HOLDING AN EXPLOSION at 20,000 fps - Smarter Every Day 156

Hey, it's me Destin. Welcome back to Smarter Every Day (SED). So excited about this video! In the last episode of SED, I showed you my transparent potato gun, at 3,000 fps during the day. 3, 2, 1. But this episode it's going to be the same transparent potato gun, meaning we can look directly into the science at night, at 20,000 fps. Google up! Science is about to happen.

What happens if I start the ignition of the potato gun from the back? Vs. What happens if I start it from the middle? I always thought it would have worked better if you go from the middle, 'cause you get even expansion of those gases. But, let's record it in high-speed, and let's see what the difference is between the two. We'll go from the back first! We got some potato juices in there. 3, 2, 1 (laughter). It's like 9 o'clock.

Alright, let's see it in high speed. It's completely awesome. We saw that it started at one end and went all the way to the end. Now, I'm gonna take this, it's a grill-ignitor. You can see the little zaps there. I've got electrodes set up in the center of the potato gun and we're going to zap it from the center, and we should get a more even combustion, right? So, let me hook it up and then we'll give it a shot. Literally.

Loading the potato gun again. 3, 2, 1 (laughter). It doesn't get old! Was that cool, or what? Ok, so check it out, so this is my combustion chamber, right? The first time I shot this thing, it was a flint-sparker on the end of this thing, right? If I hold it, you can see a twist and I get the spark. What you saw on the second time - here are my electrodes that I told you about going to hit this button you can see down in there. Can you see what I'm sparking right there in the middle? That's how I fired that thing.

When we first start the combustion from the end, it's a wave; you've got an oxygen/fuel mixture that goes all the way down to the combustion chamber. When we start right in the middle, we start to expand, we start using that fuel and using that oxygen, but we run out of oxygen before we burn all the fuel. We're going to shoot this thing from the end. We're going to zoom in with a high-speed directly on the combustion chamber and we're gonna see if we can see that flame front moving: one Mississippi, two Mississippi, three Mississippi, four Mississippi, three, two, one.

Ok, before I show you the next slow mo, I want you to understand that it has enough information in it to provide for several PhDs, so I want you to be on the lookout for a couple of key things that I find interesting.

#1 When the explosion moves towards the closed off into the combustion chamber, unless the fuel/air mixture is just perfect, sometimes you'll run out of oxygen and have unburned fuel. This creates a compressed pocket of fuel that stays unburned until it turns into a rarefaction and pulls more oxygen back in through the barrel, to burn it up.

#2 There's something that happens in combustion chambers called "sidewall cooling." Basically, the pipe itself cools the gases down below the combustion point, so it can't burn right next to the wall. This leaves more unburned fuel and actually counts for most of the pollution products caused by the engine in your car.

The slow-motion you're about to see is real. I haven't ran the speed to make it look more interesting. It's just pure awesome footage, straight out of high-speed cameras. Watch what happens, as the explosion over expands, the system has to equalize so it pulls air back in through the barrel, into the combustion chamber, which causes that fresh oxygen in the air to meet up with the remaining fuel and start the burning process all over again.

It's a pretty inefficient way to burn a fuel rich mixture; so I think my original hypothesis might be wrong. I think it's cool that when the gas squeezes down into the barrel from the combustion chamber, it has to speed up so that it can maintain the exact same flow rate.

One of the things I love about science, is the ability to look at a complex dynamic system like this and try to understand every physical phenomenon that's occurring, but equally important to me personally is the ability to look at footage like this and say "you know, that's pretty."

To summarize, what we've learned tonight, think about this: when you have a combustion chamber, we've learned that it depends not only on the localized chemistry or the stoichiometry, the fuel/air mixture at any point inside that combustion chamber; it also depends on how it's vented. It's really fascinating; the dynamics of venting a combustion chamber affect directly the ability to create localized pressures and rarefactions.

Sometimes at the end of the video, when the ad part comes up, you kind of roll your eyes, because you're not excited about it. Well, this time is different, you see, SED is supported by audible.com and they have supported me for a really long time and it's a really big deal because it's kind of expensive to afford videos like this.

So, what I'm going to do is I'm going to create an audible event so that you can remember to support SED at audible.com/smarter. We're going to create the same flame front that we did before, only we're not going to contain both sides of it; we're going to vent one side. I want you to listen to this huge audible vent and then we'll look at high speed; ready? 3, 2, 1... It's on fire!

If you want to support SED, feel free to go to audible.com/smarter. That's a very audible event, I hope you agree. Anyway, they have tons of free audiobooks. If you get a free trial at audible.com/smarter, I'm super stoked that they have chosen to support Smarter Every Day for so long. They've been a very valuable partner, and I am very appreciative.

So, if you want to support SED directly, go to audible.com/smarter, get a free trial of audiobooks; you're going to get smarter and you're going to like it. So there you go, I hope you feel like this video on your subscription. If not, that's it.

I'm Destin. You're getting smarter every day. Have a good one. (inaudible) I... going at watermelons. Yeah, and I'm recording in slow motion.

You are? Yeah. Is that cool? - That's what they all say... You don't do it at night too? Is that all right? - It's your property, do what you want to do on it. You do anything you want to do over here! You ain't going to bother me. I think it's time to go to bed.