Untitled - Aerobib - Universidad Politécnica de Madrid

208 CHAPTER 7. TURBULENT FLAMES
(a) Time exposure photograph
(b) Instantaneous Schlieren photograph
Figure 7.1: Stoichiometric natural gas-air flame, Re = 25 000, burner tube diameter =
5.08 cm (by courtesy of Bureau of Mines).
preference was given to the use of the mean surface between those internally and externally
limiting the luminous zone [3] or else to the surface where luminosity reaches
its maximum intensity [4].
Additional difficulty lies in the fact that the paths of the gas particles are not
known. Therefore it is risky to identify the fraction of unburnt mixture corresponding
to each element of the flame front. Such difficulty has often been eluded by measuring
the mean velocity obtained when the total flow rate of the mixture is divided by the
area of the flame front. The propagation velocity of turbulent flames has been measured
with these or similar techniques by Damköhler [2] , Bollinger and Williams [3],
Scurlock [4], Williams, Hottel and Scurlock [5], Karlovitz, Denniston and Wells [6],
Wohl, Shore, von Rosemberg and Weil [7], Leason [8], Bowditch [9], Wohl and Shore
[10], Mickelsein and Ernstein [11], etc.
Figure 7.2 taken from Ref. [3] shows, as an example, the results of the measurements
performed by Bollinger and Williams in mixtures of several hydrocarbons
and air. For each case the mixture giving the maximum velocity was used. This figure
also shows the laminar velocities corresponding to the same mixtures. It is seen that
the effect is maximum in the acetylene flame where the value of the laminar flame ve-