The Naked Scientists

The Naked Scientists Forum

Author Topic: which one has more powerful explosion LEL or UEL or between them ?  (Read 4245 times)

Offline taregg

  • Sr. Member
  • ****
  • Posts: 168
    • View Profile
same question


 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
LEL: Lower explosive limit
UEL: Upper explosive limit

If you have a mixture of gases, the LEL and UEL are the points where they would just support an explosion.

At concentrations below the LEL, or above the UEL, an explosion does not occur.

Your most powerful explosion should occur when the the reducing gas (hydrocarbon) and oxidizing gas (oxygen) are at stoichiometric ratios which should occur somewhere between the LEL and UEL.
 

Offline damocles

  • Hero Member
  • *****
  • Posts: 756
  • Thanked: 1 times
    • View Profile
LEL: Lower explosive limit
UEL: Upper explosive limit

If you have a mixture of gases, the LEL and UEL are the points where they would just support an explosion.

At concentrations below the LEL, or above the UEL, an explosion does not occur.

Your most powerful explosion should occur when the the reducing gas (hydrocarbon) and oxidizing gas (oxygen) are at stoichiometric ratios which should occur somewhere between the LEL and UEL.

The last sentence would be true for a thermal explosion, where propagation and magnification of the explosion depended solely on thermal feedback -- energy release as heat of reaction.

Reactions with oxygen, though, have very complicated mechanisms. They usually produce radical explosions, where the propagation and magnification arise from an exponential build up in the concentration of reactive free radicals in addition to thermal feedback.

This means that the composition that produces the most powerful explosion can be shifted away from the stoichiometric mix, and, indeed, that in some cases the stoichiometric mix is even located outside the explosive limits.

Example: methyl chloride; tabulated explosive limits for 1 atm air and 20C 10.7-17.4%. Stoichiometric mix 7.8%

 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Thanks,
So explosions aren't necessarily determined by stoichiometry.

However, I'm having troubles visualizing a situation in which the explosive strength wouldn't at least be at a local minimum at either the LEL or UEL.  The strongest explosion may not be exactly between the two, but it would have to be somewhere between the two.
 

Offline damocles

  • Hero Member
  • *****
  • Posts: 756
  • Thanked: 1 times
    • View Profile
Thanks,
So explosions aren't necessarily determined by stoichiometry.

However, I'm having troubles visualizing a situation in which the explosive strength wouldn't at least be at a local minimum at either the LEL or UEL.  The strongest explosion may not be exactly between the two, but it would have to be somewhere between the two.

That would certainly be the case nearly all of the time. And the stoichiometric mix will usually give the strongest thermal feedback. But the radical mechanisms do complicate the picture considerably, and they are not completely understood. According to the wikipedia page, for example, a state-of-the-art model for ethane combustion needs to take into account about 200 different elementary reaction steps.

I think that your statement is right -- if an explosive limit were anything other than a local minimum, why would a mixture immediately next door fail to propagate an explosion? And a stoichiometric mix will probably provide the strongest thermal feedback, although in my very limited experience explosions seem to be more violent and erratic when a fuel/air mixture is running lean.
 

Offline taregg

  • Sr. Member
  • ****
  • Posts: 168
    • View Profile
http://tkolb.net/tra_sch/Flammable/FlammableRange.gif....you can see from this  imge the powerful explosion and pressure between LEL and UEL...  Is it right
 


Offline damocles

  • Hero Member
  • *****
  • Posts: 756
  • Thanked: 1 times
    • View Profile
The shape in the diagram you have linked is quite correct except that in any particular system the peak might not be exactly in the middle, but closer to the LEL or to the UEL. It will probably be close to the middle even if it is not exactly there.
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums