Explosive limits of gases mixtures

Hello

I am searching for all kind of informations about explosive limits of
mixtures , like mixture of oxygene with other gases.
For example, what are the ratio those can be used without explosions,
under several pressure and T¤C conditions.
Can anyone share some knowledge in this area?

Benoit Bulliot

(Benoît BULLIOT) wrote in message . com...

I am searching for all kind of informations about explosive limits of
mixtures , like mixture of oxygene with other gases.
For example, what are the ratio those can be used without explosions,
under several pressure and T¤C conditions.
Can anyone share some knowledge in this area?


This is a difficult request, as there is no formal definition of "explosion
limits."

You should also be careful with the use of the term "explosion." When used
in a combustion context, "explosion" typically refers to "a run-away
chemical reaction," not the "bang & boom" that the popular usage of the word
"explosion" implies. The "explosion limit" of a combustible gas mixture
refers to critical conditions where gas will support self-sustained
reaction, e.g., a flame.

The more appropriate terms are "flammability limits" and "detonability
limits." Searching under those terms may be more productive for you.

Flammability limits are concentrations that will still support
self-sustained combustion, while detonability limits are concentrations
where a supersonic combustion wave (i.e., detonation wave) can propagate.

While you may find published values of flammability limits (e.g., for
hydrogen/air, you will often find 4% volumetric cited as the value of lower
explosion or flammability limit), but these values are somewhat arbitrary
and should not be taken as absolute. In fact, you can almost always design
a clever experiment and force the mixture to successfully burn at lower
concentrations.

The issue here is what causes limits: typically, this is a competition
between energy release (via chemical reactions) and energy loss (via heat
transfer, drag, etc.). For example, flammability limits are determined by
heat losses to the walls, heat being pulled away from the flame by buoyancy,
etc. Thus, flammability limits are usually defined by a standardized test,
such as in a 5-cm-diameter, open-ended plexiglass tube (the Coward & Jones
apparatus). Even then, the limits depend on upward vs. downward
propagation, which reflects the role of buoyancy. If you eliminate
buoyancy, then flammability limits can be much wider (as witness by the
"Flame Ball" experiments on STS-107, in which *very* diluted combustible
mixtures would weakly burn in microgravity).

Your best reference for standard flammability data at different temperatures
and pressures is the classic text by Lewis and von Elbe:

Lewis, B., and von Elbe, G.,
Combustion, Flames, and Explosions of Gases,
3rd Ed., Academic Press, 1987.

...this is available in any library.

The issue for detonability limits is similar that of flammability limits:
the limits are a function of the tube diameter. For propagation in tubes,
momentum and heat transfer losses to the tube walls will result in
detonation waves failing if the tube is made too small, although the
"cutoff" not well defined, with a lot of very messy near-limit behavior.

If your interest is detonability limits, the best data to examine is the
critical energy required to initiate detonation. For example, for
stoichiometric methane-air, the critical energy require to initiate
detonation is equivalent to about 50 kg of TNT (~200 MJ). Since we rarely
encounter energy sources this large, methane-air is not typically considered
a detonable mixture. But it *will* detonate if hit hard enough.

As you go rich or lean, the critical energy required increases dramatically,
giving a "U"-shaped dependence on critical energy with respect to
stoichiometry. In principle, *any* combustible mixture is capable of
supporting detonation, if a large enough energy source is applied. In
practice, as mixtures become very lean or rich, the critical energy
increases exponential to huge values. This can be used to define a
detonability limit, but the definition is always a bit arbitrary.

If want data on detonability limits, there is a convenient database on the
Web, complied and maintained at CalTech:

http://www.galcit.caltech.edu/detn_db/html/

This will give you data on critical energy and minimal diameter of
propagation. As long as you use gas mixtures in a tube smaller than the
minimum tube diameter and do not encounter energy sources larger than the
critical energy, there is little danger of detonation.
--
Andrew J. Higgins Mechanical Engineering Dept.
Assistant Professor McGill University
Shock Wave Physics Group Montreal, Quebec CANADA
http://www.mcgill.ca/mecheng/staff/academic/higgins/