Combustion - put on your physics cap

Prophetsnake · Jul 29, 2014

OK, here's my question. In a low pressure environment, say 4 PSI, that has been compensated with a breathable amount of O2, as they do in a spaceship, is combustion more, less or about the same. In other words, if I were to try and light up a cigarette with a match in gas that had more or less the same O2 density as you'd find at sea level on Earth, would the match and cigarette behave more or less the same? Would the absence of an inert gas make a difference?

Thanks!

robjvargas · Jul 29, 2014

I'm forgetting the math here, but I don't think you can have sufficient oxygen to breathe in a 4 PSI environment.

That said, fire doesn't need nearly as much oxygen as a breathing person.

You can *survive* (sort of) for a short time as low as 6%. So says OSHA.

On the other hand, some combustible materials, like thermite, have enough oxygen in the substance to sustain combustion even in a vacuum. So "combustion" isn't a simple question to answer.

By the way, 4 PSI is (roughly) 1/3 regular atmospheric pressure. Breathing isn't the only problem at that low a pressure. This link is more about high altitude effects, but most of them are based on a response to reduced barometric pressure.

King Neptune · Jul 29, 2014

There is at least one more question that has to be answered first: What is the local gravity. What Vargas cites assumes an Earth gravity, but things are different in low gravity where there may be no gravity to give the atmosphere pressure. The 6% might be convertible to molecules per volume, which would not depend on gravity.

cmhbob · Jul 29, 2014

You might reach out to Randy Cassingham, of This is True newsletter fame. He used to work at JPL, and I know he still has connections. His email addy is on the website. I think it's arcie@, but can't recall, and lost my email address book recently.

robjvargas · Jul 29, 2014

King Neptune said:
There is at least one more question that has to be answered first: What is the local gravity. What Vargas cites assumes an Earth gravity, but things are different in low gravity where there may be no gravity to give the atmosphere pressure. The 6% might be convertible to molecules per volume, which would not depend on gravity.

I'm fairly sure that isn't true. I'm thinking of Boyle's Law. Also of Pascal's Law.

Gravity might invoke a certain amount of pressure, but 4 PSI is 4 PSI on Earth, on Mars, on Jupiter, on Neptune, and so on.

I've seen some articles that they've tested fire in microgravity, and it burns a lot slower because the flow of oxygen through the combustion zone gets altered. But what (more or less) static pressure does is unaffected by gravity.

King Neptune · Jul 29, 2014

robjvargas said:
I'm fairly sure that isn't true. I'm thinking of Boyle's Law. Also of Pascal's Law.

Gravity might invoke a certain amount of pressure, but 4 PSI is 4 PSI on Earth, on Mars, on Jupiter, on Neptune, and so on.

I've seen some articles that they've tested fire in microgravity, and it burns a lot slower because the flow of oxygen through the combustion zone gets altered. But what (more or less) static pressure does is unaffected by gravity.

You probably are right, but both of those laws refer to gases in an enclosed space; although Boyle's law it less restricted.

Atmospheric pressure is a consequence of gravity acting on gases. If there were no gravity, then the gases would dissipate. But we don't know for certain whether the OP is about open atmosphere on an enclosed volume; although it seems to refer to open "air".

I believe that in an unenclosed volume, say the surface of a planet, the atmospheric pressure is a result of gravity, and a planet with a gravity a sixth of Earth's would have a surface pressure one sixth of Earth's atmospheric pressure, and this is roughly the case on Mars. But the OP will have to clarify the question.

Prophetsnake · Jul 29, 2014

Ok. Closed space, low gravity (moon) and 4psi but nearly 100% O2 for the people to breathe. Normal match, normal cigarette.

robjvargas · Jul 29, 2014

I still don't think 4 PSI can support breathing. Oxygen has to infuse into the blood, and I don't think 4PSI supports that.

But I'm well outside any objective proof to back that up.

Prophetsnake · Jul 29, 2014

robjvargas said:
I still don't think 4 PSI can support breathing. Oxygen has to infuse into the blood, and I don't think 4PSI supports that.

But I'm well outside any objective proof to back that up.

It can, that much I do know. It's precisely what NASA used in their early spacecraft . But it's irrelevant. It's only flAme I'm interested in here. I've seen someone trying to light up at 17,000 feet and he had a hell of. A time getting his lighter to go, but of course that was in a normal oxy/nitrogen mixture.
By the way, he did get it lit and started hacking uncontrollably after the first drag.

blacbird · Jul 29, 2014

Prophetsnake said:
I've seen someone trying to light up at 17,000 feet and he had a hell of. A time getting his lighter to go, but of course that was in a normal oxy/nitrogen mixture.
By the way, he did get it lit and started hacking uncontrollably after the first drag.

I just gotta ask: Why in Bokonon's Holy Universe is somebody trying to light up a smoke at 17,000 feet altitude?

Well, for that matter, why in Bokonon's Holy Universe is somebody trying to light up a smoke at sea level? But that's another discussion.

caw

Duncan J Macdonald · Jul 30, 2014

One would also ask if the pressure is 4 psia or 4 psig. One assumes that the OP means 4 psia, but then one remembers what "ass-u-me" means.

King Neptune · Jul 30, 2014

It fdoes appear that the OP means psia.

In that case the match and the cigarette would burn about the same as they do in open air on Earth. The biggest difference would be that the smoke would be denser and would not dissipate as quickly, because there would be less gas to dilute it and to carry it away.

wendymarlowe · Jul 30, 2014

I know a flame will burn itself out almost immediately in zero G - if there's no air flow, there's nothing to bring fresh oxygen to the area right around the flame, so it uses up whatever oxygen is immediately nearby and then sputters. I don't know what very low G would do, but I'm guessing it might have a similar effect. (Adding in a ventilation system, wind, etc. could change this.)

Kevin Nelson · Jul 30, 2014

I'm reasonably sure that the level of nitrogen would make little difference. I suspect the gravity would be more of an issue, as others have pointed out. But my guess is that with lunar gravity, the flame would still burn pretty well. I wouldn't be surprised if no one really knows for sure.

Prophetsnake · Jul 30, 2014

I just gotta ask: Why in Bokonon's Holy Universe is somebody trying to light up a smoke at 17,000 feet altitude?

Heh. I'm pretty sure Bokonon's creator would have too.
The guy smoked eighty a day and it had been more than fifteen minutes since his last. He was completely mystified by the coughing fit, incidentally. Jesus, I felt like hacking as well. We were using O2 intermittently, well, me more than him because the mask was in the way of his cigarette, of course.

Prophetsnake · Jul 30, 2014

And yes, absolute. I wouldn't ask a how long is a piece of string question. And let's eliminate G as a factor. Say it's a box, a room, on Earth. Gravity is normal. (It isn't in the story, but this is confusing the issue)
Essentially, the idea is that you have a box full of air, standard N-O2 mix. You remove enough nitrogen so that the pressure in the box which for simplicity, let's say was a bar and is now one third of that. So, you have precisely the same number of oxygen molecules as you had before but at a reduced pressure. Their spacing in relation to one another is identical. Ignore any temperature variation just to keep it simple.
Now, if we had simply reduced the pressure without changing the two gasses ratio the flash point would be lower, bu twitch exactly the same amount of oxidiser present?

King Neptune · Jul 31, 2014

Prophetsnake said:
And yes, absolute. I wouldn't ask a how long is a piece of string question. And let's eliminate G as a factor. Say it's a box, a room, on Earth. Gravity is normal. (It isn't in the story, but this is confusing the issue)
Essentially, the idea is that you have a box full of air, standard N-O2 mix. You remove enough nitrogen so that the pressure in the box which for simplicity, let's say was a bar and is now one third of that. So, you have precisely the same number of oxygen molecules as you had before but at a reduced pressure. Their spacing in relation to one another is identical. Ignore any temperature variation just to keep it simple.

My answer about made this assumption.

Now, if we had simply reduced the pressure without changing the two gasses ratio the flash point would be lower, bu twitch exactly the same amount of oxidiser present?

If you mean that you would remove air (O2 and N2) until the pressure was less than 4 PSI, then you would add a different oxidizing gas to bring the pressure back up to 4 psi and so that the air would have as much oxidizer as if it were pure O2. By necessity it would be pure O2 or O2 equivalent, so my earlier answer would apply.

Prophetsnake · Jul 31, 2014

If you mean that you would remove air (O2 and N2) until the pressure was less than 4 PSI, then you would add a different oxidizing gas to bring the pressure back up to 4 psi and so that the air would have as much oxidizer as if it were pure O2. By necessity it would be pure O2 or O2 equivalent, so my earlier answer would apply./QUOTE]

No, I meant just removing the nitrogen.

It's a low pressure high oxygen environment that contains roughly the same volume of O2 as normal earth air, it's just missing most of the neutral gasses. It's an environment meant to live in.

Combustion - put on your physics cap

Prophetsnake

robjvargas

King Neptune

cmhbob

Did...did I do that?

robjvargas

King Neptune

Prophetsnake

robjvargas

Prophetsnake

blacbird

Duncan J Macdonald

Plotting! Not Plodding!

King Neptune

wendymarlowe

writer, mother, geek

Kevin Nelson

Aspiring to authorship since 1975

Prophetsnake

Prophetsnake

King Neptune

Prophetsnake