Untitled - Aerobib - Universidad Politécnica de Madrid

12.1. INTRODUCTION 283
In fact, otherwise, some of the species would diffuse across the flame and react with
those on the other side.
In order to judge the validity of this model Hottel and Hawthorne [3] performed
some experiments to determine the composition of the gases in a diffusion flame of
hydrogen burning in the open atmosphere, taking samples of the gas at different points
of the flame by means of a special sound. Details of these experiments can be found
in the said work. Figure 12.2, taken from it, shows the results of the sounding made
in three cross sections of the flame. The results of such measurements prove the
correctness of Burke and Schumann’s model.
100
75
50
25
H 2
flame front
distance from port= 30.5 cm
Percentage in dry sample
0
100
75
50
25
0
100
O 2
H 2
N 2
N 2
axis of flame
flame front
O 2
distance from port= 22.85 cm
75
50
25
H 2
N 2
flame front
distance from port= 15.25 cm
0
O 2
0 10 20 30 40
Radial distance (mm)
Figure 12.2: Gas composition in a hydrogen diffusion flame
Burke and Schumann have successfully applied their model to the calculation
of the shape and height of laminar diffusion flames for the case where the fuel jet
discharges within a tube where an air stream moves with the same velocity than the
fuel jet, Fig. 12.3. This device eliminates the difficulties originating from momentum
transfer between the fuel and the surrounding air when their velocities are not equal.
Furthermore, for this reason it is easier to obtain a stable flame. By introducing several
drastic simplifications, to be considered further on, Burke and Schumann were able
to calculate the shape of the flame. Some of their results are outlined in Fig. 12.3.
Fig. 12.3 (a) corresponds to an overventilated flame, which closes at the axis of the