Untitled - Aerobib - Universidad Politécnica de Madrid

302 CHAPTER 13. COMBUSTION OF LIQUID FUELS
If the diameter of the droplet is larger than, for example, a few tenths of a
millimeter the phenomenon becomes more complicated. Such size is too large to
allow evaporation across the heating zone of a flame. Therefore, in this case individual
diffusion flames surrounding the droplets are produced.
For technical applications the fuel is normally introduced in the combustion
chamber by means of an atomizer producing droplets of different sizes. These droplets
spray and evaporate in contact with the atmosphere of the chamber, which is strongly
turbulent. Furthermore, in the combustors of jet engines, the gases flow at high velocity
through the combustion chamber. Generally, in such cases not enough space
is available for the complete evaporation of the fuel before it reaches the combustion
zone. Consequently when the fuel reaches the said zone it is only partially evaporated
and the mixing with the oxidizing atmosphere is incomplete. The entire process starts
with the entrance of the fuel in the combustion chamber and ends when the combustion
products are formed. The complete study of this process includes the following stages:
atomizing, spraying, mixing, evaporation and combustion of the fuel jet. Due to the
complexity of the process a study in detail can not be attempted with any probabilities
of success. Therefore, we must resort to empiricism and to the study of simplified
physical models which emphasize some of the features of the phenomenon.
Hereinafter we shall first briefly refer to the atomization, spraying and mixing
of fuel jets and then, more closely, to the combustion of isolated droplets. Finally the
evaporation of droplets with no combustion will be considered.
13.2 Atomization
A great effort has been made both theoretically and experimentally for the study of
the atomization of fuel jets [8], [9]. The process depends on:
1) The geometrical characteristics of the atomizer.
2) The physical properties of the liquid and the atmosphere in which it discharges.
3) The working conditions.
Depending on the discharge velocity of the liquid and on the conditions of the surrounding
atmosphere, several states of atomization can be observed [9]. In the discharge
at high velocity, normally produced in burners, the atomization starts at the
nozzle of the atomizer. Atomization is produced by the turbulence of the liquid, whose
radial velocities tend to brake the jet, and by the action of the atmosphere at the outlet.
Surface tension and viscosity of the liquid oppose to the jet atomization. The prob-