Untitled - Aerobib - Universidad Politécnica de Madrid

222 CHAPTER 8. IGNITION, FLAMMABILITY AND QUENCHING
8.2 Ignition
Considering a combustible mixture capable of propagating a flame, in order for this
to happen it is necessary to supply it with a certain amount of energy, located within a
small volume and reduced interval of time, to initiate the wave. The study of this process,
both theoretically and experimentally, has been the subject of numerous reports.
Among the various sources of energy that may be utilized, the most adequate is the
electrical spark because it is capable of steering great amounts of energy in reduced
spaces and times. For this reason it is the most widely used and has been the subject
of more systematic and complete experimental studies, of which a description may be
found in Refs. [1] and [2].
The fundamental problem of ignition lies on determining the minimum energy
required to ignite a mixture of given composition, at known pressure and temperature.
The results of experiments have pointed out that to each mixture it corresponds a well
defined minimum ignition energy, which increases very rapidly with the decrease in
pressure. Such energy results to be independent from the distance between electrodes,
provided this distance exceeds a given minimum value under which it grows very
rapidly and finally the flame cannot propagate.
Several theories have been attempted for the calculation of this minimum energy
with fair success. The present state of knowledge on this problem may be considered
as comparable to the situation existing 20 years ago respect to theories on
flames.
All the theories developed are based on the following model. Let us consider
a small volume V of gas to which energy H is instantaneously communicated, rising
its temperature from T 0 to T f . Starting from this instant, the gas contained in V
tends to cool, by thermal conductivity, heating the gas layers surrounding V . On the
other hand, the chemical reaction which produces in the heated mass releases heat,
which tends to compensate the cooling effect. From the balance between this two
phenomena, it will depend that the flame may progress towards a wave of the type
described in Chap. 6, or, to the contrary, that it extinguishes.
The available theories differ in the development of this idea and on the conditions
applied to determine the minimum volume V and the necessary energy H. For
instance, Lewis and von Elbe [3] assume that volume V is the one corresponding to
the quenching 1 distance and that the energy is determined by the excess enthalpy of
the flame. The validity of this concept does not appear clearly justified, although the
1 See §4.