Untitled - Aerobib - Universidad Politécnica de Madrid

274 CHAPTER 11. SIMILARITY IN COMBUSTION. APPLICATIONS
11.5 Flame stabilization
In the preceding chapter we have studied the aerodynamic field of a flow with a flame
stabilized by a holder. We verified that the expansion of the burnt gases may originate
a chocking effect which imposses a limitation to the maximum velocity of the flow
entering the chamber.
Another limitation to this velocity, bearing a great practical interest, is that impossed
by the need to fix the flame to the holder in order to maintain combustion.
In fact, it has been experimentally observed that when the velocity of the incoming
flow exceeds a certain value, the flame blows out. The blowing velocity of the flame
is determined by the composition and state of the combustible mixture and by the
characteristics of the holder. The problem arising from this circunstance holds a great
practical interest in the new propulsion systems, specially in ramjets and after-burners,
in which their limits of practical application are often determined by this effect. Consequently,
this phenomenon has been the subject of a particular interest during the last
years and abundant experimental data have been gathered on a great variety of types of
holder and on the influence of the parameters of the mixture on the blowing velocity
of the flame. An extensive bibliography on the matter can be found in the proceedings
of the congresses and colloquia on combustion celebrated in recent years.
The initial study on this problem was carried out by Scurlock [16], who pointed
out the significance of the recirculation zone that forms in the wake of the holder. Figure
11.2 taken from Ref. [17] illustrates the phenomenon. Behind the holder a recirculation
zone forms in which the flame stabilizes. The boundary of this zone consists
in a mixing zone formed by a free boundary layer which separates the unburnt gases
from the combustion products. Within this mixing zone is where the combustible
gases ignite through a process of heat and mass transport, whose characteristics are
not yet well known.
Since the presence of a recirculation zone is essential for the existence of the
flame, the holders used for this purpose are given a shape which produces a large
aerodynamic wake.
In many of the tests conducted, attempts were made to establish a correlation
between the blowing velocity of the flame, in a mixture of given composition and
temperature, and the size of the holder, characterized by a linear dimension of the
same, for example by its diameter in the case of a holder of circular section as the one
shown in Fig. 11.2. A summary of the works performed from this stand-point, up to
1953, and of the state or knowledge at the time may be found in Ref. [18].