Untitled - Aerobib - Universidad Politécnica de Madrid

13.14. COMBUSTION OF FUEL SPRAYS 325
tions demonstrate that the extinction velocity is proportional to the droplet diameter.
Experimental results confirm such prediction. Spalding also arrives to the conclusion
that, even when no convection exists, the flame extinguishes if the diameter of the
droplet is very small.
13.14 Combustion of fuel sprays
The present theory enables the calculation with good approximation of the burning
velocity of an isolated droplet under laboratory conditions. In particular, this theory
allows the study of the influence of the physical characteristics of the fuel and the
state of the surrounding atmosphere on the burning velocity of the droplet. Now these
results must be extended to the study of the combustion of fuel sprays of the type
existing in the combustion chambers of jet engines [29]. Such an extension has not
yet been achieved. In the actual state of knowledge this seems impossible due to the
interaction of the many factors of the process. In fact, for this extension to become
possible it is necessary to know in advance the distribution of droplets in the spray
and the characteristics of the surrounding atmosphere, in a system in which the fuel
is partially in the liquid phase and partially in the gaseous phase as it enters the combustion
zone. It is also necessary to take into account the interaction of droplets and
the influence of the highly turbulent motion of the gas. All this makes the theoretical
study of the problem extremely difficult. For this reason the studies made up to date
are of a highly empirical character. These studies limit themselves either to an analysis
of the influence of some fundamental parameters on the combustion efficiency of a
burner [30], or to obtain general conclusions as for the way in which several variables
can influence the process [24].
13.15 Droplet evaporation
When combustion is absent the evaporation process of a fuel spray is also a very
complicated phenomenon. This problem has only been studied empirically for some
typical cases [16]. Bahr [30], for example, has given an empirical formula to express
under given conditions the evaporated fraction as a function of the distance to the
atomizer. The Bahr formula represents a good approximation to the results obtained
from his own experiments and those performed by Ingebo [16].
As a first step towards the solution of the problem, the evaporation of isolated
droplets can be studied by the same procedure applied in the preceding paragraphs