Untitled - Aerobib - Universidad Politécnica de Madrid

11.5. FLAME STABILIZATION 277
Flame holder
geometry
D
D
1/8 inch
3/16
1/4
τ i
3.09 × 10 −4 s
2.85
2.80
D
1/4 3.00
mesh screen
D
D
1/4
3/8
1/2
3/4
1/4
3/8
1/2
3/4
2.38
2.70
2.65
2.58
3.46
3.12
3.05
3.03
D
3/4 3.05
D
3/4 2.70
Table 11.1: Ignition time for different types and dimensions of flame holders.
Since the existence of the flame is governed by the possibility of igniting the
mixture, while it travels along the mixing zone, the residence time τ r of the mixture
within this zone must be an important factor of the phenomenon. The residence time
may be expressed by the ratio
τ r = L (11.56)
V
of length L of the mixing zone to the velocity V of the gases when travelling along
it, which is approximately the velocity of the free flow. Consequently, if this interpretation
of the phenomenon is correct, the blowing velocity may be reached when τ r
reaches a limiting value τ i , below which there is no time for ignition. Furthermore,
τ i must depend exclusively on the composition and state of the mixture and on the
conditions at the free boundary layer, but not on the shape of the holder or any other
mechanical factor.
In order to check the validity of this assumption, Marble and Zukoski computed
the value of τ i for a set of quite different cases. Results are summarized in Table