The effect of multicomponent diffusion on the stability of laminar flames

Abstract

The problem of the stability of the combustion of a three-component gas mixture (fuel, oxidiser and reaction products) is formulated and studied, taking into account the exact mutual diffusion between the components. Boundaries of two regions of thermodiffusive instability TDI-1 and TDI-2 for two mixtures with widely different diffusion properties are constructed, and a comparison is made with the binary theory. The effect of the initial composition of the mixture (a weak, stoichiometric or rich mixture) on the position of the boundaries of the monotonic and oscillatory instability is studied. The problem of the thermodiffusive instability TDI of a laminar flame in case of a substantial deficiency of fuel was studied in /1–8/. The mechanism of the onset of TDI proposed in /1/ for a Lewis number L > 1 ( L = D x ; D, x are the effective diffusion coefficient and thermal conductivity) connected with the distortion of the flame front was confirmed quantitatively in /2–5/. The perturbations increase monotonically in this case (region TDI-2, the terminology is that of /3, 4/). Another mechanism responsible for the appearance of TDI was suggested in /6/, and studied within the frame-work of the formulation of the problem in /2/, analytically and numerically in /7/. This instability is oscillatory and is realized when L < 1 (TDI-1). In /8/, where the assumption used in /2–5, 7/ of the quasistationary character of the velocity of perturbed flame front is not used, satisfactory matching of the boundaries of the zones TDI-1 and TDI-2 with those obtained in /3–5, 7/ was achieved (the difference does not exceed 8%).