This publication is totally included in Chapter 4 of this PhD thesis, but it is not indicated as a reference in the main text.

No abstract

The propagation of premixed flames in adiabatic and non-catalytic planar microchannels subject to an assisted or opposed Poiseuille flow is considered. The diffusive-thermal model and the well-known two-step chain-branching kinetics are used in order to investigate the role of the differential diffusion of the intermediate species on the spatial and temporal flame stability. This numerical study successfully compares steady-state and time-dependent computations to the linear stability analysis of the problem. Results show that for fuel Lewis numbers less than unity, Le F < 1, and at sufficiently large values of the opposed Poiseuille flow rate, symmetry-breaking bifurcation arises. It is seen that small values of the radical Lewis number, Le Z , stabilise the flame to symmetric shape solutions, but result in earlier flashback. For very lean flames, the effect of the radical on the flame stabilisation becomes less important due to the small radical concentration typically found in the reaction zone. Cellular flame structures were also identified in this regime. For Le F > 1, flames propagating in adiabatic channels suffer from oscillatory instabilities. The Poiseuille flow stabilises the flame and the effect of Le Z is opposite to that found for Le F < 1. Small values of Le Z further destabilise the flame to oscillating or pulsating instabilities.

This is a postprint version of the following published document:Sánchez-Sanz, M. ; Fernández-Galisteo, D. ; Kurdyumov, V. N. Effect of the equivalence ratio, Damköhler number, Lewis number and heat release on the stability of laminar premixed flames in microchannels. Combustion and Flame, 2014, 161 (5) The effect of the equivalence ratio on the stability and dynamics of a premixed flame in a planar microchannel 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 / parametric space. The flame stability is analyzed for fuels with different diffusivity by changing the Lewis numbers in the range 0:3 6 Le F 6 1:4. On the other hand, the Lewis number of the oxidizer is kept constant and equal to unity Le O ¼ 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 / > 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 ¼ 1, a symmetric stability map around the stoichiometric mixture / ¼ 1. In all cases considered, the most stable flames are always found around the stoichiometric mixtures / ¼ 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 / ¼ 1, that trend is reversed to give flames that become more stable as the size of the channel is increased.

Abstract