Nonlinear Flame Dynamics for Laminar Flame in a Co-fow Mixing Layer with Widely Varying Premixedness: Preferential Difusion and Stoichiometry Efects D. Bhatt a,b and S. R. Chakravarthy a a Department of Aerospace Engineering & National Centre for Combustion R&D, IIT Madras, Chennai, India; b Department of Aeronautical Engineering, Veltech Rangarajan Dr.Sagunthala R&D Institute of Science and Technology, Chennai, India ABSTRACT Intrinsic oscillations arising due to thermo-difusive mechanism have been studied numerically in the literature for laminar fames having Lewis number greater than unity. In the previous work, the Lewis numbers of fuel and oxidizer are assumed to be equal and the unburnt mixture to be in stoichiometric in proportion. Here, the study is extended to relax these assumptions. In the frst part of the present work, the Lewis numbers of fuel and oxidizer are varied individually keeping the overall mixture stoichiometric. The simulations are run for laminar fames stabilized near to a cold burner in a co-fow mixing layer generated by a fuel rich stream and oxidizer rich stream. The premixedness of the unburnt mixture is varied using a premixedness parameter defned in the previous work. This parameter characterizes upstream mixture-fraction gradients such that it approaches unity for zero mixture-fraction gradient and zero when the mixture-fraction gradient is infnite. Thus, upon ignition, diferent fame structures like, planar premixed fame, triple fames and an edge fame can be observed, as this parameter is varied from zero to unity. Simulations are run to study large time behavior for various values in this range of premixedness. Intrinsic oscillations in the total reaction rate (inte- grated over the domain) are observed for a combination of values of Lewis numbers of fuel and oxidizer. These are presented as series of contour plots of amplitude of total reaction rate onto a 2D plane with Lewis number of fuel on x-axis and Lewis number of oxidizer on y-axis for various values of premixedness. It is found that the contours of constant amplitude are nearly linear for difusion fame and partially premixed fames, thus validating the efective Lewis number defnition hitherto defned for difusion fames. Hence, this efective Lewis num- ber can determine the onset of oscillations and also their magnitude. But for weakly curved triple fames, there is signifcant variation, where the constant amplitude lines are curved. Also, the amplitude of oscilla- tions observed for weakly curved triple fames is much higher than that for other fames. Apart from intrinsic oscillations the shape and structure of triple fame is also studied. It is found that premixed fame branches of a triple fame are highly asymmetric about the trailing edge difusion fame. The change in the location of triple point is also observed. In the second part of the work, the assumption of the overall stoichiometric ratio being unity is relaxed to understand the efect of ARTICLE HISTORY Received 21 December 2019 Revised 7 August 2020 Accepted 14 August 2020 KEYWORDS Preferential difusion; diferential difusion; unequal Lewis numbers; thermo-difusive instability; large Lewis number oscillations; laminar fame; triple fame CONTACT D. Bhatt davidbhatt@gmail.com Department of Aerospace Engineering & National Centre for Combustion R&D, IIT Madras, Chennai, India COMBUSTION SCIENCE AND TECHNOLOGY https://doi.org/10.1080/00102202.2020.1811699 © 2020 Taylor & Francis Group, LLC