Effect of the equivalence ratio, Damkoehler number, Lewis number and heat release on the stability of laminar premixed flames in microchannels

摘要

The effect of the equivalence ratio on the stability and dynamics of a premixed flame in a planar micro-channel with a step-wise wall temperature profile is numerically investigated using the thermo-diffusive approximation. To characterize the stability behavior of the flame, we construct the stability maps delineating the regions with different flame dynamics in the inlet mass flow rate m vs. the equivalence ratio Φ parametric space. The flame stability is analyzed for fuels with different diffusivity by changing the Lewis numbers in the range 0.3 ≤ Le_F ≤ 1.4. On the other hand, the Lewis number of the oxidizer is kept constant and equal to unity Le_O = 1. Our results show that, for very diffusive fuels, the stability of the flame varies significantly with the equivalence ratio, transitioning from stable flames for lean mixtures to highly unstable flames when Φ＞ 1. As the fuel Lewis number approaches unity, the stability behavior of the flame for lean and rich mixtures becomes more similar to give, in the equidiffusional case Le_F = 1, a symmetric stability map around the stoichiometric mixture Φ = 1. In all cases considered, the most stable flames are always found around the stoichiometric mixtures Φ = 1, when the flame instabilities are completely suppressed for very diffusive fuels Le_f ＜ 1, or are reduced to a narrow range of inflow velocities for fuel Lewis numbers equal or greater than unity. The ratio between the size of the channel and the flame thickness d turns out to be of great importance in the stability behavior of the flame. Keeping the rest of parameters constant, an increase in d for lean flames makes the flame considerably more unstable, confirming the findings of previous works. Nevertheless, as the stoichiometric ratio approaches Φ = 1, that trend is reversed to give flames that become more stable as the size of the channel is increased.