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

Abstract

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.

The differential diffusion effect of the intermediate species on the stability of premixed flames propagating in microchannels

“…Flames with q > 0.93 become stable in this case, indicating that only equivalence ratios close to the flammability limit suffer from unstable modes. A similar behaviour has been reported recently when studying the influence of the equivalence ratio in the flame stability in channels with a prescribed wall temperature and making use of an Arrhenius chemistry [50].…”

Low-dimensional modeling of flame dynamics in heated microchannels

“…This separation remains large even at the millimeter or sub-millimeter scales considered in present experiments. This evidence supports a multi-scale, or homogenization, approach (Papanicolau et al, 1978;Sánchez-Sanz et al, 2014;Castiglione et al, 2008;Bender and Orszag, 1999), in which the fast chemical process can be fairly represented through effective equations. The application of the present reduced model to a different experimental set-up is under investigation in order to assess present results and the model limitations.…”

An experimental study of methane–oxygen–carbon dioxide premixed flame dynamics in non-adiabatic cylinderical meso-scale reactors with the backward facing step

“…In this regard, it was observed in the experiments that increasing the dilution percent increased the flame inclination and its sensitivity to the imposed perturbations through the reactor wall and the internal flow field in the meso-scale reactors. These results can be somehow interpreted by considering the numerical results of Tsai [32], Kurdyumov et al [35,36], and Sánchez-Sanz et al [37]. According to Table 2, it can be concluded that, under condition in which the effective Lewis number is smaller than unity, the various flame dynamics can be promoted in the mesoscale reactors with inner diameters in the order of 50-250d l .…”