Stability of the normal flame front

Conclusions

We have investigated the instability of the normal flame in propane-air mixtures at initial pressures from 1 to 9 atm abs in a small chamber (volume 5 cc).

Using a more sensitive method than previous experimenters, we have established that the flame Reynolds number at which instability occurs, with flame radius as characteristic dimension, is of the order of 10³.

It has been shown that the average size of the cells resulting from instability decreases as the flame propagates and the pressure of the unburned mixture correspondingly increases. In this case the Reynolds number Re_c, in which the average cell dimension is taken as the characteristic dimension, remains in the (2–4) · 10² range.

The fact that Re_c remains constant indicates that this quantity characterizes the internal instability of the flame. A critical value of Re_c of the order of 10² does not contradict the theories that take into account the stabilizing effect of viscosity on flame stability.

When the instability of the normal flame becomes more or less significant, the criterion K [2] for a spherical flame passes through unity.

The criteria K and Re_c should be used to estimate the possibility of instability of normal combustion. In this case all the contradictions between theory and experiment are eliminated. If K>1, then instability should always develop in the presence of perturbations whose wavelength satisfies the relation.

$$\operatorname{Re} _c \geqslant (2 - 4) \cdot 10^2 $$

where the wavelength is taken equal to the average cell dimension.