Untitled - Aerobib - Universidad Politécnica de Madrid

276 CHAPTER 11. SIMILARITY IN COMBUSTION. APPLICATIONS
Maximum blowoff
velocity (ft/s) φ m
2.0
1.5
1.0
600
400
200
100
10
2 4 6 8 10 2 4 6 8 10
Reynolds number
3 4 5
Figure 11.3: Maximum blowoff velocity and corresponding equivalence ratio for a cylindrical
flame holder.
Maximum blowoff velocity (ft/s)
1000
800
600
400
300
200
150
100
4
2.5 4 6 8 10 1.5 2 3 4 6 8 10 5 1.5
Reynolds number
Figure 11.4: Maximum blowoff velocity for different types of flame holders (see [22] for
more details).
Both figures show clearly the existence of a transition Reynolds number, approximately
equal to 10 4 , which separates two different regions. For Re < 10 4 , the
maximum blowing velocity corresponds to a composition different from the stoichiometric
one, which furthermore varies with the Reynolds number, whilst if Re exceeds
the transition number the composition remains constant. Likewise, for Re > 10 4
the blowing velocity varies with the square root of the Reynolds number, while for
Re < 10 4 it follows a different law.
Zukoski and Marble, Ref. [17], have demonstrated that such a change is due to
the fact that the free boundary layer of the mixing zone changes from laminar to turbulent.
This transition is important because the transfer of heat and chemical species between
unburnt gases and combustion products, within the mixing zone, occurs through
laminar diffusion under subcritical conditions and through turbulence in the opposite
case.