Untitled - Aerobib - Universidad Politécnica de Madrid

226 CHAPTER 8. IGNITION, FLAMMABILITY AND QUENCHING
and graphics may be found in Ref. [1], giving the inflammability limits of several
mixtures of hydrocarbons with air, under different pressures. Ref. [3] supplies an
abridged review of the problem. The Proceedings of the Fourth International Symposium
on Combustion include, as well, several papers on the subject, Coward and
Jones, [9], reviewed the state of knowledge up to 1952, in a report including an extensive
experimental and bibliographic material. Two recent, very interesting, reviews
are those by Egorton [10] and Linnot-Simpson [11].
At present, the situation of the problem is such, that the causes for the existence
of inflammability limits are still unknown and, even more, it is questioned
whether they actually exit as quantities governed only by the state and composition
of the mixture or, to the contrary, they depend upon the characteristics of the experimental
device used. Lewis and von Elbe [3] suppose that the existence of such limits
may be due to the internal stability of the combustion wave, which then would be unstable
beyond the limits. Several attempts to study the problem from this stand-point
have been made, [12] and [13], in addition to the one by Lewis and von Elbe. Of
all, the most satisfactory is own to Werner and Rosen [14], who analyze the internal
stability of the wave with respect to small disturbances of temperature by means of a
linearization of the system of Eqs. (8.7), (8.8) and (8.9) around their stationary solution.
These authors solved the resulting system after introducing several simplifying
assumptions regarding the disturbances of the composition of the mixture and of the
mass flux. They reach the conclusion that the stability of the wave depends on the
behavior of the disturbances of Y with respect to those of T . Although this study
must be considered as uncompleted it suggests the possibility for some waves to be
internally unstable, which could explain the existence of inflammability limits. An
application of this theory to the ozone-oxygen flame is given in Ref. [15], the result
of which shows that the flame is stable for ozone rich mixtures and unstable for loan
ones. However, the experiments carried out by Streng and Grosse [16] disclose that
the flame is stable even for loan mixtures.
Recently Spalding [17] has analyzed the possible influence of heat losses of
the flame on its behavior. He reaches the conclusion that there exist two different
flame velocities, and that the smaller is unstable. When heat losses augment the velocities
approach one another and finally coincide. Spalding identifies the coincidence
of velocities with the inflammability limits. Although this work does represent an important
contribution, yet an unpublished work, being carried out at the Combustion
Laboratory of the INTA, reveals that his conclusions are not free from criticism.