Untitled - Aerobib - Universidad Politécnica de Madrid

12.1. INTRODUCTION 287
remains appreciably constant as the velocity of the jet increases. Fig. 12.6 gives
some of the results of the measurements done by Gaunce [9] with city gas in a 3
mm diameter burner. The three aforementioned zones are clearly shown in this
figure.
24
20
Distance from nozzle (inch)
16
12
8
4
Total length, on−port flames
Break point length
Flame separation begins here
Upper part of flame blows−off
0
0 50 100 150 200 250 300
Nozzle velocity (ft/s)
Figure 12.6: Effect of nozzle velocity on flame length.
Hottel and Hawthorne [3], Wohl, Gazley and Kapp [10], Yagi and Saji [13]
and Barr [5] have attempted an extension of Burke and Schumann’s method to the
prediction of the length of open flames both laminar and turbulent. Through rudimentary
approximations they obtain an expression for the flame length containing an
unknown function which they determine empirically from the results of their experiments.
Fig. 12.7 taken from Ref. [5] shows the theoretical and experimental lengths
of some laminar open flames as functions of the fuel flow rate. This figure gives an
idea on the agreement to be expected between experimental measurements and those
predicted by semi-empirical formulae. The extrapolation to values not included in
this figure is risky. A summary on the state of knowledge regarding this matter can
be found in the work by Hottel listed in Ref. [14] where additional bibliography is
included.
Lately, J. A. Fay [15] has calculated the shape and characteristics of the laminar
diffusion flame obtained when a fuel jet discharges into the open atmosphere for the
two-dimensional case and for the case with axial symmetry. Fay has taken into account
the influence of the variation of the velocity in cross direction to the jet, computing
the cross distributions of the velocities, concentrations and temperatures. The model