Untitled - Aerobib - Universidad Politécnica de Madrid

286 CHAPTER 12. DIFFUSION FLAME
at the flame tip. When Reynolds number of the jet is increased this region propagates
towards the flame base.
4) When the tip of the flame becomes turbulent its length decreases as Reynolds
number is increased until turbulence reaches its base. There on it remains constant
independently from the discharge velocity of the jet.
5) Within the laminar region the length of the flame is independent from the burner’s
diameter and for a given fuel it depends only on its flow rate. Such result agrees
with that obtained by Burke and Schumann for confined flames.
6) Within the turbulent region the length of the flame is proportional to the diameter
of the tube and independent from the fuel velocity.
Diffusion flames
Transition region
Fully developed turbulent flames
Envelope of flame heights
Height
Increasing nozzle velocity
Envelope of
break points
Figure 12.5: Change in flame type with increasing nozzle velocity.
Fig. 12.5 taken from the work by Hottel and Hawthorne summarizes the experiments
performed by these authors. This figure shows the law of variation of the height of
the flame with the velocity of the jet, and the extension of the turbulent zone. The
following states are observed:
a) Laminar. When the velocity of the jet is small. Here the length of the flame
increases, at first proportionally to the jet velocity and then slows down up to the
maximum height that limits the laminar zone.
b) Transition. The turbulent zone of the flame initiates close to the tip and when
the velocity of the jet increases it propagates towards its base. The total length
of the flame decreases when the extension of the turbulent zone increases. This
is such because the diffusivity of the reactant gases is much larger in turbulent
than in laminar state.
c) Turbulent. The whole flame is turbulent throughout its extension except within a
small zone close to the mouth of the burner. In this state the length of the flame