Sciencemadness Discussion Board

Whoosh bottle experiment

Wrecking Bereserker - 8-4-2019 at 10:32

There is an experiment of burning alcohol in a polycarbonate bottle right?
How does it work and why shouldn't we use glass bottles while plastic works?

[Edited on 9-4-2019 by Wrecking Bereserker]

unionised - 8-4-2019 at 11:24

"why shouldn't we use glass bottles while plastic works?"

MrHomeScientist - 8-4-2019 at 12:46

It's called a "woosh bottle." Research exactly how much alcohol (and what type) to use before trying it, otherwise you run the risk of explosion. Which, incidentally, is why you don't want to use glass.

BromicAcid - 8-4-2019 at 14:39

I saw this done twice, both times were with the super-thick old time glass water jugs. At the time I was like "Oh, that makes sense, it can contain the pressure." I'm guessing the polycarbonate splits rather than sends out shards then, correct?

[Edited on 4/8/2019 by BromicAcid]

Morgan - 8-4-2019 at 17:00

I think even the plastic ones are said to be good only for so many demos perchance they weaken over time.
I found that if you spark start some bottles by tucking an ignition wire inside the bottle instead lighting it from the top, it's more likely to go bang instead of whoosh, the flame front propagates somewhat suddenly so to speak. The first time I found that out sprinkling the tiniest amount of a powdery platinum on alumina catalyst inside a meager glass gallon jug with methanol for fuel. There were these little glowing talc-like dust specks softly floating down into the jug as they spontaneously reacted with the methanol vapor and then out of nowhere a sudden incredible bang. Fortunately the bottle held up.
Here's a mundane test with an exhaust tube added.

mayko - 8-4-2019 at 17:11

Polycarbonate failure:

Glass failure:

The second guy is lucky to have his eyesight....

Morgan - 8-4-2019 at 18:17

Around the 6 second mark whatever fuel/air ratio they had going was rather lackluster, but typical drums of that motif have killed people. Methanol can be quite perky when ratios to air are ideal. Low humidity and mild room temperatures help create energetic conditions. Hot containers can throw off fuel/air ratios and cooler, denser air probably helps the cascade exponentially.

"At the bomb range, the Build Team attached a drum to a wheeled dolly and ignited the methanol remotely with a road flare. Their attempts only caused the kart to move a few feet, so they returned to the workshop for small-scale testing. Reducing the fuel/air ratio allowed a slight improvement in distance, but bubbling air into the methanol (for better vapor mixing) gave a much larger increase until the kart set itself and the test track on fire. When the team attached a funnel-shaped exhaust nozzle and injected the fuel through an atomizer (creating a pulse jet engine), they could safely propel the kart to a distance of 15 ft (4.6 m)."

"Never use methanol" ha

Vomaturge - 8-4-2019 at 20:45

Another "safety tip" I've seen widely repeated is to have "enough" excess liquid to "slow down" combustion. Both of the ruptured bottle videos mayko linked to had comments blaming a (sic) "stochastic" amount of fuel vapor, as opposed to obstructed exhausts (both bottles had an obstruction in the neck), or the fact that the match fell through the plastic bottle and ignited the whole volume faster than the deflagration would have by itself. Even Flynn scientific has a video with a warning about not using enough liquid.
Calculated with data from Wikipedia, engineering toolbox and chemicools gas mol calculator, methanol will form a perfectly oxygenated mixture when it makes up 12.3% of the air by volume. To do that in a 18.9l bottle at 20C requires 3.9ml* of alcohol to vaporize. Air saturated with it at 101.3 kPa and 20C will contain 12.85% methanol. For ethanol the values are 6.54%, 3ml*, and 5.95% respectively, and for 100% isopropyl alcohol they are 4.46%, 2.7ml*, and 4.4%. For methane, propane,butane and dimethyl ether measured in gas phase, the perfect concentrations and volume for a 18.9 l container are 9.5%/1.8L, 4%/760ml, 3.1%/590ml, and 6.5%/1.23L (White Rain hairspray has DME and a bit of EtOH, and based on small scale tests 15 seconds of spraying should make a flammable mixture in that size vessel. Not sure how much is the perfect amount)
I have had what was supposedly the stoichiometric amount of methanol fail to ignite when sprayed in a closed container, but 5 times that amount worked just fine. The excess made it easier to coat the walls and get enough evaporation. But depending on the precise ambient temperature and the time it has to evaporate, this could make the reaction either more or less vigorous. So concentration is not equal to saturation, but rather (evaporation rate-ventilation)*(patience of the experimenter).
Excess alcohol can create that really cool pulse jet effect after the initial burn.
It can also melt the bottle. Or, if your 30 seconds of shaking a a few hundred excess ml of cold alcohol failed to save you from "stochastic" disaster (I guess that is a somewhat appropriate word for how why and when whoosh bottles become bang bottles) or if the bottle is on it's side, there can be a flaming alcohol spill. The glass bottle the teacher shattered was a mild case.
Oxygen balance rant over.
My own stupid mistake, which I beg you not to repeat, is don't light it with a match, and certainly not a lighter. Use a long stick. Even with a 1L bottle, that was an instant race change operation for my index finger. And not on the first time I did it. The YT experts who are holding matches haven't been slow to withdraw their arm, or had an extra fast reaction, or had a flame deflected toward them yet.
*maybe reduce the liquid amounts by about 10% if you heat the whole device with hot water to 50C to vaporize all the fuel. That should compensate for the lower air density at elevated temperature.

[Edited on 9-4-2019 by Vomaturge]

j_sum1 - 9-4-2019 at 03:43

Quote: Originally posted by mayko  
lass failure:

The second guy is lucky to have his eyesight....

The full comment by Explore Chemistry is worth a read.
Nice observation skills.

Wrecking Bereserker - 9-4-2019 at 08:43

Quote: Originally posted by j_sum1  
Quote: Originally posted by mayko  
lass failure:

The second guy is lucky to have his eyesight....

The full comment by Explore Chemistry is worth a read.
Nice observation skills.

Why isn't glass a pressure vessel?i mean what is a pressure vessel?
Are these explosions because of the air expands inside rapidly or somekind of waves?

Why doesn't the flame travel to the inside of the alcohol burner through the wick causing a whoosh bottle?

[Edited on 9-4-2019 by Wrecking Bereserker]

Morgan - 9-4-2019 at 10:15

As pointed out, its not a good idea to occlude the opening of a bottle restricting the flow. If that was a meter stick, the flat side with tape and match or whatever could have very well cut off a sizable amount of the exhaust area. If you freeze the video at the 18 second mark, you can see a large orange feathery flame off-center of vertical following the stick 3/4 of its length up
That half full translucent jug on the desk with the orangy liquid kind of looks like it could be a nitro fuel product, typically a nitromethane/methanol blend. But that's only a guess. Maybe liquid soap, who knows.
It's conceivable the thick glass could have been previously stressed or just near instantly heat stressed to crack at some point, maybe around the neck at the first hint of the whoosh-bang event. Even borosilicate can't always be trusted.
When the flames die down a bit, they take on a blue color just as he walks in front of the flames. It's not certain if there were trace residues in the bottle before or on the desk but barring adding oxygen to the bottle I don't know of any common fuel that burns as perky as straight methanol. I've tried ether, acetone, gasoline, nitro/methanol blends, ethyl and isopropyl alcohol, and you'd be hard pressed to get the fuel/air ratios just right, the alcohols more forgiving. Methanol has a wide window in which it will burn rich or lean.
I had a can of ethyl ether and could never even get a jam jar jet to run using it. But also I've had it unexpectedly fireball in front of me when pouring some on the ground and striking a match standing near the fumes, it was a very cool day.
To sum up, it could be some other fuel, but don't discount the potential of methanol to shatter a glass bottle such as the one seen.
Anyway notice the oblique orangy flame at the 18 second mark if you can freeze it just right and the flame color as he walks in front of them around the 37 second mark. I should say it takes some patience to get it to freeze or stop it just as the orange feathery flames appear around 18 seconds up the stick.

Another event

[Edited on 9-4-2019 by Morgan]

MrHomeScientist - 9-4-2019 at 10:57

I believe BromicAcid is right in that the plastic splits while the glass shatters. Thus the glass bottle is a much more dangerous situation of flying shrapnel, rather than the plastic which stays mostly intact. Also, the plastic bottle can stretch a little from the pressure and absorb some of the energy, making a container failure less likely.

Morgan - 9-4-2019 at 11:46

One thing about some plastic bottles is that the bottom will flex so much so that the bottle jumps up in the air and if you have excess fuel it may spill out lighting something on fire.

Vomaturge - 9-4-2019 at 12:18

@jsum yeah, that guy had a fair understanding what happened. And he explained more than I did, in less words:)

@Morgan, good eye seeing that color change in the fire on the table. My guess is 70:30 isopropyl alcohol:water or similar. That will make a slightly sooty flame, but as it burns out you basically get less and less fuel and more steam. Lucky thing there wasn't much of it, and it mostly stayed on the table.

@Bereserker for a flame to spread through an opening, that opening must be large enough that the hot gases don't lose all their heat to the walls.
There is not a big enough hole in or around the burners wick to let the flames pass. Especially not with convection also pushing the flame upwards.

What's a good pressure vessel? Not a 5 gallon glass or plastic water jug, apparently. Most of the time the expanding hot flames vent without too much pressure, but any fuel that can make a whoosh can potentially burst the bottle IMO. What if your alcohol is a hair warmer or cooler one time, creating a more powerful fuel ratio? Or you get a turbulent flame, or a spark falls from your igniter and causes an abnormally fast burn? Or your bottle has a defect or scratch that weakens it? These are variables that are tough to control.
The idea of using a plastic bottle, and a limited amount of alcohol (not more than 30ml, I'd recommend, and drain excess after mixing) is just to reduce the danger (sharp fragments, splashes of burning liquids) if it does burst. Doing it outside with a lab partner and safety glasses might help too, in the event of an accident.

[Edited on 9-4-2019 by Vomaturge]