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Does Combustion Create a Vacuum?

  1. Mar 13, 2014 #1
    I've heard that fire creates a vacuum due to the consumption of oxygen. Obviously, combustion results in other gases being released (CO2 and H2O vapor). Some have said that there is no vacuum; others have said there is in fact a partial vacuum. I assume that it depends on the relative densities of the gases (i.e. how does the density of the oxygen + fuel compare to the density of the byproducts).

    But if a burning fire does not create a vacuum, what causes the expansion and contraction cycle which leads to the observation of smoke "puffing" through holes and cracks when a room has the potential for a backdraft fire? Is it just that the gases in the room are cooling and contracting, which draws in fresh air, leads to more combustion, and then produces heat and expansion with the cycle repeating? Or does combustion produce a partial vacuum in some instances?
     
  2. jcsd
  3. Mar 14, 2014 #2

    maajdl

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    Gold Member

    The "amount of gases" (in moles) does not usually change much during combustion.
    But the increased temperature makes the gas expand, its density decreases and its volume increases.
    Lower density implies a buoyancy force.
     
  4. Mar 14, 2014 #3
    It depends on what the products of the combustion are and what is burning.

    I've seen this done with a strip of magnesium. Magnesiumoxide is a solid, so the oxygen will be consumed and no new gases are produced.

    Burning carbon won't do this, because you get one molecule of CO2 for each oxygen molecule consumed.
     
  5. Mar 14, 2014 #4
    I suppose it would be helpful to shrink the box down to a manageable size so we can all think inside the box and still be on the same page. Let's assume we are talking about a fire burning with a carbon-based fuel, say paper.

    If I can distill two potentially separate issues:

    1. Since carbon is being liberated from the solid to a gas, does this change the pressure of gas in the air? I realize that the "room" will still contain the same number of atoms when considering the amount of fuel and amount of air, but since there is a change of state for some of those atoms I am wondering if there is also a change in density. I suppose (feel free to rebut) that the change in density which is caused by the change in temperature is far greater and may therefore render any other changes in the total gas volume irrelevant. Thoughts?

    2. There is still the mystery of the puffing observed in backdraft (fires which have been deprived of oxygen because they are in a sealed environment but remain at a temperature which is higher than the flash point of the fuel); what causes this?

    Also, maajdl, I understand that a change in density will cause a buoyancy force, but where are you going with that thought? I'd like to follow you, but I can't see the light at the other end.

    Thank you, maajdl and willem2, for the thought and replies thus far.
     
  6. Mar 15, 2014 #5
    Why do you not logically try to attempt to see what happens, with what maajdi has stated, rather than have it all explained to you.

    Here are some questions for you to help you out:
    What effect does buoyancy have on objects?
    What does a lighter object, or material of lesser density do when immersed in a denser material, such as a piece of wood in water?
    What does the wood do if pushed down below the surface and released?

    Would a less dense gas behave the same way if immersed in a denser gas?

    How does this relate to the gases produced in a fire and the surrounding air?
    Can this explain the way some people talk about fire producing a vacuum?
     
  7. Mar 15, 2014 #6
    Here are some more questions:

    What are the three items necessary combustion?
    If one or all of the conditions are not met, does the fire continue to burn?

    Of what are the hot gases in the backdraft scenario that you describe composed?
    What is missing to make these hot gases not burn ( if the gases really are a fuel )?
    If the hot gases are inside the hot room, what is outside the hot room?
    At the cracks and holes what do you think is happening?
     
  8. Mar 15, 2014 #7
    Thank you 256bits, for correcting my apathy ever so gently. I really do appreciate your challenge.

    The bouyant force will allow the gas with greater density to sink in the room, thereby causing the gas with lower density to rise. This will, to some extent, set up convection currents. Interestingly, the convection currents themselves may generate areas of low pressure in the room if there are obstructions in the flow (Bernouli's Principle), although I do not know whether they would be significant.

    Low density gas may have two distinct causes. First, the fire will warm the gases closest to it and will cause thermal expansion in the gas. This is the cause of ordinary convection currents which we observe in campfires, fireplaces, and wood stoves even, which currents are the cause of vacuums at the base of the fire.

    Second, different gases have different densities because of their chemical makeup. The density of O2 at NTP is 1.429 kg/m^3; density of CO2 is 1.977 kg/m^3 (source: http://www.engineeringtoolbox.com/gas-density-d_158.html). This means that the CO2 byproduct will tend to stay low in the room while O2 is forced toward the ceiling (which may explain why fire fighters are trained to vent rooms by breaching high).

    So CO2 is denser than O2 and there would be a partial vacuum created by combustion of carbon based materials. But, I conclude that the expansion due to heat is greater than the potential loss in density due to the change in chemistry. Any vacuum caused in a container or room which is almost entirely sealed has as its major cause the subsequent contraction of cooling air.

    The fire, in a backdraft situation will cause the volume of gas in the room to expand, forcing air and smoke out of any opening (cracks, holes, gaps, etc.). This process will continue until the fire runs out of one of the three necessary items (heat, oxygen, fuel). (Note, this may seem counterintuitive in an ordinary fire where a flow exists because we see air being forced in continually at the base of the fire.) While structure fires do not generally lack oxygen, it could happen in a sealed room where heat is contained and plenty of fuel is available. Air would be pushed out until the fire reached its critical point at which time the density of air in the room would stabilize. From that point, the room would begin to cool down which would produce lower density, creating a partial vacuum. The vacuum would be filled with air from the "hallway," which presumably is denser, colder, and has more oxygen.

    The colder, denser, oxygen-rich air would expand as it heats up inside the room. This may be partially responsible for the puffing, but would not likely sustain the condition. The introduction of oxygen into the room would allow further combustion, producing additional heat, returning the fire to its previous state (hot, but lacking oxygen).

    I've written too much already. I apologize for my many words as I think out loud.

    Thoughts?
     
  9. Oct 10, 2015 #8
    I have been wondering about the same phenomenon too.It came to my attention when i attended a lecture on crankcase explosion in marine diesel engines, where the lubricating oil mist burns to create a partial vacuum which is held as the culprit that causes the devastating secondary explosion.How on earth is the burning of lubricating oil mist cause this vacuum?The same engine uses the high pressure generated by burning gases in closed spaces to work.I was told by some that the consumption of oxygen causes this vacuum but it makes no sense as the same amount of carbon dioxide and water vapor are created which compensate for the loss in volume.Another argument was that the product is denser than the reactant(oxygen) but that too doesn't hold as any two gases occupy the the same volume when equal moles of both are taken in the same temperature and pressure(Avogadro's law I think).Then there is the heat output of the reaction which should cause a rise in temperature and pressure.So clearly the burning of lub-oil in this case should create higher pressures.
    One possible solution is that the higher pressures created by the primary explosion causes some of the gases to escape,while the remaining hot gases inside gradually cools after the fire subsides and create this low pressure.It's like the classical shrinking bottle trick where one would shake the bottle with few drops of some volatile fuel,drop a lighted match into it and close the mouth of the bottle.The bottle would then shrink due to the partial vacuum created inside(the trick is mostly done using nearly empty liquor bottles).Applying the same principle in the crankcase the high pressures of the primary explosion should be causing some of the gases to escape and the rest of it should be gradually cooling afterwards to create the vacuum.But escaping the crank case is difficult for the gases even at higher pressures since the case is supposedly airtight.I am guessing the relief valve lets out the gases when there is a sudden rise in pressure.Then remaining gases cool to create this partial vacuum.
    I thought of sharing this idea when I read the above articles.If anyone has some more ideas on the subject please let me know.......
     
  10. Oct 11, 2015 #9
    Get a pan and put 1/2" water in it. Take a small wad of newspaper, put it in the pan on a stand (I used a paper clip) and light it. Immediately put a clear glass over it so the open end is below the water line. The burning paper will quickly go out. What does the water do? (Hint: prepare to be amazed.)
     
  11. Oct 13, 2015 #10
    There is heat exchange between the gas in the room and the material in fire for the combustion to continue but the whole temperature in the room keep raising. Combustion causes the density of gas in room to decrease and cause the turbulence of gas in the room to cause the movement of gas to the fire and from the fire and cause oxygen to be absorbed into the room and CO and so on to be puffed out of the room.
     
  12. Oct 13, 2015 #11
    Related to this question, would an explosion create a vacuum as the air is forced away from the epicentre? At least very temporarily at least.
     
  13. Oct 13, 2015 #12
    At the moment of explosion,there is materials and gas emitted out of the epicenter and so there is not vacuum there at that moment.
     
  14. Oct 20, 2015 #13
    I'm guessing that he was asking about a time shortly after the explosion. Outward moving gasses will have momentum, and so will continue moving outward after the pressure has equalized, thus, pulling vacuum at the epicenter of the explosion. Makes sense to me, but I'm not an explosives expert so there are likely factors I'm not thinking of.
     
  15. Oct 20, 2015 #14
    At the moment of explosion, there is big outward momentum on materials and gases from the explosion center away outwards. I think it is difficult to create the vacuum at the center. There should be outward momentum gradient and so pressure gradient from the center away. And how many forces is needed to push and pull all air out of the center? It is not likely to create the vacuum.
     
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