Smallest possible flame ?

[Externet]

As a flame is a chemical reaction, am posting here; hope it is a right
place.

Remember learning a long ago that there is a size limit for a flame, I
believe cannot be smaller than ~3mm, no matter how small (a wick) or
what is burning. What is the theory/fact about it ? Would a spark/
ambers arrester also block the propagation of a flame ?

[m***@aol.com]

Post by Externet
As a flame is a chemical reaction, am posting here; hope it is a right
place.
Remember learning a long ago that there is a size limit for a flame, I
believe cannot be smaller than ~3mm, no matter how small (a wick) or
what is burning. What is the theory/fact about it ? Would a spark/
ambers arrester also block the propagation of a flame ?

A lot depends on your definition of a flame. Assume here that a flame
is a self sustaining combustion reaction in open air that utilizes
atmospheric oxygen and a volatilized fuel source. There a few problems
that you run into with very small flame.

Foremost heat would disperse very rapidly from the flame undercutting
the ignition of further material. A higher atmospheric temperature
would decrease the minimum size of flame while a lower temp would
increase it. If the ambient temperature was the same as the ignition
temperature this factor would allow a minimum flame size of zero.

At room temp, the temperature of ignition and the amount of energy
released would effect the minimum size. The lower the ignition
temperature of the fuel air mix and the more energetic the reaction,
the smaller the flame could be. Sticking to fuels that don't supply
their own oxygen, perhaps cyclopropane or acetylene would be near
ideal. I have seen candle flames pushing ~3mm and wax is not as
volatile as these two gases so I think that they could get
significantly smaller in a carefully controlled stream.

The stability of the fuel air mix is probably one of the most
important practical issues. A slight breeze can easily change the gas
mix and carry away enough heat to break the self sustaining reaction.
A sealed chamber would be necessary. At a small enough size the air
circulation generated by the flames heat itself might disperse the
flame. This circulation could be decreased by raising the ambient
temperature or making the flame in a lower gravity environment. At
very small flame sizes and high temperatures, diffusion itself would
probably be enough to disperse reaction products and mix reactants.

[Bill Penrose]

Post by Externet
As a flame is a chemical reaction, am posting here; hope it is a right
place.
Remember learning a long ago that there is a size limit for a flame, I
believe cannot be smaller than ~3mm, no matter how small (a wick) or
what is burning. What is the theory/fact about it ? Would a spark/
ambers arrester also block the propagation of a flame ?

While you're at it, imagine the smallest possible bubble, another
conundrum in chem engineering.

Dangerous Bill

[Marvin]

Post by Externet
As a flame is a chemical reaction, am posting here; hope it is a right
place.
Remember learning a long ago that there is a size limit for a flame, I
believe cannot be smaller than ~3mm, no matter how small (a wick) or
what is burning. What is the theory/fact about it ? Would a spark/
ambers arrester also block the propagation of a flame ?

There is a temperature gradient between the core of the
flame, where the reactions take place, and the surroundings
at (often) room temperature. The physics of energy flow
over the gradient sets a limit on how small that distance
can be. I'd guess that the minimum distance is related to
the mean-free-path of a molecule in the flame.