Untitled - Aerobib - Universidad Politécnica de Madrid

8.4. QUENCHING 227
8.4 Quenching
So far studies have dealt with flames propagating through unlimited mixtures, not
taking into account the influence of the proximity of walls,
A set of phenomena exist, of great importance, relative to the behavior of
flames close to walls which have been the subject of such attention in recent years.
Among those phenomena are quenching, blow-off and flash-back.
Experiments prove that when the diameter of a tube full of a combustible gas
is sufficiently small, it is impossible for a flame to propagate through it. This phenomenon,
which has been recognized for a long time and which has been applied to
the prevention of explosions, is known as “quenching”.
The way in which the walls of the tube act to prevent flame propagation is
not well understood at present, The most direct explanation is to attach the effect to
the cooling of the flame due to the proximity of the walls. However, it is though
that chemical action of the wall when acting as chain breaker may also be significant.
Analogously to what happened for some time with flame theory, if one considers either
cooling or diffusion as the only acting effect, a “thermal” or “diffusive” theory will
be obtained. Examples of thermal theories are those by Lewis-von Elbe [1] and by
Kármán-Millán [18], and on diffusion theories the one by Simon-Bellos [19].
The difference between this case and the unlimited flame theory lies on the fact
that both the formulation of the equation and boundary conditions and their solution
are far more difficult. Hence, the solutions may only be attempted through drastic
simplifications.
A great number of measurements have been carried out with the purpose of
determining the quenching distance in ducts of different cross-sections and the influence
on it, of the mixture. For an additional study of this question as well as on the
phenomena of blow-off and flash-back we refer the reader to Refs. [20] through [23].
In particular references [20] and [23] include an extensive bibliography.
References
[1] Lewis, B. and von Elbe, G.: Combustion, Flames and Explosions of Gases.
Academic Press, New York, 1951.
[2] Lewis, B, Pease, R. N. and Taylor, H,S.: Combustion Processes. Vol. II of High
Speed Aerodynamics and Jet Propulsion, Princeton University Press, 1956.