Untitled - Aerobib - Universidad Politécnica de Madrid

Chapter 7
Turbulent flames
7.1 Introduction
There is experimental evidence that turbulence increases the propagation velocity of
the flame through a combustible mixture. This is a very important fact in technical
applications since it allows a considerable reduction of the space and time required to
burn a given mass. The influence of turbulence over combustion was first recognized
by Mallard and Le Chatelier in 1883 [1]. However the attempts made to determine the
causes of this influence as well as to estimate quantitatively its value are quite recent.
The basic problem lies in determining the propagation velocity of the flame through a
combustible mixture in turbulent motion knowing the characteristics of the turbulence
and the state and composition of the mixture.
From the experimental stand-point, several techniques are available for the determination
of the flame velocity. However the basis for such techniques are not as
solid as those applied to the case of laminar flames and the measurements taken are
not as numerous or systematic. Among them, the technique more commonly used
consists in photographing a flame, obtained for example in a bunsen burner, then measuring
the area of the combustion front and dividing it by the flow rate of the burner
as is done for the case of laminar flames. One of the difficulties of such techniques
is the fact that the combustion front of a turbulence flame is not well defined since
the long exposure photographs show a thick luminous zone (see Fig. 7.1a) whereas
a short exposure “Schlieren” photograph reveals a very irregular and wrinkled structure
(see Fig. 7.1b). These circumstances impose the introduction of an arbitrariness
in estimating the flame area. Damköhler [2], for instance, adopted as surface of the
turbulent combustion front the one that limits internally the luminous zone. Later on,
207