideally will play the fundamental frequency of this tube about 200 hertz i've chosen a subwoofer because it should do really well playing in that 200hz frequency range with high amplitude and the back of the speaker is contained in its own separate container so that very little sound will escape and the refrigerator will operate relatively quietly similar to the foam bead experiment playing the fundamental frequency of this resonator will give us a sound wave with two nodes one at the speaker and one at the closed end these are the places where as we've seen compression is greatest as the sound wave impacts the node and is forced to spring backward with each spike in pressure the air also increases in temperature but as soon as the pressure is released the energy spreads out again and the temperature returns to normal there's no lasting change to the overall temperature of the system imagine what might happen if when the gas is compressed and still hot we were able to extract some of its energy before it expands again when the gas does expand back to a normal pressure it will no longer have the same amount of energy that it started with and so it ends up slightly colder with each pulse of sound we could collect a little more heat and in that way end up with significantly colder air inside this pipe than outside thermo-acoustic refrigeration one way to make this hypothetical process a reality is to try to absorb some of the heat from the compressed sound wave into a solid material which is placed near one end of the tube this material needs to allow a sound wave to pass through it without causing too much obstruction but at the same time it needs to be in close contact with the air in order to be effective at absorbing its heat the typical way to do this is to use a stack of very thin parallel plates of plastic or metal with a tiny gap between layers cut into the shape of a cylinder so it fits into the resonator tube parallel plates formed with this much precision are a little bit tricky to make at home so instead i'm using a spiral wound cylinder made from plastic cut from the side of recycled bottles even though this is wound in a spiral shape i'll still refer to it as a stack as i describe its purpose this stack which is composed of many layers with only about a 0.5 millimeter air gap between them is placed into the resonator tube somewhere between 1 8 to one quarter of the way down from the closed end now when a sound wave travels down this tube and begins compressing near the node a small amount of its heat will be absorbed into the layers of the stack mostly on the side that faces the node because that's where compression is highest as it springs backward and decompresses it cools off and on the way it passes by the lower portion of the stack where now since the air is colder it's able to pull heat out of the plastic we end up with a temperature gradient with the top of the stack becoming hot while the low side becomes cold we can see this temperature gradient develop when i turn on the refrigerator thanks to a little piece of thermochromic tape which i've attached along the length of the stack the tape becomes black where cold and changes to blue where it's warmest this concept for thermoacoustic refrigeration relies on the stack being made from a material with fairly low heat conductivity or else the heat will migrate from the hot end of the stack down to the cold side faster than the sound wave can maintain the difference in temperature that's why i've used layers of plastic instead of metal you may remember in my last video i mentioned another youtuber blade attila who helped me solve a few problems i had with my thermoacoustic engine and he continued to be of great assistance with the construction of these refrigeration systems the way i made these plastic stacks uses one of his brilliant methods to keep the layers separated by a consistent air gap i take a little sheet of my plastic and poke a few lines of holes with a pin this creates a row of identical bumps on the other side to hold the layers apart when they're rolled up and inserted into a little cylinder which i've made to fit perfectly in the resonator tube the spacing of the layers can even be changed by using a different diameter pin to poke the holes which creates different size bumps blade attila suggested this was most easily done with a sewing machine but the results from poking the holes by hand still gave me a great result check out blade's channel later on if you're interested in more thermoacoustics he's got some great stuff and he was a big help in making this project happen [Music] so when the stack is placed inside of the resonator we end up with both a source of heat and a source of cold on opposite sides but to make use of the temperature on either side requires that we attach some sort of heat sink in order to move the heating or cooling potential somewhere outside of the system you could rely on the heat and cold simply being transferred through the wall of the resonator tube but that's very inefficient instead you can make something like this my stack is sandwiched between two copper rings each with a highly conductive heat pipe extending up and out of the top plug which closes the top of the resonator attached to the copper rings is an aluminum mesh which makes thermal contact with the ends of the stack so heat is more efficiently transferred from the plastic to the heat pipes where it can be carried up and out of the system in this way we can direct the heating and cooling potential of this system to anywhere we like just by directing the hot and cold pipe to different locations now these heat exchangers are mostly just for demonstration purposes to show you the basic concept of how the heat flow of a thermoacoustic system can be practically used in reality my window screen method of making thermal contact with the stack doesn't work very well and instead of traveling all the way up and out the top it would be a lot better if these heat pipes stuck straight out the side of the resonator tube the efficiency is not good in this configuration but hopefully it helps you understand how a system like this works by pumping heat out through one pipe and drawing heat in through another all the way down to the cold end of the stack [Music] something else you might have considered by now is that i mentioned there are two nodes created by the fundamental frequency in a tube like this instead of having the stack positioned up here could we instead move it closer to the other node which is at the speaker and yes we could do that in fact we could have two stacks in this tube at the same time one at each end both stacks would behave in the same way the only difference being that the lower stack would have the hot and cold sides flipped as the hot side would now face down toward the node that it's closer to this gives us an interesting new option instead of piping the heat from both stacks out of the resonator tube directly we could instead take the hot end from one stack and pipe it into the cold end of the other this can potentially cool down the hot end of the first stack so it never actually goes very far above room temperature at all allowing the cold end to plummet even further to temperatures much lower than a single stack could ever achieve meanwhile the lower stack where all of the excess heat is being piped to will become a much hotter heat source to be taken advantage of i haven't completed a dual stack model of that kind to show you in this video but it's a concept that's been known for quite a while to both lower the temperature that can be produced by these systems and increase the hot end temperature now one last model that i will show you briefly is a combination of several thermoacoustic concepts in my last video i demonstrated how a tube can be made to start resonating at its fundamental frequency with a precise application of heat now there is a category of thermoacoustic refrigeration device which instead of using a speaker to produce a sound wave uses the same mechanism as this singing tube heat alone causes a sound wave to begin resonating inside of the system and by placing a stack further along you get refrigeration with no moving parts no speaker no wires you just apply heat to one part of the device and you get cooling somewhere else it's incredible i've just been playing around with this model for a short while now which should operate using the singing tube concept but i need a little more time to get it working in both of my videos so far on thermoacoustics we have only looked at devices which use a standing sound wave created in systems with straight pipes and simple geometry there is another category of both thermoacoustic engines and refrigerators which use traveling sound waves moving through a loop standing wave designs are impressive all on their own but the best efficiency and most impressive results have been achieved by this new category which i am just beginning to learn about if enough of you are still interested to go deeper into this subject traveling wave engines and refrigeration devices will certainly be something we take a close look at in the future in the meantime i'd like to thank my patreon supporters for continuing to fund these projects by doing so you allow me to dedicate a lot more resources into making the gadgets and experiments for videos like this one thanks for that and thank you to everyone else for just watching and leaving me comments i really do value what you have to say and i read all of them your feedback is likely to determine how far i go with this particular series so if you like it please let me know that's it for this video thank you for watching i'll see you next time

Channel: NightHawkInLight

Views: 253,850

Rating: 4.9739499 out of 5

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Id: kkBBkQ8jFRY

Length: 19min 28sec (1168 seconds)

Published: Thu Mar 18 2021

Reddit Comments

[u/nvaus]

This video is part two of a series on thermoacoustics, but it's standalone in the sense that you don't need to watch the first part to make sense of this one. The topics covered in each video are different. Hopefully that fits in the rules about multi part videos vs related series.

[u/controlzee]

This guy was absolutely outstanding. You spoke articulately and he didn't rush. I wound up watching the entire thing and then another.

[u/MaddyJean]

I just watched both of these and loved them!

[u/MightySamMcClain]

It must have been pretty good bc I'm supposed to be working and still watched the entire episode hehe

[u/moofyre]

I never thought I'd watch so.ething like this from.front to back seeing that I know nothing about such things or my.hatred of physics, but this was so effing cool,and the dude who presented the material in such a way that I learned from it.

Thanks! I'm always learning and you're helping the path!