High pressure temperature rise and smoke

[motioncity]

this is what I did: I attached a hose to an empty plastic bottle, sealed it with plastic paste and put a cork in it. When i turned the water on the pressure started builiding up. What I noticed was that when it reaches about 2 bar it gets a lot hotter. I don't know why, do you? When the cork finally shoots out at about 2.5 - 3 bar , there is this grey smoke coming out of my bottle and it smells a little bit like chlorine gas, what is this smoke and where does it come from?

you can watch a QuickTime movie http://users.pandora.be/k-a-d/P1010016.MOV"

 

[Kurdt]

First of all nice movie. pressure and temperature are related by a linear equation pv=nrt. when the volume is fixed and the pressure rises to balance the equation something on the left must increase aswell. In this case its temperature.

Secondly to address the gas. I know part of the water treatment process is to put chlorine in water to kill germs and the likes and make it safe to drink. I imagine the high pressure you put the water under forced it to separate from any remaining chlorine gas. Also fluorine is put into water in some areas to maintain healthy teeth, and seeing as fluorine is of the same family i'd guess it would have a chlorine type smell.

 

[Q_Goest]

Hi Kurdt, the ideal gas law equation is part of it, but there's too many unknowns to resolve it from that. You need to use the polytropic forumula and make some assumption about heat transfer. If compression were isentropic, the polytropic exponant would be the ratio of specific heats. If heat transfer takes place, that value drops until it eventually becomes an isothermal process and the exponant becomes 1. Raising the pressure from 0 to 2 atmospheres results in the air getting quite hot if there's no heat transfer and we assume isentropic conditions. In this case, the temperature is on the order of 260 F (from 70F).

Regarding the chlorine smell, I don't know what that is, but I'd not suspect the water, nor fluorine. It may be coming from the plastic as the temperature may be hot enough to react with air.

 

[Hootenanny]

The solubility of gasses decreases when a solution is heated, so it is possible that the chloride ions were coming out of solution when the temperature was increased. In the US drinking water contains about 0.2ppm of dissolved chlorine, on average the human nose can detect about 0.01ppm of chlorine, therefore if only 5% of the chlorine came out of solution it would produce a detectable oder.

-Hoot

 

[Q_Goest]

Hoot, interesting thought. But if that were true, I'd have to believe that boiling water would give off a distinctive chlorine odor.

 

[Kurdt]

In England I can smell the chlorine in our water straight from the tap in a glass. If i boil it I cannot smell the chlorine perhaps because of the overwhelming volume of steam involved aswell.

 

[Kurdt]

Actually just to add. Friction of the flowing water could add to the heat felt on the bottle. I know that its not exactly as simple a system as I at first indicated but I thought it was an adequate explanation for the purpose.

As for the gas, I have looked at the video again and noticed the foreign gas only appears after the pressure is released (i.e. when the cork goes). If memory serves me correctly i read somewhere that water under high pressure retains all the gases dissolved in it but if there is a sudden drop in pressure those gases are easily released. I'm no expert on fluid dynamics at all but maybe Q_Goest could give some details.

 

[motioncity]

Actually just to add. Friction of the flowing water could add to the heat felt on the bottle.

I should have pointed out that I could only feel the gas getting hotter. When I held the bottle at the bottom like in the video, I didn't feel any heat at all. If you would like me to film anything else a little more detailed, let me know and I will post the movie here. After repeating the experiment in a windless environment it was also obvious that the gas is considerably heavier than air because it lowered to the ground immediately, so Cl2 would be a fine candidate. I've also noted that the exerted gas is not hotter than regular air that day, so that must mean the temperature drops really quickly upon release of the cork.

The cork can currently go really high (I shot all the way over my house). I volunteered to be shot at and it did hurt; which means the cork has some kinetic energy. I don't know a way to measure the exit (not average) speed of the cork but I doubt there is any way. If anyone has a neat idea for tweaking the cork gun, let me know.

 

[Kurdt]

Well you could explain the gas getting hotter by the fact that the bottle was not empty to start off with and so you managed to fill about half the bottle with water before it blew the cork. Water in this situation is fairly incompressible so the gas law would explain that. The bottom will be cold because you're adding cold water to the bottom all the time. The gas produced i can only imagine is chlorine from the water due to the sudden air pressure change over the water.

 

[russ_watters]

The "smoke" is water vapor, but I don't know what could cause the smell.

The rapid air pressure change causes water vapor in the air to condense. The Navy launches torpedoes with 1200psi air: http://www.de220.com/Armament/Torpedoes/Torpedo%20Photos/Mk32-Launching-Mk46.jpg

 

[kleinjahr]

Russ is right.

Smell may be caused by the plastic heating up. If it is chlorine, you'll know it right quick, as it "bites" your nose.