Untitled - Aerobib - Universidad Politécnica de Madrid

Chapter 13
Combustion of liquid fuels
13.1 Introduction
The present chapter is devoted to the study of some of the problems of the combustion
of liquid fuels. In certain cases surface reactions can take place in the liquid phase, for
example, in the combustion of hypergoles. On the contrary, when a fuel burns in an
oxidizing atmosphere the reaction takes place in the gaseous phase. Thereby, the fuel
evaporation must preceded combustion. In such case, if the mixing of the fuel vapours
and the oxidizing gas takes place before the chemical reaction, a flame produces of
the type studied in chapter 6. On the other hand, if mixing and combustion take place
simultaneously, a diffusion flame is obtained, similar to those studied in chapter 12.
This is the type of combustion considered in the present chapter. Khudiakov [1], [2]
has studied the characteristics of the diffusion flame that forms on the free surface of
a fuel burning in the atmosphere. Spalding [3] has also studied this problem taking
into account the influence of free and forced convection. He made this study as an
application of his uniform method [4] for the study of all the mass transport processes
(absorption, evaporation, condensation and combustion).
Of further technical interest is the case where fuel forms droplets of small
diameter. Fuel mists formed by droplets of very small diameter (smaller than a few
hundredths of a millimeter) burn forming a flame similar to the premixed gas flames
[5], [6]. In particular, in the said mists, like in the premixed gas flames, the propagation
velocity of the flame, its flammability limits, etc., are well defined magnitudes. Their
values are close, but not equal, to those corresponding to premixed gas flames [7].
The difference is probably due to the droplets evaporation effect. The diameter of the
droplets is so small that the evaporation produces within the heating zone of the flame,
so that the fuel reaches the reaction zone in gaseous phase.
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